Infrared nano-imaging reveals the surface metallic plasmons in

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Infrared nano-imaging reveals the surface metallic plasmons in topological insulator Jian Yuan, Weiliang Ma, Lei Zhang, Yao Lu, Meng Zhao, Hongli Guo, Jin Zhao, Wenzhi Yu, Yupeng Zhang, Kai Zhang, Hui Ying Hoh, Xiaofeng Li, Kian Ping Loh, Shaojuan Li, Chengwei Qiu, and Qiaoliang Bao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00568 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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Infrared nano-imaging reveals the surface metallic plasmons in topological insulator Jian Yuan†,ǁ, Weiliang Ma†,ǁ, Lei Zhang‡,ǁ, Yao Lu†, Meng Zhao┴, Hongli Guo#, Jin †

Zhao#, Wenzhi Yu , Yupeng Zhang ┴

†

§,

┬

£

, Kai Zhang , Hui Ying Hoh ‡

§,

£

¶

, Xiaofeng Li ,

†

Kian Ping Loh , Shaojuan Li , *,Cheng-Wei Qiu , * and Qiaoliang Bao , £, * †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, 215123, Jiangsu, P. R. China ‡

Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117583, Singapore

┴

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117583, Singapore #

Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, University of Science & Technology of China, Hefei 230026, P. R. China

§

College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China,

£

Department of Materials Science and Engineering, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton, Victoria 3800, Australia. ┬

i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, Jiangsu, P. R. China ¶

College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, Jiangsu, P. R. China ǁ

These authors contributed equally to this work.

*

Corresponding author(s): Qiaoliang Bao (Q.B.): [email protected], [email protected]; Cheng-Wei Qiu (C.Q.): [email protected]; and Shaojuan Li (S.L.): [email protected].

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ABSTRACT Surface plasmons make a high degree of localization of electromagnetic fields achievable at the vicinity of metal surfaces. Topological insulators (TIs) are a family of materials which are insulating in the bulk but have metallic surfaces caused by the strong spin−orbit coupling. Surface plasmons supported by the surface state on topological insulators have attracted incredible interests from ultra violet to mid-infrared frequencies. In this work, we experimentally investigate the near-field properties of Bi2Te3 nanosheets using scattering-type scanning near-field optical microscopy (s-SNOM). The s-SNOM tip enables to detect significantly enhanced intensity in its near field at precisely controlled positions with regards to Bi2Te3 structure. With the help of highly position-selective excitation and high-pixel real-space mapping, we discover near-field patterns of bright outside fringes which are associated with its surface-metallic, plasmonic behavior at mid-infrared frequency. Thereby, we experimentally demonstrate that the scattered signal responses and near-field amplitudes of outside fringes can be tailored via mechanical (sheet thickness of Bi2Te3), electric (electrostatic gating), and optical (incident wavelength) fashions. The discovery of outside fringes in TI nanosheets may enable the development of strongly enhanced light–matter interactions for quantum optical devices, mid-infrared (MIR) and terahertz detectors or sensors.

KEYWORDS: Bismuth telluride, surface plasmon, near-field optical microscopy, infrared nanoscopy


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A topological insulator is a quantum system with an insulating electronic gap in the bulk and gapless surface metallic states. The gapless states are associated with massless Dirac fermions. A great deal of interest has been attracted to investigate the electronic and optical properties.1, 2 What’s more, TIs surface states spontaneously provide a 2D metallic system which set a new stage for investigating plasmonic behavior beyond noble metal. A special type of surface plasmon (SP) supported by massless Dirac electrons, was observed in graphene structures,3, 4 which has drawn tremendous interests for various applications in sensing5, 6 and photodetectors7, 8 due to its ultra-compact field confinement, electrical tunability, and low loss.9-12 In particular, the Dirac plasmon supported by the topologically protected surface state is immune from scattering processes and nonmagnetic defects, which is incredibly attractive for optoelectric devices.13 Lately, Dirac plasmon was demonstrated in topological insulators (TIs), a promising material with a core-shell metallic surface state originating from spin-orbit coupling.2,

13, 14

The plasmonic resonance in TI

materials could cover a broad frequency range from ultra violet to infrared by controlling the chemical composition,15, 16 which is inaccessible using conventional noble metals and graphene. Technically, localized plasmon modes can be readily excited by far field illumination. However, either special setup or structure engineering is necessary to fill the momentum gap between SPs and free space light, e.g., a prism, sharp or periodic structures can be used for this propose.11 In addition, to excite a pure surface plasmon , we also need to suppress the contribution from bulk by using low-energy photon excitation or thin samples.15,

