2-Fabry-Pérot resonators to plasmonic nano-antennas.

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Enhanced single-molecule spectroscopy in highly confined optical fields: from k/2-Fabry–Pe ´ rot resonators to plasmonic nano-antennas Andreas M. Kern,* Dai Zhang, Marc Brecht,† Alexey I. Chizhik,‡ Antonio Virgilio Failla,§ Frank Wackenhut and Alfred J. Meixner* While single-molecule fluorescence from emitters with high quantum efficiencies such as organic dye molecules can easily be detected by modern apparatus, many less efficient emission processes such as Raman scattering and metal luminescence require dramatic enhancement to exceed the single-particle detection limit. This enhancement can be achieved using resonant optical systems such as plasmonic particles or nanoantennas, the study of which has led to substantial progress in understanding the interaction of quantum emitters with their electromagnetic environment. This review is focused on the advances in measurement techniques and potential applications enabled by a deeper understanding of fundamental optical interaction processes occurring between single quantum systems on the nanoscale. While the affected phenomena are numerous, including molecular fluorescence and also exciton luminescence and Raman scattering, the interaction itself can often be described from a unified point of view. Starting from a

Received 10th October 2013 DOI: 10.1039/c3cs60357a

single underlying model, this work elucidates the dramatic enhancement potential of plasmonic tips and nanoparticles and also the more deterministic influence of a Fabry–Pe ´rot microresonator. With the extensive knowledge of the radiative behavior of a quantum system, insight can be gained into nonradiative factors

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as well, such as energy transfer phenomena or spatial and chemical configurations in single molecules.

1 Introduction The aim of this article is to review the advances made in the last decade in enhancing optical single-molecule detection efficiency and spectroscopy in the condensed phase by controlling the electromagnetic environment of a single quantum emitter. The focus is placed on emitters very close to a metal nanoparticle or enclosed in an optical microresonator, and the common physical concepts are presented from a unified point of view. We will consider both luminescence of single emitters, such as single molecule fluorescence or luminescence of single quantum dots, and single-molecule Raman scattering since both processes often occur in parallel and the ways in which they are enhanced are closely connected.

¨bingen, Institute of Physical and Theoretical Chemistry, Eberhard Karls University Tu ¨bingen, Germany. E-mail: [email protected], Auf der Morgenstelle 18, 72076 Tu [email protected] † Present address: Zurich University of Applied Science (ZHAW), School of Engineering, Technikumstrasse 9, 8400 Winterthur, Switzerland. ‡ Present address: Group of Biophysics and Complex Systems, Third Institute of ¨ttingen, Friedrich-Hund-Platz 1, 37077 Go ¨ttingen, Physics, Georg-August University Go Germany. § Present address: UKE Microscopy Imaging Facility, University of Hamburg, Martinistrasse 52, 20246 Hamburg, Germany.

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Fluorescence and photoluminescence are essentially the same processes in which a molecule, an atom or, in more general terms, a quantum system is resonantly excited by absorption of a photon and in response emits again a photon of somewhat less energy or longer wavelength.1 This process can easily be described by a system with four electronic energy levels as outlined in Fig. 1(a). Initially, the system is in thermal equilibrium with its surroundings, described by state X0. By absorbing a photon it is electronically excited into the short lived level X1 0 – in the case of a molecule this can be a higher electronic level or a vibronic level. From X1 0 , it relaxes very quickly to the lowest electronically excited state X1. By reemission of a photon the system reaches level X0 0 which, in the case of a molecule, is a higher excited vibrational state of the ground state X0. From here the molecule quickly thermally equilibrates to reach level X0 and the process can start anew. For most molecules, the vibrational energies are so large that they are not excited in thermal equilibrium at room temperature. In this case, X0 is the electronic and vibrational ground state, whereas in X0 0 , one or more vibrations are excited. Similarly, the levels X1 and X1 0 reflect the first electronically excited state with additional vibrations excited in X1 0 . The latter state is described by the term ‘‘vibronic’’ which is a combination of the words ‘‘vibration’’ and ‘‘electronic’’. Of course, X1 0 can also be a

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higher electronic level than X1, but emission of fluorescence photons generally occurs from the first electronically excited state, an observation that is known as Kasha’s rule.2 Exceptions to Kasha’s rule are known,3 but as of yet are of minor interest to optical single-molecule spectroscopy. If the system is not a molecule but an atom, ion, or defect instead, local vibrational modes in the host material can absorb the excess energy. In this case, the process of electronic excitation by a photon and subsequent emission of a longer wavelength photon is denoted by the more general term luminescence, rather than fluorescence. More detailed information can be found in the excellent textbooks by J. R. Lakowicz1 or, on the single-molecule level, by C. Gell et al.4 Raman scattering is another type of inelastic optical interaction. It is a very inefficient process and only competes with photoluminescence when the incident radiation is close to or in

resonance with an electronic transition in the quantum system.5 Surface enhanced Raman scattering (SERS), however, discovered in the late 1970s, must today be accepted as the model system of an optical process which can be dramatically enhanced by controlling the local electromagnetic environment.6 Even so, detection of the first single molecule Raman scattering was reported, independently by Kneipp7 and Nie,8 more than ten years after the first proof of detection of single molecule fluorescence. In the years after, it became clear that to fully describe single molecule SERS, effects beyond electromagnetic enhancement were necessary,9 a current topic even today.10 Nevertheless, it has been shown that in many cases, a purely electromagnetic description accurately reproduces the observed Raman enhancement near plasmonic structures.6,11,12 An excellent detailed introduction to SERS is given in the textbook by E. Le Ru et al.13

Andreas M. Kern received his Diploma in Physics in 2007 from the University of Freiburg, Germany. For his PhD he joined the group of Prof. O. J. F. Martin at the Swiss Federal Institute of Technology (EPFL) in Lausanne, where he stayed until 2011. He is currently a postdoctoral research associate at the Institute of Physical and Theoretical Chemistry, Eberhard-Karls ¨bingen, Germany. University, Tu Andreas M. Kern His research interests include optical single-molecule spectroscopy, quantum–plasmonic hybrid systems and numerical modelling techniques.

Dai Zhang obtained her PhD in 2004 from Nanjing University, P. R. China. She is a permanent faculty member and leads the ‘‘Parabolic Mirror Nanooptics’’ subgroup at the Institute of Physical and Theoretical Chemistry, Eberhard-Karls ¨bingen, Germany. University, Tu Her main research topics are high-resolution optical spectroscopy, as well as developing new types of optical and electrical Dai Zhang microscopy techniques. Her research interests include investigating the morphology-related photophysical, photochemical and transport processes in organic optoelectric materials, the linear/non-linear optical behavior of plasmonic nanostructures, as well as the electromagnetic coupling in hybrid systems.

Marc Brecht studied Physics at ¨bingen and the the University Tu FU Berlin (1992–1997). For his PhD studies he joined the group of Prof. Lubitz at the TU Berlin (1997–2001). As a Post-Doc he moved into the field of optical spectroscopy and set up a lab for single-molecule spectroscopy in the group of Prof. Bittl at the FU Berlin. In 2009 he finished his habilitation. Also in 2009 he was awarded a Heisenberg Fellowship Marc Brecht and moved to the University of ¨bingen. In 2013 he was a visiting Professor at the UTT Troyes, Tu France. Since September 2013 he holds a position at the ZHAW Winterthur, Switzerland.

Alexey I. Chizhik received his BSc and MSc from St. Petersburg State University (St. Petersburg, Russia). He earned his PhD from the Institute of Physical and Theoretical Chemistry at the Eberhard-Karls University ¨bingen as a member of the Tu group of Prof. Alfred J. Meixner. He is currently an Alexander von Humboldt fellow in the group of Prof. Jo¨rg Enderlein at the Georg ¨ttingen, August University, Go Alexey I. Chizhik Germany. His research interest is mainly focused on the interaction of various types of quantum emitters with plasmonic nanostructures and its application in super-resolution imaging.

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Fig. 1 Jablonski scheme of a luminescent quantum system in free space (a, b) and under the influence of a photonic system (c). Disregarding absorption from X0 directly to X1, the scheme in (a) can be simplified to (b), omitting the transitional level X0 0 . Solid and dashed lines represent radiative and nonradiative processes, respectively.

Ordinary Raman scattering of radiation from a molecule, i.e. illuminated far from an electronic transition, also starts from the electronic ground state X0 in thermal equilibrium with its environment, or in an excited vibrational state X0 0 . Different from photoluminescence, the molecule does not necessarily need to be in resonance with the photon energy for excitation. In this case, X1 0 is not a truly excited electronic level but can be described by a linear combination of all excited states the molecule can occupy and hence is often termed a ‘‘virtual level’’. Furthermore, the reemission of light occurs directly from this virtual state X1 0 either to a vibrationally excited electronic ground state (X0 0 ), or directly to X0 in thermal equilibrium with the environment. In the first case the energy of the scattered photon is lower than the incident photon, the energy difference used to excite the molecular vibration. This process is called Stokes-shifted or ordinary Raman scattering. The second process, in which the scattered photon possesses a higher energy than the incident photon, is called anti-Stokes Raman scattering. This situation can occur if the sample temperature is higher and scattering molecules are present with excited vibrations X0 0 of the electronic ground state. Scattering can then also occur from a molecule in the state X0 0 , returning to X0 after scattering. The intensity ratio of the Stokes and anti-Stokes lines in a Raman spectrum is hence a

Alfred J. Meixner received his Diploma in Chemistry in 1984 and his PhD in 1988 from the Swiss Federal Institute of Technology (ETH). He was a Postdoc at IBM Almaden Research Center in California and earned his Habilitation in Physics from the University of Basel in Switzerland in 1996. His current position is Full Professor of Physical Chemistry and Director of the Institute Alfred J. Meixner of Physical and Theoretical ¨bingen, Chemistry at the Eberhard-Karls University in Tu Germany. His current research interests are optical singlemolecule spectroscopy and plasmon-enhanced near-field optical microscopy.


