pubs.acs.org/NanoLett Letter Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials Igor Ilyakov,* Alexey Ponomaryov, David Saleta Reig, Conor Murphy, Jake Dudley Mehew, Thales V.A.G. de Oliveira, Gulloo Lal Prajapati, Atiqa Arshad, Jan-Christoph Deinert, Monica Felicia Craciun, Saverio Russo, Sergey Kovalev,* and Klaas-Jan Tielrooij* Cite This: Nano Lett. 2023, 23, 3872−3878 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nano- second time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz- induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime. KEYWORDS: terahertz radiation, frequency conversion, ultrafast thermal emission, graphene, electrical gating, metamaterial T he current and future needs of society for high-frequency communication, imaging, and sensing have prompted great interest in THz photonics and optoelectronics research carrier frequencies will be deployed. An efficient and tunable mechanism of converting THz waves to the visible or near- infrared domain is thus highly desirable, yet currently elusive. and development.1,2 Particularly interesting applications exist Gapless 2D materials with massless charge carriers, such as in fields ranging from homeland security to quality inspection graphene and topological insulators, are highly promising for and from astronomy to wireless communication. The THz THz optoelectronics due to the combination of high carrier range is however a technologically less developed part of the mobility and low electronic heat capacity of the 2D massless electromagnetic spectrum, due to its location between the realms of electronics and optics. electron gas. Indeed, graphene has been studied and exploited The conversion of signals between different frequencies quite extensively in the THz regime. For example, it has been provides a powerful approach to exploit the advantageous used for efficient THz detection,6−8 active ultrafast modu- properties of specific frequency regions. For example, in the lation,9,10 and THz harmonic generation.11−14 Two-dimen- method of electro-optical sampling a THz field is encoded sional materials furthermore offer the possibility to control onto the polarization of near-infrared light, which is then their properties through electrostatic gating. This has, for efficiently measured with conventional Si detectors.3 Another example, enabled electronic control over frequency conversion example is imaging applications, where recent studies showed through harmonic generation in graphene and semiconducting efficient photodetection by upconverting broad-band THz layered materials13,15−19 and electrical control of the photo- light to near-infrared light using a cryogenic quantum well luminescence of graphene excited by near-infrared light.20 device4 and graphene.5 Frequency conversion is of particular importance in communication technologies, where optical interconnects provide the crucial link between electronic Received: February 8, 2023 signals that are processed in the devices of end users and Revised: April 24, 2023 optical signals that are transported through optical fibers that Published: April 28, 2023 provide a higher information transmission capacity than electronic channels. Similar transducers will likely be required for 6G wireless communication systems, where (sub-)THz © 2023 The Authors. Published by American Chemical Society https://doi.org/10.1021/acs.nanolett.3c00507 3872 Nano Lett. 2023, 23, 3872−3878 Nano Letters pubs.acs.org/NanoLett Letter Graphene is therefore an interesting candidate material system for tunable THz-to-visible frequency conversion. Several studies have shown that graphene can emit broad- band light in the visible range upon excitation by ultrashort pulses, typically in the near-infrared range. This effect was ascribed to radiative recombination,21 thermal emission,20,22,23 and collisions between electrons and holes.24 Graphene emission in the visible range has also been observed upon excitation by intense single-cycle THz pulses,25 where the observed broad-band emission was attributed to Landau− Zener interband transitions during THz excitation. A clear understanding and experimental validation of the mechanism behind THz-induced visible emission from graphene is however currently lacking. In this work we provide a demonstration of ultrafast, gate- tunable conversion of THz light to UV−visible light and find that our results are in agreement with a thermal radiation mechanism from the heated electron system of graphene. This mechanism