17

To exploit potential

applications, it would be indispensable to visualize their plasmonic near-field interaction, however, it is still largely unexplored for TI family materials. Infrared scanning near-field optical microscope (SNOM) is an ideal tool to fulfill the above requirement.18 Besides the low energy source, the sharp tip can provide enough momentum to match the momentum gap.19 At last, the nanometer scale tip apex would also beat diffraction limit and enables real space imaging of SP at subwavelength scale. This technique was ever used to study one type of TI materials antimony telluride (Sb2Te3),20 but unfortunately, SPs were not observed because the


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surface states are most probably masked by the bulk effects in very thick crystals (~300

nm).

Most

recently,

localized

plasmonic

mode

in

single

crystal

Bi1.5Sb0.5Te1.8Se1.2 was investigated at visible range by s-SNOM. Typical dipolar and higher-order plasmonic field distributions were explicitly observed.21 It is noteworthy that the excitation of SPs at mid-infrared wavelength has not been observed in real space even though it has been indirectly verified by the spectroscopic approaches.22 In another aspect, the ability to control plasmonic mode via electrostatic voltage is one of the most important advantages that make graphene plasmon an interesting and promising candidate for future electronic and photonics devices.10,23 So far, electrically tunable plasmons were mainly achieved in graphene structures. However, it would be nontrivial to develop alternative plasmonic materials with similar or even superior tunability at other frequency ranges. To this regard, TI could be an interesting alternative candidate as their surface states can also be electrically tuned by gate bias.24, 25 In this work, surface states of a typical TI, bismuth telluride (Bi2Te3) spontaneously provide a 2D metallic system and we investigated the plasmonic behavior using a scattering-type SNOM (s-SNOM) equipped with infrared laser. As the topological surface states are normally observed in TI crystals with certain thickness,26 we chose Bi2Te3 nanoplaletes with thickness in the range from 5 to 25 nm so as to suppress the bulk effect. It is interesting to observe strong near-field intensity within the crystal as well as notable bright outside fringe surrounding the crystal edge, Unlike graphene plasmon which is strictly limited to one atomic layer, the bright outside fringe in TI was found to be dependent on sample thickness since it is originated from the interaction between the tip and the edge. In addition, the near field amplitude of the outside fringe can be controlled upon electrostatic gate and tuned by changing incident light wavelength. The distinct plasmonic behaviour in TI may afford new opportunities to develop MIR and terahertz detectors or sensors. RESULTS AND DISSCUSSION The schematic setup for measuring the scattering near field signal is shown in Figure


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1a. Specifically, a metallized tip (antenna) converts the incident IR light into a strongly confined near field at the tip apex, which provides the necessary momentum to launch radially emanating plasmons in TI nanosheets. The backscattered radiation is recorded simultaneously with the topography. The tip is vibrated vertically and the detected signal is demodulated at a higher harmonic n (n=3 in this work) of the tip vibration frequency, yielding background-free scattering signal. Figure 1b shows a pseudo 3D topography image of an 18-nm-thick single crystal Bi2Te3. The near-field scattering signal shows in Figure 1c, which excited by a mid-infrared (MIR) source with a wavelength of 11.086 µm. The sample was grown by a hydrothermal method.15,27 A hexagonal shape with smooth surface and sharp edges were obviously observed. In particular, there is sharp field peak around the sample edge. For a close comparison, both the profile height and normalized scattering signal are plotted in Figure 1d. Here S3 and S3 (SiO2) are the third-order demodulated harmonics of near-field amplitude measured from the Bi2Te3 sample and SiO2 substrate, respectively. Compared with SiO2 substrate, the scattering amplitude at the top surface of the flake is enhanced to 300%, which is stronger than 150−200% enhancement at graphene surface.4 Surprisingly, the scattering signal increases sharply at the edge of the Bi2Te3 flake, and decays exponentially into the exterior space, which indicates a strong evanescent feature. The distance between the apex of the sharp peak and the crystal edge is 40 nm. As the diameter of the tip is ~ 20 nm, it is further confirmed that the bright fringe is outside of the crystal surface. This observation is in good agreement with previous report on black phosphorus, Bi2Te3 and MoS2 where there is a surface metallic layer.28 While the s-SNOM tip is close to the edge of the sample, a gap hot-spot field is produced between the tip and the sample edge, reminiscent of hot spots in metal nano-plasmonics. The field in the hot spot depends on the negative permittivity of the surface metallic state.28, 29 In comparison with graphene structures with atomic thickness, the plasmonic response at TI materials arises from both bulk electrons and surface charge carriers, as that being reported in ultraviolet and visible range.15-17, 21 The band gap energy of the bulk Bi2Te3 is ~0.165 eV (i.e., λ0 = 7.515 µm). Therefore, no contribution from


