Transient GaAs Plasmonic Metasurfaces at Terahertz Frequencies

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Transient GaAs Plasmonic Metasurfaces at Terahertz Frequencies Yuanmu Yang, N. Kamaraju, Salvatore Campione, Sheng Liu, John L. Reno, Michael B. Sinclair, Rohit P. Prasankumar, and Igal Brener ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00735 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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Transient GaAs Plasmonic Metasurfaces at Terahertz Frequencies Yuanmu Yang,1,2* N. Kamaraju,3 Salvatore Campione,1,2 Sheng Liu,1,2 John L. Reno,1,2 Michael B. Sinclair,1 Rohit P. Prasankumar,3 Igal Brener1,2* 1 2

Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

Center for Integrated Nanotechnologies (CINT), Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

3

Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA *Corresponding authors: [email protected] or [email protected]

Abstract: We demonstrate the ultrafast formation of macroscopic terahertz (THz) metasurfaces through all-optical creation of spatially modulated carrier density profiles in a deep-subwavelength GaAs film. The switch-on of the transient plasmon mode, governed by the GaAs effective electron mass and electron-phonon interactions, is revealed by structured-optical pump THz probe spectroscopy, on a time scale of 500 femtoseconds. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz. Furthermore, we numerically demonstrate that the metasurface presented here can be generalized to more complex architecture for realizing functionalities such as perfect absorption, leading to a 30 dB modulation depth. The platform also provides a pathway to achieve ultrafast manipulation of infrared beams in the linear and potentially, nonlinear regime. Keywords: ultrafast plasmonics; semiconductor; metasurface; perfect absorber

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Plasmonic metasurfaces are composed of metallic elements that usually exhibit effective electric and/or magnetic polarizabilities. Such polarizabilities arise from the excitation of coherent charge motion and the associated plasmon modes confined in deep subwavelength volumes, leading to strong light-matter interactions1,2. Rich new physics can emerge when photons, electrons and phonons interact cohesively in such condensed matter systems and on ultrafast timescales. Noble metals such as gold and silver are the most commonly adopted plasmonic materials at optical frequencies, but their carrier densities and associated plasmonic properties are not tunable by either electrical or optical perturbations. Moreover, common metals behave like perfect electric conductors in the technologically relevant infrared or terahertz (THz) ranges of the electromagnetic spectrum, leading to a finite light penetration depth and limited interaction between light and the electron plasma3. As an alternative, doped semiconductors, such as Si4,5, III-V materials6–8, and transparent conducting oxides9–11, can be much more versatile, with their plasma frequency ωp tunable from the near- to the far-infrared spectral range depending on the adopted material system and the doping density. In addition, plasmonic metasurfaces formed directly from doped semiconductors can enjoy wide and dynamic tunability in their optical properties using external optical and/or electrical stimuli. In the THz regime, graphene12,13 and GaAs/AlGaAs quantum wells14 supporting two-dimensional electron gases (2DEGs) have been used to form plasmonic metasurfaces capable of modulating THz waves by electrostatic gating. However, it remains a major challenge for graphene-based metasurfaces to achieve small plasmon damping, especially for large area and patterned samples, and a wide spectral modulation range. For quantum well-based structures, the operation is limited to cryogenic temperatures. Therefore, hybrid systems involving both 2DEGs and metallic metasurfaces have been introduced for enhanced device performance15. All-photoinduced

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metasurfaces directly imprinted on GaAs or Si wafers have also previously been demonstrated for polarization or diffraction control at THz frequencies. However, in most previous work16–20, the GaAs or Si materials are subject to an intense optical pump, thereby simply acting as electric conductors. Minimum spectral tunability upon photoinduced changes in carrier density has been observed because the spectral position of the plasmonic grating mode is only weakly dependent on ω p . In addition, the ultrafast dynamics of the THz plasmonic mode formation due to photoinduced carriers have not been well studied. Here, we construct a transient plasmonic metasurface on a semiconductor GaAs thin film by creating a spatially varying carrier density profile with a structured photoexcitation. The ultrafast formation of the transient plasmon mode on a sub-picosecond (ps) timescale following hot electron thermalization and electron-lattice cooling is revealed by broadband structuredoptical-pump THz-probe (s-OPTP) spectroscopy. Using different optical pump fluences, we can change the carrier density, and therefore ωp of the GaAs metasurface. By utilizing uniquely the (localized) electric dipole instead of the (non-localized) grating mode of the metasurface, we experimentally demonstrate wide tuning of the electric dipole resonance of the metasurface, from 0.5 THz to 1.7 THz. We further show in numerical simulation that adapting more complex designs exhibiting perfect absorption can further enhance the modulation depth to 30 dB. The sample under investigation is shown in Fig. 1(a) and consists of a 1-µm-thick GaAs film followed by a 1-µm-thick Al0.55Ga0.45As barrier and bonded to a sapphire substrate. Here we utilized one specific wafer available to us, the GaAs film is n-doped to 8×1015/cm3, which is insufficient to create a plasmonic response at THz frequencies in the absence of additional photoinduced carriers, and the Al0.55Ga0.45As barrier layer also does not play a major role. Under