works as follows: incident THz radiation is absorbed by the mobile charge carriers in graphene via Drude absorption. The carriers in the electronic system rapidly exchange energy, resulting in a distribution with an elevated electron temperature. This leads to thermal radiation in the visible and near-infrared for temperatures in the range of a few thousand Kelvin. We study three different graphene samples (see Supporting Information Note 1 for details). Sample A (see Figure 1a) consists of single-layer graphene grown by chemical vapor deposition (CVD) on a 1 mm thick quartz glass substrate. The graphene layer is in contact with two metal electrodes that act as source and drain, allowing for measurement of the resistance. Two gate electrodes at opposite lateral edges of the graphene sheet are in contact with the polymer electrolyte LiClO4:poly(ethylene oxide), but not with the graphene sheet, thus allowing for electrostatic gating. By applying a gate voltage on the order of 1 V, we can shift the Fermi energy EF up to around 0.4 eV.13 Sample B consists of a few-layer graphene film grown by CVD and intercalated with ferric chloride (FeCl3) (see Supporting Information Note 1).26 Hole doping from FeCl3 increases the carrier density in the graphene layers, while leaving this material highly transparent to visible and near-infrared light.27 We estimate the Fermi energy in Sample B to be 0.55 eV (see Supporting Information Figure S1). Finally, Sample C is a grating-graphene metamaterial sample, where CVD graphene is covered by Figure 1. (a) Schematic of the THz-induced photoemission metallic strips with a metal width of 18 μm, separated by gaps generation from an electrically gated graphene sample (Sample A). with a width of 2 μm. This implies that 90% of the graphene Drain (D) and source (S) electrodes are in contact with the graphene area is covered by metal, and there is 10% active graphene area. layer. An electrolyte gate covers the graphene layer and gate (G) This Sample C leads to increased light−matter interaction electrodes. Applying an electrical voltage to the electrolyte shifts the Fermi energy in graphene. (b) Temporal dependence of the observed inside the gap, where the field enhancement factor is emission (thin red line represents individual data points; thick red line approximately 5, and to an overall increased absorption by a represents the smoothed values) measured by time-correlated single factor of 2.5.12 photon counting. The detected fwhm is very close to the instrument In our experiments, we excite these samples using single- response function of 210 ps (black dashed line), determined by using cycle THz pulses centered around 0.5 THz and collect emitted incident laser pulses at 800 nm with a duration of 30 fs. Inset: electro- photons over an angle of approximately 10° (see Supporting optically detected time profile of the THz pump-pulse intensity. (c) Information Note 2 for additional information about the We control the THz-induced emission in the visible range using the experiments). The peak THz fluence is on the order of 176 μJ gate voltage, where the emission magnitude (in counts/s) changes by cm−2, which is incident on the graphene samples at an angle of more than 30 times. We simultaneously measured the graphene resistance by applying a small bias voltage between S and D electrodes 45°. We first detect the optical emission using time-correlated and the measuring current (see top horizontal axis). The highest single-photon counting (TCSPC) and Sample A. We observe resistance occurs at the Dirac point for VD = 0.9 V, where THz visible emission with a duration of around 240 ps (full-width at absorption is reduced, resulting in a lower electron temperature and half-maximum, fwhm), which is similar to the instrument therefore weaker emission in the visible region. The green-dashed response function (see Figure 1b). This shows that THz- horizontal line is a background level detected signal when the induced visible emission is only present during a very short substrate with electrolyte and without graphene is used. 3873 https://doi.org/10.1021/acs.nanolett.3c00507 Nano Lett. 2023, 23, 3872−3878 Nano Letters pubs.acs.org/NanoLett Letter time scale after interaction with a THz pulse, which is emission spectra using gate-tunable Sample A. The THz- interesting, because it allows for THz-to-visible light induced emission spectra are very broad and extend beyond conversion at high modulation frequencies. the experimentally observed range (1.5−4 eV). We describe We next demonstrate electrostatic control over visible light these spectra by blackbody radiation using Pem(ν)= C·(hν)2/[1 emission upon excitation by THz pulses (see Figure 1c). We + exp(hν/(2·Te)]2, where Pem is the emitted power, ν is the observe an on/off ratio of more than 30 for the visible emission emitted photon frequency, h is Planck’s constant, Te is the from Sample A, while the electrical resistance of the graphene graphene sheet electronic temperature, and C is a constant that layer changes by around a factor of 3. The number of emitted captures the emission and collection efficiencies of the photons increases upon increasing the doping level. This experimental setup. Importantly, all experimental spectra can dependence is qualitatively different from that observed for be described by thermal emission with a fixed amplitude C and visible emission induced by ultrashort near-infrared pulses a temperature Te that depends on the Fermi energy. Figure 2b instead of THz pulses. In that case, the emission gradually shows the obtained temperatures. decreases upon increasing the Fermi energy, followed by a We test the validity of the thermal emission mechanism by steep decrease when the Fermi energy exceeds half of the comparing the obtained temperatures with a calculation of the photon energy of the excitation pulse.20 In both cases, we electron temperature of graphene upon THz light absorption. understand the trends intuitively from the Fermi-level- In this model (see Supporting Information Note 3), all energy dependent light absorption. For near-infrared excitation, the that is absorbed from THz radiation is directly converted into steep decrease in the emission is due to Pauli blocking, which electronic heat. The temperature that is reached is governed by the electronic heat capacity, where we use the regime Te > TF = strongly limits interband absorption for elevated Fermi energy. EF/kB, with TF being the Fermi temperature and kB the For THz excitation, absorption occurs due to the intraband Boltzmann constant.28 In this regime, the heat capacity does Drude response. Upon increasing the doping level the number not depend on the Fermi energy and the only parameter that of free carriers increases, which leads to increased THz changes with EF is the THz absorption, which depends on the absorption. sheet conductivity σ. Since we simultaneously perform gate- In order to study these qualitative trends more quantita- dependent electrical conductivity measurements through the tively, we examine the emission spectra as a function of Fermi source-drain contacts of our sample (see Figure 1b), we obtain energy. In Figure 2a, we show the experimentally obtained the Fermi-energy-dependent absorption from the sheet conductivity using the thin-film approximation (see Supporting Information Note 3). The resulting electron temperatures from this simple thermodynamic model agree well with the experimental data, which adds credibility to the assignment of the THz-induced emission to an electronic heat effect. Since we observe that increasing the Fermi energy leads to stronger THz-induced visible emission, we study if we can further increase the emission by using a highly doped FeCl3 intercalated few-layer graphene sample (sample B). We also study if we can increase the emission using a grating-graphene metamaterial with enhanced light−matter interaction. We prepared Sample C, which consists of a graphene sheet covered by metallic stripes, as described in ref 12. Figure 3a shows schematics of Samples B and C. For these measurements, we vary the incident THz power and measure the collected number of emitted photons, as shown in Figure 3b. We observe a monotonous increase, where the optical emission scales roughly quadratically with incident THz power. The observed emission from Samples B and C is stronger than the emission from the gate-tunable sample (Sample A, taken at Vg − VD = −1.8 V), which we ascribe to the higher Fermi energy (Sample B) and to an enhanced light−matter interaction (Sample C). We note that the Fermi energy in Sample B is 0.55 eV, which could affect interband transitions below ∼1.1 eV through Pauli blocking. Since we examine spectra well above 1.5 eV, we are not sensitive to these effects. At the Figure 2. THz-induced emission spectra and corresponding radiation highest THz power, we find that the grating-graphene temperatures. (a) Experimental photoemission spectra at different metamaterial emits up to 2 orders of