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interband excitation will be involved in the above experiment. In order to further verify the origin of the sharp peak of the near-field signal, scattering signals at the cross section (red dashed line in Figure 1b and 1c) were collected under the excitation of other three wavelengths 11.286, 7.380 and 4.493 µm (i.e., with photon energy below, close to and above the band gap energy of the bulk Bi2Te3, respectively), as shown in Figure 1e. Similarly, the near field at the top surface was greatly enhanced for all three wavelengths, in comparison with the counterpart from bare SiO2 substrate. However, the field peak out of the edge disappeared under the excitation of 4.493 µm. The above observations provide an important clue about the origin of the near-field signal at MIR frequency. The contribution from surface electrons should play a crucial role in generating the bright field fringe. For excitation wavelength of 11.286 µm, because the photon energy is insufficient to compensate the overlarge band gap, the strong field can only be attributed to the excitation of surface electrons of the TI sample. However, as the photon energy increase (i.e., shorter wavelength), the interband transition is ignited, and bulk mode starts to contribute the measured signal. At last, it even overpasses the signal from surface electrons at excitation wavelength of 4.493 µm, which results in the disappearance of bright fringe in scattering signal. Bi2Te3 is a typical 3D TIs which exhibit nontrivial metallic surface states due to the strong spin−orbit coupling (SOC) effect.26, 30 To clarify the contribution of surface states, we calculated the electronic band structure of a 9 quintuple-layer (QL) Bi2Te3 flake with SOC (see our previous work for more details).15 As shown in Figure 1f, Density functional theory calculations show that all the observed plasmon modes are related to the strong spin−orbit coupling induced surface states of Bi2Te3. The topological surface states of Bi2Te3 obtained are making the surface to be metallic. Therefore, we can assume that the near-field resonance at longer wavelength mainly arises from the surface plasmons of spontaneously produced surface metallic state. Subsequently, we further performed wavelength−dependent measurements on a Bi2Te3 sample with a step-like profile, as shown in Figure 2. The sample is a single crystal grown by chemical vapor deposition (CVD) and the thickness at region A and


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region B are 25 and 15 nm (see Figure 2a), respectively. Figure 2b-d show the normalized near-field intensities obtained at three selected wavelengths. The fringes at the sample edges were undoubtedly observed for all the wavelengths in spite of limited resolution, as shown in Figure 2e. Notably, the near-field amplitude is linearly dependent on the incident wavelength, as shown in Figure 2f. In particular, the relative contrast between two regions (i.e., S3,A/S3,B = ηA/ηB, where η is the near-field contrast factor) remains ~1.25 for all the wavelength. According to the finite-dipole method, which can effectively explain the scattering signal measured with s-SNOM, 1/η ~ 1/β = (εs+1)/(εs−1), where β is the electrostatic reflection factor and εs is the effective dielectric constant of the target sample. Therefore, stronger near-field scattering at longer wavelength is expectable due to the slight increase of real part of total dielectric constant for longer wavelength (Figure S3 in Supplementary Information (SI)). One may note that the higher amplitude occurs at thicker region, which will be discussed later. The near-field response of TI materials can be effectively tuned via applying a gate bias. A gold electrode was evaporated on the top side of Bi2Te3 flakes, while both the metalized tip and surface of Bi2Te3 are at ground (GND) potential during the gate-tuning experiment, as that has been done in graphene.11,

12

By varying the

back-gate voltage Vg, the near-field optical amplitude was changed accordingly, as depicted in Figure 3a-c. Figure 3d shows more profiles of scattering amplitude extracted at the position labeled by bright dashed in Figure 3a. It is noteworthy that the field peak outside the crystal is always clearly resolved while varying the gate bias. As back-gate voltage varies from −40 V to +40 V, the scattering signal at the top surface decreases by ~10. Such a signal variation could be attributed to the variation of carriers. Such a weak dependence on back-gate voltage case be explained theoretically. In fact, the concentration of 2D carriers at a conducting surface layer can be taken into account by a Drude-term,17 yielding a dielectric function of Bi2Te3 flake given by