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equilibrium condition, the complex dielectric function of GaAs, ε GaAs , can be expressed by the Drude model as:

ε GaAs = ε ∞ −

ω 2p ω 2 + iωγ

(1)

,

where ε ∞ is the high frequency permittivity and γ is the electron scattering rate. The plasma frequency of GaAs is related to the carrier density n by:

ω p = ne2 ε 0 m* ,

(2) *

where ε 0 is the vacuum permittivity, e is the electron charge, and m = 0.067m0 is the effective electron mass at the GaAs conduction band minima (with m0 being the free electron mass). To probe the macroscopic plasmonic response of the semiconductor thin film, additional in-plane momentum provided by a prism or grating is required to couple the plasmon modes to the normally incident light. Alternatively, as in our case, a GaAs metasurface can be created by spatially modulating the carrier density, as schematically depicted in Fig. 1(a). Our experimental system is based on a 1 kHz, 800 nm, 35 femtosecond (fs) Ti-sapphire amplifier, whose output beam was split into three paths for THz generation, THz detection and optical pumping, respectively. The ultrafast temporal evolution of the plasmon mode in the photo-excited GaAs metasurface can then be revealed by electro-optic sampling of the transmitted THz pulse as a function of the sampling time τ and by varying the relative time delay between the THz pulse and the optical pump pulse t D (Fig. 1b). During the s-OPTP measurements, special care is taken to ensure that the variable THz delay stage is located in the THz generation beam path, such that at each optical pump delay, every portion of the entire THz transient experiences an identical delay time following the near-IR pump pulse21. The time resolution of the measurement is then 4 ACS Paragon Plus Environment

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only limited by the pump pulse duration. The carrier density modulation in the GaAs thin film is realized by routing the optical pump beam through a one-dimensional (1D) grating-shape chrome mask and a 4-f imaging setup, such that the 1D grating pattern (30 µm period, 50% duty cycle) can be directly imaged from the mask to the GaAs thin film with 1:1 magnification. The dimension of the grating is chosen such that the electric dipole resonance of the doped GaAs metasurface can fall between the spectral range of ~0.5 THz to 2.0 THz. We chose a 1D metallic grating as the mask for simplicity, and the mask can be engineered to have more complex forms. And we emphasize that the plasmonic response of the transient GaAs metasurface is due to a localized resonance instead of a grating effect. The pump beam intensity profile in the GaAs film is measured by placing a silicon charge-coupled device (CCD) at the original sample position, and is presented in the inset of Fig. 1(b). The THz field is polarized perpendicular to the 1D grating. Fig. 2(a) shows the photoinduced change in the transmission of the THz waveform through the GaAs metasurface as a function of the pump delay t D , with the pump fluence fixed at 6 µJ/cm2 (corresponding to a carrier density of ~1.2×1017/cm3); Fig. 2(b) shows the relative change in the Fourier transformed spectral response ∆T T . The power transmittance is defined as , where

and

are the Fourier transformed transmitted THz

electric fields of the sample and reference pulses, respectively. Note that due to such a definition, the transmittance of the sample can be greater than 1 if any anti-reflection behavior for the GaAs epi-layer may be present. In the absence of the optical pump pulse, a slight increase in transmittance with increased frequency can be observed in the transmittance spectrum, which can be attributed to the absorption of pre-existing free carriers in the GaAs film. The transmittance spectra of the GaAs metasurface vary drastically within 500 fs upon 5 ACS Paragon Plus Environment

