Composition Control of Plasmon–Phonon Interaction Using

May 24, 2016 - The topological quantum-phase transition induced by varying the ... is (Bi1–xInx)2Se3, where the SOC-dependent quantum phase can be ...
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Letter pubs.acs.org/journal/apchd5

Composition Control of Plasmon−Phonon Interaction Using Topological Quantum-Phase Transition in Photoexcited (Bi1−xInx)Se3 Sangwan Sim,†,# Jun Park,†,# Nikesh Koirala,‡ Seungmin Lee,† Matthew Brahlek,‡,Δ Jisoo Moon,‡ Maryam Salehi,∥ Jaeseok Kim,† Soonyoung Cha,† Ji Ho Sung,§,¶ Moon-Ho Jo,§,¶,⊥ Seongshik Oh,‡,□ and Hyunyong Choi*,† †

School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea Department of Physics and Astronomy, ∥Department of Materials Science and Engineering, and □Institute for Advanced Materials, Devices and Nanotechnology, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, United States § Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), ¶Division of Advanced Materials Science, and ⊥Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Pohang 790-784, Korea ‡

S Supporting Information *

ABSTRACT: Plasmonics is a technology aiming at light modulation via collective charge oscillations. Topological insulators, where Dirac-like metallic surfaces coexist with normal insulating bulk, have recently attracted great attention in plasmonics due to their topology-originated outstanding properties. Here, we introduce a new methodology for controlling the interaction of a plasmon with a phonon in topological insulators, which is a key for utilizing the unique spectral profiles for photonic applications. By using both static and ultrafast terahertz spectroscopy, we show that the interaction can be tuned by controlling the chemical composition of (Bi1−xInx)2Se3 microribbon arrays. The topological quantum-phase transition induced by varying the composition drives a dramatic change in the strength of the plasmon−phonon interaction. This was possible due to the availability of manipulating the spatial overlap between topological surface plasmonic states and underlying bulk phonons. Especially, we control the laser-induced ultrafast evolution of the transient spectral peaks arising from the plasmon−phonon interaction by varying the spatial overlap across the topological phase transition. This study may provide a new platform for realizing topological insulator-based ultrafast plasmonic devices. KEYWORDS: topological insulators, plasmonics, ultrafast terahertz spectroscopy, plasmon−phonon interaction, topological phase transition

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As a possible methodology for controlling the plasmon− phonon interaction, we consider manipulation of the material’s topological character by tuning its chemical composition. This methodology is based on the quantum phase transition20−26 (QPT) between topological and nontopological phase (Figure 1b). It is well established that, in a topological phase (i.e., normal TI), the spin−orbit coupling (SOC) is strong enough to drive band inversion in the bulk, leading to TSS formation. On the other hand, the TSS cannot be formed in a nontopological phase due to the weak SOC. The QPT between these two phases has been realized in recent studies through engineering the SOC. A representative example is (Bi1−xInx)2Se3, where the SOC-dependent quantum phase can be controlled by manipulating the In atom (x) concentration.23 Figure 1b describes the QTP in (Bi1−xInx)2Se3. While the strong SOC sustains the existence of the TSS (red regions) at x < 0.06, the system enters into the nontopological phase due to

topological insulator (TI) is a quantum system where massless Dirac electron states are formed on the surface of normal insulating bulk (Figure 1a).1−4 Recently, TIs are of particular interest in plasmonics5−9 owing to their outstanding properties;10−19 since the electrons in the topological surface states (TSSs) are protected from backscattering, the plasmon mode of TSS electrons exhibits a significantly low damping character and strong robustness to temperature variations.10,13 More importantly, recent studies have shown that the plasmon strongly interacts with bulk phonon modes in the terahertz (THz) range, which leads to unique spectral profiles that can be utilized in various applications.10,11,13 The strong plasmon−phonon interaction also has been found to play a pivotal role in the ultrafast modulation of the plasmon profiles.14 Due to such importance, research efforts have been devoted to control the interaction via various methods, such as manipulation of size and shape of plasmonic structures and modulation of topological states by using a magnetic field.10,11,13 However, in order to manipulate the optical properties in practice, a higher degree of freedom is required in designing the optimum plasmonic system. © XXXX American Chemical Society

