1500
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Enhanced third-harmonic generation in Si-compatible epsilon-near-zero indium tin oxide nanolayers Antonio Capretti,1,2 Yu Wang,1 Nader Engheta,3 and Luca Dal Negro1,4,* 1
Department of Electrical and Computer Engineering and Photonics Center, Boston University, 8 Saint Mary’s Street, Boston, Massachusetts 02215, USA 2
3
CNR-SPIN, Complesso MonteSantangelo, Via Cinthia, 80126 Napoli, Italy
Department of Electrical and Systems Engineering, University of Pennsylvania, 200 South 33rd Street, Philadelphia, Pennsylvania 19104, USA 4 Division of Materials Science and Engineering, Boston University, 15 Saint Mary’s Street, Brookline, Massachusetts 02446, USA *Corresponding author: [email protected] Received January 21, 2015; revised February 23, 2015; accepted February 23, 2015; posted February 24, 2015 (Doc. ID 232986); published March 27, 2015 We experimentally demonstrate enhanced third-harmonic generation from indium tin oxide nanolayers at telecommunication wavelengths with an efficiency that is approximately 600 times larger than crystalline silicon (Si). The increased optical nonlinearity of the fabricated nanolayers is driven by their epsilon-near-zero response, which can be tailored on-demand in the near-infrared region. The present material platform is obtained without any specialized nanofabrication process and is fully compatible with the standard Si-planar technology. The proposed approach can lead to largely scalable and highly integrated optical nonlinearities in Si-integrated devices for information processing and optical sensing applications. © 2015 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (160.4330) Nonlinear optical materials; (310.7005) Transparent conductive coatings. http://dx.doi.org/10.1364/OL.40.001500
The search for materials with efficient optical nonlinearities has been boosted by the recent research on plasmonic and metamaterial platforms, which have a variety of applications, including nonlinear frequency mixing [1,2], electrical modulation [3–5], and signal processing [6]. Epsilon-near-zero (ENZ) media [7], which are typically obtained by appropriately designing metal-dielectric metamaterials that exhibit near-zero permittivity, have been proposed to manipulate electromagnetic fields at the nanoscale. ENZ media have applications ranging from energy squeezing [8], imaging [9], scattering [10,11], and nonlinear optics [12–15]. In particular, efficient optical nonlinearities have been theoretically predicted in ENZ media [12–15]. The peculiar features of ENZ media have been investigated to achieve enhanced bistability in Kerr nonlinear devices [14], and switching in active optoelectronic devices [16]. Recently, phase mismatch-free propagation has been demonstrated experimentally [17]. Moreover, it has been theoretically predicted that ENZ metamaterials can significantly enhance harmonic generation. As a matter of fact, a large discontinuity of the normal electric field component can be obtained at the longitudinal oscillation frequency of a generic Lorentz medium, giving rise to a strong enhancement in the optical nonlinearities [13,18]. However, the current metamaterial concepts for the near-infrared and optical frequencies [19–21] are limited by the use of metallic components, which present large extinction losses. These losses severely limit practical implementations, especially for nonlinear optics, where materials capable of withstanding high temperatures and electric fields are needed. Moreover, the need for threedimensional manufacturing limits the compatibility with silicon (Si) technology, thus often increasing technology costs. Recently, transparent conductive oxides, such as indium tin oxide (ITO), and nitride-based thin films have 0146-9592/15/071500-04$15.00/0