measure of the sample temperature and can be used as a spectroscopic means to deduce the sample temperature by Boltzmann’s law. To complete the picture we must add that one speaks of Rayleigh scattering when the incident and the scattered photon have the same energy, i.e. the scattering is elastic. Furthermore, it is important to note that the incident and the re-radiated electromagnetic waves have a fixed phase relationship or coherence. If the light incident on a molecule has nearly the same energy as an electronic transition in the molecule, the observed Raman signal can increase by many orders of magnitude.5,14 This effect is called resonance Raman scattering, its greater cross section due to a larger amplitude of the oscillating dipole induced in the molecule when excitation is near or in resonance. Besides a larger scattering cross section, resonance Raman scattering exhibits a different dependence on its surroundings than conventional Raman scattering. In contrast to fluorescence, Kasha’s rule does not apply to resonance Raman scattering – the Stokes and anti-Stokes lines appear centered around the full incident photon energy, even when excitation is close to higher electronic levels. In contrast to Raman scattering, the excitation and emission processes in photoluminescence are typically incoherent, the respective electromagnetic waves having no fixed phase relation. Instead, the state X1 has a distinct excited state lifetime describing the statistical decay probability of the excited quantum system. In addition, the emission of a photon during a luminescence process always starts from the electronically excited level we have termed X1, independent of the physical process that led the molecule to this level. Besides optical excitation, the molecule could be excited by impact, e.g. with an electron, or by electrons injected through the junction of a semiconductor leading to electroluminescence, e.g. in a lightemitting diode (LED). Due to these differences, luminescence emission and Raman scattering are often distinguished as different optical processes in the spectroscopic literature. However, the essential physical processes for absorption and emission of radiation by a molecule from an electromagnetic point of view are very similar in nature and the observable signal enhancement can thus be conveniently rationalized from a common point of view.15–17 The emission of radiation by a molecule close to a metallic surface may differ considerably from the emission of a molecule in free space, i.e. in a homogeneous dielectric medium

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with a dimension larger than the wavelength. This is true for both fluorescence emission and Raman scattering. It is convenient to distinguish three different regimes in terms of the distance between the molecule and the metallic object or interface.18,19 The first is when the molecule is so close to the metal surface that the molecular orbitals and the orbitals of the nearest metal atoms overlap. In this case we may no longer speak of an excited molecule, since an electronic excitation, although still localized around the molecule, is considerably modified from the ‘‘free molecule’’ and involves, to a considerable extent, the energy levels of the metal atoms. A suitable term to describe this situation, introduced by A. Otto in the context of explaining chemical enhancement of SERS, is ‘‘electronically excited surface complex’’.20 The next regime could be called the optical near-field regime, in which the molecule can still be described by its transition dipole moment. In this regime the distance between the molecule and the interface is small compared to the wavelength, typically on the order of 1–30 nm, but large compared to the dimensions of the molecule. The molecule’s orbitals are thus separated from the interface and its electronic structure is only weakly perturbed. Electromagnetically, the molecule can be viewed as infinitely small, its field distribution that of a point-like dipole emitter. In this regime, the dominant electric field component of the emitter is evanescent, decaying in free space as a function of the distance. At this scale, the metallic interface can significantly modify the boundary conditions of the electromagnetic field near the molecular emitter, exerting a strong influence on the absorption and emission of light. This important effect is a consequence of the ability of the metallic structure to couple to the near field of the emitter. If, at the same time, the metallic structure can efficiently couple to the optical far field, it can function as an antenna, both directing incident light to the near field of the emitter and promoting the radiation of emitted light. In particular, carefully tuned metallic nanoparticles with sizes on the order of the illumination or emission wavelength can assume this role and are therefore called plasmonic nano-antennas.21,22 In the third regime, the distance between the molecule and the interface is on the order of half a wavelength or greater. At this scale, the evanescent field has sufficiently decayed so that it can be neglected compared to the propagating far field emitted from the molecule. In this regime, the periodic character of the propagating waves can be used to evaluate frequency-specific influence on the radiative properties of a quantum system. As the magnitude of the far field is much lower than that of the near field, electromagnetic manipulation must utilize manywave interference to obtain effects comparable to those observed in the near field. For example, a far-field resonator would have to use highly reflective mirrors to induce the same effect on a molecule as a near-field plasmonic resonator. While the first regime listed above entails a modification of the electronic structure of the molecules under study, the enhancement effects in the near-field and far-field regimes are purely electromagnetic in nature. These are thus the cases we will study in detail in this article. A series of groundbreaking studies demonstrate the different effects of both near-field and

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far-field enhancement on single molecules using highly original approaches: in a first set of experiments, performed by Drexhage23 and later Barnes,24 thin dielectric layers were deposited on a silver mirror, acting as a spacer between the mirror and subsequently deposited molecular emitters.25 A Langmuir–Blodgett technique was used to control the thickness of the dielectric films with nanometer precision. More recently, Sandoghdar26 followed a similar, but more dynamic approach: the authors attached a metal sphere to the end of a glass fiber tip and approached it to a sample containing molecular emitters, using a feedback loop to control the distance on a nanometer scale (see Section 3.1 and Fig. 10). In this study, the oscillatory behavior in the far field and the evanescent nature of the near-field are reflected in the molecules’ lifetime, and the transition between the nearfield and far-field regimes can clearly be seen. In the following sections, we will review the different means to modify the radiative properties of a quantum system using a resonant system. In particular we will study the effect of nearfield resonators in the form of plasmonic particles as well as far-field l/2-microresonators. First, a theoretical model will be presented that can be used to describe the quantum system and the influence of an external resonant system. We will describe in detail how numerical simulations can be used to predict the behavior of different resonators on many types of quantum systems. In the second part of the paper, a review will be given of various experimental systems that have been used to take control of the radiative properties of a quantum system with the goal of maximizing the yield or the control of the system’s emission of light.

2 Unified theoretical description As discussed in the introduction, many processes contribute to the interaction of a quantum system with its background. In most cases, however, the electromagnetic enhancement caused by the optical properties of its environment is sufficient to accurately predict a quantum system’s behavior when placed in a modified medium. In particular, the electromagnetic enhancement of the wide range of emission processes listed in the introduction can be regarded from a unified point of view.15–17 In addition, easily accessible numerical simulations can then be used to predict and explain the enhancement potential. In this section, the established approaches for describing the electromagnetic enhancement of fluorescence, luminescence and conventional as well as resonance Raman scattering will be presented in a form that allows a single model to predict the enhancement of all of the emission processes. Combining the model with numerical simulations, Section 2.6 will show examples of this approach applied to model systems often used in experiments. Finally, a review of modern theoretical approaches going beyond electromagnetic effects will be given. 2.1

Rate equation model

In many cases, the quantum system, e.g. atom, molecule or quantum dot, can be described using a simple three-level system16 as


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shown in Fig. 1(b). Initially in the ground state X0, the system can be excited to the intermediate state X 0 following absorption of a photon at the excitation wavelength lexc. Immediately, the system decays nonradiatively to the excited state X1 from which it may decay radiatively, emitting a photon at the emission wavelength lem, or nonradiatively. The state X0 0 in Fig. 1(a) can be omitted if its lifetime is sufficiently short and absorption from state X0 directly to X1 is not considered. The rates g at which these processes occur are defined by two values: the rate constants k and the populations of the corresponding initial states. For example, excitation occurs at a rate of gexc = X0kexc while radiative decay occurs at grad = X1krad. Assuming that the intermediate state X 0 is occupied only for infinitesimally short times, one can assume X0 + X1 = 1. The excited state dynamics can then be written as a simple differential equation, : X1 = gexc gnr grad (1) = (1 X1)kexc X1(knr + krad).

(2)

Here, the dot denotes a derivative in time. 2.2

Fig. 2 (a) Schematic of an experimental setup showing incident light in blue, emitted light propagating to the far field in green and evanescent light in red. (b) The three subsets of the local density of optical states in the kx–ky-plane: incident modes rinc, evanescent emission modes rev and propagating, radiative emission modes rpr.

The first subset of modes, rinc, is important when describing the absorption of light by the quantum system. In energetic equilibrium with its surroundings, an optical system’s modes exhibit a population Z proportional to the incident illumination intensity I0. As the photon density f in the resonator is given by the mode density times the mode population,

Modifying the transition rates

The rate constants k are parameters of the quantum system and depend on its energetic and geometric structure and symmetry. However, they are also influenced by the coupling of the system to its environment. For example, the nonradiative decay through knr is often influenced by the coupling of a system to phonons in the matrix in which it is embedded. In optical studies, the most important interaction mechanisms are the absorption and emission of light by the quantum system. The coupling to the optical field is governed by the local density of optical states (LDOS) r(r) at the position of the quantum system. This value describes the density of states a photon can populate in an optical system before being absorbed by or after being emitted from a molecule located at the position r. Taking control of r(r), one can directly influence both the excitation and the radiative decay of the molecule under study. While the LDOS is purely a property of the space surrounding the quantum system, the fraction of those states actually participating in the different optical interactions depends on the transitions’ properties. Therefore, one must differentiate between three subsets of the LDOS,19,27 defined by their spectral and geometric properties as illustrated in Fig. 2. The first subset is given by the density of states that can be occupied by the incident light, i.e. those corresponding to the incident wavelength lexc and momentum. This fraction is called rinc. The next subset describes those states that can be populated by a photon emitted from the quantum system at the emission wavelength lem. Those with a real-valued wave vector component kz, i.e. in the observation direction, are in subset rpr. These are propagating states reaching the far field, the population of which can be detected in a far-field microscope. The states with complex-valued kz, on the other hand, are evanescent and decay exponentially in space, their energy in the far field tending to zero. This fraction is called rev.


f = rincZ,

(3)

the excitation rate constant kexc = fsabs is proportional to the excitation mode density rinc. Here, sabs is the quantum system’s absorption cross section. The influence of the resonant system on the excitation process can thus be described by the enhancement factor Fexc, Fexc ¼

kexc rinc I ¼ ¼ ; k0exc r0inc I0

(4)

where r0inc is the density of modes populated by the incident light in free space. As rinc is proportional to the photon density and thus the intensity I in the resonator, Fexc is equal to the intensity enhancement caused by the resonator at the position of the quantum system. The subset rpr describes modes that match the quantum system’s emission wavelength and reach the far field, and can thus be called radiative modes. They participate in and thus influence the radiative relaxation of the quantum system according to Fermi’s golden rule,28,29 krad ¼

2p jWfi j2 rpr ; h

(5)

where Wfi is the transition matrix element, i.e. the magnitude of the molecule’s emission dipole moment. In free space, only propagating modes exist and thus the radiative enhancement factor Frad (with respect to free space) can be introduced as Frad ¼

krad rpr ¼ ; k0rad r0em

(6)

where r0em = 4p2/( hclem2) is the emission LDOS in free space, with c the speed of light in vacuum. The modes in the subset rev are also populated by photons emitted from the quantum system. Their extent, however, is limited and thus the energy transferred to them cannot be

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detected from the far field. Instead, ohmic losses may induce non-radiative decay processes leading to energy dissipation in the near field.30 This dissipation process is called photonic quenching and so one can define the quenching factor Fq analogous to Frad,