magnitude more photons gate voltages (dots) together with blackbody radiation spectra (lines), than the gate-tunable graphene. where the electron temperature is the only freely adjustable In order to understand these results quantitatively, we again parameter, while the amplitude scaling factor is kept fixed for all describe the experimentally obtained spectra using blackbody measurements. The optical spectra are normalized by the response curve of the spectrometer. (b) The electron temperatures obtained radiation with a fixed amplitude C and variable temperature Te from the blackbody emission spectra (dots) and from a thermody- (see Figures 3c,d). We note that the spectra in Figure 3d are namic model of THz-induced electron heating (dashed line; see taken over a narrower photon energy range because of Supporting Information Note 3), as a function of the gate voltage (VD emission from the metal strips at higher photon energies (see = 0.9 V). Supporting Information Figure S2). Figure 3e,f shows the 3874 https://doi.org/10.1021/acs.nanolett.3c00507 Nano Lett. 2023, 23, 3872−3878 Nano Letters pubs.acs.org/NanoLett Letter Figure 3. Increasing the THz-induced visible emission. (a) Schematic of the THz-induced photoemission generation from an intercalated few-layer graphene sample (Sample B) and a grating-graphene metamaterial sample (Sample C). (b) THz-induced visible emission in different samples for increasing incident THz pulse energy. The gated graphene data point is taken at Vg − VD = −1.8 V. (c, d) Emission spectra and corresponding blackbody radiation temperatures for Samples B (c) and C (d) as a function of the incident THz pulse energy. The temperature is the only freely adjustable parameter for the curves corresponding to each sample. The optical spectra are normalized by the response curve of the spectrometer. (e, f) The electron temperatures obtained from the blackbody emission spectra (dots) and from a thermodynamic model of THz-induced electron heating (dashed line; see Supporting Information Note 3), as a function of THz pulse energy for Sample B (e) and Sample C (f). obtained temperatures from the blackbody radiation spectra, blackbody radiation spectra, for a range of incident powers and together with the results from our thermodynamic model. a range of Fermi energies, and the obtained temperatures are in Again, we use the limit Te > TF and fix EF to the value obtained agreement with those from a simple thermodynamic model. from independent measurements on each of the samples (see This leads to the conclusion that the observed emission is Supporting Information Note 3). This means that the only likely the straightforward result of THz-induced thermal parameter that varies is the absorbed power. For Sample C, we emission from the heated electron system. This is in agreement use a grating-induced absorption enhancement of 2.5, based on with earlier observations of efficient THz-induced electronic the simulations in ref 10. The observed agreement again adds heating in graphene29 and thermal emission of the electron credibility to the assignment of the THz-induced emission as a system of graphene upon excitation by ultrashort pulses in the consequence of THz-induced electronic heating. near-infrared.20,22,23 We will now briefly discuss the results in terms of the The speed of this THz-to-visible conversion mechanism is mechanism that is responsible for THz-induced visible light promising for data communication applications that require emission, followed by a discussion of the speed and efficiency GHz, or even THz, modulation frequencies. We observe a of this conversion mechanism, as well as possible technologies temporal response with a subnanosecond time scale, which is that could be enabled by this mechanism. The experimental an upper limit because it is limited by the temporal response spectra of all three samples can be described accurately by function of our detection system. Indeed, we expect that 3875 https://doi.org/10.1021/acs.nanolett.3c00507 Nano Lett. 2023, 23, 3872−3878 Nano Letters pubs.acs.org/NanoLett Letter emission occurs within a ps time window�the time it takes for signatures of saturation or threshold. Our findings thus pave THz heated electrons to dissipate their energy to phonons.30 the way for a range of THz photonic technologies that require Thus, this mechanism could allow for ultrahigh-speed control of THz-to-visible energy conversion at exceptionally modulation of THz-to-optical conversion with a bandwidth high