gathered significant attention as alternative materials for plasmonic and metamaterials applications. These materials show lower losses compared to the traditional noble metals, such as Au and Ag, in the near-infrared and visible spectral ranges [22,23]. In addition, they can withstand the high temperature loads and electric fields that arise due to nanoscale confinement. However, the potential of these materials as tailored ENZ media with efficient nonlinear frequency conversion in Si still needs to be addressed. Here, we demonstrate third-harmonic generation (THG) from ITO nanolayers with an efficiency that is ∼600 times larger than crystalline silicon. The enhanced optical nonlinearity is driven by the ENZ condition at telecommunication wavelengths. The proposed material platform is obtained without the need for any specialized nanofabrication, and it is fully compatible with Si technology. We deposited ITO thin films by magnetron sputtering at room temperature on Si substrates using an ITO target in a Denton Discovery 18 confocal target system. The thickness of all the samples was fixed at 37 5 nm. The base pressure was 10−7 Torr, and the Ar gas flow was 12 sccm. We performed post-deposition rapid thermal annealing processes, as detailed in Table 1, thus tuning the optical dispersion properties, due to the increased free carrier density in the material. The electric permittivity εω of the fabricated samples was experimentally measured using variable-angle Table 1. ITO Samples’ Annealing Parameters λENZ (nm) 1270 1390 1550
ℏωENZ
ε2 at ENZ
Annealing Temp. (°C)
Annealing Time (min)
0.98 0.89 0.80
0.52 0.48 0.61
750 550 350
30 30 60
© 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
1501
spectroscopic ellipsometry. It was fitted with excellent accuracy using the standard Drude–Sommerfeld model: εω ε1 ω iε2 ω ε∞ −
ω2p ; ω2 iΓω
(1)
where ε∞ is the high frequency limit of εω; ωp is the plasma angular frequency, and Γ describes the charge carrier collision rate. The real ε1 and the imaginary ε2 parts of the permittivity are shown in Figs. 1(a) and 1(b), respectively. All of the samples feature a wavelength of zero ε1 , denoted as λENZ , which can be tuned in the range between 1270 and 1550 nm by means of post-deposition annealing conditions. Moreover, the value of ε2 at λENZ is in the range of 0.48–0.61 (see Table 1). This range is significantly smaller than that of traditional metals in the telecommunication window [24]. It is important to note that the ENZ condition simultaneously requires ε1 0 (corresponding to the screened plasma frequency) and ε2 < 1. Both conditions are necessary to enhance the normal electric field component inside the layer, which, as we demonstrate in this work, boosts the nonlinear optical processes. We calculated the amplitude of the electric field as a function of the incident angle and wavelength, using the measured permittivity and the analytical solution for the reflection and transmission of waves in a planar layered structure [25]. Specifically, we considered a free-space (m 0)/ITO (m 1)/Si (m 2) geometry with a plane wave illuminating an ITO slab on a Si substrate. The generic medium m is characterized by its wavenumber km and impedance ζm . The spatial coordinates are the same as in [25], with the zˆ axis perpendicular to the surface. The electric field for the p-polarization is: k Em x;y;z ζm Am m;z 1− R~ m;m1 e2ikm;z dm km;z z ˆx km km;x 2ikm;z dm km;z z ~ 1 Rm;m1 e ˆz eikm;x x−km;z z ; km (2) where the wavevector km in medium m and its spatial components (the x, y, and z subscripts) are determined by Snell’s law. The expressions for Am and R~ m;m1 are given in [25]. The amplitudes of the normal component E z of the internal field are displayed in Figs. 2(a) to 2(c) for a p-polarized incident plane wave. E z is maximized in correspondence with the ENZ wavelength for each sample.
Fig. 1. (a) Real and (b) imaginary parts of the electric permittivity for three ITO nanolayers where λENZ 1270 (blue), 1390 (green), and 1550 nm (cyan).
Fig. 2. Amplitude of the internal electric field component E z normal to the film surface as a function of the incident angle and wavelength for the ITO nanolayers when λENZ (a) 1270, (b) 1390, and (c), 1550 nm. Reflectivity as a function of the incident angle and wavelength for the ITO nanolayers when λENZ (d) 1270, (e) 1390, and (f), 1550 nm.