Special Issue: Nonlinear and Ultrafast Nanophotonics Received: January 12, 2016

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DOI: 10.1021/acsphotonics.6b00021 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) Illustration of the surface plasmon in a TI thin film. Collective oscillations of TSS electrons (red regions) at the top and bottom surfaces are coupled via Coulomb interaction. (b) Schematics describing the quantum phase transition in (Bi1−xInx)2Se3. While the TSS wave functions penetrate into the bulk more deeply with increasing In concentration (x) in the topological phase (x < 0.06), there exists no TSS in the nontopological phase (x ≥ 0.06).

Figure 2. (a) Optical microscopy image of the microribbon array sample for x = 0.00. Polarization of the THz field (blue line) is perpendicular to the microribbon axis (blue arrow), which is vertically incident on the plane of the sample. (b) Description of the plasmon− phonon interaction model. Red continuum-like states and gray discrete state represent the plasmon and the phonon, respectively. v, g, and w are coupling parameters (see main text). (c) THz extinction spectra of (Bi1−xInx)2Se3 microribbon arrays for several x values (open circles). The normalized bare plasmon responses (red dotted lines) are obtained through the plasmon−phonon interaction model analysis (black solid fit lines). Gray lines are bare phonon resonances extracted from the THz response of the unpatterned samples. (d) Corresponding plasmon line width as a function of In concentration. (e) Line width of the bare phonon in unpatterned samples (black circles) and v parameters of the patterned microribbon arrays (blue squares).

the weak SOC when x is larger than a critical value (∼0.06).23−25 It is important to note that, in the topological phase (x < 0.06), the TSS wave functions penetrate into bulk more deeply with increasing x, resulting in an increased TSS− bulk spatial overlap.23−25 This interesting property hints toward a novel strategy for controlling the plasmon−phonon interaction because their spatial overlap can be manipulated, considering plasmons and phonons are originated mainly from the TSS and bulk, respectively.10,14 In this work, by means of both static and ultrafast timeresolved THz spectroscopy,27 we demonstrate that the plasmon−phonon interaction can be tuned by controlling the chemical composition of (Bi1−xInx)2Se3 microribbon arrays. The TSS−bulk spatial overlap depending on the composition plays a key role in tuning the plasmon−phonon interaction. In static x-dependent measurements, the plasmon−phonon interaction is enhanced when the TSS−bulk overlap is increased with increasing x in the topological phase (x < 0.06), whereas it abruptly decreases with vanishing TSS (x ≥ 0.06). Ultrafast time-resolved THz measurements are performed to confirm the effect of the TSS−bulk overlap on the plasmon−phonon interaction. Upon photoexcitation, while coupling between the plasmon and the phonon is reduced when the TSS−bulk overlap is small (x = 0.00), the coupling is robustly maintained when the TSS−bulk overlap is large (x = 0.04). We provide compelling experimental evidence by presenting time-dependent transient extinction spectra. These results underscore the crucial role of the TSS−bulk overlap in determining the plasmon−phonon interaction, possibly for controlling the spectral profile in potential ultrafast TI-based photonic devices. We first explore static THz responses of (Bi1−xInx)2Se3 microribbon arrays (Figure 2a) in order to understand the xdependent plasmon−phonon interaction (Figure 2b).10,28,29 The width and the thickness of the ribbons are 7 μm and 50 nm, respectively, and the array period is 14 μm. Details in the sample fabrication and the THz measurements are described in Methods. In Figure 2c, we plot the measured x-dependent extinction spectra (black circles) with noninteracting phonons (gray lines) obtained from unpatterend TI samples for

