Letter pubs.acs.org/NanoLett
Single Photons On-Demand from Light-Hole Excitons in StrainEngineered Quantum Dots Jiaxiang Zhang,† Yongheng Huo,*,† Armando Rastelli,‡ Michael Zopf,† Bianca Höfer,† Yan Chen,† Fei Ding,*,† and Oliver G. Schmidt† †
Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstraße 20, 01069, Dresden, Germany Institute of Semiconductor and Solid State Physics, Johannes Kepler University of Linz, Altenbergerstraße 69, Linz 4040, Austria
‡
S Supporting Information *
ABSTRACT: We demonstrate for the first time on-demand and wavelength-tunable single-photon emission from light-hole (LH) excitons in strain engineered GaAs quantum dots (QDs). The LH photon emission from tensile-strained GaAs QDs is systematically investigated with polarization-resolved, power-dependent photoluminescence spectroscopy, and photoncorrelation measurements. By integrating QD-containing nanomembranes onto a piezo-actuator and driving single QDs with picosecond laser pulses, we achieve triggered and wavelength-tunable LH single-photon emission. Fourier transform spectroscopy is also performed, from which the coherence time of the LH single-photon emission is studied. We envision that this new type of LH exciton-based single-photon source (SPS) can be applied to realize an all-semiconductor based quantum interface in distributed quantum networks [Phys. Rev. Lett. 2008, 100, 096602]. KEYWORDS: Light-hole, quantum dot, single-photon source, PMN-PT, coherence time, nanomembrane
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By employing the large discrepancy between the g-factors of electrons and LHs confined in a semiconductor quantum well, Kosaka and co-workers recently demonstrated a coherent state transfer from classical light onto electron spins, which represents a breakthrough toward an all-semiconductor quantum interface application.14,15 However, the study is based on a conventional semiconductor quantum well with valence-band ground-states (GS) of dominant heavy-hole (HH) type, whereas the LH states are lower in energy. Rapid relaxation of holes from the LH states to the HH states prevents formation of the LH exciton and therefore results in a difficulty of using this strategy to achieve the inverse electronspin-to-photon interconversion. In addition, a quantum well is a two-dimensional system and does not allow storage and generation of single qubits. In this context, a QD with LHGS appears very appealing, because it may allow for bidirectional interconversion between the states of single
n demand single-photon sources (SPSs) are key building blocks for many proposed quantum communication and computation technologies.1,2 The main reason is that single photons can be used as fast and stable quantum information carriers for coherent interconversion from one physical form of a quantum bit (qubit) to another. This plays a crucial role in building a quantum interface between spatially distant nodes in a distributed quantum network.3−5 So far, such a single-photon based quantum interface has been predominantly implemented in atomic systems.6−9 Self-assembled quantum dot (QD) based SPSs are one of the promising candidates for photonic quantum interface applications. First, QDs are excellent quantum emitters which can provide stable and bright single-photon emission and are compatible with standard semiconductor processing technologies.10,11 Second they can be simultaneously used as hosts for stationary qubits, i.e., electron and hole spins, which can be stored and manipulated locally.12 Hence, self-assembled QDs can be used to build an all-semiconductor based quantum interface, from which the polarization state of single photons and the electron spin states can be coherently interconverted.13 © XXXX American Chemical Society
Received: September 30, 2014 Revised: December 1, 2014
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stationary and flying qubits. To date, LH ground-state excitons have been realized in a pyramidal GaAs/AlGaAs quantum dotin-dot heterostructure;16 however, the studied system shows broad LH emission spectra (>2 meV) probably due to charged impurities. This hampers their practical applications in the field of quantum information technologies. Recently bright and sharp LH-exciton emission has been reported from GaAs/ AlGaAs QDs with valence-band ground state of dominant LH character,17 but until now triggered LH single-photon emission with tunable wavelength, which is critical for above-mentioned quantum interface applications, has not been demonstrated. In this Letter, we report on-demand and wavelength-tunable LH single-photon emission from tensile-strained GaAs QDs. The former property confirms the zero-dimensional character of our emitters, while the latter may allow interfacing different QDs for remote entangling electron-spins. First we present the typical photon emission from a LH neutral exciton, which is characterized by three orthogonally polarized emission lines. Then we demonstrate triggered LH single-photon emission via second-order time correlation measurements. We also show that the emission wavelength of LH single photons can be dynamically and precisely tuned in a wide range by means of an externally induced strain field. Moreover, the coherence time of the LH single photons was investigated via a single-photon interference with a Michelson interferometer. The sample studied in this work consists of a GaAs/AlGaAs QD heterostructure embedded in two prestressed InAlGaAs layers. Figure 1a shows the as grown sample structure fabricated by solid-source molecular beam epitaxy (MBE). It includes a 140-nm-thick QD-containing nanomembrane on top of a 100nm-thick Al0.75Ga0.25As sacrificial layer. In situ Ga droplet etching was used to create GaAs QDs within AlxGa1−xAs barriers (dot density = |1/2, ±1/2> and |Jlh, Jlh,z> = |1/2, ±1/2> (see Figure 1c). In the total spin angular momentum representation Jz = Jlh,z + Se,z, these single-particle states give rise to four LH exciton states | +1> = |+1/2,+1/2>, |+0> = |+1/2,−1/2>, |−0> = |−1/2,+1/2> and |−1> = |−1/2,−1/2>. The presence of spin exchange interaction due to the anisotropy of the confinement potential leads to the LH excitonic eigenstates17 |B1> = 1/√2 (|+1> − |−1>), |B2> = 1/√2 (|+1> + |−1>), |B3> = 1/√2 (|−0> + | +0>) and |D3> = 1/√2 (|−0> − |+0>). The eigenstates |B1>, | B2>, and |B3> are optically allowed bright