On the contrary, the internal electric field component E x tangent to the surface features an intensity that is reduced down to ∼0.5. We also remark that the maximum of E z is achieved for the incident angles at ∼45°, corresponding to the quasi-Brewster angle of the ITO material, which is limited by optical losses. This is further demonstrated by the calculated reflectances in Figs. 2(d) to 2(f), which feature the minimum in correspondence to the maximum of E z . The amplitude of the electric field component E z is up to 1.8 times higher than the incident field at telecommunication wavelengths. In contrast, the internal field in traditional noble metals in the same spectral range is much lower, because of the large reflection coefficients. Inside ITO thin films, the field amplitude is affected by the ENZ condition and the reduced material losses (ε2 ∼ 0.5), leading to the remarkable enhancement of nonlinear optical processes, such as THG. Moreover, ITO thin films benefit from material transparency at visible wavelengths, which enables the efficient extraction of the generated light. In order to experimentally demonstrate the relevance of the ENZ condition for the control and enhancement of optical nonlinearities, we characterized the ITO nanolayers using third-harmonic generation spectroscopy. As excitation, we use the idler output (1000–1700 nm) of an optical parametric oscillator pumped by a Ti:sapphire laser, which delivers 150 fs pulses with a center wavelength of 820 nm at a repetition rate of 81 MHz. The incident beam has an average power of 10 mW. It is focused by means of a 20× microscope objective, with an estimated peak power density of 40 MW∕cm2 . A 1000 nm long-pass filter on the excitation line removes any spurious signal. The THG signals are collected through a 30 mm lens, coupled into a monochromator (Cornerstone 260) through a 180 mm lens, and detected by a lock-in
1502
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
amplifier (Oriel Merlin) that is coupled to a low-light photomultiplier tube (PMT, Oriel Instrumentation 77348). The collected light is filtered by a 1000 nm short-pass filter in front of the monochromator. The incident angle was kept at 45° for all acquisitions. In order to demonstrate that the THG signals are generated in the fabricated ITO films, we substituted the investigated samples with a bulk Si substrate. In this case, no signal is detected under the same excitation power. In Fig. 3(a), we show the representative THG spectra collected under the p-polarized incidence for the sample in which λENZ 1390 nm. The intensity exhibits a maximum if the incident wavelength is equal to λENZ , and it scales cubically with the incident power, as shown in the inset of Fig. 3(b). These findings directly demonstrate that the enhanced electric field component E z achieved at the tailored λENZ can be utilized to boost the THGemitted intensities. The behavior of the THG intensity with respect to the incident wavelength is summarized in Fig. 4 for all samples. The THG intensities show a characteristic trend that approximately peaks at the corresponding λENZ in each sample. We notice that the THG curves are not symmetrical. This is mainly due to the lack of symmetry in the linear optical response around λENZ , i.e., ε1 is negative for λ > λENZ (plasmonic), and positive for λ
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
Enhanced third-harmonic generation in Si-compatible epsilon-near-zero indium tin oxide nanolayers Antonio Capretti,1,2 Yu Wang,1 Nader Engheta,3 and Luca Dal Negro1,4,* 1
Department of Electrical and Computer Engineering and Photonics Center, Boston University, 8 Saint Mary’s Street, Boston, Massachusetts 02215, USA 2
3
CNR-SPIN, Complesso MonteSantangelo, Via Cinthia, 80126 Napoli, Italy
Department of Electrical and Systems Engineering, University of Pennsylvania, 200 South 33rd Street, Philadelphia, Pennsylvania 19104, USA 4 Division of Materials Science and Engineering, Boston University, 15 Saint Mary’s Street, Brookline, Massachusetts 02446, USA *Corresponding author: [email protected] Received January 21, 2015; revised February 23, 2015; accepted February 23, 2015; posted February 24, 2015 (Doc. ID 232986); published March 27, 2015 We experimentally demonstrate enhanced third-harmonic generation from indium tin oxide nanolayers at telecommunication wavelengths with an efficiency that is approximately 600 times larger than crystalline silicon (Si). The increased optical nonlinearity of the fabricated nanolayers is driven by their epsilon-near-zero response, which can be tailored on-demand in the near-infrared region. The present material platform is obtained without any specialized nanofabrication process and is fully compatible with the standard Si-planar technology. The proposed approach can lead to largely scalable and highly integrated optical nonlinearities in Si-integrated devices for information processing and optical sensing applications. © 2015 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (160.4330) Nonlinear optical materials; (310.7005) Transparent conductive coatings. http://dx.doi.org/10.1364/OL.40.001500