Fq ¼

kq r ¼ ev : k0rad r0em

(7)

In free space or for macroscopic resonators, near-field effects can be neglected and rev = 0, thus there is no quenching. In this case, the decay rate of the quantum system’s excited state is only influenced by the radiative enhancement factor Frad, which is then called the Purcell factor.31 Named after E. M. Purcell, an early pioneer of quantum optics, this factor is the essential quantity for describing the interaction of a quantum system with the modes of its macroscopic surroundings. Without quenching, Purcell derived an expression for LDOS in a resonator depending on the latter’s quality factor and mode volume. While it must be modified to include quenching in systems involving the near field, the Purcell factor remains the decisive parameter for molecules in a l/2-microresonator. Inserting the modified rate constants into eqn (2), the excited state dynamics in a photonic system can be written as : X1 = (1 X1)k0excFexc X1k0rad(F1 (8) 0 1 + Frad + Fq). Here the fluorescence quantum yield of the molecule or quantum system in free space, F0 ¼

k0rad ; þ k0nr

k0rad

(9)

has been used. 2.3

Solving the rate equation

i.e. a ‘‘good’’ emitter.32 For weak illumination, kexc E 0, the total rate of radiative decay gtot then becomes gtot ¼ k0exc Fexc

Frad : Frad þ Fq

(12)

One can see that the emitted fluorescence is a product of the enhanced excitation and the quantum yield of the external resonant system, Fext ¼

Frad : Frad þ Fq

(13)

In this case, gtot is linearly dependent on the excitation rate and thus the incident illumination, and it is called the linear regime.16 For high incident intensities, on the other hand, the radiative decay rate becomes gtot = k0radFrad.

(14)

In this case, the fluorescence rate does not depend on the incident intensity as the quantum system is saturated in the excited state X1. In this regime, which is called the saturation regime,16 fluorescence enhancement is given solely by the enhancement of the radiative decay rate. Finally, for low fluorescence quantum yields F0 { 1, the total emission rate gtot tends towards gtot = k0excF0FexcFrad,

(15)

called the lossy regime.16 The three regimes introduced above are depicted in the enhancement map shown in Fig. 3. The horizontal axis shows the effect of fluorescence quantum yields F0 of the quantum system from 106 1 while the vertical axis represents the illumination strength in the range of k0exc/k0rad = 102 104, both axes scaled logarithmically. The enhancement regimes are shown for three typical resonant systems: a plasmonic particle,

As the incident intensity I and thus kexc can be a function of time, there is no general solution to the differential equation in eqn (8). In the case of continuous wave (CW) illumination, however, simplifications can be made leading to an intuitive analytical result. Under CW illumination, I is constant and the system will quickly reach equilibrium, so the steady-state : solution to eqn (8) can be considered: X1 = 0. The population probabilities for the excited states can then be written as X1 ¼

k0exc Fexc

þ

k0rad

k0 F exc1 exc : F0 1 þ Frad þ Fq

(10)

The measured value in most fluorescence applications is the intensity of light emitted from the quantum system, i.e. the total rate of radiative decay, gtot ¼ X1 k0rad Frad ¼

2.4

k0exc k0rad Fexc Frad : (11) k0exc Fexc þ k0rad F1 0 1 þ Frad þ Fq

Enhancement regimes

For certain cases, the result in eqn (11) can be further simplified. A first assumption is to assume a unity quantum yield F0 = 100%,

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Fig. 3 Enhancement regimes for three typical resonant systems: a plasmonic particle (solid lines), a plasmonic tip (dashed lines) and a l/2-microresonator (dotted lines). Enhancement factors taken from numerical simulations.16,33–35 Approximate fluorescence quantum yields of Au luminescence, diindenoperylene (DIP), mCherry and Rhodamine 6G (R6G) are shown as vertical dash-dotted lines.36–39


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Table 1 Enhancement factors for three typical resonant systems, taken from numerical calculations16,33–35

Resonant system

Fexc

Frad

Fq

Plasmonic particle Plasmonic tip l/2-microresonator

50 4 104 10

80 4 104 3

500 4 105 0

´rot microresonator. The a plasmonic tip and a l/2-Fabry–Pe corresponding enhancement factors F, shown in Table 1, were taken from numerical calculations.16,33–35 Quantum yields of selected quantum systems are drawn as dash-dotted lines,36–39 demonstrating which enhancement regimes can be expected under what conditions. For each regime, a radiative enhancement factor G can be defined, given by the total emission rate gtot of the resonatorcoupled system divided by the emission rate in free space (Fexc = 1, Frad = 1, Fq = 0). The factors G for the three regimes are given by linear regime: G = FexcFext,

(16)

saturation regime: G = Frad,

(17)

lossy regime: G = FexcFrad.

(18)

One can see that quenching, while a major limitation in the linear regime, plays no role in intrinsically inefficient40 emitters or in very strong illumination. 2.5

Raman scattering

Besides enhancing luminescence emission, resonant systems have been widely used to enhance Raman scattering, e.g. in surface-enhanced Raman scattering (SERS). As Raman scattering is a coherent process, the rate equation model presented above cannot be directly applied to predict its electromagnetic enhancement. Instead, a classical model describing the induced dipole moment in the Raman active molecule can be used. First, the polarizability a of the molecule is written as a(t) = a0 + a1 cos(Ot),

(19)

where a0 and a1 are constants and O describes the frequency of the molecule’s corresponding vibrational mode. Given an incident illumination of the form Einc(t) = E0eiot, the induced dipole moment p in the molecule is given by -

-

p(t) = a(t)Einc(t)

~0 eiot þ 1 a1 E ~0 eiðoþOÞt þ a1 E ~0 eiðoOÞt : ¼ a0 E 2

(20) (21)

The first term in eqn (21) can be interpreted as the elastic Rayleigh scattering originating from the molecule and the two remaining terms as the anti-Stokes and Stokes lines of its Raman spectrum. Considering only the Stokes term pS, the power emitted from the induced dipole can be written as -

PS = C|pS|2r 2

-

2

= Ca1 |ES| r


(22) (23)

where C = m ho2c2/(12p) is a constant. Immediately, one can see that the power PS is proportional to the incident intensity I p |E0|2 and the LDOS r. Coupling the Raman-active molecule to a resonant system, enhancement processes similar to those for luminescence enhancement occur. First, according to eqn (4), the incident intensity is enhanced by the factor Fexc. Second, the total LDOS will be modified by the factor Frad + Fq. As only the power radiated to the far field is of interest, the total Raman enhancement can thus be written as41 G = FexcFrad,

(24)

which is identical to the case of luminescence enhancement in the lossy regime. Interestingly, quenching plays no role in the enhancement of Raman scattering.42 This is due to the fact that there is no excited state population that can be depleted by nonradiative decay. While the population of evanescent modes does in fact occur, it has no effect on the energy scattered into the radiative modes. Instead, the additional, evanescent modes into which light can be scattered increase the scattering cross section of the molecule, drawing more energy out of the incident light. Only if scattering into nonradiative modes is so strong that the depletion of the incident light is no longer negligible will the Raman enhancement abate. In resonance Raman scattering, quenching plays a nonnegligible role as the coherent oscillations of the molecular dipole excited by the incident field are stronger and can couple to modes of a nearby plasmonic particle.16,42 Typically, the dephasing rate k of a molecular dipole excited near-resonantly is much larger than the decay rate constant ktot = knr + krad of the near-resonant transition’s excited state.43 In this case, the enhancement of the Raman process behaves similar to that of a fluorescent system with an intrinsic quantum yield of F0 = 2ktot/k.15 The enhancement of Raman scattering is thus equivalent to that of a lossy fluorophore, with conventional Raman scattering corresponding to the limit F0 - 0. 2.6

Quantitative calculations

To predict the influence of a resonant system on luminescence and Raman scattering, first the enhancement factors F must be determined. As presented in Section 2.2, the factors F depend on different subsets of the LDOS at the position of the quantum system and can thus be derived from electromagnetic calculations. Depending on the resonant system considered, either an analytical or a numerical approach can be taken. In this section, we will present calculations for a metal l/2-microresonator and a plasmonic particle. To compute the excitation enhancement Fexc, the simplest approach is often to calculate the intensity enhancement at the position of the quantum system for the appropriate illumination, e.g. plane wave illumination or a focused beam.44,45 The enhancement factor Fexc is then simply given by the ratio of the intensity with and without the resonator. The radiative and non-radiative decay enhancement factors Frad and Fq can be calculated through the power radiated from a resonator containing a quantum emitter and the power

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Fig. 4 Scheme for calculating the radiative and absorption enhancement factors Frad and Fq of a resonant system, represented here by a gold nanoparticle (GNP).

absorbed in the resonator, respectively. The power radiated to the far field, Prad, is obtained by integrating the Poynting vector S over a surface containing the emitter and the entire resonator, shown as Orad in Fig. 4, ð ~ rÞ dA: ~ Sð~ (25) Prad ¼ Orad

Analogously, the power absorbed by the resonator is given by the integral over a surface containing only the dissipative elements of the resonator, ð ~ rÞ dA: ~ Sð~ (26) Pabs ¼ Oabs

P0dip

emitted by a point dipole in free space can be The power computed analytically, P0dip ¼

o4 m 2 p ; 12pc

(27)

where m is the magnetic permeability of the surrounding media. The enhancement factors can then be computed using the relations Frad ¼

Prad ; P0dip

Fq ¼

Pabs : P0dip

(28)

2.6.1 Microresonator. One approach to modifying the ´rot photonic environment of a dipole emitter is the Fabry–Pe microresonator, shown schematically in Fig. 5. This resonator consists of two metallic mirrors of thickness d1 and d2, separated by a dielectric of thickness L. The resonance condition for these boundaries demands that all reflected waves interfere constructively inside the resonator. Neglecting phase shifts during reflection, this is fulfilled if the length L if the resonator is an integer multiple n of half the wavelength of light between the mirrors. For n = 1, 2, 3 this is shown in Fig. 5. A l/2´rot resonator with n = 1, i.e. the microresonator is a Fabry–Pe mirror separation L = l/2 is half the wavelength. For this simple system, the density of states can be calculated analytically, e.g. using the transfer matrix method (TMM).46 In this approach, the field between the mirrors is expanded in plane waves with propagation components kx,kz, for which the reflection and interference behavior can easily be computed. Two types of modes in the resonator can be excited.

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Fig. 5 Geometry of a Fabry–Pe ´rot microresonator comprising two mirrors of thickness d1 and d2 separated by a distance L. First, second and third order longitudinal modes are indicated by green curves.