comparison (see Supporting Information).10 At x < 0.06, we clearly observe that each spectrum shows distinguishable maxima both above and below the phonon resonance near 1.2−1.4 THz and 2−2.2 THz, respectively. Such profiles agree well with previous studies and can be understood by the destructive/constructive Fano-type30 interference of the phonon with the plasmon excitation.10,11,13,14 Note that, in the nontopological phase (x ≥ 0.06), the lower peak is not clearly distinguishable due to the largely broadened plasmon profiles (see below). To obtain a deeper understanding of the measured spectra, we performed an analysis using the plasmon−phonon interaction model following ref 10. As illustrated in Figure 2b, this model accounts for the Fano-type interaction between continuum-like plasmon (red) and discrete phonon (gray) states.28,29 The corresponding coupling strength can be gauged by the parameter v. The parameter g describes the excitation probability of the plasmon with a THz electric field. Thus, it can be approximated by g = (Γplasmon)0.5 (where Γplasmon is the plasmon line width) via Fermi’s golden rule. Although v can also be assumed to be the square root of the phonon line width in a similar manner, we will show that this simple relation is not valid due to the effect of TSS−bulk overlap on the plasmon− phonon interaction. The remaining coupling parameter w measures the coupling strength between the ground and the noninteracting phonon states (see Supporting Information).28,29 By fitting this model to measured spectra (black B

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Figure 3. (a) Schematic of the ultrafast optical-pump THz-probe spectroscopy. (b) Normalized dynamics of the transient THz field under 1.55 eV optical excitation at x = 0.00 (blue) and x = 0.04 (red). (c, d) Transient extinction spectra (open circles) with bare plasmon resonances (dotted lines) obtained from the plasmon−phonon interaction model analysis (solid fit lines) for several pump−probe time delays at x = 0.00 (c) and x = 0.04 (d). Filled circles are maxima, and dashed black lines are guides to the eye. (e, f) Time-resolved dynamics of the plasmon center frequency (e) and v parameter (f) at x = 0.00 (blue squares) and x = 0.04 (red squares). Note that, for these two samples, different excitation fluences (55 μJ cm−2 for x = 0.00 and 180 μJ cm−2 for x = 0.04) were used to induce comparable maximum transient photocarrier density nmax ≈ 2.7 × 1019 cm−3 (see Supporting Information).

< 0.06 (Figure 2c and d), which cannot explain the sharp rise in v in the topological phase (Figure 2e). For the phonon mode, the change in Γphonon can influence v, as mentioned above.28,29 However, our calculation shows that the increase in TSS−bulk spatial overlap makes about 4 times larger contribution to the change of v, compared with Γphonon, confirming the dominant role of TSS−bulk overlap in determining the plasmon−phonon interaction in the TI (see Supporting Information). In order to show more clearly the role of TSS−bulk overlap on the plasmon−phonon interaction, we compare two representative ultrafast dynamics of the samples with the smallest (at x = 0.00) and the largest (at x = 0.04) TSS−bulk overlap. For this purpose, we employ a standard optical-pump THz-probe spectroscopy, where an optical short pulse with a photon energy of 1.55 eV and duration of 50 fs serves as a pump for all measurements (Figure 3a). Before analyzing spectral responses, it is instructive to compare the time scales of these samples. As shown in Figure 3b, the transient change in the THz field (ΔE) at x = 0.04 is much faster than that measured at x = 0.00. We attribute the rapid dynamics at x = 0.04 to the indium-substitution-induced increase in the decay rate. When the photoexcitation takes place in a defect-rich sample, the carrier relaxation is accelerated; thereby increasing x results in the faster decay kinetics (see Supporting Information for further discussion and numerical analysis).25 This understanding is corroborated by a recent theoretical work showing that the indium atoms in (Bi1−xInx)2Se3 form isolated defect-like clusters instead of homogeneous substitution of the bismuth atoms.26 We now explore the dynamics of spectral profiles under ultrafast optical excitation. Figure 3 shows transient extinctions (open circles) with normalized bare plasmon line shapes (dotted lines) extracted from the plasmon−phonon interaction model (solid fit lines) for several pump−probe time delays (τ) at x = 0.00 (Figure 3c) and x = 0.04 (Figure 3d), respectively. Corresponding fitting parameters are plotted in Figure 3e and f (see Supporting Information for the fitting parameters). We observe that optical excitation leads to the photodoping-

