Session J35: Topological Insulators: Optics

Far-infrared magneto-spectroscopy study of ultrathin Bi$^{2}$Se$^{3}$ layers in high magnetic field

We present a far-infrared (FIR) magneto-spectroscopy study of thin Bi2Se3 layers. Transmittance and reflectance measurements are performed in the Faraday geometry at 4.2 K and in a magnetic field up to 17.5 T. The thin samples (much less than 1 micron) are stabilized on the Scotch tape, which enable us to obtain reliable FIR transmission signals. A pronounced electron-phonon coupling, i.e., the coupling of a FIR phonon of Bi2Se3 with the continuum free-carrier spectrum, is observed and strongly enhanced by the applied magnetic field. The phonon lineshape is asymmetric and can be fit by a Fano formula. After examining the fitting parameters, we find no systematic broadening of the lineshape in our measurements.   

Terahertz and magneto-optical studies of HgTe films

We present a frequency-domain infrared, time-domain terahertz, and magneto- optical study of thin films of high-mobility HgTe. The conductivity at low frequency and a wide range of temperature is described, and clear cyclotron effects are observed at low temperature and frequency. The context of these findings to the possible 3D topological phase will be discussed.   

Magneto-Optics in the search for the topological insulating state

We present infrared spectroscopic data for the thermoelectric material Bi-Sb at the insulating composition, taken in magnetic field applied both in the Voight and Faraday geometries, in order to gain insight into the recently discovered topological insulating state. We observe a metallic plasma edge that displays significant blue shift at high temperatures. The form of the plasma edge shows broadening suggestive of sample inhomogeneities. The plasma edge is remarkably sensitive to the applied magnetic field, resulting in a complicated spectral lineshape. We compare these results with magneto-optics of the topological insulator Bi$_{2}$Se$_{3}$.\footnote{A. D. LaForge, \textit{et. al.}, Phys. Rev. B 81, 125120 (2010).}   

Magneto-optical spectra of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$ in magnetic fields up to 18 Tesla

Topological insulators are a novel class of materials that behave as insulators in the bulk, but have conducting states on the surface. Studies of their behavior in magnetic field is an important avenue towards understanding their complex properties. We will report the results of our magneto-optical measurements of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$ in magnetic fields up to 18 Tesla. In all three compounds we detect magnetic-field induced changes in optical properties, which are most pronounced around the plasma edge. The induced changes are much bigger in Bi$_2$Se$_3$ than in Bi$_2$Te$_3$ and Sb$_2$Te$_3$.   

Optical properties of novel topological insulators

The recent discovery of topological insulators (TI) with the spin-polarized Dirac-like electrons on their surfaces have attracted a lot of attention and theoretical and experimental research efforts. One of the fascinating properties of TIs is the presence of the linear magneto-electric coupling which is responsible for coupled charge-spin modes in dynamical response. We will report our results on the search for such modes together with the conventional optical properties of materials predicted to be TI such as 5d oxide Na$_{2}$IrO$_{3}$, SmB$_{6}$, and Bi$_{2}$Se$_{3}$ and other materials. Measurement results include optical reflectance and/or transmission in the frequency range 5 - 50,000 cm$^{-1}$ and Faraday or Kerr rotation and circular dichroism at selected THz frequencies.   

Terahertz Kerr and Reflectivity Measurements on the Topological Insulator Bi$_2$Se$_3$

We report the first terahertz Kerr measurements on bulk crystals of the topological insulator Bi$_2$Se$_3$ with and without Fe doping at 4\,K and magnetic fields up to 8\,T. Transport evidence and characterization of the surface states will be presented. By employing a gate that creates a small depletion layer, the optical signals from the surface state carriers are modulated with no contribution arising from the bulk carriers. The real and imaginary parts of the Kerr angle yeild the transport scattering rate, spectral weight, and mass of the surface state carriers. FTIR and magneto-optical measurements characterize the bulk carriers. Comparisons with ARPES and other transport measurements will be discussed.   

Magneto-optical and Magneto-electric Effects of Topological Insulators in Quantizing Magnetic Fields

Topological insulators show novel magneto-electric effect when the surface Dirac cone dispersion is gapped by a weak Zeeman field. In this talk, we present our study of the magneto-optical and magneto-electric effects of a thin-film topological insulator in the presence of a strong quantizing magnetic field. We find that low-frequency magneto-optical properties depend only on the sum of top and bottom surface Dirac-cone filling factors, whereas the magneto-electric response depends only on the difference. The Faraday rotation is quantized in integer multiples of the fine structure constant $\alpha$ and the Kerr effect exhibits a full-quarter rotation. Strongly enhanced cyclotron-resonance features appear at higher frequencies that are sensitive to the filling factors of both surfaces. When the product of the bulk conductivity and the film thickness in $e^2/h$ units is small compared to $\alpha$, magneto-optical properties are only weakly dependent on accidental doping in the interior of the film.   