First, modes corresponding to plane waves propagating inside the resonator have real-valued kx and kz and can be described by a real-valued angle y between the propagation direction and the z-axis. These modes are defined by the condition kzL = np and represent the modes shown in Fig. 5. For partially reflective mirrors, these modes can propagate to the far field where they can be detected, e.g. in a microscope. Second, surface plasmons propagating along the mirrors can be excited for complex kz, i.e. for evanescent waves in the resonator. These modes cannot propagate to the far field as illustrated in Fig. 2 but, instead, lead to quenching of a quantum system and are in general undesired. Due to the evanescent behavior of the plasmon modes, they can only be excited for emitters very close to the mirrors’ surfaces. As this represents an unfavorable condition for the excitation of propagating modes, emitters are generally separated from the mirrors’ surfaces with spacer layers and thus plasmonic quenching does not generally pose experimental problems47 and will not be considered in this paper. As the resonance of the microresonator’s modes is sensitive to the angle y of the exciting light, the difference between excitation LDOS rexc and propagating emission LDOS rpr becomes important. A plane wave impinging on the resonator at a single angle y can be enhanced in the resonator by up to a factor of Fexc = (1 R)1, where R is the mirrors’ reflectivity and (1 R)1 E 40 for silver mirrors. For the emission enhancement factor Fpr, however, one must consider that the quantum system can emit in a range of angles y, and so the enhancement factor must be integrated over the appropriate angular spectrum,35 leading to a value of Fpr r 3. Fig. 6 shows the enhancement factors Fexc and Fpr calculated for d1 = d2 = 50 nm using TMM simulations, placing the emitter in the center of the cavity. The wavelength-dependent permittivity of silver used to model the mirrors was taken from experimental data.49 Immediately, one can see the difference in the magnitude of the two enhancement processes. In addition, the emission enhancement shows a broadband behavior as a range of emission wavelengths can populate resonator modes with different angles y, whereas the incident plane-wave


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Fig. 6 Plane-wave excitation and radiative emission enhancement factors Fexc and Fpr calculated for a microresonator with silver mirrors of thickness d1 = d2 = 50 nm using the transfer matrix method, for fixed wavelength l = 450 nm (a) and fixed mirror spacing L = 325 nm (b). The emitter was assumed to be in the center of the resonator. The dashed lines show where the cavity is resonant for y = 0. Data taken from ref. 48.

illumination has a fixed angle. The resonance at n = 2 is not visible in either case as the emitter lies exactly in a node of the corresponding mode (see Fig. 5). One can see that the l/2microresonator offers the possibility to enhance or inhibit the radiative decay of a quantum system in a well-defined and predictable manner. 2.6.2 Plasmonic nanoparticle. Another way to modify the interaction of a quantum emitter with light is to place it in the proximity of a plasmonic nanoparticle. Depending on the size, shape and material of a metal nanoparticle, resonant modes can be excited around the particle corresponding to collective oscillations of the particle’s mobile electrons coupled to the surrounding electromagnetic field. For coinage metals such as gold and silver, these resonances can efficiently be excited at optical frequencies and can thus be used to modify atomic and molecular transitions with wavelengths in the optical regime. The influence of a plasmonic nanoparticle on a radiative quantum system is nearly the same as for the microresonator, with two very important differences: while the modes exploited in the microresonator are propagating, the plasmonic particle’s

modes are evanescent, and so the quantum emitter must be located very close to the plasmonic particle. The second difference, consequently, is the importance of quenching modes near the plasmonic particle, which are not only present but often limiting.50 For simple geometries such as spheres or ellipsoids,32,51 the enhancement factors F can be calculated using an analytical approach. For realistic structures such as the nanoparticle depicted in Fig. 7, no analytical solution can be found and one must instead rely on numerical methods. Many simulation approaches such as the finite element method (FEM),52,53 finite-difference time-domain (FDTD)54,55 and boundary element method (BEM)44,56 have been shown to yield accurate values for the plasmonic enhancement factors. For the particle shown in Fig. 7, the enhancement factors F were computed using numerical simulations with a surface integral equation (SIE) approach.16,57 The red and yellow colors around the particle show the intensity enhancement in its near field when illuminated by a plane wave traveling along k and polarized along E as shown in the figure. One can see that very close to the particle, the intensity distribution is influenced strongly by the exact realistic shape of the particle. Even at only ten

Fig. 7 Structure and illumination geometry of a realistic gold nanoparticle studied by numerical simulation. The yellow and red colors display the nearfield intensity enhancement according to the scale in Fig. 8. The hatched area represents the subspace plotted in Fig. 8 and 9. The inset shows the nearfield intensity maximum as a function of the illumination wavelength. Data taken from ref. 16.


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Fig. 8 Simulated enhancement factors Fexc, Frad and Fq around a realistic gold nanoparticle on resonance (l = lres) and for wavelengths Stokes-shifted to the blue (l = la) and red (l = ls) by 1000 cm1. Data taken from ref. 16.

2.7

Fig. 9 Simulated enhancement factor G in the linear and lossy regimes for on-resonant excitation. Scales are as in Fig. 8 but for the range specified in the panels. Data taken from ref. 16.

nanometers from the surface, however, the field becomes largely independent of the particle’s geometry.58 The inset shows the behavior of the maximum intensity in the plasmonic particle’s near-field, exhibiting a resonance at lres = 595 nm. The enhancement factors F in the plane through the particle’s center (see the hatched area in Fig. 7) obtained from the simulations are shown in Fig. 8. To account for the Stokes shift between excitation and emission wavelengths and the resulting differences in enhancement, the factors F were calculated on resonance (lres) and Stokes-shifted by 1000 cm1 towards blue (la) and red (ls) wavelengths. As both the quantum system and the plasmonic particle emit in a dipolar pattern, reciprocity states that the excitation and radiative emission enhancement are identical, Fexc = Frad, for a given wavelength. Once obtained, the enhancement factors F can be used to calculate the total emission enhancement G in the different enhancement regimes. For the linear and lossy regimes, the factor G is plotted in Fig. 9. One can see that the magnitude of enhancement can change dramatically between regimes, varying from approximately 10 in the linear regime to more than 105 in the lossy regime. Second, one can see that the highest enhancement in the linear regime is obtained by placing the quantum system a few nanometers from the particle surface, while in the lossy regime the enhancement rises monotonously when approaching the particle. This is due to plasmonic quenching that inhibits enhancement in the linear regime but not in the lossy regime.

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Beyond electromagnetic enhancement

At the single molecule level, very high enhancement factors are required in order to obtain measurable signals from Raman active molecules or inefficient luminescent particles. As can be seen above, this is often the case very close to a resonant plasmonic particle. In such configurations, effects beyond classical electromagnetic interaction can take place. For a molecule chemisorbed on the surface of a nanoparticle, i.e. its orbitals sharing electrons with the nanoparticle, direct charge transfer from the molecule’s excited state to the metal can occur, dramatically modifying the quantum system’s energetic configuration. For the most part, this charge transfer entails a rapid and nonradiative energy transfer to the metal, leading to extremely efficient fluorescence quenching.59 While this effect prevents the observation of fluorescence, it enables the detection of weaker underlying effects such as Raman scattering.9 Even without conductive charge transfer, molecules physisorbed on a metal surface can strongly interact with the latter’s electron spill-out: though restricted to a metal structure’s geometry by coulomb interaction with the background ions, the metal’s electrons experience a quantum repulsion originating in the Pauli exclusion principle which forbids two electrons from occupying the same quantum state at the same time.60 This leads to a finite electron probability density even outside the metallic structure’s geometry. Besides interacting with nearby molecules, the electronic spill-out beyond a metal structure’s surface can also modify the coupling to a second metallic object. This case is of great interest in the field of plasmonic enhancement as particle–dimer or tip– substrate systems are often used to create localized ‘‘hot spots’’ of very large field and LDOS enhancement. From a purely classical point of view, in a system containing a geometric singularity, e.g. two spheres touching at a single point, a plasmon traveling towards the singularity would never arrive, instead experiencing theoretically infinite energy compression and field enhancement.61 This intuitively unphysical case is contradicted by the finite spatial extent of the wave functions of the electrons taking part in the plasmon resonance. Instead, the metal’s dielectric


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response must be considered nonlocal – a metallic particle cannot be polarized by an electric field which spatially oscillates on a scale much smaller than the wavelength of a Fermi electron.62,63 This nonlocality effectively smoothes geometric singularities, leading to more physical behavior. In particular, it leads to the gradual formation of a charge transfer plasmon (CTP)64 tunneling between two nearly touching, approaching metallic particles. From this motivation, the coupling of subnanometer-spaced plasmonic particles has recently been the focus of intense study, using analytical approaches,60–63 a specular electron reflection model,65 time-dependent density functional theory,66 or experimental investigation.67 These nonlocality effects effectively limit the field enhancement as well as induce a blue-shift of the coupled system’s plasmon resonances and are reported to become observable for very small (o10 nm) particles separated by gaps up to about 3 Å. Deviations from classical expectations have also been reported for single nanoparticles, quantum effects modifying the structure’s dielectric function to lead to a significant blue shift of the observed plasmon resonance.68 For an excellent overview of recent developments in quantum plasmonics, the reader is referred to ref. 69 and the references therein. In recent years, a further type of plasmon-enhanced chemical interaction has been observed and exploited: ‘‘Hot’’ electrons, excited to non-thermal energy distributions in metals by the decay of surface plasmons, have been shown to readily take part in photochemical reactions at the boundaries of noble metal particles.70,71 At first glance this is surprising as the decay of electron–hole pairs excited in metals is accepted to be extremely fast. However, the ‘‘hot’’ electrons and holes excited by decaying plasmons have been shown to couple to the orbitals of adsorbed molecules, dwelling long enough to induce photochemical changes,72 or couple through the Schottky-barrier to conduction-band states of an adjacent semiconductor.73

3 Experimental applications In this section, demonstrations of the previously introduced electromagnetic modification will be presented in a range of different applications. First, examples will be given which confirm the presented theory and investigate the magnitude and predictability of the obtained effect. Then applications from different fields will be reviewed which utilize resonators to improve the efficiency of the studied process. 3.1

Dipole emitter near a metallic mirror

A straightforward and demonstrative approach to influencing the electromagnetic background of an emitter is to place it close to an infinitely large flat mirror. The effect on the emitter’s field can easily be calculated analytically in this case.74 In addition, it is possible to fabricate this geometry with extremely high accuracy, either using thin dielectric films to control the mirror– emitter separation,23 or by using nanometer-precision actuators to control the position of the mirror itself.26 In the case of the latter, as presented by Buchler et al., one single molecule can be