lines in Figure 2c), we obtain bare line shapes of plasmons (red dotted lines in Figure 2c) and quantitative information about the plasmon−phonon interaction (see Supporting Information for other fitting parameters). First, we discuss the response of the bare plasmon. As shown in Figure 2d, the Γplasmon (blue squares) of the bare plasmon abruptly increases at x ≥ 0.06, which can be understood by the vanishing TSS across the QPT. In contrast to the TSS plasmon in the topological phase (x < 0.06), the plasmon excitation in the nontopological phase (x ≥ 0.06) stems from the topologically trivial two-dimensional electron gas31 (2DEG) that is formed by band bending at surfaces.13 Because the 2DEG electrons have no topological protection from backscattering, their collective mode should exhibit a large damping rate, which agrees well with our observation of the sudden increase in Γplasmon and the broad spectral profiles in the nontopological phase.13 We now discuss the x-dependent plasmon−phonon interaction based on the measured v parameters. When the Fano interaction between the continuum and the discrete states is relatively weak, v simply increases with increasing the line width of the discrete state.29 In our measurements, however, although the line width of the discrete phonon mode (Γphonon) monotonically increases with increasing x, v reaches its maximum at x = 0.04, as shown in Figure 2e. This behavior clearly indicates that there is strong plasmon−phonon interaction at x = 0.04. More importantly, considering the largest TSS−bulk overlap occurs at x = 0.04, this result provides strong evidence that the strength of the plasmon−phonon interaction can be controlled by manipulating their spatial profiles. Note that this interpretation is in good agreement with our previous THz study of unpatterned (Bi1−xInx)2Se3, where an increased TSS−bulk overlap invokes a strong Fano-type interaction between the Drude-like TSS excitation and the phonon near 2 THz.25 Of course, v can be affected by other x-dependent factors, such as center frequencies and line widths of the bare plasmon and phonon states. For the bare plasmon, however, both the center frequency and the line width show almost no change at x C