Photo-Induced Kerr Rotation in the Bulk and Surface of a Topological Insulator

We report ultrafast bulk- and surface-sensitive optical pump-probe spectroscopy from Bi$_{2}$Se$_{3}$ (111). Using second harmonic generation, we demonstrate that the bulk and surface electronic contributions can be separated and exhibit qualitatively different relaxation dynamics. Ultrafast surface-sensitive optical measurements reveal a large photo-induced Kerr rotation from the surface that is dependent on the helicity of the circularly polarized excitation pulse. We will discuss the microscopic origin of this observation and how it relates to the strong spin-charge coupling on the surface of a topological insulator.~   

Infrared studies of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$

In this study we have used infrared spectroscopy to probe the electrodynamic response of topological insulators Bi$_2$Te$_3$, Sb$_2$Te$_3$ and Bi$_2$Se$_3$. Infrared spectra are collected over a broad frequency and temperature range. The results reveal similar spectra in all three compounds, with well defined plasma edge located in the far-infrared part of the spectrum. However there are some important differences in the temperature evolution of the spectra. Namely, as temperature decreases the plasma edge shifts to lower frequencies in Bi$_2$Se$_3$, whereas in Bi$_2$Te$_3$ and Sb$_2$Te$_3$ it shifts to higher frequencies.   

Surface optical study of topological insulator Bi2Se3

We report the observation of optical surface second harmonic generation from the (111) surface of Bi2Se3 using ultrafast laser pulses. We demonstrate that second harmonic generation is sensitive to both the surface crystal structure as well as the surface carrier density, which we tune through surface molecular doping. Protected nodes in the second harmonic circular dichroism spectroscopy provide a method to study time-reversal symmetry breaking effects from a single surface in a contact free manner.   

Ultrafast carrier and phonon dynamics in Bi$_{2}$Se$_{3}$ crystals

Ultrafast time-resolved differential reflectivity of Bi$_{2}$Se$_{3}$ crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes: a sub-ps and a few-ps electron-phonon relaxation process due to different phonon modes and a defect-induced charge trapping process. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy. We argue that the observed behavior is likely due to the presence of Se vacancies. Further, weaker charge trapping process and vanishing air doping effect are observed in magnetically doped samples, supportive of our argument. Part of this work is published on Appl. Phys. Lett. 97, 182102(2010).   

Photo-induced currents in the topological insulator Bi2Se3

We report the observation of photo-induced currents in micro-devices built from exfoliated single crystals of Bi2Se3. Our experiments are performed using ultrashort laser pulses at 800 nm and in the absence of an applied bias. We find that the induced currents scale quadratically with laser field strength, confirming their second order nature. We will present the temperature dependence of these second order currents and discuss their microscopic origin.   

Circular photogalvanic effect on topological insulator surfaces: Berry curvature-dependent response

Strong spin-orbit coupling is commonly exploited for generating electric currents using circularly-polarized light. We study, theoretically, the direct current generated by circularly-polarized light on the surface of a topological insulator, focusing on the part that reverses on switching the light-helicity. Interestingly, the dominant current, due to an interband transition, is controlled by the Berry curvature of the surface bands. This extends the connection between photocurrents and Berry curvature beyond the quasiclassical approximation where it has been shown to hold. Explicit expressions are derived for the (111) surface of the topological insulator Bi$_2$Se$_3$ where we find significant helicity-dependent photocurrents when the rotational symmetry of the surface is broken by an in-plane magnetic field or a strain. Moreover, the dominant current grows linearly with time until a scattering occurs, which provides a means for determining the scattering time. DC spin density is generated on the surface as well, and is also dominated by a linear-in-time, Berry curvature-dependent contribution.   

Raman studies of irradiation-induced defects in thin flakes of Bi2Se3 and related materials

We report a Raman spectroscopy study of exfoliated Bi2Se3 flakes of various thicknesses after exposure to irradiation by lasers, electron-beam and oxygen plasma. We observe little effect of irradiations on Raman spectra of thicker ($>$50 nm) Bi2Se3 flakes, which exhibit characteristic Raman peaks at $\sim $130 cm-1 and $\sim $170 cm-1 similar to bulk Bi2Se3. However, spectra from irradiated thinner ($<$20 nm) flakes show the appearance of an extra Raman peak ($\sim $250 cm-1) and attenuation and broadening of the peaks at $\sim $130 cm-1 and $\sim $170 cm-1. This additional peak is not seen in flakes exposed to electron-beam irradiation and lower-power lasers. We interpret the new peak in the Raman spectra as due to irradiation-induced disorder. We also performed similar Raman studies on Bi2Te3, Sb2Se3 and Sb2Te3.   

Tuning optical and electrical transport properties of Bi2Se3 with Ca

We have systematically tuned the carrier type and density in Bi$_{2}$Se$_{3}$ single crystals by introducing a calcium dopant. By controlling Ca-concentration x in Ca$_{x}$Bi$_{2-x}$Se$_{3}$, a minimum carrier density of $\sim $ $1 \times 10^{17}$ cm$^{-3}$ is achieved in both n- and p-type materials. The Fourier transform infrared (FTIR) measurements were carried out in samples with different doping levels to obtain the inter-band transition energy, sample thickness, and the plasma frequency. The band gap and reduced effective mass of carriers were determined from the relation between the inter-band transition energy and carrier density. The undoped samples show a high electron density ($\sim $ $5 \times 10^{18}$ cm$^{-3})$ and the electrical resistivity shows a typical metallic behavior. At high magnetic fields (up to 14 T), the undoped samples show the Shubnikov-de Haas oscillations. Near the compensation point or x=1.2{\%}, the electrical resistivity shows an insulating behavior with a low temperature saturation. This work was supported in part by DOE and NSF.