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Fig. 10 Lifetime of two single molecules modified by proximity to a metallic mirror. The horizontal axis is the distance between the molecule and the mirror. Solid and outlined symbols show behavior when approaching and retracting from the mirror. Solid lines are fits to the analytical model. Reprinted figure with permission from ref. 26: B. C. Buchler, T. Kalkbrenner, C. Hettich and V. Sandoghdar, Phys. Rev. Lett., 2005, 95, 063003. Copyright 2005 by the American Physical Society.

studied while actively changing the electromagnetic background, e.g. by varying the position of the mirror. Fig. 10 shows the measured lifetime of two single molecules (blue and red symbols, respectively) as a function of their distance to the mirror. The same influence was measured while approaching and retracting from the mirror (solid and outlined symbols, respectively), supporting the assumption that the observed effect was purely electromagnetic and demonstrating the mechanical stability of the experimental setup. The blue and red curves are fits to the analytical model obtained for this simple system. The fluorescence quantum yield describes the probability of emission of a photon upon return of an emitter from the excited state to the ground state. This value plays a key role in numerous applications involving fluorescence, photovoltaics, energy transfer or lasing.1 The quantum yield F0 can be represented as the ratio of the radiative rate krad to the total rate krad + knr as defined in eqn (9). Changing the distance to the mirror modifies the radiative transition rate of the molecule while leaving the nonradiative rate unaffected. Therefore, employing a theoretical model of the expected change of krad as a function of the mirror spacing and measuring the full excited-to-ground state transition rate krad + knr as determined in a lifetime measurement, one can extract the absolute value of a fluorophore’s quantum yield. As the lifetime modulation depth also depends on the orientation of the molecule with respect to the mirror, the obtained quantum yield is only as accurate as the orientation can be determined. Buchler et al. measured the emitter’s tilt angle y by recording the molecule’s angular emission pattern, resulting in a quantum yield of 0.9 o F0 o 1. 3.2

Emitters in a microresonator

Experimentally, a simple and efficient way of modifying the local mode density around an emitter is placing it inside an optical resonator.75 As introduced in the previous section, the spontaneous emission rate of an embedded emitter is proportional to the photonic mode density inside a resonator and the

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associated vacuum fluctuations of the optical field and can, hence, be either inhibited76 or resonantly enhanced.77 Since each cavity mode is associated with a specific wavelength, the coupling of a broadband emitter to a cavity results in a spectral and angular redistribution of its emission with respect to free space. Cavity-induced changes in the emission and absorption spectra have been theoretically predicted for a single molecule embedded within a spherical metal resonator.78,79 Experimentally, the effect of tuning a single molecule spectrum has been shown by Steiner et al., who investigated single isolated perylene dye molecules in a planar metal microcavity for different mirror spacings and, hence, for different electromagnetic mode structures.80 A few years later, Chizhik et al. used a metal subwavelength microcavity with variable mirror separation for tuning the ratio of distinct vibronic transition probabilities in a single molecule.81 Recently, using the same tunable cavity, the authors changed the electromagnetic field density around a single SiO2 nanoparticle, which resulted in controllable tuning of its fluorescence.82 In particular, a redistribution of the fluorescence spectrum and a modification of the excited state lifetime of the same individual SiO2 nanoparticle at different cavity lengths were demonstrated. Fig. 11(a) shows the energy diagram of the SiO2 nanoparticle and the resonator configuration for selecting the zero-, one- or two-phonon assisted recombination channels. In (b–d), the emission spectra of the nanoparticle embedded in the resonator are shown (red curves), demonstrating a redistribution of the recombination channels to account for the latter’s modified lifetimes. The black curves show a simulation obtained by redistributing the emission channels according to the calculated Purcell factor (blue dashed lines) of the microcavity, combined with an angular response function (blue dotted lines) representing the microscope objective’s apodization. In (e–g), the transient response of the same three configurations as in (b–d) is shown, exhibiting a distinct modification of the excited-state lifetime, evidence of an actual redistribution of the emission channels in contrast to spectral filtering of the emission. A tunable optical resonator, in combination with a radially polarized laser beam,83,84 has been also used for determining the three-dimensional orientation of single molecules immobilized inside a cavity with an accuracy of up to 11, exploiting the dipolar transition moment.85 Conversely, by placing an isotropic emitter inside a tunable cavity and scanning it through the focal area of a radially polarized laser beam, Gutbrod et al. could measure the emitter’s longitudinal position with a precision of l/60.86 While the cavity-induced modulation of the radiative rate has been broadly investigated for various types of emitters, a new application has recently emerged. It has been shown that by measuring the change of the fluorescence lifetime of a single fluorophore placed inside the cavity at different cavity lengths, one can extract the quantum yield of an individual fluorophore. This approach has been successfully applied for measuring the quantum yield of a single dye molecule.87 In the resonator geometry, the white-light transmission spectrum offers an intrinsic measure of the cavity’s resonance wavelength. In addition,

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Fig. 11 (a) Diagram of zero- (k0), one- (k1), and two- (k2) phonon-assisted recombination on a defect state in the band gap of SiO2. By tuning the mirror distance of the resonator, its modes can be set to match the freespace spectral emission maxima of zero-phonon and phonon-assisted transitions. (b–d) Measured (red curves; gray solid curves) and calculated (black curves) photoluminescence spectra of a single SiO2 nanoparticle placed inside the microresonator at the cavity lengths displayed in (a). The blue dotted curves are calculated angular sensitivity functions to account for the apodization of the microscope objective. The dashed blue curves represent the calculated Purcell factor. The grey shaded area shows a typical photoluminescence spectrum of a single SiO2 nanoparticle in free space. The inset in (c) shows the irreversible photoluminescence bleaching of the nanoparticle (scanning direction up-down). The dashed circle represents the area where the nanoparticle is centered. (e–g) Transients of the same single SiO2 nanoparticle inside the microresonator as used for recording the spectra, acquired at the same cavity lengths as in (b–d). All transients were fitted using a monoexponential decay function (red lines). Adapted from ref. 82.

the orientation of the emitter could accurately be assessed using higher order laser modes,85 allowing a precise determination of the quantum yield F0. It could be shown that different molecules immobilized on a SiO2 surface under the polymer film exhibit a broad distribution of quantum yield values, whereas radiative transition rates do not change significantly from molecule to molecule. This reflects the heterogeneous local nature of the host, which determines the nonradiative relaxation of an excited molecule via interaction with the local chemical environment.


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´rot microresonators, coupling of quantum Besides Fabry–Pe emitters to cavities in photonic crystals (PC) has recently become a topic of avid interest:88,89 in these resonators, interference at periodically distributed structural defects creates a band gap in the otherwise optically transparent material. This approach yields cavities with high quality factors Q B 104 and small mode volumes Vm B (l/n)3, resulting in a lifetime modification of >10, compared to o3 for metallic microcavities, due to the additional lateral confinement in the PC resonators. The fixed geometric configuration, however, makes PC cavities inherently difficult to tune over large wavelength ranges.90 The theoretical predictions in Section 2.6.1 and the experiments above assume that the emitter’s transition dipole moment is fixed in space. For rapidly rotating emitters, however, the LDOS must be averaged over all possible orientations, significantly changing the emitters’ coupling to the electromagnetic environment as compared to a fixed dipole moment. This was demonstrated by Chizhik et al., who placed a dye strongly diluted in various solvents between ´rot microcavity.91 It could be shown that the mirrors of a Fabry–Pe while for slow rotators, the average lifetime can exceed the free space lifetime for specific cavity configurations, the average lifetime for rapidly rotating molecules will always be smaller than the free space lifetime. 3.3 Light-harvesting systems in interaction with plasmonic nanostructures The function of photosynthetic complexes has attracted a large interest in the scientific community since the essential properties were discovered in the 18th century by Joseph Priestley among others (for a review see ref. 92). The high efficiency, robustness and fast energy transfer are astonishing achievements of evolution. Most photosynthetic complexes exhibit quantum yields close to 100% and energy yield around 60%.93 Combining light-harvesting systems with an additional antenna is a promising approach to enhance the photoinduced signal. A further advantage is that the nanoparticles can serve as both optical antenna and acceptors of photo-electrons. 3.3.1 Principles of light harvesting and excitation-energy transfer. The conversion of solar energy by photosynthetic proteins is one of the most important biological processes in which the transfer of excitation energy plays an essential role.94 The objective of photosynthesis is to produce energy in a stable, storable form generated by photo-induced charge separation occurring in the reaction center of the complexes. Photosynthetic reaction centers are surrounded by many chromophores, most of them chlorophylls. These chromophores are part of the protein complex containing the reaction center or are bound in additional protein complexes, dedicated to light harvesting, that funnel the harvested energy to the protein complexes holding the reaction center.95 All of these complexes have a high concentration of chromophores that absorb solar photons. The harvesting of light energy starts with the absorption of a photon; in a first step the absorbed energy is stored in short lived excited states of the chromophores. This excitation energy is then transferred to the reaction center, in which the charge separation takes place.95


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Fig. 12 Scheme of the excitation transfer in photosynthetic complexes. After absorption of a photon, the excitation energy is transferred to the reaction center, where the charge separation is initiated.