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ε஻௜మ ்௘య (ω) = ߝஶ (1 −

ఠ೛ మ ఠ ା௜ఠఊ

)

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(1)

with

߱௣ =

ଵ

௡௘ మ

ට ଶగ௖ ௠∗ ఌ

(2)

బ ఌಮ

where c is the speed of light, e is the elementary charge, and ε଴ is the vacuum

permittivity, ߱௣ is the plasma frequency that depends on the free-carrier concentration n as: ߱௣ ~݊.

In formula (1), the only variable parameter is the free charge carrier concentration n. When applying a negative bias (Vg = 40 V), the free-carrier concentration n is downshifted via the injection of holes. The concentration of free carrier (electron) decreases, which leads to an increase in dielectric constant of surface state. On the contrary, the scattering amplitude reaches a 268% enhancement when applying a positive bias (Vg = +40 V), which upshifts the free-carrier concentration n via the injection of electrons. Since the topological state of Bi2Te3 samples only contributes from the surface electrons within a ~2 nm thick thin layer. Therefore, the weak optical signal from the surface state is thus submerged in the counterparts from bulk contributions for thicker flakes. In order to sort out surface signal, reducing the total thickness of Bi2Te3 flakes is one of the most efficient way.26 Here, we also investigated the dependence of near-field intensity on the thickness of Bi2Te3 flakes. Figure 4a-d show topological profile of Bi2Te3 crystals with thicknesses ranging from 5 to 25 nm, while Figure 4e-h show the corresponding normalized near-field amplitude images at the fixed excitation wavelength of 10.708 µm. It is clear to see the near-field amplitude increases for thicker flakes, which is consistent with the observations in Figure 2. The extracted profiles of near-field amplitude in these crystals are shown in Figure 4i. Interestingly, the near-field amplitude is almost linearly dependent on thickness, as shown in the inset of Figure 4i. Furthermore, the bright outside fringes of Bi2Te3


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flakes are always observed and the amplitude also shows a linear dependence on the sample thickness. In order to study the decay of the edge fringe, the edge profiles were zoomed in and fitted with an exponential function. It is found that the decay length, defined as the distance at which the amplitude is decayed to 1/e of the peak amplitude, is almost the same for all the samples with different thicknesses considering the errors (see Figure S1h in SI). Based on above observation, we conclude that the optical near field signals of all the surfaces has the same origin, which is due to the oscillation of intrinsic free carriers or phonons.20,

31

Such observation is also verified by our

simulations, as shown in Figure 4j, near-field intensities at the surface of Bi2Te3 films with an increasing thickness shows the same trend as the scattering signal in experiment. The linear dependence of intensity on thickness is also verified, as shown in the inset of Figure 4j. When the s-SNOM tip is close to the sharp edge of the metallic sample, a gap hot-spot field is produced between the tip and the sample edge, reminiscent of resonant field in metal nanoplasmonics.28 The resonant field in nano-gap between metallic nanostructure is extremely sensitive to the excitation frequency and gap-width.32, 33 Mode coupling in the nano-gap also plays a significant role in remodeling the resonant field for particular applications. To further explore the coupling effect between edge modes on Bi2Te3 samples, we engraved a long and narrow gap with a depth of 25 nm in an 80-nm thick Bi2Te3 flake, which is fabricated by mechanical exfoliation. The resonant field in nano-gap between metallic nanostructure is extremely sensitive to the excitation frequency and gap-width.32, 33 Here, the gap width gradually varies from 120 to 100 nm, as indicated by dashed lines in Figure 5a. A pseudo 3D mapping shows the scattering signal measured at 10.708 µm, where the strong field is localized at the gap region. For a closer comparison, the localized field at three gap positions is plotted in Figure 5g-i. When illuminating at 10.708 µm, the localized field shows two split peaks at wider gap (blue curve), whereas two peaks gradually merge together as the gap gets narrower (red and black curves). In contrast, only one peak is observed under the excitation at long


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wavelength 11.286 µm. Therefore, the field distribution is sensitive to both gap width and wavelength. Such feature can be well attributed to the exponential decay of edge mode, as shown in Figure S1h. The localized field in the gap can be considered as a superimposition of two edge modes. Wider gap results in a less field overlap and the two peaks thus remains independent. As the gap becomes narrower, field overlap area increases and then presents only one peak. However, for excitation at longer wavelength, larger decay length makes the fields of two edge modes overlapping with each other even at wider gap. Therefore, there is always one peak under excitation at longer wavelength.