excitonic states, while |D3> is an optically forbidden dark excitonic state. Provided that their relevant energy differences, known as fine structure splitting (FSS), are large, the bright eigenstates can be thought as electrical dipoles aligned along the [110], [1−10], and [001] crystal axis, respectively.26 These states will be mapped onto the linear polarizations of emitted photons in optical measurements. The B1 and B2 eigenstates can couple to in-plane linearly polarized light, and the corresponding emission lines are denoted here as I1 and I2, while the B3 eigenstate can only couple to out-of-plane (//z) linearly polarized light, denoted as I3. By means of a top PL collection (//z) and a cleaved-edge PL collection geometry (⊥z), the unique optical properties of LH exciton emission can be determined.16,17 In the present study the QDs were intentionally grown to have an asymmetric shape so that the optical axis of the QDs is tilted away from the [001] crystal axis by about 6°. Therefore, all three orthogonal bright excitonic emission lines are accessible with an objective placed normal to the sample surface (//z) (see more details in Supporting Information). For optical measurements, the studied QD device (see Figure 1b) was mounted in a helium flow cryostat, and all of the measurements were carried out at 5 K. The QDs were optically excited either by a frequency-doubled Nd:YVO4 continuous wave (CW) laser with a wavelength of 532 nm or by a pulsed diode laser with ∼50 ps pulse duration and 80 MHz repetition rate. The PL from the QD was collected normal to the sample surface by a 50× microscope objective with numerical aperture of 0.42 and then directed to a 50:50 nonpolarizing beam splitter which splits the PL into two beams. Each beam was sent to a 750 mm focal length spectrometer and detected by a liquidnitrogen-cooled Si-CCD camera with a spectral resolution of about ∼20 μeV. For the second-order time correlation measurements, two high efficiency single-photon counting avalanche photodiodes (SPADs) were used after each spectrometer. The electrical signals corresponding to the
Figure 1. (a) Schematic illustration of the sample structure, which contains a GaAs/AlGaAs QD layer sandwiched between In0.2Al0.4Ga0.4As stressor layers. (b) Sketch of a strain-tunable LH QD device. (c) Relevant single electron and hole levels in a single tensile-strained GaAs QD after the nanomembrane is released from the substrate. The energies of the LH and HH states are reversed by the biaxial tension provided by the stressor layers. The arrows show the optical selection rules which are responsible for the optically allowed transitions between confined states (eR and eL stand for righthand and left-hand circular polarized emission), and the numbers represent the projection of the total angular momentum of a single electron or a single hole along the quantization axis. B
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Figure 2. (a) Polarization-resolved spectra of the emission of LH neutral excitons confined in a representative tensile-strained GaAs QD. (b) Dependence of the integrated peak intensities of in-plane (I1 and I2) and out-of-plane (I3) emission lines on the excitation power, P. (c) Crosscorrelation measurements between I1 and I2 (upper panel) and between I1 and I3 (lower panel).
Figure 3. Triggered and wavelength tunable LH single-photon emission from dot A, which is excited with a pulsed diode laser at a repetition rate of 80 MHz. The center wavelength is at 632 nm. (a) Autocorrelation measurement of the in-plane polarized emission line of LH exciton at a wavelength of 792.93 nm when no electric field is applied to the PMN-PT. (b) Wavelength tuning of the LH single-photon emission as a function of Fp. A positive (negative) electric field corresponds to a compressive (tensile) strain field. (c, d) Autocorrelation measurements at different strainshifted wavelengths.
side, indicating that this line is mostly polarized in the vertical direction (see Supporting Information). To further characterize the LH exciton emission, we performed excitation powerdependent measurement, and the results are plotted in Figure 2b. The power law, I ∝ Pα, is used to fit the experimental data and exponents of the power-law (α), corresponding to the I1 + I2 and I3 are estimated to be 1.08 ± 0.02 and 1.17 ± 0.02. Such linear dependences on the excitation power support the assumption that all of the three emission lines stem from the neutral excitonic transitions. Further, cross-correlation measurements between each component were carried out. In Figure 2c, the lower panel shows the cross-correlation between the two in-plane polarized emission lines, I1 and I2, and the upper panel is the cross-correlation between the in-plane and the outof-plane polarized emission lines, I1 and I3. Pronounced
photon detection events on each SPAD were sent to a timecorrelated single photon counting module (PicoQuant, Picoharp300), from which a histogram g(2)(τ) as a function of delay time (τ) was recorded. In addition, by inserting a flip mirror in the optical path, the PL signal could be alternatively sent to a Michelson interferometer, which allows for measuring the coherence length of the LH single-photon emission. The QDs were first excited with the CW laser, and a polarization-resolved measurement was performed by rotating a λ/2 wave plate placed before a polarizer in front of the spectrometer. Representative polarization-resolved spectra of the LH photon emission from dot A are shown in Figure 2a. The plot clearly shows three linearly polarized emission lines, which are attributed to I1, I2, and I3 respectively. I3 becomes the dominant spectral feature when PL is collected from the sample C
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Figure 4. (a) Spectrum of the LH photon emission from dot B under pulsed excitation. The excitation intensity is 2.0 W/cm−2. The inset shows a sketch of the Michelson interferometer. (b) Visibility of the interference as a function of optical path difference ΔL and corresponding delay time τ. The insets show two interference fringes at ΔL = 0 mm and ΔL = 15 mm. (c) Lifetime of the in-plane (green dashed line in upper panel) and out-ofplane (violet dashed line in lower panel) polarized LH single-photon emission. Single exponential functions (solid red lines) are used to fit the experimental data.