An illustration of the excitation energy transfer within a photosynthetic complex is shown in Fig. 12. In many cases, a deviation from the simple ladder-type transfer is observed; e.g. in Photosystem I, several chlorophylls form energetic traps that are, nevertheless, important for the excitation energy transfer of the whole complex (for reviews see ref. 96–98). Photosystem I is one promising candidate for bio-solar applications;99–101 within its extent of several nanometers, a photovoltage of B1 V can be generated102 with the quantum yield approaching 100%; as a consequence, nearly all photons absorbed are converted into a charge-separated state that is stable for B100 ms. The low potential reductant that is produced is favorable e.g. for H2 evolution.100 Despite the excellent quantum yield and large photovoltage, a single Photosystem I monolayer absorbs only a portion of the incident sunlight, limiting the applicability. As a consequence, coupling photosynthetic complexes to plasmonic nanoparticles serving as additional optical antennae is a promising approach. In recent experiments, Grimme et al. succeeded in transferring a photo-electron from a Photosystem I complex to a nearby nanoparticle.100 The nanoparticle was covalently linked via a molecular wire to the Photosystem I, enabling electron transfer and subsequent H2 production.100,101 In 2012, Mershin et al. reported that Photosystem I could function as a light harvester and charge separator in solar cells that were self-assembled on nanostructured semiconductors.103 Both publication nicely show that constructs of Photosystem I and nanostructures hold a certain potential for bio-solar applications, making them promising candidates despite the current disadvantages such as low efficiency and high production cost. 3.3.2 Interactions of plasmons with photosynthetic proteins. One of the first experiments on individual photosynthetic proteins in interaction with plasmonic nanostructures was carried out by Mackowski et al.104 The fluorescence emission of single light-harvesting peridinin–chlorophyll–protein (PCP) complexes deposited on a silver island film (SIF) was studied.105 On the SIF sample, fluorescence emission was enhanced up to 18-fold as a result of the interaction between the proteins and the plasmonic Ag-nanoparticles. The PCP complexes remained intact in the presence of the metal film and showed reduced photobleaching when deposited on the SIF sample, which was attributed to an increased number of photocycles. It was shown that the increase of the fluorescence emission due to the Ag nanoparticles was much stronger than the quenching. The observed fluorescence

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Fig. 13 Fluorescence intensity scans of Photosystem I (a), Photosystem I with Au nanoparticles (b), and Photosystem I on a silver island film (c): the three spectra for each sample type are from different individual Photosystem I complexes. Fluorescence intensity scans were acquired with an integration time of 2 ms per pixel. Plots (a–c) exhibit the same color scale ranging from 0 to 105 counts per second for comparison of the data. Spectra (d–f) were taken with an acquisition time of 40 s. Figure adapted from ref. 111.

enhancement was attributed mainly to an increase in absorption; nevertheless, the authors mentioned the possibility that the presence of Ag nanoparticles (NPs) can affect the energy transfer between the chromophores in the PCP complex. The combination of Photosystem I with plasmonic nanostructures has recently been investigated at the single-molecule level; the experiments were performed with Au NPs and on SIF106 as well as on Fischer patterns.107,108 Due to a low fluorescence quantum yield and strong photodegradation of Photosystem I at room temperature, the experiments were carried out at cryogenic temperatures.109,110 In Fig. 13, single-molecule experiments performed on Photosystem I, Photosystem I with Au NPs and Photosystem I on SIF are shown. The top row (a–c) shows fluorescence intensity scans taken under identical experimental conditions, plotted in the same scale. An intensity increase of the sample with Au NP or SIF is clearly visible. Representative emission spectra for each of the sample types are given in Fig. 13(d–f). The spectra are composed of characteristic contributions, appearing at different wavelength positions in the emission spectrum of Photosystem I (for further details see ref. 98 and 110). The increased signal-tonoise ratios in the spectra with Au NP and SIF reflect the enhanced fluorescence emission. Based on data taken from individual Photosystem I complexes or Photosystem I complexes in interaction with plasmonic nanostructures, the quantitative enhancement factors can be calculated. For individual

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Photosystem I complexes on SIF, the enhancement factors can be as large as 36, whereas the average enhancement factors are 2.2 on Au Fischer patterns, 5.7 on Ag Fischer patterns, 7 on SIF and 9 with Au NPs.108,111,112 In addition the shape of the emission spectrum is changed by the interaction of the proteins with the nanoparticles.108,111,112 Plasmonic interaction generally depends on the spectral resonance conditions of the chromophores and the surface plasmons, their intersystem distance and their relative orientations,16,32 as described in Section 2.6.2. In addition, the electric field close to the plasmonic NP is highly heterogeneous:58,113,114 apart from hot-spot positions with very high field enhancements, positions with vanishing field contributions are also found. These properties are in line with the observed broad distribution of enhancement values found on the different nanostructures.108,111,115 Among the most influential parameters on the enhancement factor is the emitter– particle separation.40 Depending on the properties of the emitter and the geometry of the particle, an enhancement maximum can be found for emitters located a few nanometers from the particle’s surface. Further away, the emitter–particle coupling becomes too weak and closer, the enhancement is canceled by quenching. Assuming a homogeneous distribution of proteins over the surface of the nanostructures, a large fraction of the proteins will be too far away from the nanostructures for enhancement. In the studied samples of Photosystem I with metallic nanostructures, however, nearly all Photosystem I complexes were affected by the nanostructures, indicating a preferred location close to the metal surfaces. The distance distribution of lightharvesting system II (LH2) deposited on top of a surface spread with Au-nanoparticles was recently investigated by Beyer et al. in detail.115 They, as well, found clear deviation from a homogeneous distribution of the LH2 complexes over the surface of the sample and thus concluded that LH2 tended to stick to the surface of the Au-nanoparticles. The distribution of the chromophores in these huge protein complexes is a further point that must be considered on the near-field scale. For example, the shape of a Photosystem I complex can be approximated by a cylinder measuring about 20 nm in diameter and approximately 5 nm in height.116 Assuming a similar distance-dependent enhancement as predicted theoretically, some chlorophylls of Photosystem I will be located very close to the metal surface and will thus be quenched; other chlorophylls will be in the distance range for maximum enhancement. As a consequence, the emission of certain chlorophylls within one Photosystem I complex will be switched off whereas the emission of other chlorophylls will be enhanced. This leads to a remarkable change in the shape of the emission spectrum of Photosystem I close to metallic NP compared to uncoupled Photosystem I.108,111,115 Until now, the orientation of the protein complexes in the investigated samples was not well-defined with respect to the plasmonic structures, but instead the protein complexes were assumed to be randomly distributed. As long as the pigments within a multichromophoric system are assumed to be non-interacting,


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taking the average over an ensemble of spectra should, in this case, result in uniform enhancement. In contrast, experiments with Photosystem I on plasmonic nanostructures showed a strong deviation from linear enhancement in their spectra.108,111,115 The reason for the deviation from uniform enhancement can be found in the coupling between the individual chromophores inside a single protein complex.111 The chlorophylls in photosynthetic proteins are coupled to ensure fast and efficient energy transfer to the reaction centers.96 The energy transfer efficiency depends on the spectral overlap, spatial separation, and orientation of the involved chromophores.117 Their specific arrangement leads to a characteristic set of transition rates and thus to preferred energy transfer pathways117,118 (see Fig. 14). The interaction between coupled chromophores and plasmonic ¨rster-interaction structures is, however, known to alter Fo distances between chromophores: in donor–acceptor pairs, ¨rster radius has been shown to increase from 8.3 nm to the Fo 13 nm as a result of plasmonic interaction.119 Assuming that this energy transfer process also takes place in photosynthetic proteins, the exciton distribution of almost all chromophores will change. It is thereby most likely that additional energy transfer pathways will be formed. One consequence is that chromophoric subunits not participating in the native state will be involved if the system is coupled to a plasmonic nanoparticle. In other cases, chromophores which were previously

Fig. 14 Scheme for visualization of the excitation energy transfer pathways in an antenna system of a photosynthetic protein. Without plasmonic interaction: specific coupling conditions between the chromophores lead to a characteristic set of transition rates indicated by gray arrows. Clocks indicate the respective excited state lifetimes. With plasmonic interaction: the set of transition rates is modified through plasmonic coupling, which is distance- and orientation-dependent indicated by the ruler and black arrows, respectively. Additional excitation energy transfer pathways (red) illustrate the origin of the modified system response. Figure adapted from ref. 111.


Review Article

almost non-fluorescent can become fluorescent. This situation is illustrated in Fig. 14. The change in the excitation energy transfer pathways represents an important modification of the original protein function. Based on the discussed experiments, the design of future bio-nano hybrids must carefully account for changes in the protein function caused by a modified density of optical states. 3.4

Luminescence

Many of the aspects used to describe molecular fluorescence also apply to photoluminescence (PL) in metals. In particular, the excitation and decay of electronic states is very similar in both processes: in metals, the states shown in Fig. 1 are not discrete molecular levels, but rather electronic states within the metal’s band structure. Similar to fluorescence, photons can be absorbed by the energetically lower electrons, exciting electron– hole pairs called ‘‘excitons’’. The excitons decay statistically after a characteristic lifetime, typically on the order of 100 fs.120 This extremely fast decay is dominated by nonradiative recombination of the electron–hole pairs which leads to an extremely low fluorescence quantum yield36 on the order of F0 B 1010. It is thus not surprising that photoluminescence of metals is typically regarded as an inherently weak process. At the same time, however, its low quantum yield places metal photoluminescence in the lossy enhancement regime introduced in Section 2.4. Plasmonic coupling of photoluminescence should therefore be able to dramatically enhance the luminescence yield. Intriguingly, coupling of photoluminescence emission from gold nanoparticles to the plasmonic modes of the very same particle is so efficient that the luminescence of even tiny particles can be easily detected.121 Since the last decade, several scientific groups have been studying the luminescence of gold nanorods (GNRs). The detection and imaging of GNRs’ one and two photon luminescence (1/2PL) has already been reported and exploited, e.g. in bio-oriented experiments.122–124 Plasmonic coupling has indeed been shown to dramatically enhance the luminescence intensity, increasing the quantum yield by several orders of magnitude.125,126 Only recently, however, has a deep insight into the physics underlying this process been achieved.126–129 One important discovery was that the 1PL in GNRs is a fully plasmon-mediated process, i.e. excitons can directly be excited by and decay into the plasmonic modes of a GNR. This is illustrated by the spectral link between the GNR’s plasmon modes and its photoluminescence: 1PL emission, usually spread across a wide spectral range at energies lower than excitation, exhibits two distinct maxima when originating from a GNR. These two emission peaks correspond to the GNR’s two plasmon modes – one along its short axis, i.e. the transversal plasmon mode (TPM), and the other along its main axis, i.e. the longitudinal plasmon mode (LPM). Luminescence in these two emission bands is spectrally separated and occurs around 520 nm when originating from the TPM (using e.g. 488 nm excitation) or in the red/infrared spectral range when originating from the LPM. The latter emission band’s maximum wavelength shows a clear dependence on the GNR’s aspect ratio, as does the maximum of the LPM130 (see Fig. 15(a)).

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Fig. 15 Comparison between elastic scattering and 1PL of GNRs. (a) Ensemble extinction spectra of GNRs in solution. Increasing the aspect ratio causes a monotonous red shift and intensity increase of the LPM with respect to the TPM. (b) 1PL luminescence spectra of individual GNRs excited by 488 nm laser light. Increasing the aspect ratio causes a monotonous red shift of the LPM while its intensity follows a Gaussian trend (blue line). Adapted with permission from F. Wackenhut, A. V. Failla and A. J. Meixner, J. Phys. Chem. C, 2013, 117, 17870–17877. Copyright (2013) American Chemical Society.