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induced blue shift of ωplasmon in both samples (Figure 3e), which follows the transient ΔE dynamics (Figure 3a).14 Although maximum shifts of ωplasmon in these samples are comparable, their extinction spectra show somewhat different features. As shown in Figure 3c (at x = 0.00), the gap between the lower and the higher peaks (filled circles) shrinks and recovers along with the changes of ωplasmon (see dashed black guidelines to the eye and dotted bare plasmon lines). It is known that the frequency interval between the maxima decreases with decreasing the Fano interaction between broad and discrete states in such a given condition.29 Thus, together with the decrease in the v parameter (blue squares in Figure 3f), the observed shrinkage of the gap between maxima at x = 0.00 can be attributed to the photoinduced reduction of the plasmon−phonon interaction. This is because the photoexcitation-induced increase in the carrier temperature reduces the electron−phonon coupling.32 Alternatively, the reduced plasmon−phonon interaction may arise from the photodoping of 2DEG and consequently increase the contribution of 2DEG to the plasmon.14 This is particularly the case where the electron−phonon coupling for the 2DEG33 is generally smaller than that for the TSS electrons.34 In contrast, when x = 0.04, although the maximum change in ωplasmon is comparable with that in the x = 0.00 sample, the size of gap between the maxima (filled circles in Figure 3d) and v (red squares in Figure 3f) is almost unchanged, indicating that the plasmon−phonon interaction is not reduced. This implies that the large TSS− bulk spatial overlap at x = 0.04 induces the robustness of the interaction, which sustains the Fano-type characteristic interaction even under strong photoexcitation. Therefore, this result provides a new methodology for controlling the laserinduced ultrafast evolution of the interaction-driven spectral peaks. Note that we have excited the x = 0.04 sample with a much higher pump fluence (180 μJ cm−2) than the x = 0.00 sample (55 μJ cm−2) to induce similar maximum transient photocarrier densities (nmax ≈ 2.7 × 1019 cm−3 for both samples) and plasmon shift (see Figure 3e) (see Supporting Information). For a deeper understanding of this phenomenon, we consider the contribution of TSS to the plasmon shift. Our prior work showed that,14 although the incident pump photons are absorbed primarily by the bulk electrons, their corresponding surface plasmon mode in the unconfined bulk is far beyond our THz window. This means that the observed plasmon shift under photoexcitation should be understood by the photocarrier transition from the bulk to the confined surface states, namely, the TSS and the bulk, and the subsequent photodoping of these states. In the normal TI at x = 0.00, the photoinduced plasmon shift mainly arises from the photodoping of 2DEG, due to the low efficiency of the photocarrier transition from the bulk to the TSS.14 However, the bulk-to-TSS transition is activated with increasing the TSS−bulk overlap,25 implying that the contribution of TSS to the plasmon mode at x = 0.04 should be larger than that at x = 0.00. Considering the electron−phonon coupling for TSS is stronger than that for 2DEG,33,34 the observed behavior of v under photoexcitation at x = 0.04 partially stems from the increased contribution of TSS to the plasmon mode. Finally, we compare the nmax-dependent extinction of these two samples. Figure 4a and b display the measured extinction spectra at τ = 6 ps (for x = 0.00) and τ = 3 ps (for x = 0.4), respectively. The corresponding ωplasmon (v) is shown in Figure 4c (Figure 4d). At x = 0.00, an increase in nmax enlarges the

Figure 4. (a, b) Transient extinction spectra (open circles) of x = 0.00 (a) and x = 0.04 (b) samples for several nmax at T = 6 ps and T = 3 ps, respectively (see Supporting Information). Dotted lines are the line shapes of the bare plasmons obtained from the plasmon−phonon interaction model analysis (solid fit lines). Filled circles are the spectral maxima, and dashed black lines are guides to the eye. (c, d) Corresponding plasmon center frequency (c) and v parameter (d) for x = 0.00 (blue squares) and x = 0.04 (red squares) samples, respectively.

photoinduced blue shift of ω plasmon (Figure 4c) and simultaneously weakens the plasmon−phonon interaction, as revealed by the shrinkage of the gap between maxima (filled circles in Figure 4a) and by the decreased v (Figure 4d). On the contrary, at x = 0.04, the size of the gap between maxima (filled circles in Figure 4b) and v (Figure 4d) is almost unchanged despite a considerable shift of ωplasmon (Figure 4c). These features clearly indicate the robustness of the plasmon−phonon interaction when the TSS−bulk overlap is large. This result further again confirms a significant role of TSS−bulk overlap on controlling the strength of the plasmon−phonon interaction. In summary, we have shown that the Fano-type plasmon− phonon interaction in TIs can be tuned by controlling the chemical composition of (Bi1−xInx)2Se3 microribbon arrays. The composition-dependent TSS−bulk spatial overlap plays a crucial role in controlling the plasmon−phonon interaction. In particular, by using time-resolved THz spectroscopy, we have found that the robustness of the plasmon−phonon interaction under optical excitation is determined by the TSS−bulk overlap, governing the ultrafast time-dependent positions of the interaction-driven spectral peaks. The composition control of the plasmon−phonon interaction can be easily hybridized with other methods, such as size/shape control of plasmonic patterns and modulation of topological states by using a magnetic field. 10,11,13 Thus, our study is expected to significantly enhance the applicability of TI plasmonic systems for novel THz modulators, considering the Fano-type interaction offers an ideal profile for manipulating the light spectra.30