Such a tunability is demonstrated with our LH QDs in Figure 3b. An electric field Fp was applied to the PMN-PT actuator to generate an in-plane strain field on the QDs, and a series of PL spectra from the dot A were recorded as a function of Fp. Compared to the emission wavelength at Fp = 0 kV/cm, the LH single-photon emission can be blue-shifted (red-shifted) when a positive (negative) electric field is applied.23−25 The total wavelength was tuned by 3 nm (6 meV in energy) as Fp varied from −20 to 40 kV/cm. In the meantime, autocorrelation measurements were carried out for the wavelength-shifted LH emission. At Fp = −20 and 40 kV/cm, we observe that the wavelength of the in-plane polarized LH exciton emission is shifted to 794.11 and 791.07 nm, respectively. The autocorrelation results at these two wavelengths are shown in Figure 3c and d, from which g(2)(0) = 0.22 ± 0.10 and 0.18 ± 0.08 are found. These results indicate that the optical properties of our LH SPS can be accurately tuned, and stable singlephoton emission during the tuning process is preserved. One of the important parameters for single-photon qubits is a long coherence time, as this not only determines the maximum number of possible qubit operations, but also the efficiency of the coupling between independent quantum systems.4,28 In our work, the coherence time of the triggered LH single-photon emission was studied by a Michelson interferometer inserted in the PL optical path. The interferometer consists of two arms. One arm is equipped with a fixed retroreflector, and the other one is equipped with a retroreflector, which is translated by the combination of a mechanical translation stage and a piezodriven stage (see Figure 4a). The interference signal was acquired by the spectrometer, and the integrated interference intensity as a function of the optical path difference ΔL was recorded.29−31 The results for a typical LH QD (dot B) are shown in Figure 4. Figure 4b shows interference fringe visibility of the in-plane polarized LH photon emission lines from dot B (green rectangular area is the integration window of 0.12 nm). The visibility is defined as V = (Imax − Imin)/(Imax + Imin), where Imax and Imin are the maximum and minimum integrated peak intensities from the interference fringes. We find that the visibility is as high as 95% at ΔL = 0 mm, see the left inset in Figure 4b and decreases with increasing ΔL. When ΔL = 15 mm, the interference fringes become extremely weak, and the visibility drops to about 9% (see the right inset of Figure 4b). After acquiring the visibilities at different positions defined by
symmetric antibunching dips at zero-time delay for both correlations g(2)(0) = 0.33 and 0.23 are found, which suggest that these emission lines are from different recombination channels of the same neutral exciton state in the studied QD. The above observations provide evidence of single photon emission from a LH neutral exciton with three emission lines. Of most interest is the pulsed optical operation of our device. This allows controlling the time of generation of excitons and thereby realizing triggered LH single-photon emission. In order to achieve this, the diode laser was used to drive the QDs, and meanwhile, the second-order time correlation function was measured under pulsed excitation. Figure 3a shows the unnormalized autocorrelation function of the in-plane polarized exciton emission, marked with the red arrow in Figure 3b at Fp = 0 kV/cm. The quantum nature of LH photon emission is revealed by the significantly suppressed peak at zero-time delay. The normalized correlation function for this peak shows g(2)(0) = 0.20 ± 0.12. This finite value represents a multiphoton emission probability that likely arises from the weak background emission from the sample rather than the QD. In addition to the vanishing peak at zero-time delay, periodic peaks in the autocorrelation measurement are also observed. This provides an unambiguous signature of a triggered LH single-photon emission. Interfacing remote nodes in a quantum network requires the initialization of qubits in the same state and the communication between nodes via indistinguishable single photons.27 Unlike natural atoms or ions, the inhomogeneous spectral broadening of self-assembled QDs is a big challenge for the realization of this scheme. In Figure 2 we have studied the preparation of excitons with LH ground state in a semiconductor QD, which can be used for the coherent state transfer from photon qubit to electron qubit, as already shown by Kosaka et al. with a quantum well.14 The basic ingredient for this scheme is a substantially larger in-plane g-factor for the LH state compared to the electron, which is the case for our QDs [B. J. Witek et al. (in preparation)]. In Figure 3a we demonstrate the triggered emission from such a LH exciton, which suggests that, after local manipulation of the spin qubit, the quantum information can be re-emitted as photon qubit.13 Now, one of the last elements for a LH-based reversible quantum interface is a “tuning knob” for the ground state energy levels of different QDs, which facilitates the direct state transfer from the photon qubit emitted by one QD to the electron qubit in another QD. D
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Notes