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photoexcitation, and remain nearly unaltered at later times. Line scans of the metasurface transmittance at increasing pump probe delay time are shown as colored dots in Fig. 2(c). At a negative delay of -1 ps, a nearly flat Drude response is observed due to the background free carrier absorption. Within ~500 fs following the arrival of the optical pump pulse, the plasmon mode gradually builds up with a slight shift of spectral position and narrowing of resonance linewidth. The decay of the mode is relatively long (~1 ns), governed by the bulk recombination rate of electron hole pairs in GaAs21,22. The long decay time can be useful in situations, for example, when the detection of long coherence-time dynamics is required, although we note that the recombination rate can also be engineered to be much shorter by low temperature epitaxial growth23 or by utilizing superlattice ErAs/GaAs nanostructures24. The physics behind the ultrafast dynamics of the photo-excited GaAs metasurface is schematically illustrated in Fig. 2(d). According to eq. (2), the plasma frequency ω p in GaAs is related to the electron effective mass. Upon photoexcitation, the initial non-equilibrium hot electron distribution quickly thermalize into a hot Fermi distribution

25

. Due to the non-

parabolicity of the Γ- valley of the conduction band of GaAs26, the effective mass of the photo-

( )

excited electrons is strongly dependent on the electron distribution function f E,t

as

determined by21: (3)

where

is the Planck constant, E is the energy, and k is the wave vector of the electrons. The

effective mass of the photoexcited electrons is largest immediately following the thermalization, and gradually decreases as the electron loses its energy and decays to the conduction band minima by electron-phonon coupling21,25,27. The change in the effective mass alters the plasma 6 ACS Paragon Plus Environment

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frequency ω p in GaAs, and concomitantly the spectral position of the plasmon mode of the GaAs metasurface. During the electron-phonon cooling process, the scattering rate γ is anticipated to decrease as well, which can lead to a narrowing in the metasurface transmittance spectra. Such a physical picture is manifested in our experimental results. In Fig. 2(e), we show the plasma frequency ω p and scattering rate γ at each individual pump-probe delay time fitted from numerical simulations, where clear trends of the predicted increase of ω p and decrease of

γ can be observed. The electron-phonon cooling time is estimated to be about 500 fs with a single exponential fit to the trend of both ω p and γ . We rule out other possibilities that may also contribute to the ultrafast dynamics of the GaAs metasurface, such as velocity overshoot and intervalley scattering by the Gunn effect. Because the Γ- and the L-conduction band minima in GaAs are separated by 290 meV, the kinetic energy of the photoexcited electrons (130 meV) is too small to drive any significant amount of photo-excited carriers directly into the L valley. Another way to scatter the electrons in GaAs to the L-valley is through the Gunn effect21. However, we measured the applied THz electric field to be 0.8 kV/cm, which is much smaller than the required field of ~3 kV/cm. We also confirmed that the metasurface is working in the linear regime by measuring its response with the THz intensity reduced by a factor of three, and did not see any significant change of the spectral response of our GaAs metasurface. In Fig. 3(a), we show the numerically simulated transmittance of the photoexcited GaAs metasurface with 30 µm grating period as a function of the plasma frequency, assuming a perfectly square photoexcitation profile in the x-direction and a uniform carrier distribution in the 7 ACS Paragon Plus Environment

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z-direction. The simulated plasmon mode linearly blue shifts with increasing plasma frequency. As shown in Fig. 3(b), the electric dipole-like and tightly confined field distribution further verifies the localized nature of the resonance. To gain physical understanding of the resonance and its dependence on the plasma frequency ωp, we recall that for an electric dipole plasmonic antenna, the resonance frequency is governed by28:

L=

where L is the cavity length,

λeff 2

a1 + a2 =

2

ωp ω res

,

(4)

λeff is the effective wavelength of light in the plasmonic cavity,

and a1 and a2 are constants related to the exact geometry of the antenna. The resonant frequency