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METHODS Sample Preparation. High-quality epitaxial thin films of (Bi1−xInx)2Se3 were grown on 1 cm × 1 cm × 0.5 cm Al2O3 D

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ACKNOWLEDGMENTS S.S., J.P., S.C., J.K., and H.C. were supported by the National Research Foundation of Korea (NRF) through the government of Korea (MSIP) (Grant Nos. NRF-2015R1A2A1A10052520, WCI-2011-001), Global Frontier Program (2014M3A6B3063709), the Yonsei University Yonsei-SNU Collaborative Research Fund of 2014, and the Yonsei University Future-Leading Research Initiative of 2014. J.H.S. and M.-H.J. were supported by the Institute for Basic Science, Korea, under contract number IBS-R014-G1-2016-a00. N.K., M.B., J.M., M.S., and S.O. were supported by the NSF (EFMA1542798, DMREF-1233349, and DMR-1308142) and the Gordon and Betty Moore Foundation’s EPiQS Initiative (GBMF4418).

(0001) substrates by molecular beam epitaxy (MBE) using an SVTA-designed MBE chamber, whose base pressure was below 2 × 10−10 Torr. Greater than 99.999% pure Bi, In, and Se sources from Alfa Aesar were thermally evaporated using Knusden cells for the film growth. Following the two-step growth method developed at Rutgers University,35,36 the initial 3 QL of film was deposited at 135 °C and then annealed to 300 °C, at which temperature the rest of the film was deposited. Film growth was performed in a Se-rich environment with the ratio of combined Bi and In flux to Se flux maintained at ∼1:10, and the Se shutter was kept open at all times during the growth. Single-phase two-dimensional growth mode was determined using in situ reflection high-energy electron diffraction (RHEED). Bi and In sources were calibrated using an in situ quartz crystal microbalance and ex situ Rutherford backscattering in order to accurately determine In concentration and thickness of the films. These two calibration methods combined offer accuracy to within 1% of the desired In concentration.23,35,36 The microribbon arrays were patterned using AZ5214 (1.5 μm in thickness), which was baked at 90 °C for 1 min. This was followed by the dry etching process using a reactive ion etcher (40 sccm, SF6, 10 mTorr, and 45 W). The etching rate was 18 nm min−1. The AZ5214 was cleanly removed by acetone. THz Spectroscopy. THz spectroscopy measurements were based on a 250 kHz Ti:sapphire-regenerative amplifier laser system (Coherent RegA 9050). Pulses of 1.55 eV with a duration of 50 fs were used to excite samples in the timeresolved measurements and also used to generate and detect THz pulses via optical rectification and electro-optic sampling, respectively, in a pair of ⟨110⟩ ZnTe crystals. In both static and time-resolved measurements, the extinction spectrum was obtained by 1 − T, where the T is the transmittance of the microribbon array samples. The samples were maintained at 7 × 10−6 Torr and 78 K in a cryostat for all measurements.