the mechanical stage, a coherence length (or coherence time) is extracted by fitting the curve with a single exponential function V(τ) = c exp(−τ/T2), where the coherence time T2 is found to be 34 ± 2.3 ps. From the inverse Fourier transform, a homogeneous line width of about 32 μeV is deduced. It is noticeable that the coherence time reported here for our LH single-photon emission is comparable to that of HH singlephoton emission from GaAs QDs grown by the similar droplet method.31,32 Time-resolved PL measurements were used to determine the lifetime T1 for the LH single-photon emission; see open symbols in Figure 4c. By fitting the decay curves with a single exponential decay function, a lifetime of 636 ps for inplane polarized and 670 ps for out-of-plane polarized emission lines was extracted. The similar radiative decay times for both polarized emission lines of the LH photon emission further support their common origin as discussed above. The large difference between T1 and T2 implies that a strong dephasing process occurs in the sample. This is ascribed to the nonresonant excitation, which leads to the creation of free charge carriers in the vicinity of the QD interacting with carriers confined in the QD.30,33 Recent studies have shown that such dephasing processes can be substantially suppressed under resonant excitation conditions, and thereby the coherence time can be drastically enhanced.34,35 In summary, we reported a novel triggered and wavelengthtunable LH single-photon source based on well-designed tensile-strained GaAs QDs. The LH photon emission, characterized by three linearly polarized emission lines, is explicitly verified in a series of PL measurements, including polarization-resolved, power-dependent, and photon-correlation measurements. The triggered single-photon emission from the LH neutral exciton is manifested in the second-order time autocorrelation measurements. By integrating the LH QDcontaining nanomembrane onto a PMN-PT actuator, we have achieved a broad wavelength tuning for the triggered LH singlephoton emission. The observed low multiphoton emission probabilities g(2)(0) values imply that the energy tuning by the externally induced strain fields does not deteriorate the emission properties. In addition, the coherence time of the LH single-photon emission has been studied by Michelson interferometry for the first time, and a coherence time of about 34 ps for our LH exciton is found. Further improvements, including growth optimization and resonant excitation, are necessary to improve the coherence of LH single photons from such QDs. We envision that, combining with an external magnetic field in the Voigt geometry, the demonstrated triggered LH SPS opens up a possibility to realize an allsemiconductor two-way quantum interface between single photons and spin qubits.13
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported financially by BMBF QuaHL-Rep (Contracts No. 01BQ1032 and 01BQ1034) and the European Union Seventh Framework Programme 209 (FP7/2007-2013) under Grant Agreement No. 601126 210 (HANAS). J.X.Z. was supported by China Scholarship Council (CSC, No. 2010601008). The authors thank S. Böttner and J. W. Deng for fruitful discussions, E. Zallo, P. Atkinson and Ch. Deneke for helpful discussions in sample growth and fabrication, and B. Eichler, R. Engelhard, and S. Harazim for the technical support.
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(1) O’Brien, J. L.; Furusawa, A.; Vučković, J. Nat. Photonics 2009, 3, 687. (2) Knill, E.; Laflamme, R.; Milburn, G. J. Nature 2000, 409, 46. (3) Duan, L. M.; Lukin, M. D.; Cirac, J. I.; Zoller, P. Nature 2001, 414, 413. (4) Kimble, H. J. Nature 2008, 453, 1023. (5) Gisin, N.; Thew, R. Nature 2007, 1, 165. (6) Cirac, J. I.; Zoller, P.; Kimble, H. J.; Mabuchi, H. Phys. Rev. Lett. 1997, 78, 3221. (7) Van Enk, S. J.; Cirac, J. I.; Zoller, P. Phys. Rev. Lett. 1997, 78, 4293. (8) Matsukevich, D. N.; Kuzmich, A. Science 2004, 22, 663. (9) Stute, A.; Casabone, B.; Brandstätter, B.; Friebe, K.; Northup, T. E.; Blatt, R. Nat. Photonics 2013, 7, 219. (10) Michler, P.; Kiraz, A.; Becher, C.; Schoenfeld, W. V.; Petroff, P. M.; Zhang, L.; Hu, E.; Imamoglu, A. Science 2000, 290, 2282. (11) Yuan, Z.; Kardynal, B. E.; Stevenson, R. M.; Shields, A. J.; Lobo, C. J.; Cooper, K.; Beattie, N. S.; Ritchie, D. A.; Pepper, M. Science 2002, 295, 102. (12) Imamoglu, A.; Awschalom, D. D.; Burkard, G.; DiVincenzo, D. P.; Loss, D.; Sherwin, M.; Small, A. Phys. Rev. Lett. 1999, 83, 4204. (13) Vrijen, R.; Yablonovitch, E. Phys. E 2001, 10, 569. (14) Kosaka, H.; Shigyou, H.; Mitsumori, Y.; Rikitake, Y.; Imamura, H.; Kutsuwa, T.; Arai, T.; Edamatsu, K. Phys. Rev. Lett. 2008, 100, 096602. (15) Kosaka, H.; Mitsumori, Y.; Rikitake, Y.; Imamura, H. Appl. Phys. Lett. 2007, 90, 113511. (16) Troncale, V.; Karlsson, K. F.; Pelucchi, E.; Rudra, A.; Kapon, E. Appl. Phys. Lett. 2007, 91, 241909. (17) Huo, Y. H.; Witek, B. J.; Kumar, S.; Cardenas, J. R.; Zhang, J. X.; Akopian, N.; Singh, R.; Zallo, E.; Grifone, R.; Kriegner, D.; Trotta, R.; Ding, F.; Stangl, J.; Zwiller, V.; Bester, G.; Rastelli, A.; Schmidt, O. G. Nat. Phys. 2014, 10, 46. (18) Seidl, S.; Kroner, M.; Högele, A.; Karrai, K.; Warburton, R. J.; Badolato, A.; Petroff, P. M. Appl. Phys. Lett. 2006, 88, 203113. (19) Ding, F.; Singh, R.; Plumhof, J. D.; Zander, T.; Křaṕ ek, V.; Chen, Y. H.; Benyoucef, M.; Zwiller, V.; Dörr, K.; Bester, G.; Rastelli, A.; Schmidt, O. G. Phys. Rev. Lett. 2010, 104, 067405. (20) Plumhof, J. D.; Krapek, V.; Ding, F.; Jons, K. D.; Hafenbrak, R.; Klenovsky, P.; Herklotz, A.; Dorr, K.; Michler, P.; Rastelli, A.; Schmidt, O. G. Phys. Rev. B 2011, 83, 121302R. (21) Kuklewicz, C. E.; Malein, R. N.; Petroff, P. M.; Gerardot, B. D. Nano Lett. 2012, 12 (7), 3761. (22) Trotta, R.; Atkinson, P.; Plumhof, J. D.; Zallo, E.; Rezaev, R. O.; Kumar, S.; Baunack, S.; Schroter, J. R.; Rastelli, A.; Schmidt, O. G. Adv. Mater. 2012, 24, 2668. (23) Kumar, S.; Trotta, R.; Zallo, E.; Plumhof, J. D.; Atkinson, P.; Rastelli, A.; Schmidt, O. G. Appl. Phys. Lett. 2011, 99, 161118. (24) Trotta, R.; Zallo, E.; Oritix, C.; Atkinson, P.; Plumhof, J. D.; Brink, J. V. D.; Rastelli, A.; Schmidt, O. G. Phys. Rev. Lett. 2012, 109, 147407.