In fact, the spectral position of the emission bands is wholly independent of the excitation wavelength and can be regarded as a property of the GNR alone. These observations strongly suggest that the 1PL of GNRs is a plasmon-mediated process and thus imply a relation between plasmon-mediated 1PL and GNRs’ elastic scattering.131,132 Two important observations, however, point to significant differences between 1PL and elastic scattering at GNRs: first, a red/infrared 1PL band is visible even when exciting the GNR at its TPM wavelength (l B 520 nm). The energy difference between the modes indicates an inelastic process. In fact, the red/infrared 1PL emission from the LPM mode is substantially more intense than the light elastically scattered from the particle’s long axis. Second, the strength of the red/infrared 1PL band depends on the GNR’s aspect ratio in a non-monotonous way (see Fig. 15). Measurements show that the intensity of the red/ infrared 1PL band is maximal only when the GNR’s LPM shows a spectral overlap with the exciton band.129 This is possible only for limited values of the GNR’s aspect ratio. Though even more detailed investigation might be necessary, the results shown recently in ref. 129 demonstrate that 1PL in GNRs can be induced by two independent physical channels. The first can be understood as a 4 step process, i.e. (1) excitation of the TPM, (2) conversion of the TPM into an exciton, (3) decay of the exciton via LPM excitation and (4) luminescence emission of the LPM. The second originates from the direct excitation of an excitonic state by the incident light, followed by its decay into the LPM and, finally, the luminescence emission.

4 Enhanced Raman spectroscopy 4.1

Introduction

Raman spectroscopy is a non-destructive method which is able to provide a molecular ‘‘fingerprint’’ of unknown samples without the use of fluorescence markers. It finds popular applications in a wide range of fields ranging from fundamental research to industrial quality control. Using Raman spectroscopy for the purpose of trace analysis such as optical single molecule detection, however,

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may seem out of reach. While single-molecule detection based on fluorescence microscopy has been readily observed, microscopy based on Raman scattering poses much greater difficulties since the corresponding cross sections are orders of magnitude smaller than those of fluorescence processes: typical cross sections for Raman scattering by small, non-resonant molecules such as water lie in the range of 1030 cm2. For much larger dye molecules, e.g. malachite green isothiocynide (MGITC), Raman cross sections of up to about 1028 cm2 are expected. For resonant Raman scattering, i.e. when the energy of the incident light lies close to that of an electronic transition in the molecule, an increase in the effective cross section by another few orders of magnitude is observed for large dye molecules. To reach the single molecule detection level of around 1016 cm2, however, additional enhancement is required. As discussed in Section 2.5, the Raman process depends on the LDOS and, as a consequence, can directly be influenced by the presence of a nearby plasmonic nanoparticle or tip. More specifically, it can be calculated by considering the electromagnetic field enhancement due to the plasmonic nanosystem for the excitation as well as the scattering process. For Stokes shifts that are small compared to the plasmon resonance width, an enhancement factor proportional to the fourth power of the local electromagnetic field can be predicted theoretically41 and realized experimentally, giving rise to a giant enhancement in the Raman peak intensities. In addition, the background of the Raman spectra bears the signatures of fluorescence or photoluminescence emission from the excited molecules, or the molecule–substrate complexes, and hence further insight into the electronic transitions between defined energy states can be achieved. Surface-enhanced Raman spectroscopy (SERS) and tipenhanced Raman spectroscopy (TERS) represent the two most successful enhanced spectroscopic techniques, both allowing single molecule detection. In the last few years, several excellent review articles have extensively presented the progress in SERS and TERS – for a recent review see ref. 133. In the following section, we will shortly present some highlights of applying SERS and TERS to single-molecule spectroscopy and imaging and show some fascinating examples of TERS-spectroscopy of thin molecular films with ultra-high spatial resolution.


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4.2

SERS

The enhancement of Raman signals from pyridine, a small conjugated organic molecule, deposited on rough silver structures was first observed by Fleischmann et al. in 1974, who mistook it for an effect of the increased surface area.134 Three years later, Jeanmaire and Van Duyne135 as well as Albrecht and Creighton136 independently described SERS as the genuinely enhanced process it is. The latter group measured an enhancement factor of approximately 105 and mentioned the important role of surface plasmons in the excitation process. SERS is explained by the following three enhancing mechanisms:7,137–139 An electromagnetic enhancement due to the resonant excitation of localized surface plasmons in the metallic substrate (see Section 2.5). A chemical enhancement due to charge-transfer complexes between the molecules and the substrate.140,141 Resonant excitation in the case of surface-enhanced resonant Raman scattering (SERRS). 4.3

SERS substrates

4.3.1 Plasmonic materials. Classic SERS substrates are made from gold, silver, or copper. Gold is chemically more stable than silver and copper, both of which react with sulfur or oxygen under ambient conditions. The popularity of these materials in SERS largely arises from their localized surface plasmon resonances covering the spectral range from the visible to the near infrared, and from the corresponding electromagnetic enhancement useful for many practical applications. To achieve highly enhanced Raman scattering in the ultraviolet regime, other metallic materials, such as aluminum, rhodium and ruthenium, can be used. For example, no SERS enhancement can be observed for SCN anions adsorbed at Ag with an excitation wavelength of 325 nm radiation. However, an enhancement factor up to 102 was theoretically and experimentally demonstrated using ruthenium spheroids as a SERS substrate.142 In order to create and to tune the localized surface plasmon resonance, SERS substrates of different geometries, shapes and compositions have been reported. Covered nanospheres (film over nanospheres, FONs),143 3D metallic voids,144 core–shell particles,145–148 metallic superstructures and hybrid materials149–151 have been used for SERS. Detailed information about designing SERS-active plasmonic nanostructures is well-documented in a variety of recent review articles.152–155 4.3.2 Nonmetallic materials. Recently, SERS for non-metallic materials such as graphene,156–158 semiconductors159 and quantum dots has been demonstrated. The Raman enhancement from graphene is suggested to be due to charge transfer bands, resonances with interband transitions and p–p stacking interactions. Electromagnetic enhancement in this case is still under debate. It has therefore been suggested to be an ideal system to separately investigate the electromagnetic enhancement and the chemical enhancement processes.156 Compared to metals, graphene as a SERS substrate shows its advantages in terms of easy preparation, low cost, and biocompatibility. Though the Raman enhancing mechanisms are not yet fully clarified,


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these non-metallic materials demonstrate new concepts for SERS in research and application. For example, cavity-enhanced Raman scattering (CERS) was theoretically predicted by overlapping the Raman scattered radiation with local surface ‘‘hot spots’’ which are generated by whispering-gallery-modes.160 This concept was recently demonstrated by a remarkable SERS sensitivity achieved using TiO2-shell based spherical resonators.161 4.4

Single molecule SERS

Different approaches can be used to perform SERS at the single molecule limit. One of the classic methods is to prepare samples from an ultralow-concentration analyte solution (typically 109 mol l1 and lower) either by spin-coating it over a substrate with plasmonic nanostructures or by adding it to a colloidal solution of respective metal nanoparticles. After the molecules have adsorbed or chemisorbed on the surface of the metal structures, the optical measurement can be performed – for single-molecule detection and spectroscopy typically a confocal microscope is used. The single-molecule detection limit in SERS measurements becomes apparent if only a few of the nanoparticle clusters or structures show a Raman signal. Since the SERS signal predominantly originates in the so-called ‘‘hot spots’’, i.e. locations at which the strongest electromagnetic field enhancement on the metal nanostructures occurs,162 the statistical spatial distribution of the Raman scattered light is not a sufficient criterion to prove that the single-molecule limit has been reached. When single Raman scatterers are indeed resolved, the measured signal usually shows considerable intensity fluctuations along with reversible spectral changes such as line-splitting or the temporary appearance of additional lines, occurring on the time scale of seconds.163,164 Such fluctuations result from thermally activated molecular motion close to a local energy minimum and are frozen out at cryogenic temperatures;165 at ambient temperatures they disappear with higher concentrations due to ensemble averaging. A further relatively simple and general test for singlemolecule resolution uses a mixture of equal amounts of two different analytes which exhibit SERS signals that can easily be distinguished. By reducing the concentration of the analyte solution, the single-molecule limit can easily be assessed when hot spots start to appear that show only one of the two SERS signals. More comprehensive discussions can be found in ref. 10 and 166 and the citations therein. 4.5

TERS

To produce electromagnetic ‘‘hot spots’’ in a controlled manner, structures containing planar nanogaps have been tailored as effective SERS substrates.167 They can be obtained using nanofabricated, mechanically controllable break junction,168 templated electroplating,169 or electromigration.170 The nanometer sized gap between two closely spaced nanoparticles or nanoelectrodes supports efficient plasmonic coupling under optimized laser illumination and polarization conditions.171 Apart from the Raman spectroscopic information, nanogap configuration also allows one to study electronic transport or single molecule conductance in a combined manner.

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Tip enhanced Raman scattering (TERS), which is based on a similar fundamental principle of planar nanogap SERS, is a fairly new technique for optical near-field spectroscopy, in combination with scanning probe microscopy.172 In the TERS configuration, the geometry of a planar nanogap is turned upside down: one side of the nanogap is a planar surface supporting the sample molecules, the other side is a sharp metallic tip which approaches the molecules from above. Towards the end of the conical tip, its diameter becomes smaller than the penetration depth of the excitation field into the metal. The electric field of the incident radiation induces a plasmon oscillation at the tip. If the incident electric field is oriented along the tip’s axis, the ‘‘lightning rod’’ effect causes a maximum of the oscillating surface charge at the sharp apex and thus an enhanced local field. The response of the sample or substrate to the tip-enhanced field can be explained with the concept of mirror charges, i.e. the localized polarization below the apex acts back on the tip. The tip and the sample thus form a coupled oscillator system influencing each other via the electric field in the nanogap and the magnetic field around the tip apex. In the following, the field distribution formed by the nanogap will be referred to as the gap mode in analogy ´rot resonator. The gap width, to the field enclosed in a Fabry–Pe however, has a dimension much smaller than l/2. The field of TERS has recently been actively reviewed (see ref. 173–175). The concept of this type of optical microscopy technique, however, is much more general: any local optical material property that influences the gap mode becomes accessible in the far field signal and hence offers the possibility for performing optical spectroscopy with ultrahigh spatial resolution, be it elastic scattering,176 inelastic scattering such as Raman177 or tip enhanced luminescence.178,179 Confining farfield energy to the near field and, conversely, transmitting the local mode to the far field, gap modes play an essential role in both the excitation and detection of molecular optical processes. As has been shown in Section 2.2, a molecule in the gap can experience both an increased excitation intensity and emission efficiency with respect to free space. In particular, optical processes with a low quantum yield, e.g. luminescence from nanometer-sized protrusions on flat Au surfaces180 or Raman scattering from adsorbed molecules, can be enhanced by many orders of magnitude, enabling the spectroscopic detection of even a monolayer of small molecules adsorbed on atomically smooth single-crystalline metal surfaces.181–184 In addition, due to the tight confinement of the gap mode, the enhancement just below the tip apex is significantly higher than for the rest of the sample in the detection focus, which is one of the essential concepts of how near-field optical microscopy circumvents the optical diffraction limit. In this manner, near-field optical microscopy successfully achieves both high sensitivity and high resolution simultaneously: the sharp metal tip is the key element that turns TERS into a highly versatile combination of Raman spectroscopy and topographical scanning probe microscopy. Controlling the interaction of the tip and the sample is thus a crucial task that has to be mastered. In this sense, the field has greatly profited from the introduction