The search for materials with efficient optical nonlinearities has been boosted by the recent research on plasmonic and metamaterial platforms, which have a variety of applications, including nonlinear frequency mixing [1,2], electrical modulation [3–5], and signal processing [6]. Epsilon-near-zero (ENZ) media [7], which are typically obtained by appropriately designing metal-dielectric metamaterials that exhibit near-zero permittivity, have been proposed to manipulate electromagnetic fields at the nanoscale. ENZ media have applications ranging from energy squeezing [8], imaging [9], scattering [10,11], and nonlinear optics [12–15]. In particular, efficient optical nonlinearities have been theoretically predicted in ENZ media [12–15]. The peculiar features of ENZ media have been investigated to achieve enhanced bistability in Kerr nonlinear devices [14], and switching in active optoelectronic devices [16]. Recently, phase mismatch-free propagation has been demonstrated experimentally [17]. Moreover, it has been theoretically predicted that ENZ metamaterials can significantly enhance harmonic generation. As a matter of fact, a large discontinuity of the normal electric field component can be obtained at the longitudinal oscillation frequency of a generic Lorentz medium, giving rise to a strong enhancement in the optical nonlinearities [13,18]. However, the current metamaterial concepts for the near-infrared and optical frequencies [19–21] are limited by the use of metallic components, which present large extinction losses. These losses severely limit practical implementations, especially for nonlinear optics, where materials capable of withstanding high temperatures and electric fields are needed. Moreover, the need for threedimensional manufacturing limits the compatibility with silicon (Si) technology, thus often increasing technology costs. Recently, transparent conductive oxides, such as indium tin oxide (ITO), and nitride-based thin films have 0146-9592/15/071500-04$15.00/0
gathered significant attention as alternative materials for plasmonic and metamaterials applications. These materials show lower losses compared to the traditional noble metals, such as Au and Ag, in the near-infrared and visible spectral ranges [22,23]. In addition, they can withstand the high temperature loads and electric fields that arise due to nanoscale confinement. However, the potential of these materials as tailored ENZ media with efficient nonlinear frequency conversion in Si still needs to be addressed. Here, we demonstrate third-harmonic generation (THG) from ITO nanolayers with an efficiency that is ∼600 times larger than crystalline silicon. The enhanced optical nonlinearity is driven by the ENZ condition at telecommunication wavelengths. The proposed material platform is obtained without the need for any specialized nanofabrication, and it is fully compatible with Si technology. We deposited ITO thin films by magnetron sputtering at room temperature on Si substrates using an ITO target in a Denton Discovery 18 confocal target system. The thickness of all the samples was fixed at 37 5 nm. The base pressure was 10−7 Torr, and the Ar gas flow was 12 sccm. We performed post-deposition rapid thermal annealing processes, as detailed in Table 1, thus tuning the optical dispersion properties, due to the increased free carrier density in the material. The electric permittivity εω of the fabricated samples was experimentally measured using variable-angle Table 1. ITO Samples’ Annealing Parameters λENZ (nm) 1270 1390 1550
ℏωENZ
ε2 at ENZ
Annealing Temp. (°C)
Annealing Time (min)
0.98 0.89 0.80
0.52 0.48 0.61
750 550 350
30 30 60
© 2015 Optical Society of America
April 1, 2015 / Vol. 40, No. 7 / OPTICS LETTERS
1501
spectroscopic ellipsometry. It was fitted with excellent accuracy using the standard Drude–Sommerfeld model: εω ε1 ω iε2 ω ε∞ −
ω2p ; ω2 iΓω
(1)
where ε∞ is the high frequency limit of εω; ωp is the plasma angular frequency, and Γ describes the charge carrier collision rate. The real ε1 and the imaginary ε2 parts of the permittivity are shown in Figs. 1(a) and 1(b), respectively. All of the samples feature a wavelength of zero ε1 , denoted as λENZ , which can be tuned in the range between 1270 and 1550 nm by means of post-deposition annealing conditions. Moreover, the value of ε2 at λENZ is in the range of 0.48–0.61 (see Table 1). This range is significantly smaller than that of traditional metals in the telecommunication window [24]. It is important to note that the ENZ condition simultaneously requires ε1 0 (corresponding to the screened plasma frequency) and ε2 < 1. Both conditions are necessary to enhance the normal electric field component inside the layer, which, as we demonstrate in this work, boosts the nonlinear optical processes. We calculated the amplitude of the electric field as a function of the incident angle and wavelength, using the measured permittivity and the analytical solution for the reflection and transmission of waves in a planar layered structure [25]. Specifically, we considered a free-space (m 0)/ITO (m 1)/Si (m 2) geometry with a plane wave illuminating an ITO slab on a Si substrate. The generic medium m is characterized by its wavenumber km and impedance ζm . The spatial coordinates are the same as in [25], with the zˆ axis perpendicular to the surface. The electric field for the p-polarization is: k Em x;y;z ζm Am m;z 1− R~ m;m1 e2ikm;z dm km;z z ˆx km km;x 2ikm;z dm km;z z ~ 1 Rm;m1 e ˆz eikm;x x−km;z z ; km (2) where the wavevector km in medium m and its spatial components (the x, y, and z subscripts) are determined by Snell’s law. The expressions for Am and R~ m;m1 are given in [25]. The amplitudes of the normal component E z of the internal field are displayed in Figs. 2(a) to 2(c) for a p-polarized incident plane wave. E z is maximized in correspondence with the ENZ wavelength for each sample.