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REFERENCES

(1) Hasan, M. Z.; Kane, C. L. Colloquium: Topological Insulators. Rev. Mod. Phys. 2010, 82, 3045−3067. (2) Fu, L.; Kane, C. L.; Mele, E. J. Topological Insulators in Three Dimensions. Phys. Rev. Lett. 2007, 98, 106803. (3) Moore, J. E. The Birth of Topological Insulators. Nature 2010, 464, 194−198. (4) Ando, Y. Topological Insulator Materials. J. Phys. Soc. Jpn. 2013, 82, 102001. (5) Maier, S. A.; Atwater, H. A. Plasmonics: Localization and Guiding of Electromagnetic Energy in Metal/dielectric Structures. J. Appl. Phys. 2005, 98, 1−10. (6) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (7) Oulton, R. F.; Sorger, V. J.; Zentgraf, T.; Ma, R.-M.; Gladden, C.; Dai, L.; Bartal, G.; Zhang, X. Plasmon Lasers at Deep Subwavelength Scale. Nature 2009, 461, 629−632. (8) Hess, O.; Pendry, J. B.; Maier, S. A.; Oulton, R. F.; Hamm, J. M.; Tsakmakidis, K. L. Active Nanoplasmonic Metamaterials. Nat. Mater. 2012, 11, 573−584. (9) Sun, S.; He, Q.; Xiao, S.; Xu, Q.; Li, X.; Zhou, L. Gradient-Index Meta-Surfaces as a Bridge Linking Propagating Waves and Surface Waves. Nat. Mater. 2012, 11, 426−431. (10) Di Pietro, P.; Ortolani, M.; Limaj, O.; Di Gaspare, A.; Giliberti, V.; Giorgianni, F.; Brahlek, M.; Bansal, N.; Koirala, N.; Oh, S.; Calvani, P.; Lupi, S. Observation of Dirac Plasmons in a Topological Insulator. Nat. Nanotechnol. 2013, 8, 556−560. (11) Autore, M.; D’Apuzzo, F.; Di Gaspare, A.; Giliberti, V.; Limaj, O.; Roy, P.; Brahlek, M.; Koirala, N.; Oh, S.; García de Abajo, F. J.; Lupi, S. Plasmon-Phonon Interactions in Topological Insulator Microrings. Adv. Opt. Mater. 2015, 3, 1257−1263. (12) Glinka, Y. D.; Babakiray, S.; Johnson, T. A.; Lederman, D. Thickness Tunable Quantum Interference between Surface Phonon and Dirac Plasmon States in Thin Films of the Topological Insulator Bi2Se3. J. Phys.: Condens. Matter 2015, 27, 052203. (13) Autore, M.; Engelkamp, H.; D’Apuzzo, F.; Gaspare, A. Di; Pietro, P. Di; Vecchio, I. Lo; Brahlek, M.; Koirala, N.; Oh, S.; Lupi, S. Observation of Magnetoplasmons in Bi2Se3 Topological Insulator. ACS Photonics 2015, 2, 1231−1235. (14) Sim, S.; Jang, H.; Koirala, N.; Brahlek, M.; Moon, J.; Sung, J. H.; Park, J.; Cha, S.; Oh, S.; Jo, M.-H.; Ahn, J.-H.; Choi, H. Ultra-High Modulation Depth Exceeding 2,400% in Optically Controlled Topological Surface Plasmons. Nat. Commun. 2015, 6, 8814. (15) Politano, A.; Silkin, V. M.; Nechaev, I. A.; Vitiello, M. S.; Viti, L.; Aliev, Z. S.; Babanly, M. B.; Chiarello, G.; Echenique, P. M.; Chulkov, E. V. Interplay of Surface and Dirac Plasmons in Topological Insulators: The Case of Bi2Se3. Phys. Rev. Lett. 2015, 115, 216802. (16) Raghu, S.; Chung, S. B.; Qi, X.-L.; Zhang, S.-C. Collective Modes of a Helical Liquid. Phys. Rev. Lett. 2010, 104, 116401. (17) Stauber, T. Plasmonics in Dirac Systems: From Graphene to Topological Insulators. J. Phys.: Condens. Matter 2014, 26, 123201.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b00021. Plasmon−phonon interaction model, static THz responses of the unpatterned samples, static THz responses of the microribbon arrays, details of ultrafast THz responses of the microribbon arrays, details of the In-concentration-dependent photocarrier dynamics (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (H.-Y. Choi): [email protected]. Present Address Δ

Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States. Author Contributions #

S. Sim and J. Park contributed equally to this work.

Notes

The authors declare no competing financial interest. E

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ACS Photonics

Letter

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DOI: 10.1021/acsphotonics.6b00021 ACS Photonics XXXX, XXX, XXX−XXX