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of the piezoelectric tip–sample shear-force distance control, based on a resonantly vibrating quartz tuning fork that holds the tip over a non-conducting sample.185,186 Nowadays, atomic force microscopy (AFM)187 and scanning tunneling microscopy (STM)188 are also used to control the tip–sample distance. The optical resolution obtained in TERS is determined by the size of the tip apex and values down to 10–20 nm are achievable, a regime far below the Rayleigh limit of conventional optical microscopy.189 The TERS effect makes it possible to study a few or even single resonant molecules, as was shown by Steidtner et al. when measuring a single cresyl blue molecule.190 In the field of biomolecular studies, Bailo et al. recently obtained highly reproducible TERS spectra along a single poly(cytosine) RNA strand.187 In recent years, a booming development of TERS has been witnessed in a range of fields, e.g. single-molecule TERS imaging with a resolution of approximately 0.5 nm,191 locally induced catalytic processes monitored at the nanoscale with TERS,192 optical nanocrystallography193 and single-molecule junction conductance.194 4.5.1 Single molecule detection. Single-molecule sensitivity of TERS has been reported recently by several research groups. Pettinger et al. first demonstrated TER spectroscopy and microscopy of single brilliant cresyl blue (BCB) molecules adsorbed on gold single-crystal substrates.190 The Van Duyne group recently reported single molecule TERS of two isotopologues of Rhodamine 6G (R6G)195 and sub-nanometer molecular resolution images of copper phthalocyanine (CuPc) in an ordered adlayer on Ag(111) that are correlated with TERS.196 The Tian and Ren groups investigated the conductance of a single-molecule junction using ‘‘fishing-mode’’ TERS (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature.194 One of the milestones of single molecule TERS imaging is the recent publication by Zhang et al.191 Their work revealed Raman spectral imaging with a spatial resolution below one nanometer, resolving the inner structure and surface configuration of a single mesotetrakis(3,5-di-tertiarybutylphenyl)-porphyrin (H2TBPP) molecule (see Fig. 16). This is achieved by spectrally matching the resonance of the nanocavity plasmon to the molecular vibronic transitions, particularly the downward transition responsible for the emission of Raman photons. This matching is made possible by the extremely precise tuning capability provided by scanning tunneling microscopy. Experimental evidence suggests that the highly confined and broadband nature of the nanocavity plasmon field in the tunneling gap is essential for ultrahighresolution imaging through the generation of an efficient doubleresonance enhancement for both Raman excitation and Raman emission. Zhang et al. pointed out that their technique not only allows for chemical imaging at the single-molecule level, but also offers a new way to study the optical processes and photochemistry of single molecules. 4.5.2 Single-strand DNA. The application of TERS to the study of biological structures is of great interest due to the method’s high sensitivity, nanometer spatial resolution and the lack of complicated labeling – for recent reviews see ref. 173 and 197.


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Fig. 16 TERS mapping of a single H2TBPP molecule on Ag(111). (a) Representative single-molecule TERS spectra on the lobe (red) and centre (blue) of a flat-lying molecule on Ag(111). The TERS spectrum on the bare Ag about 1 nm away from the molecule is also shown, in black (120 mV, 1 nA, 3 s). (b) The top panels show experimental TERS mapping of a single molecule for different Raman peaks (23 23, B0.16 nm per pixel), processed from all individual TERS spectra acquired at each pixel (120 mV, 1 nA, 0.3 s; image size: 3.6 3.6 nm2). The bottom panels show the theoretical simulation of the TERS mapping. (c) Height profile of a line trace in the inset STM topograph (1 V, 20 pA). (d) TERS intensity profile of the same line trace for the inset Raman map associated with the 817 cm1 Raman peak, integrated over 800–852 cm1. Figure from ref. 191, reprinted with permission from Macmillan Publishers Ltd: Nature, 2013, 498, 82–86, copyright 2013.

Many biological molecules, such as DNA or RNA, have large cross-sections and allow extremely sensitive detection, even down to the single-molecule level. Bailo et al. have demonstrated experiments on a single-strand RNA homopolymer of cytosine (poly(C)) and have shown the potential of single-base identification, i.e. sequencing, when the tip is scanned along the strand.187 For their TERS measurements, RNA strands were immobilized on a mica surface via phosphate groups by adding Mg2+ cations to the sample. Fig. 17 shows the TERS measurements from 7 locations along the RNA strand. The TERS spectra were carefully assigned to the fingerprint of cytosine with significant fluctuations in band intensities and slight shifts of the band positions. The spectral intensity changes were likely related to a minuscule variation of the tip–sample distance. The shift of the band positions was suggested to be due to the molecular orientation differences in a fixed RNA strand with respect to the scanning tip. In addition, the polarization condition and the electromagnetic field distribution could also have induced the spectral fluctuations. The authors proposed


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Fig. 17 TERS experiment along an RNA strand. (a) Topography image showing seven adjacent spots corresponding to the positions of the TERS experiments and one additional spot for the reference measurement (position 8). (b) Raman spectra of the positions in (a). Copyright r 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Reprinted from ref. 187.

that this principle could be applied to the sequencing of peptides and proteins in a general fashion.198 4.5.3 Single-wall carbon nanotube and single-layer graphene sheet. Hartschuh et al. reported the first TERS investigation of single-wall carbon nanotubes (SWNTs) with a spatial resolution of better than 15 nm.199 The simultaneous near-field photoluminescence, Raman scattering and topography were imaged by detecting the light intensity transmitted by a band-pass filter and measuring the feedback signal200 of the tip–sample distance control electronics as a function of sample position. Fig. 18 demonstrates their recent work on correlated topography, tip-enhanced photocurrent imaging, and Raman G-band imaging of a SWNT fixed between two electrodes.201 The spatial pffiffiffi resolution of the Raman image is about 19 nm, which is 1= 2 smaller than that of the tip-enhanced photocurrent imaging. The authors pointed out that the technique can be used to study the direction and decay length of the band bending at the contacts of metallic carbon nanotube devices and to probe local variations in the electronic structure along a single tube that cannot be detected by classic confocal microscopy. Tip-enhanced spectroscopic methods have also been proven to be effective in identifying the number of graphene layers, as well as in studying intermolecular interactions, molecular orientations and symmetry distortions of graphene.202 Saito et al. have reported TERS analysis of n-graphene layers.203 A continuous

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Fig. 18 Antenna-enhanced imaging of photocurrent fluctuations along a single CNT. (a) Topography image. The drain and source electrodes appear at the top and at the bottom of the image. (b and c) Antenna-enhanced photocurrent and Raman G-band image. The scale bar is the same as in panel (a). Figure adapted from ref. 201.

change of intensity and peak shift of the G-band as well as the second order overtone of the defect mode 2D-band across the layer boundaries have been observed. The authors have suggested a near-field optical method to estimate the number of graphene layers and the thickness distribution in ultrathin graphite flakes. Snitka et al. have presented TERS imaging of single-layer and multilayer graphene flakes with tips fabricated by flattening an Au microwire.204 An enhancement of up to 106 of the D and G band of graphene was reported, and, by careful control of the tip–sample pressure, a spectral shift in the G band could be induced. In addition, the peak fluctuation in the 2D band also suggests the local stress distribution produced due to the graphene–tip interaction. The number of layers and stress analysis in 2-D imaging allow nondestructive identification of graphene layers, a critical issue for the evaluation of this material in future device applications. TERS signatures of pristine, defective, and contaminated/H-terminated single-layer graphene have also been reported by Stadler et al. with a resolution of less than 12 nm.205

5 Conclusions The coupling of plasmonic particles to quantum systems is a process that can be observed and exploited in an enormous range of applications. This is largely due to the fact that the interaction in these hybrid systems is so fundamental in nature. While a plethora of approaches has been presented to describe plasmonic coupling in many different experimental configurations, even a simple model based on a molecular rate equation and the optical density of states, coupled by the energy balance of the quantum emitter and the plasmonic structure, is sufficient to predict the behavior of most systems. The extent and possibilities of plasmonic coupling as predicted by the rate equation model are greater than often realized. While fluorescence enhancement has been accepted as a model application of plasmonic nanoparticles, the details and physical insight have only recently emerged, explaining why the same nanoparticle can enhance fluorescence from one molecule while completely quenching another. In addition, other physical processes such as the enhanced photoluminescence of gold nanoparticles, while physically quite different, can be considered with the same

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approach as a fluorescent dye molecule. Finally, having understood the optical interaction processes, even the problem of a molecule in the extremely enhanced and localized light field in the gap between a scanning near-field tip and a substrate can be tackled with the very same equations. While the goal of plasmonic enhancement is often to increase the measured signal to the maximum possible, other situations are encountered as well. For example, when coupled to lightharvesting complexes, a plasmonic particle can be used to quench states in certain energy transfer channels, e.g. those that might damage the complex or reduce its quantum efficiency. Along the same lines, an optical microresonator can be used to deterministically influence a quantum system: by modifying the radiative properties of a molecule in a reproducible and well-defined way, insight into the optically hidden nonradiative decay channels can ¨rster energy be gained, including electron–phonon coupling and Fo transfer. Insight into optical effects beyond classical electromagnetics combined with the extremely controlled and selective analytical and manipulatory tools given by scanning-probe techniques may give access to a multitude of new applications in the field of photochemistry at the single-molecule level. In conclusion, nanoscale resonant systems can be used to efficiently couple the measurable optical far field to the otherwise inaccessible near field. A thorough understanding of this process can explain phenomena in artificial and natural hybrid systems and elevate the spatial and spectral performance of modern micro-spectroscopy to new levels.

Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft (Grants ME 1600/5-3, ME 1600/12-2, ME 1600/13-1, DFG-SPP 1391 program) and its Heisenberg-Program (Grants BR 4102/1-1 and BR 4102/2-1).

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