Session Y31: Focus Session: Topological Insulators: Synthesis & Characterization - Optical & THz Spectroscopy

Topological surface state dispersion measured using THz magneto-ellipsometry

We present a magneto-optical study of the three-dimensional topological insulator, strained HgTe. Using polarization-sensitive time-domain THz spectroscopy in a magnetic field, reliable information on the Drude weight and cyclotron resonance frequency severely constrain the details of surface state dispersion within 1meV of the Fermi level. Details of the technique and its prospect for future observation of axion electrodynamics using THz spectroscopy will also be discussed.   

Terahertz Kerr Measurements of the Surface States on n- and p-type Bi$_2$Se$_3$ Topological Insulators

We report terahertz magneto-optical characterization of a single surface state on bulk crystals of n-type Bi$_2$Se$_3$ and p-type Bi$_2$Se$_3$ doped with Mg as well as Sm, breaking time reversal symmetry and opening a gap at the Dirac point. A gate is used to apply an electric field which creates and modulates a small depletion layer. The modulated Fermi energy of the surface state produces differential optical signals with no contribution arising from bulk carriers. The real and imaginary parts of the differential Kerr angle yield the transport scattering rate, spectral weight, and mass of the surface state carriers. Comparison with ARPES and other transport measurements will be discussed.   

Nonlinear optical responses to circularly polarized lights of the surface state of a topological insulator

Recent photoelectron spectroscopy experiments have revealed the presence of the Dirac cone on the surface of the topological insulator and its spin-splitting due to the spin-orbit interaction. In general, on spin-orbit coupled systems, electric fields induce spin polarizations as linear and nonlinear responses. Here we investigate the inverse Faraday effect on the surface of the topological insulator. The inverse Faraday effect is a non-linear optical effect where a circularly polarized light induces a dc spin polarization. We employ the Keldysh Green's function method to calculate the induced spin polarization and discuss its frequency dependence. In particular, in the low frequency limit, our analytical result gives the spin polarization proportional to the frequency and the square of the lifetime. As for the finite frequency regime, we employ numerical methods to discuss the resonance due to interband transitions. We also discuss the photogalvanic effect, where an illumination of a circular polarized light generates the dc charge current. Lastly, we evaluate those quantities with realistic parameters.\\[4pt] [1] T. Misawa, T. Yokoyama, S. Murakami, Phys. Rev. B84, 165407 (2011).   

Optical properties of highly resistive Bi$_2$Te$_2$Se

Topological insulators are characterized by strong spin-orbit coupling, producing metallic surface states while the bulk remains insulating. Experimental observations of surface states in transport and optical measurements have been obstructed by bulk contributions due to sample defects, placing the Fermi level in the bulk energy gap. Bi$_2$Te$_2$Se (BTS), a three-dimensional topological insulator, has recently been shown to exhibit a bulk resistivity larger than any other topological insulator measured to date. However, quantum oscillations are still observed, and attributed to the surface states. In this talk I will present the optical properties of BTS, obtained by broadband FTIR spectroscopy combined with ellipsometry, over a range of 20 - 45,000 $cm^{-1}$, as well as the Raman spectrum of BTS.   

Quasiparticle dynamics of topological insulator Bi2Se3 studied by time and angle-resolved photoemission spectroscopy

Topological insulator is a new state of matter that hosts spin helical surface states that may be important for future spintronic applications. Even though the conical bandstructure of the helical Dirac fermions is well established, elastic and inelastic scattering rates of their excitations are less well known because conventional transport or optical techniques can not easily separate surface contribution from bulk effects. Here we use time and angle-resolved photoemission spectroscopy to study the surface electron time-domain dynamics of a prototypical topological insulator Bi$_{2}$Se$_{3}$. We observe non-Fermi liquid behavior of quasiparticles both above and below the Fermi level. Our result suggests that surface-bulk scattering can play an important role in the transport properties of topological insulator.   

Ultrafast Bulk and Surface Dynamics of Bi2Se3 Measured by Time-Resolved ARPES

We investigated the nonequilibrium carrier dynamics of the topological insulator Bi$_2$Se$_3$ using femtosecond time- and angle- resolved photoemission spectroscopy. Optical excitation leads to a metastable population of bulk carriers due to the presence of the bandgap. We discuss the coupling of these carriers to the Dirac surface state, which results in a long-lived nonequiilibrium surface carrier distribution. We will comment on the implications for topological insulator device applications.   

Optical and magneto-optical spectra of Bi$_{1-x}$Sb$_x$ with x = 0.015

Bismuth and its alloys with antimony have attracted attention in recent years due to possible realization of topological insulating state. In this study we have used infrared and magneto-optical spectroscopies to probe the electrodynamic response of bismuth doped with 1.5 $\%$ of antimony. The spectra will be presented for temperatures down to 5 K, and in magnetic fields as high as 18 Tesla. The results reveal strong magneto-optical activity, especially around the plasma minimum in reflectance. These findings will be compared and contrasted with magneto-optical results on pure bismuth.   