ω res is thereby linearly proportional to ω p at a fixed L , confirming the trend we observe numerically in Fig. 3(a). On the contrary, when the pump fluence is increased to ~100 µJ/cm2, the plasma frequency of GaAs can increase beyond 50 THz, at which point the GaAs film starts to resemble closely a perfect electric conductor at THz frequencies. For a GaAs metasurface with 100 µm grating period, the resonance frequency converges to 1 THz, which agrees with the plasmonic grating equation29. Fig. 3(d) presents the typical electric field profile at the resonance, which exhibits clearly a non-localized nature. By working in the regime of localized resonance at relatively low pump fluence, we expect to achieve a much wider spectral tuning range comparing with the cases where large pump fluence is used17,30. The localized nature of our structure can also be verified by controlled numerical simulations, varying either the duty cycle or the period of the structure while keeping the rest of the parameters fixed. As depicted in Fig. 3(e), when we vary the duty cycle of the resonator while maintaining a fixed period, we observe a significant red-shift of the resonant position as the width of the photoexcited GaAs stripe is increased. On


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the contrary, when the width of the photoexcited GaAs resonator is fixed and the period is varied as shown in Fig. 3(f), no significant change of the resonant position can be observed. Fig. 4(a) shows the measured power transmittance spectra of the photo-excited GaAs metasurface as a function of the optical pump fluence. The measurements are performed at room temperature with the THz probe pulse arriving 50 ps after the optical pump, such that the GaAs film can be assumed to be in the quasi-equilibrium condition. At a moderate pump fluence of 2 µJ/cm2, a tiny dip in transmittance emerges at a frequency of ~0.5 THz. Further increasing the pump fluence leads to a much more pronounced resonance shifting to higher frequencies. At a pump fluence of 14 µJ/cm2, the resonance shifts to 1.7 THz. To the best of our knowledge, such a dynamic frequency tuning range is one of the widest among all active THz and infrared devices reported so far31–34. The measured results qualitatively agree with the numerical simulations in Fig. 3(a), aside from wider spectral broadening. In contrast to the assumption of a uniform carrier distribution in the GaAs layer along the z-direction, in the realistic case, the finite skin depth of GaAs at 800 nm (~1 µm) leads to an exponential decay of carrier densities in the film along the z-direction, leading to an inhomogeneous broadening of the resonance linewidth. In Fig. 4(b), we present the numerical simulation results in which the non-uniform carrier density profile in the GaAs film along the zdirection is carefully taken into account. In these simulations, the GaAs plasma frequency ω p and scattering rate γ are treated as adjustable parameters to fit the experimental data. The fitted transmittance spectra are in good agreement with the experiments. The fitting parameters ω p and γ as a function of the pump fluence are shown in Fig. 4(c). We find that with the increase in pump fluence, not only ω p but also γ increases. The increase of ω p can be attributed


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straightforwardly to the increased carrier density according to Eq. (2), whereas we attribute the increased plasmon damping to an increase in electron-electron and electron-phonon scattering at higher carrier densities. We further perform control experiments with the shadow mask removed from the optical pump path to show that the spatial modulation is required to achieve the observed tuning. As illustrated in Fig. 4(d), with a homogeneous pump profile, only a monotonic decrease of the sample transmittance is observed with increasing pump fluence as a result of increased free carrier absorption. We notice that one major drawback with the current design is that the plasmonic resonance is relatively shallow with a quality- (Q-) factor of ~1, leading to a small modulation depth. One way to circumvent this issue is to use semiconductor materials with higher electron mobility, such as InAs or InSb, such that the plasmon damping can be reduced. Yet another approach is to look for alternative metasurface designs. In Fig. 5(a), we show that a highly ndoped GaAs back mirror (with a carrier concentration of 5×1018/cm3or similar) and an undoped AlGaAs spacer layer can be designed beneath the photo-doped GaAs layer to construct monolithically a metasurface perfect absorber. By photo-doping the top GaAs layer, the impedance of the structure can be engineered to match closely to that of the free space, leading to perfect absorption35,36. In Fig. 5(b), we plot the numerically simulated reflectance of the photoexcited GaAs metasurface as a function of the plasma frequency, with the assumption of a non-uniform carrier density profile in the GaAs film along the z-direction to imitate the realistic situation. We observe that the reflectance of the structure to be greater than 90% in the undoped case, and reaches close to 0 with ωp = 40 THz at 1.15 THz. In Fig. 5(c), we present two line scans of the sample reflectance spectrum plotted in linear and logarithmic scales, respectively, from which we observe a 30 dB modulation depth at 1.15 THz and a resonance Q-factor of 3.7.