speeds. up to hundreds of gigahertz, which is interesting for information and communication technologies. We note that high-speed switching can be achieved with a back-gate or top- ■ * ASSOCIATED CONTENT sı Supporting Information gate31 or by hybridizing electrolyte top-gating with back- The Supporting Information is available free of charge at gating.32 We note that the conversion benefits from an elevated https://pubs.acs.org/doi/10.1021/acs.nanolett.3c00507. electron temperature, which benefits from the use of large peak powers. For THz pulses that are longer than the cooling time, Experimental methods, THz intensity estimation, sample the efficiency decreases for a given average THz power. preparation, theoretical calculations of electron temper- Sample C has the highest optical emission, which ature, characterization of Sample B, and optical emission corresponds to a 10−10 THz-to-visible energy ratio. Since the from a metamaterial grating sample without graphene collection angle in our experiment is roughly 1/400 of the (PDF) whole sphere, the actual conversion efficiency is significantly higher�approximately 10−8. Importantly, the power depend- ence of the emission scales quadratically with THz power without any signatures of saturation. Thus, we expect to ■ AUTHOR INFORMATION Corresponding Authors achieve higher THz-to-visible conversion efficiencies under Igor Ilyakov − Institute of Radiation Physics, Helmholtz- higher incident THz fields. Furthermore, by using metamate- Zentrum Dresden-Rossendorf, 01328 Dresden, Germany; rial structures with larger field and absorption enhance- orcid.org/0000-0002-5928-7996; Email: i.ilyakov@ ment,33−36 we expect to reach efficient conversion even for hzdr.de lower incident THz powers, well below the μW range. We note Sergey Kovalev − Institute of Radiation Physics, Helmholtz- that the absence of saturation is in contrast to recent studies of Zentrum Dresden-Rossendorf, 01328 Dresden, Germany; THz harmonic generation using grating-graphene metamate- orcid.org/0000-0002-2290-1016; Email: s.kovalev@ rials,12,37 which we understand as follows. To increase incident hzdr.de power, electronic cooling via phonon emission slows down as Klaas-Jan Tielrooij − Catalan Institute of Nanoscience and shown in ref 30, which limits harmonic generation, as this Nanotechnology (ICN2), BIST and CSIC, Barcelona 08193, relies on the ultrafast heating−cooling dynamics.37 In contrast, Spain; Department of Applied Physics, TU Eindhoven, 5612 we expect that THz-induced visible emission benefits from AZ Eindhoven, The Netherlands; orcid.org/0000-0002- slower electronic cooling via phonons, as this currently acts as 0055-6231; Email: klaas.tielrooij@icn2.cat an efficient competing pathway for thermal emission. Interestingly, suspended graphene demonstrates a 1000-fold Authors enhancement in thermal radiation efficiency compared with Alexey Ponomaryov − Institute of Radiation Physics, substrate-supported graphene, when both are excited with DC Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, electrical current,38−40 which could also be a promising Germany approach to enhance the THz-to-visible conversion efficiency. David Saleta Reig − Catalan Institute of Nanoscience and Finally, we point out that we could use nanophotonic Nanotechnology (ICN2), BIST and CSIC, Barcelona 08193, structures in order to enhance and control the coupling Spain; orcid.org/0000-0003-3189-2331 efficiency of hot carriers to far-field radiation, through Conor Murphy − Centre for Graphene Science, University of plasmon−electron and plasmon−phonon interactions.41 Such Exeter, Exeter EX4 4QF, U.K. an approach can also lead to more narrow-band radiation, as Jake Dudley Mehew − Catalan Institute of Nanoscience and shown for mid-infrared emission from graphene coated with Nanotechnology (ICN2), BIST and CSIC, Barcelona 08193, nanodisks and excited by ultrashort near-infrared light pulses.42 Spain; orcid.org/0000-0002-8859-9374 Regarding potential technologies based on this ultrafast and Thales V.A.G. de Oliveira − Institute of Radiation Physics, controllable THz-to-visible conversion mechanism, there are Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, several opportunities related to information and communica- Germany; orcid.org/0000-0002-4886-0654 tion technologies. For example, since sixth generation (6G) Gulloo Lal Prajapati − Institute of Radiation Physics, wireless communication systems will likely employ frequency Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, bands