CONCULUSION In summary, we have experimentally investigated the near-field properties of Bi2Te3 flakes with different thicknesses and shapes using s-SNOM at mid-infrared frequencies. A bright near-field outside fringe was clearly observed only when the energy of the incident light is lower than the bulk band gap of Bi2Te3. Based on the systematic investigation of the wavelength- and thickness-dependent responses of the scattered signals, we attributed the observed near-field features to the interaction between the surface metallic state of Bi2Te3 samples and the tip, which were further verified by numerical simulations. The realization of tunable fringes on Bi2Te3 flakes upon an external bias will afford potential applications for novel tunable terahertz detectors or sensors.

METHODS Sample preparation. Bi2Te3 samples in our experiments were obtained with different methods including mechanical exfoliation, CVD and solvothermal method. In a typical solvothermal process, polyvinylpyrrolidone (0.3 g), Bi2O3 (0.5 mmol), TeO2 (1.5 mmol) and 2 mL of NaOH solution (5 molL-1) were added in ethylene glycol (18


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ml). The resulting suspension was transferred to an autoclave and kept at 200℃ for 4 hrs. After cooling to room temperature, the products were collected by centrifugation and washed several times with distilled water, absolute ethanol and isopropyl alcohol. In mechanical exfoliation method, the Bi2Te3 flakes with different thicknesses were transferred to SiO2/Si substrate using the standard mechanical exfoliation method from purity bulk Bi2Te3 crystal purchased from Smart Elements. In CVD process, Bi2Te3 flakes were directly grown on SiO2/Si substrate. Bi2Te3 powder with the weight of 0.2~0.5 g was put in a rail boat as source, and the source was placed in the middle of heating zone, and the substrates were put in the downstream and apart from the source 20~23 cm. The growth of Bi2Te3 was under the protection of argon gas and kept at 500℃ for 600~1800 seconds. Infrared nano-imaging. In our experiments, we utilized a commercial s-SNOM (NeaSNOM, NeaSpec.com) to realize high-resolution nanoscale real-space imaging of Bi2Te3 flakes. Our s-SNOM was built based on an atomic force microscope (AFM) operated in tapping mode with a tapping frequency of ~ 270 KHz and a tapping amplitude ∆z = 30 nm. A CO2 laser (Edinburgh Instruments, wavelength: 10.7 ~ 11.4 µm) and mid-IR quantum cascade lasers (Daylight Solutions, wavelength: 4.493 µm, 7.380 µm) were equipped for wavelength−dependent nano−imaging experiment. The IR laser beam was focused on a metalized AFM tip (NanoWorld Arrow NCPt) using a parabolic mirror. The sharp tip with a diameter of ~ 10 nm can focus the incident light into a small spot. When the tip approaches the Bi2Te3 flakes, a highly confined field at the apex can effectively excite the surface resonant modes by fulfilling the momentum match condition with near field. Then the backscattered light from tip-sample interaction can be collected by a pseudo-heterodyne detection to reflect the topography and near-field images of the resonant information at nanoscale, which is only determined by the tip size.34 The implement of gate-tuning experiment is similar to that of graphene plasmons nano-imaging.11, 12 Numerical simulation. The numerical simulation was carried out with Lumerical


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Finite-Difference Time-Domain (FDTD). Perfect matched layers (PMLs) were used along all the axes. A Si spheroid nanoparticle with semi-axes 5, 5 and 100 nm, respectively, was coated with a layer of 2-nm-thick Pt to mimic the s-SNOM tip. A broadband Gaussian beam was focused onto the tip/sample with an oblique angle 30o. The dielectric constant of SiO2, Si and Pt were extracted from experiment data.35 The Bi2Te3 flakes were modeled as a bulk material coated with a two nanometers thick surface for separation of the contributions from bulk state and surface state. The dielectric constant of Bi2Te3 was obtained by fitting the measured absorption data from sample produced with solvothermal method. Detailed parameter fitting was described in Supporting Information. The substrate SiO2 was infinite thick and the thickness of Bi2Te3 was set the same as corresponding samples.