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Future electrically tunable THz devices enabled by directly modulating the GaAs carrier density and plasmon modes can be envisioned for compact solid-state THz modulators. The structured illumination technique may also prove as a powerful tool for the investigation of lightmatter interactions, including ultrafast dynamics, in a variety of delicate systems. Finally, the transient GaAs metasurface offers a great platform for the investigation of nonlinear plasmon dynamics of doped semiconductors under intense THz irradiation, which may lead to a new playground for nonlinear THz optics37. Furthermore, the proof of concept presented here could be extended to mid-IR dynamic metasurfaces by proper scaling of the fabricated structures and pump excitation fluences, while the issue of plasmon damping due to the finite electron mobility can also be largely relieved.

The authors thank P. Q. Liu of Sandia National Laboratories for stimulating discussions and M. R. C. Williams of Los Alamos National Laboratory for assistance in the sample measurements. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.


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Figures and Legends

Fig. 1. (a) Schematic of a transient GaAs metasurface induced by a structured femtosecond optical pump beam and the dimensions of the structure (in µm). (b) Schematic of the s-OPTP configuration by integrating a 4-f imaging system into the optical pump path of a conventional OPTP setup. The inset shows the pump beam profile on the GaAs metasurface, measured by replacing the sample with a silicon CCD.


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Fig. 2. (a) Electric field change ∆ETHz induced by optical photoexcitation versus the pump-probe delay time t D and the real THz trace τ. (b) Relative THz transmittance change ( ∆T T ) as a function of t D at 6 µJ/cm2 pump fluence. (c) Line-scans of the THz transmittance spectra versus

t D . Each set of traces is vertically offset by 0.5 for clarity. Symbols show the experimental data and lines show the numerical fit. (d) Schematic illustration of ultrafast dynamics in the GaAs metasurface, consisting of photoexcitation, hot electron thermalization and electron-phonon coupling. (e), Extracted Drude model parameters ( ω p and γ ) as a function of t D , together with exponential fits.


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Fig. 3. (a) 2D colormap of the numerically simulated transmittance of the GaAs metasurface with 30 µm grating period as a function of the GaAs plasma frequency. The white dashed line is a guide to the eye of the location of the electric dipole resonance. (b) The electric field magnitude Abs(E) in log scale at the electric dipole resonance (noted as the star in panel a). (c) 2D colormap of the numerically simulated transmittance of the GaAs metasurface with 100 µm grating period as a function of the GaAs plasma frequency. (d) The electric field magnitude Abs(E) in log scale at the non-localized resonance (noted as the star in panel c). (e) Simulated spectral transmittance as a function of the duty cycle of the photoexcited GaAs resonator with a fixed lattice constant. (f) Simulated spectral transmittance as a function of the period of the photoexcited GaAs resonator with a fixed resonator width.


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Fig. 4. (a) Measured transmittance spectra of the photo-imprinted GaAs metasurface as a function of pump fluence. (b) Simulated transmittance spectra of the photo-imprinted GaAs metasurface as a function of pump fluence, accounting for inhomogeneous carrier profiles. The inset shows the electric field magnitude Abs(E) in linear scale at the electric dipole resonance with a pump fluence of 6 µJ/cm2. (c) Drude model parameters ( ω p and γ ) extracted by fitting the numerical simulation to the experimental data. (d) Measured transmittance spectra of the homogeneously pumped GaAs metasurface as a function of pump fluence.


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Fig. 5. (a) Schematic and dimension (in µm) of a transient GaAs metasurface perfect absorber. (b) 2D colormap of the numerically simulated reflectance of the GaAs metasurface perfect absorber as a function of the GaAs plasma frequency. (c) Line-scans of the THz spectral reflectance after photoexcitation in log scale. The inset shows the same graph but with the reflectance in linear scale.


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For Table of Contents Use Only

Transient GaAs Plasmonic Metasurfaces at Terahertz Frequencies Yuanmu Yang, N. Kamaraju, Salvatore Campione, Sheng Liu, John L. Reno, Michael B. Sinclair, Rohit P. Prasankumar, Igal Brener

Schematic of a transient GaAs metasurface induced by a structured femtosecond optical pump beam and the dimensions of the structure. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz.


Transient GaAs Plasmonic Metasurfaces at Terahertz Frequencies

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