ASSOCIATED CONTENT
S Supporting Information *
Expanded discussion on the sample growth, QDs morphology investigation, and photon collection efficiency of the LH photon emissions. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. E
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Single Photons On-Demand from Light-Hole Excitons in StrainEngineered Quantum Dots Jiaxiang Zhang,† Yongheng Huo,*,† Armando Rastelli,‡ Michael Zopf,† Bianca Höfer,† Yan Chen,† Fei Ding,*,† and Oliver G. Schmidt† †
Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstraße 20, 01069, Dresden, Germany Institute of Semiconductor and Solid State Physics, Johannes Kepler University of Linz, Altenbergerstraße 69, Linz 4040, Austria
‡
S Supporting Information *
ABSTRACT: We demonstrate for the first time on-demand and wavelength-tunable single-photon emission from light-hole (LH) excitons in strain engineered GaAs quantum dots (QDs). The LH photon emission from tensile-strained GaAs QDs is systematically investigated with polarization-resolved, power-dependent photoluminescence spectroscopy, and photoncorrelation measurements. By integrating QD-containing nanomembranes onto a piezo-actuator and driving single QDs with picosecond laser pulses, we achieve triggered and wavelength-tunable LH single-photon emission. Fourier transform spectroscopy is also performed, from which the coherence time of the LH single-photon emission is studied. We envision that this new type of LH exciton-based single-photon source (SPS) can be applied to realize an all-semiconductor based quantum interface in distributed quantum networks [Phys. Rev. Lett. 2008, 100, 096602]. KEYWORDS: Light-hole, quantum dot, single-photon source, PMN-PT, coherence time, nanomembrane
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By employing the large discrepancy between the g-factors of electrons and LHs confined in a semiconductor quantum well, Kosaka and co-workers recently demonstrated a coherent state transfer from classical light onto electron spins, which represents a breakthrough toward an all-semiconductor quantum interface application.14,15 However, the study is based on a conventional semiconductor quantum well with valence-band ground-states (GS) of dominant heavy-hole (HH) type, whereas the LH states are lower in energy. Rapid relaxation of holes from the LH states to the HH states prevents formation of the LH exciton and therefore results in a difficulty of using this strategy to achieve the inverse electronspin-to-photon interconversion. In addition, a quantum well is a two-dimensional system and does not allow storage and generation of single qubits. In this context, a QD with LHGS appears very appealing, because it may allow for bidirectional interconversion between the states of single
n demand single-photon sources (SPSs) are key building blocks for many proposed quantum communication and computation technologies.1,2 The main reason is that single photons can be used as fast and stable quantum information carriers for coherent interconversion from one physical form of a quantum bit (qubit) to another. This plays a crucial role in building a quantum interface between spatially distant nodes in a distributed quantum network.3−5 So far, such a single-photon based quantum interface has been predominantly implemented in atomic systems.6−9 Self-assembled quantum dot (QD) based SPSs are one of the promising candidates for photonic quantum interface applications. First, QDs are excellent quantum emitters which can provide stable and bright single-photon emission and are compatible with standard semiconductor processing technologies.10,11 Second they can be simultaneously used as hosts for stationary qubits, i.e., electron and hole spins, which can be stored and manipulated locally.12 Hence, self-assembled QDs can be used to build an all-semiconductor based quantum interface, from which the polarization state of single photons and the electron spin states can be coherently interconverted.13 © XXXX American Chemical Society
Received: September 30, 2014 Revised: December 1, 2014