ACKNOWLEDGEMENTS

We acknowledge the support from the Youth 973 program (2015CB932700), the National Key Research & Development Program (No. 2016YFA0201902), the National Natural Science Foundation of China (No. 61604102, 51290273 and 91433107, 11404372), ARC (DP140101501 and FT150100450), the Natural Science Foundation of Jiangsu Province (No. BK20130328), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Collaborative Innovation Center of Suzhou Nano Science and Technology. Q. Bao acknowledges support from the Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET) (project number: CE170100039). ASSOCIATED CONTENT Supporting Information

Supporting

Information

Available: This


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material is available free of charge via the Internet at http://pubs.asc.org AUTHOR INFORMATION

Corresponding Author Qiaoliang Bao , Cheng-Wei Qiu and Shaojuan Li Tel: (+61)-3-99054927; Fax: (+86)-512-65882846; Tel: (+86)-512-65882337 E-mail: [email protected], [email protected], [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Jian Yuan, Weiliang Ma and Lei Zhang contributed equally to this work.


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Figure captions: Figure 1. Experimental setup and comparison of scattering signal and topography profile of Bi2Te3 flake. (a) Schematic of experimental setup for collecting s-SNOM signal. (b) Pseudo color images of AFM. Red dashed lines here indicate the positions of the topography at cross section shown in d (c) Pseudo color images of s-SNOM signal. The imaging wavelength is λ0 = 11.086 µm. Red dashed lines here indicate the s-SNOM signal at cross section shown in d and e. (d) Comparison of normalized scattering amplitude and its corresponding topography profile at the same position. (e) Scattering signal of the same sample illuminated by incident light with different wavelengths. (f) Calculated band structure of a 9 quintuple-layer (QL) Bi2Te3 (~9 nm) with strong spin−orbit coupling (SOC) effect. DP, BCB and BVB represent for Dirac point, bulk conduction band and bulk valance band, respectively. Figure 2. Wavelength dependence of scattering signal. (a) The AFM topography of the Bi2Te3 flake with step-like profile. (b-d) The scattering signal measured at three representative wavelengths. Scale bar: 1 µm. (e) The normalized scattering signal at different wavelength. The data were measured at the same position, as labeled by black dashed lines in (b-d). (f) Wavelength dependence of averaged scattering signals and their relative ratio at the areas A and B. Figure 3. Tunable scattering signal via gate voltage. (a-c) Mapping of normalized scattering signal for Bi2Te3 flakes with a thickness of 14 nm at wavelength of 11.086 µm. Scale bar: 400 nm. (d) Normalized scattering signal extracted at different gate voltages. The data were measured at the same position, as labeled by white dashed line in (a). Both the metalized AFM tip and the surface of Bi2Te3 flake were at ground potential. (f) The averaged scattering signal at the top surface of Bi2Te3 flake at varying gate voltages. Figure 4. Thickness dependence of scattering amplitude. (a-d) AFM profiles and (e-f) corresponding mapping of normalized scattering signal of Bi2Te3 flakes with different thicknesses. The wavelength of incident laser is 10.708 µm. All the scale bar is 200 nm. (i) The scattering amplitude extracted at the position labeled by white dashed lines in (e-f). Inset shows averaged normalized scattering amplitude above top surface and at the peak out of


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edge. (j) Numerical near-field |E|2 along the sample surface for Bi2Te3 flakes and near-field |E|2 at peak as a function of sample thickness (up-right inset). The bottom left inset schematically shows model configuration in simulation, in which the yellow line refers to the conductive surface, blue region stands for Bi2Te3 bulk and the gray region corresponds to the SiO2 substrate. The red and greed dashed lines indicate the moving trace of AFM tip and edge of Bi2Te3 flake, respectively. Figure 5. Scattering signal from a Bi2Te3 gap. (a) AFM topography images of the Bi2Te3 flake with a total thickness 80 nm. Scale bar: 400 nm. (a) The topography profiles at three positions in (a). (c) Simulation infrared near-field signal images of Bi2Te3 gap. The gap profile is outlined by green dashed lines. (d-f) The pseudo 3D scattering signal mapped at 10.708, 11.086 and 11.286 µm, respectively. (g-i) The scattering amplitude extracted at the position labeled by dashed lines in (d-f).