above 100 GHz, it could be interesting to have a way to Germany; orcid.org/0000-0001-7111-4206 convert signals from telecom photons to THz photons and vice Atiqa Arshad − Institute of Radiation Physics, Helmholtz- versa. Interestingly, the peak of the THz-induced thermal Zentrum Dresden-Rossendorf, 01328 Dresden, Germany emission spectra that we obtained occurs around 1 eV, which is Jan-Christoph Deinert − Institute of Radiation Physics, close to the most important telecom bands (1550 nm = 0.8 Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, eV). These spectra are determined by the charge carrier Germany; orcid.org/0000-0001-6211-0158 temperature, which can be controlled either by THz intensity Monica Felicia Craciun − Centre for Graphene Science, or by electrical gating. Thus, this mechanism could be a University of Exeter, Exeter EX4 4QF, U.K. promising avenue toward THz-to-telecom interconnects. The Saverio Russo − Centre for Graphene Science, University of observed effect can be also used for spatiotemporal detection Exeter, Exeter EX4 4QF, U.K.; orcid.org/0000-0002- of THz wavepackets, as the THz-induced light emission occurs 9699-4681 on a picosecond time scale and the emitted radiation scales Complete contact information is available at: linearly with instantaneous THz intensity without any https://pubs.acs.org/10.1021/acs.nanolett.3c00507 3876 https://doi.org/10.1021/acs.nanolett.3c00507 Nano Lett. 2023, 23, 3872−3878 Nano Letters pubs.acs.org/NanoLett Letter Author Contributions Degl’Innocenti, R. Ultrafast modulation of a THz metamaterial/ I.I. conceived the idea for the research. I.I., A.P., and S.K. graphene array integrated device. Appl. Phys. Lett. 2022, 121, designed and performed the experiments with contribution No. 091102. from T.V.A.d.G.O., G.L.P., A.A., and J.-C.D. D.S.R. fabricated (10) Dai, Z.; Manjappa, M.; Yang, Y.; Tan, T. C. W.; Qiang, B.; Han, and characterized the graphene-grating sample, supervised by S.; Wong, L. J.; Xiu, F.; Liu, W.; Singh, R. High Mobility 3D Dirac Semimetal (Cd3As2) for Ultrafast Photoactive Terahertz Photonics. K.-J.T. C.M., S.R., and M.F.C. fabricated and characterized the Adv. Funct. Mater. 2021, 31, 2011011. highly doped few-layer graphene sample, based on the idea (11) Hafez, H. A.; Kovalev, S.; Deinert, J. C.; Mics, Z.; Green, B.; from J.D.M. and K.-J.T. K.-J.T. performed the theoretical Awari, N.; Chen, M.; Germanskiy, S.; Lehnert, U.; Teichert, J.; Wang, modeling. I.I., K.-J.T., and S.K. analyzed the data and wrote the Z.; Tielrooij, K. J.; Liu, Z.; Chen, Z.; Narita, A.; Müllen, K.; Bonn, M.; manuscript. All authors discussed the results and commented Gensch, M.; Turchinovich, D. Extremely efficient terahertz high- on the manuscript. harmonic generation in graphene by hot Dirac fermions. Nature 2018, Notes 561, 507−511. The authors declare no competing financial interest. (12) Deinert, J.-C.; Iranzo, D. A.; Pérez, R.; Jia, X.; Hafez, H. A.; Ilyakov, I.; Awari, N.; Chen, M.; Bawatna, M.; Ponomaryov, A. N.; ■ ACKNOWLEDGMENTS Parts of this research were carried out at ELBE at the Germanskiy, S.; Bonn, M.; Koppens, F. H. L.; Turchinovich, D.; Gensch, M.; Kovalev, S.; Tielrooij, K.-J. Grating-Graphene Meta- material as a Platform for Terahertz Nonlinear Photonics. ACS Nano Helmholtz-Zentrum Dresden-Rossendorf e.V., a member of 2021, 15, 1145−1154. the Helmholtz Association. S.R. and M.F.C. acknowledge (13) Kovalev, S.; Hafez, H. A.; Tielrooij, K.-J.; Deinert, J.-C.; Ilyakov, financial support by the EPSRC (EP/V048163/1, EP/ I.; Awari, N.; Alcaraz, D.; Soundarapandian, K.; Saleta, D.; V052306/1) and the Leverhulme Trust. Germanskiy, S.; Chen, M.; Bawatna, M.; Green, B.; Koppens, F. H. L.; Mittendorff, M.; Bonn, M.; Gensch, M.; Turchinovich, D. ■ ABBREVIATIONS THz, terahertz; CVD, chemical vapor deposition; TCSPC, Electrical tunability of terahertz nonlinearity in graphene. Sci. Adv. 2021, 7, eabf9809. (14) Kuznetsov, K. A.; Tarasenko, S. A.; Kovaleva, P. M.; Kuznetsov, time-correlated single-photon counting; fwhm, full-width at P. I.; Lavrukhin, D. V.; Goncharov, Y. G.; Ezhov, A. A.; Ponomarev, half-maximum; 6G, sixth generation D. S.; Kitaeva, G. Kh. Topological Insulator Films for Terahertz ■ REFERENCES (1) Dhillon, S. S.; Vitiello, M. S.; Linfield, E. H.; Davies, A. G.; Photonics. Nanomaterials 2022, 12, 3779. (15) Ross, J. S.; Wu, S.; Yu, H.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J.; Mandrus, D. 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