Fig. 1. (a) Real and (b) imaginary parts of the electric permittivity for three ITO nanolayers where λENZ 1270 (blue), 1390 (green), and 1550 nm (cyan).
Fig. 2. Amplitude of the internal electric field component E z normal to the film surface as a function of the incident angle and wavelength for the ITO nanolayers when λENZ (a) 1270, (b) 1390, and (c), 1550 nm. Reflectivity as a function of the incident angle and wavelength for the ITO nanolayers when λENZ (d) 1270, (e) 1390, and (f), 1550 nm.
On the contrary, the internal electric field component E x tangent to the surface features an intensity that is reduced down to ∼0.5. We also remark that the maximum of E z is achieved for the incident angles at ∼45°, corresponding to the quasi-Brewster angle of the ITO material, which is limited by optical losses. This is further demonstrated by the calculated reflectances in Figs. 2(d) to 2(f), which feature the minimum in correspondence to the maximum of E z . The amplitude of the electric field component E z is up to 1.8 times higher than the incident field at telecommunication wavelengths. In contrast, the internal field in traditional noble metals in the same spectral range is much lower, because of the large reflection coefficients. Inside ITO thin films, the field amplitude is affected by the ENZ condition and the reduced material losses (ε2 ∼ 0.5), leading to the remarkable enhancement of nonlinear optical processes, such as THG. Moreover, ITO thin films benefit from material transparency at visible wavelengths, which enables the efficient extraction of the generated light. In order to experimentally demonstrate the relevance of the ENZ condition for the control and enhancement of optical nonlinearities, we characterized the ITO nanolayers using third-harmonic generation spectroscopy. As excitation, we use the idler output (1000–1700 nm) of an optical parametric oscillator pumped by a Ti:sapphire laser, which delivers 150 fs pulses with a center wavelength of 820 nm at a repetition rate of 81 MHz. The incident beam has an average power of 10 mW. It is focused by means of a 20× microscope objective, with an estimated peak power density of 40 MW∕cm2 . A 1000 nm long-pass filter on the excitation line removes any spurious signal. The THG signals are collected through a 30 mm lens, coupled into a monochromator (Cornerstone 260) through a 180 mm lens, and detected by a lock-in
1502
OPTICS LETTERS / Vol. 40, No. 7 / April 1, 2015
amplifier (Oriel Merlin) that is coupled to a low-light photomultiplier tube (PMT, Oriel Instrumentation 77348). The collected light is filtered by a 1000 nm short-pass filter in front of the monochromator. The incident angle was kept at 45° for all acquisitions. In order to demonstrate that the THG signals are generated in the fabricated ITO films, we substituted the investigated samples with a bulk Si substrate. In this case, no signal is detected under the same excitation power. In Fig. 3(a), we show the representative THG spectra collected under the p-polarized incidence for the sample in which λENZ 1390 nm. The intensity exhibits a maximum if the incident wavelength is equal to λENZ , and it scales cubically with the incident power, as shown in the inset of Fig. 3(b). These findings directly demonstrate that the enhanced electric field component E z achieved at the tailored λENZ can be utilized to boost the THGemitted intensities. The behavior of the THG intensity with respect to the incident wavelength is summarized in Fig. 4 for all samples. The THG intensities show a characteristic trend that approximately peaks at the corresponding λENZ in each sample. We notice that the THG curves are not symmetrical. This is mainly due to the lack of symmetry in the linear optical response around λENZ , i.e., ε1 is negative for λ > λENZ (plasmonic), and positive for λ