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Figure 1. Experimental setup and comparison of scattering signal and topography profile of Bi2Te3 flake. (a) Schematic of experimental setup for collecting s-SNOM signal. (b) Pseudo color images of AFM. Red dashed lines here indicate the positions of the topography at cross section shown in d (c) Pseudo color images of s-SNOM signal. The imaging wavelength is λ0 = 11.086 µm. Red dashed lines here indicate the s-SNOM signal at cross section shown in d and e. (d) Comparison of normalized scattering amplitude and its corresponding topography profile at the same position. (e) Scattering signal of the same sample illuminated by incident light with different wavelengths. (f) Calculated band structure of a 9 quintuple-layer (QL) Bi2Te3 (~9 nm) with strong spin−orbit coupling (SOC) effect. DP, BCB and BVB represent for Dirac point, bulk conduction band and bulk valance band, respectively.


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Figure 2. Wavelength dependence of scattering signal. (a) The AFM topography of the Bi2Te3 flake with step-like profile. (b-d) The scattering signal measured at three representative wavelengths. Scale bar: 1 µm. (e) The normalized scattering signal at different wavelength. The data were measured at the same position, as labeled by black dashed lines in (b-d). (f) Wavelength dependence of averaged scattering signals and their relative ratio at the areas A and B.


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Figure 3. Tunable scattering signal via gate voltage. (a-c) Mapping of normalized scattering signal for Bi2Te3 flakes with a thickness of 14 nm at wavelength of 11.086 µm. Scale bar: 400 nm. (d) Normalized scattering signal extracted at different gate voltages. The data were measured at the same position, as labeled by white dashed line in (a). Both the metalized AFM tip and the surface of Bi2Te3 flake were at ground potential. (f) The averaged scattering signal at the top surface of Bi2Te3 flake at varying gate voltages.


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Figure 4. Thickness dependence of scattering amplitude. (a-d) AFM profiles and (e-f) corresponding mapping of normalized scattering signal of Bi2Te3 flakes with different thicknesses. The wavelength of incident laser is 10.708 µm. All the scale bars are 200 nm. (i) The scattering amplitude extracted at the position labeled by white dashed lines in (e-f). Inset shows averaged normalized scattering amplitude above top surface and at the peak out of edge. (j) Numerical near-field |E|2 along the sample surface for Bi2Te3 flakes and near-field |E|2 at peak as a function of sample thickness (up-right inset). The bottom left inset schematically shows model configuration in simulation, in which the yellow line refers to the conductive surface, blue region stands for Bi2Te3 bulk and the gray region corresponds to the SiO2 substrate. The red and greed dashed lines indicate the moving trace of AFM tip and edge of Bi2Te3 flake, respectively.


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Figure 5. Scattering signal from a Bi2Te3 gap. (a) AFM topography images of the Bi2Te3 flake with a total thickness 80 nm. Scale bar: 400 nm. (a) The topography profiles at three positions in (a). (c) Simulation infrared near-field signal images of Bi2Te3 gap. The gap profile is outlined by green dashed lines. (d-f) The pseudo 3D scattering signal mapped at 10.708, 11.086 and 11.286 µm, respectively. (g-i) The scattering amplitude extracted at the position labeled by dashed lines in (d-f).


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Title: "Infrared nano-imaging reveals the surface metallic plasmons in topological insulator" Author(s): Jian Yuan; Weiliang Ma; Lei Zhang; Yao Lu; Meng Zhao; Hongli Guo; Jin Zhao; Wenzhi Yu; Yupeng Zhang; Kai Zhang; Hui Ying Hoh; Xiaofeng Li; Kian Ping Loh; Shaojuan Li; Cheng-Wei Qiu and Qiaoliang Bao.

In this work, we experimentally investigate the surface plasmonic properties of Bi2Te3 nanosheets using scattering-type scanning near-field optical microscopy (s-SNOM). With the help of highly position-selective excitation and high-pixel real-space mapping, we discover near-field patterns of bright outside fringes which are associated with its surface-metallic, plasmonic behavior at mid-infrared frequency. And we experimentally demonstrate that the scattered signal responses of outside fringes can be tailored via mechanical (sheet thickness of Bi2Te3), electric (electrostatic gating), and optical (incident wavelength) fashions. The discovery of surface plasmons in TI nanosheets may enable the development of strongly enhanced light–matter interactions for quantum optical devices, mid-infrared (MIR) and terahertz detectors or sensors.


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Infrared nano-imaging reveals the surface metallic plasmons in

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