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stationary and flying qubits. To date, LH ground-state excitons have been realized in a pyramidal GaAs/AlGaAs quantum dotin-dot heterostructure;16 however, the studied system shows broad LH emission spectra (>2 meV) probably due to charged impurities. This hampers their practical applications in the field of quantum information technologies. Recently bright and sharp LH-exciton emission has been reported from GaAs/ AlGaAs QDs with valence-band ground state of dominant LH character,17 but until now triggered LH single-photon emission with tunable wavelength, which is critical for above-mentioned quantum interface applications, has not been demonstrated. In this Letter, we report on-demand and wavelength-tunable LH single-photon emission from tensile-strained GaAs QDs. The former property confirms the zero-dimensional character of our emitters, while the latter may allow interfacing different QDs for remote entangling electron-spins. First we present the typical photon emission from a LH neutral exciton, which is characterized by three orthogonally polarized emission lines. Then we demonstrate triggered LH single-photon emission via second-order time correlation measurements. We also show that the emission wavelength of LH single photons can be dynamically and precisely tuned in a wide range by means of an externally induced strain field. Moreover, the coherence time of the LH single photons was investigated via a single-photon interference with a Michelson interferometer. The sample studied in this work consists of a GaAs/AlGaAs QD heterostructure embedded in two prestressed InAlGaAs layers. Figure 1a shows the as grown sample structure fabricated by solid-source molecular beam epitaxy (MBE). It includes a 140-nm-thick QD-containing nanomembrane on top of a 100nm-thick Al0.75Ga0.25As sacrificial layer. In situ Ga droplet etching was used to create GaAs QDs within AlxGa1−xAs barriers (dot density = |1/2, ±1/2> and |Jlh, Jlh,z> = |1/2, ±1/2> (see Figure 1c). In the total spin angular momentum representation Jz = Jlh,z + Se,z, these single-particle states give rise to four LH exciton states | +1> = |+1/2,+1/2>, |+0> = |+1/2,−1/2>, |−0> = |−1/2,+1/2> and |−1> = |−1/2,−1/2>. The presence of spin exchange interaction due to the anisotropy of the confinement potential leads to the LH excitonic eigenstates17 |B1> = 1/√2 (|+1> − |−1>), |B2> = 1/√2 (|+1> + |−1>), |B3> = 1/√2 (|−0> + | +0>) and |D3> = 1/√2 (|−0> − |+0>). The eigenstates |B1>, | B2>, and |B3> are optically allowed bright excitonic states, while |D3> is an optically forbidden dark excitonic state. Provided that their relevant energy differences, known as fine structure splitting (FSS), are large, the bright eigenstates can be thought as electrical dipoles aligned along the [110], [1−10], and [001] crystal axis, respectively.26 These states will be mapped onto the linear polarizations of emitted photons in optical measurements. The B1 and B2 eigenstates can couple to in-plane linearly polarized light, and the corresponding emission lines are denoted here as I1 and I2, while the B3 eigenstate can only couple to out-of-plane (//z) linearly polarized light, denoted as I3. By means of a top PL collection (//z) and a cleaved-edge PL collection geometry (⊥z), the unique optical properties of LH exciton emission can be determined.16,17 In the present study the QDs were intentionally grown to have an asymmetric shape so that the optical axis of the QDs is tilted away from the [001] crystal axis by about 6°. Therefore, all three orthogonal bright excitonic emission lines are accessible with an objective placed normal to the sample surface (//z) (see more details in Supporting Information). For optical measurements, the studied QD device (see Figure 1b) was mounted in a helium flow cryostat, and all of the measurements were carried out at 5 K. The QDs were optically excited either by a frequency-doubled Nd:YVO4 continuous wave (CW) laser with a wavelength of 532 nm or by a pulsed diode laser with ∼50 ps pulse duration and 80 MHz repetition rate. The PL from the QD was collected normal to the sample surface by a 50× microscope objective with numerical aperture of 0.42 and then directed to a 50:50 nonpolarizing beam splitter which splits the PL into two beams. Each beam was sent to a 750 mm focal length spectrometer and detected by a liquidnitrogen-cooled Si-CCD camera with a spectral resolution of about ∼20 μeV. For the second-order time correlation measurements, two high efficiency single-photon counting avalanche photodiodes (SPADs) were used after each spectrometer. The electrical signals corresponding to the
Figure 1. (a) Schematic illustration of the sample structure, which contains a GaAs/AlGaAs QD layer sandwiched between In0.2Al0.4Ga0.4As stressor layers. (b) Sketch of a strain-tunable LH QD device. (c) Relevant single electron and hole levels in a single tensile-strained GaAs QD after the nanomembrane is released from the substrate. The energies of the LH and HH states are reversed by the biaxial tension provided by the stressor layers. The arrows show the optical selection rules which are responsible for the optically allowed transitions between confined states (eR and eL stand for righthand and left-hand circular polarized emission), and the numbers represent the projection of the total angular momentum of a single electron or a single hole along the quantization axis. B
dx.doi.org/10.1021/nl5037512 | Nano Lett. XXXX, XXX, XXX−XXX
Nano Letters
Letter
Figure 2. (a) Polarization-resolved spectra of the emission of LH neutral excitons confined in a representative tensile-strained GaAs QD. (b) Dependence of the integrated peak intensities of in-plane (I1 and I2) and out-of-plane (I3) emission lines on the excitation power, P. (c) Crosscorrelation measurements between I1 and I2 (upper panel) and between I1 and I3 (lower panel).