Magneto-reflectance of Bi$_2$Se$_3$ in 18 Tesla fields

Magneto-optical measurements can provide valuable information in studies of topological insulators. We will report magneto-reflectance spectra of Bi$_2$Se$_3$ in magnetic fields up to 18 Tesla. Magnetic-field induced changes in reflectance are most pronounced around the plasma minimum and around the 60 cm$^{-1}$ mode. Such large magneto-optical activity is unusual for a phonon, and might indicate coupling to collective modes of magnetic origin. Model fits will provide deeper insight into evolution of optical functions with magnetic field.   

Photo-induced Chiral Edge Current in the 3D Topological Insulator Bi$_{2}$Se$_{3}$

We perform scanning photocurrent (PC) microscopy measurements on the 3D topological insulator Bi$_{2}$Se$_{3}$, and observe spatially separated PC peaks of opposite sign, localized along the sample edges. The physics origin of this experimentally observed two-way PC is attributed to the chiral nature of the TI surface states. We show that the coupling between the optical field and the topological surface states yields a spin population imbalance along the edges of the surface, leading to the first order chiral edge charge current observed in the experiment. Further experimental observations of the weak polarization and strong temperature dependence of the PC agree with the theoretical predications. The PC fades away at a low temperature indicating a mean-free path of $\sim $ 3 $\mu $m for topologically protected surface spins.   

Terahertz studies and colossal Kerr rotation in the topological insulator Bi$_2$Se$_3$

We report a study of high quality MBE grown Bi$_2$Se$_3$ topological insulator thin films using time domain terahertz spectroscopy (TDTS). We explicitly demonstrate the 2D character of the response by studying films of different thicknesses. In addition, we take advantage of a unique feature of TDTS that allows to measure the time structure of the THz pulses. In this way we measure the Faraday and Kerr rotation angles in a single experiment. We find an unprecedentedly large value of the Kerr rotation that is due to the cyclotron resonance of the 2D Dirac fermions \footnote{R. Vald\'es Aguilar, \textit{et al}. Arxiv:1105.0237.}. We will also show results on the effect of exposure of the thin films to atmospheric conditions for prolonged periods of time.   

Size effect in Bi2Se3 Topological Insulator micro and nanoparticles

3D Topological insulators are electronic materials that have a semiconducting bulk interior but a metal-like (zero-gap) surface. We present the investigations of optical transitions in bulk and nanoscale structures of the Topological Insulator Bi$_{2}$Se$_{3}$, using optical absorption spectroscopy over a range of energies between 1.1 eV and 6.5 eV. Bi$_{2}$Se$_{3}$ micro and nanoparticles were obtained by a sequence of ultrasonication steps developed by our group for this purpose. Investigations were performed on particles of different size scales ranging from 10 microns to sub-25 nanometers. While the bulk samples demonstrated expected absorption properties, the smallest (sub-25 nm) particles showed the appearance of a number of new high-energy absorption edges. The fabrication, detailed characterization, and possible reasons for the appearance of the high-energy excitation states will be discussed within the framework of its electronic band-structure description   

Temperature-Dependence in Optical Properties of Topological Insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$

Topological Insulators are materials with insulating bulk states and conducting states on their edge or surface. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we studied the temperature-dependence of the optical properties of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ single-crystals from 5~K to 300~K in the terahertz regime ($0.4$ THz to $3.0$ THz). We observed a spectral weight shift from low frequencies to frequencies above 1.0 THz in the real conductivity at temperatures below $\sim$100 K.   

Efficient THz emission from a topological insulator surface

Bi$_{2}$Se$_3$ is a 3D topological insulator (TI) recently confirmed by the ARPES.\footnote{Hsieh et al. Nature 460, 1101 (2009).} Direct optical probe of its metallic surface states is, however, hindered by the remnant Drude response of the bulk material. Second-order nonlinear optical techniques with their surface specificity provide unique opportunities for studying surface electronic transitions in TIs such as Bi$_{2}$Se$_3$ with bulk inversion symmetry.\footnote{Hsieh et al. Phys. Rev. Lett. 106, 057401 (2011).} Here we demonstrate efficient THz emission from the surface of Bi$_{2}$Se$_3$ under the excitation of a femtosecond optical pulse. The emission arises from optical rectification of the optical pulse at the TI surface and the transient current within the surface depletion region. By spectrally resolving the emission under different pump and emission polarizations, we separate the different contributions. Effects arising from just a few atomic layers of the sample surface due to resonance enhancement of the quasi-real optical transitions between the surface electronic states will be discussed.