Figure 3. Triggered and wavelength tunable LH single-photon emission from dot A, which is excited with a pulsed diode laser at a repetition rate of 80 MHz. The center wavelength is at 632 nm. (a) Autocorrelation measurement of the in-plane polarized emission line of LH exciton at a wavelength of 792.93 nm when no electric field is applied to the PMN-PT. (b) Wavelength tuning of the LH single-photon emission as a function of Fp. A positive (negative) electric field corresponds to a compressive (tensile) strain field. (c, d) Autocorrelation measurements at different strainshifted wavelengths.
side, indicating that this line is mostly polarized in the vertical direction (see Supporting Information). To further characterize the LH exciton emission, we performed excitation powerdependent measurement, and the results are plotted in Figure 2b. The power law, I ∝ Pα, is used to fit the experimental data and exponents of the power-law (α), corresponding to the I1 + I2 and I3 are estimated to be 1.08 ± 0.02 and 1.17 ± 0.02. Such linear dependences on the excitation power support the assumption that all of the three emission lines stem from the neutral excitonic transitions. Further, cross-correlation measurements between each component were carried out. In Figure 2c, the lower panel shows the cross-correlation between the two in-plane polarized emission lines, I1 and I2, and the upper panel is the cross-correlation between the in-plane and the outof-plane polarized emission lines, I1 and I3. Pronounced
photon detection events on each SPAD were sent to a timecorrelated single photon counting module (PicoQuant, Picoharp300), from which a histogram g(2)(τ) as a function of delay time (τ) was recorded. In addition, by inserting a flip mirror in the optical path, the PL signal could be alternatively sent to a Michelson interferometer, which allows for measuring the coherence length of the LH single-photon emission. The QDs were first excited with the CW laser, and a polarization-resolved measurement was performed by rotating a λ/2 wave plate placed before a polarizer in front of the spectrometer. Representative polarization-resolved spectra of the LH photon emission from dot A are shown in Figure 2a. The plot clearly shows three linearly polarized emission lines, which are attributed to I1, I2, and I3 respectively. I3 becomes the dominant spectral feature when PL is collected from the sample C
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Letter
Figure 4. (a) Spectrum of the LH photon emission from dot B under pulsed excitation. The excitation intensity is 2.0 W/cm−2. The inset shows a sketch of the Michelson interferometer. (b) Visibility of the interference as a function of optical path difference ΔL and corresponding delay time τ. The insets show two interference fringes at ΔL = 0 mm and ΔL = 15 mm. (c) Lifetime of the in-plane (green dashed line in upper panel) and out-ofplane (violet dashed line in lower panel) polarized LH single-photon emission. Single exponential functions (solid red lines) are used to fit the experimental data.
Such a tunability is demonstrated with our LH QDs in Figure 3b. An electric field Fp was applied to the PMN-PT actuator to generate an in-plane strain field on the QDs, and a series of PL spectra from the dot A were recorded as a function of Fp. Compared to the emission wavelength at Fp = 0 kV/cm, the LH single-photon emission can be blue-shifted (red-shifted) when a positive (negative) electric field is applied.23−25 The total wavelength was tuned by 3 nm (6 meV in energy) as Fp varied from −20 to 40 kV/cm. In the meantime, autocorrelation measurements were carried out for the wavelength-shifted LH emission. At Fp = −20 and 40 kV/cm, we observe that the wavelength of the in-plane polarized LH exciton emission is shifted to 794.11 and 791.07 nm, respectively. The autocorrelation results at these two wavelengths are shown in Figure 3c and d, from which g(2)(0) = 0.22 ± 0.10 and 0.18 ± 0.08 are found. These results indicate that the optical properties of our LH SPS can be accurately tuned, and stable singlephoton emission during the tuning process is preserved. One of the important parameters for single-photon qubits is a long coherence time, as this not only determines the maximum number of possible qubit operations, but also the efficiency of the coupling between independent quantum systems.4,28 In our work, the coherence time of the triggered LH single-photon emission was studied by a Michelson interferometer inserted in the PL optical path. The interferometer consists of two arms. One arm is equipped with a fixed retroreflector, and the other one is equipped with a retroreflector, which is translated by the combination of a mechanical translation stage and a piezodriven stage (see Figure 4a). The interference signal was acquired by the spectrometer, and the integrated interference intensity as a function of the optical path difference ΔL was recorded.29−31 The results for a typical LH QD (dot B) are shown in Figure 4. Figure 4b shows interference fringe visibility of the in-plane polarized LH photon emission lines from dot B (green rectangular area is the integration window of 0.12 nm). The visibility is defined as V = (Imax − Imin)/(Imax + Imin), where Imax and Imin are the maximum and minimum integrated peak intensities from the interference fringes. We find that the visibility is as high as 95% at ΔL = 0 mm, see the left inset in Figure 4b and decreases with increasing ΔL. When ΔL = 15 mm, the interference fringes become extremely weak, and the visibility drops to about 9% (see the right inset of Figure 4b). After acquiring the visibilities at different positions defined by
symmetric antibunching dips at zero-time delay for both correlations g(2)(0) = 0.33 and 0.23 are found, which suggest that these emission lines are from different recombination channels of the same neutral exciton state in the studied QD. The above observations provide evidence of single photon emission from a LH neutral exciton with three emission lines. Of most interest is the pulsed optical operation of our device. This allows controlling the time of generation of excitons and thereby realizing triggered LH single-photon emission. In order to achieve this, the diode laser was used to drive the QDs, and meanwhile, the second-order time correlation function was measured under pulsed excitation. Figure 3a shows the unnormalized autocorrelation function of the in-plane polarized exciton emission, marked with the red arrow in Figure 3b at Fp = 0 kV/cm. The quantum nature of LH photon emission is revealed by the significantly suppressed peak at zero-time delay. The normalized correlation function for this peak shows g(2)(0) = 0.20 ± 0.12. This finite value represents a multiphoton emission probability that likely arises from the weak background emission from the sample rather than the QD. In addition to the vanishing peak at zero-time delay, periodic peaks in the autocorrelation measurement are also observed. This provides an unambiguous signature of a triggered LH single-photon emission. Interfacing remote nodes in a quantum network requires the initialization of qubits in the same state and the communication between nodes via indistinguishable single photons.27 Unlike natural atoms or ions, the inhomogeneous spectral broadening of self-assembled QDs is a big challenge for the realization of this scheme. In Figure 2 we have studied the preparation of excitons with LH ground state in a semiconductor QD, which can be used for the coherent state transfer from photon qubit to electron qubit, as already shown by Kosaka et al. with a quantum well.14 The basic ingredient for this scheme is a substantially larger in-plane g-factor for the LH state compared to the electron, which is the case for our QDs [B. J. Witek et al. (in preparation)]. In Figure 3a we demonstrate the triggered emission from such a LH exciton, which suggests that, after local manipulation of the spin qubit, the quantum information can be re-emitted as photon qubit.13 Now, one of the last elements for a LH-based reversible quantum interface is a “tuning knob” for the ground state energy levels of different QDs, which facilitates the direct state transfer from the photon qubit emitted by one QD to the electron qubit in another QD. D
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Notes
the mechanical stage, a coherence length (or coherence time) is extracted by fitting the curve with a single exponential function V(τ) = c exp(−τ/T2), where the coherence time T2 is found to be 34 ± 2.3 ps. From the inverse Fourier transform, a homogeneous line width of about 32 μeV is deduced. It is noticeable that the coherence time reported here for our LH single-photon emission is comparable to that of HH singlephoton emission from GaAs QDs grown by the similar droplet method.31,32 Time-resolved PL measurements were used to determine the lifetime T1 for the LH single-photon emission; see open symbols in Figure 4c. By fitting the decay curves with a single exponential decay function, a lifetime of 636 ps for inplane polarized and 670 ps for out-of-plane polarized emission lines was extracted. The similar radiative decay times for both polarized emission lines of the LH photon emission further support their common origin as discussed above. The large difference between T1 and T2 implies that a strong dephasing process occurs in the sample. This is ascribed to the nonresonant excitation, which leads to the creation of free charge carriers in the vicinity of the QD interacting with carriers confined in the QD.30,33 Recent studies have shown that such dephasing processes can be substantially suppressed under resonant excitation conditions, and thereby the coherence time can be drastically enhanced.34,35 In summary, we reported a novel triggered and wavelengthtunable LH single-photon source based on well-designed tensile-strained GaAs QDs. The LH photon emission, characterized by three linearly polarized emission lines, is explicitly verified in a series of PL measurements, including polarization-resolved, power-dependent, and photon-correlation measurements. The triggered single-photon emission from the LH neutral exciton is manifested in the second-order time autocorrelation measurements. By integrating the LH QDcontaining nanomembrane onto a PMN-PT actuator, we have achieved a broad wavelength tuning for the triggered LH singlephoton emission. The observed low multiphoton emission probabilities g(2)(0) values imply that the energy tuning by the externally induced strain fields does not deteriorate the emission properties. In addition, the coherence time of the LH single-photon emission has been studied by Michelson interferometry for the first time, and a coherence time of about 34 ps for our LH exciton is found. Further improvements, including growth optimization and resonant excitation, are necessary to improve the coherence of LH single photons from such QDs. We envision that, combining with an external magnetic field in the Voigt geometry, the demonstrated triggered LH SPS opens up a possibility to realize an allsemiconductor two-way quantum interface between single photons and spin qubits.13
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported financially by BMBF QuaHL-Rep (Contracts No. 01BQ1032 and 01BQ1034) and the European Union Seventh Framework Programme 209 (FP7/2007-2013) under Grant Agreement No. 601126 210 (HANAS). J.X.Z. was supported by China Scholarship Council (CSC, No. 2010601008). The authors thank S. Böttner and J. W. Deng for fruitful discussions, E. Zallo, P. Atkinson and Ch. Deneke for helpful discussions in sample growth and fabrication, and B. Eichler, R. Engelhard, and S. Harazim for the technical support.
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ASSOCIATED CONTENT
S Supporting Information *
Expanded discussion on the sample growth, QDs morphology investigation, and photon collection efficiency of the LH photon emissions. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. *E-mail: [email protected]. E
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