Developments in THz range ellipsometry

Ellipsometry is a technique whereby the measurement of the two orthogonal polarization components of light reflected at glancing incidence allows a characterization of the optical properties of a material at a particular frequency. Importantly, it obviates the need for measurement against a standard reference sample, and so can provide reliable spectroscopic information even when surface morphology is unknown, of marginal quality and/or a reference is unavailable. Although a standard technique in the visible range, it has not been widely applied in the Terahertz (THz) spectral range despite its potential utility. This is largely because of the technical difficulties that these frequencies present. This review details recent progress in the implementation of THz range ellipsometry. We discuss a variety of configurations including various kinds of laboratory and facility based sources using both continuous wave and pulsed spectroscopic methods. We discuss the general problems encountered when trying to import the methodologies of visible range ellipsometry to the THz range and give examples of where the technique has been successful thus far.Comment: Review article to appear in `Journal of Infrared, Millimeter, and Terahertz Waves'; 27 pages, 17 figure

Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here we lower the chemical potential of three-dimensional (3D) Bi$_2$Se$_3$ films to $\sim$ 30 meV above the Dirac point, and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 T, we observed quantized Faraday and Kerr rotations, whereas the DC transport is still semi-classical. A non-trivial Berry phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine structure constant based on a topological invariant of a solid-state system.Comment: A shortened version has been published in Science. Discussion on AC quantum Hall effect without involving edge states is adde

Charge Transfer and Charge Transport on the Double Helix

We present a short review of various experiments that measure charge transfer and charge transport in DNA. Some general comments are made on the possible connection between 'chemistry-style' charge transfer experiments that probe fluorescence quenching and remote oxidative damage and 'physics-style' measurements that measure transport properties as defined typically in the solid-state. We then describe measurements performed by our group on the millimeter wave response of DNA. By measuring over a wide range of humidity conditions and comparing the response of single strand DNA and double strand DNA, we show that the appreciable AC conductivity of DNA is not due to photon assisted hopping between localized states, but instead due to dissipation from dipole motion in the surrounding water helix.Comment: 7 pages, 3 figure

A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNb2_2O6_6 investigated by high resolution time-domain terahertz spectroscopy

Kink bound states in the one dimensional ferromagnetic Ising chain compound CoNb$_2$O$_6$ have been studied using high resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schr\"odinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near two times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states -- two pairs of kinks on neighboring chains

Nonvolatile Solid-State Charged-Polymer Gating of Topological Insulators into the Topological Insulating Regime

We demonstrate the ability to reduce the carrier concentration of thin films of the topological insulator (TI) Bi2Se3 by utilizing a novel approach, namely non-volatile electrostatic gating via corona charging of electret polymers. Sufficient electric field can be imparted to a polymer-TI bilayer to result in significant electron density depletion, even without the continuous connection of a gate electrode or the chemical modification of the TI. We show that the Fermi level of Bi2Se3 is shifted towards the Dirac point with this method. Using THz spectroscopy, we find that the surface chemical potential is lowered into the bulk band gap (~ 50 meV above the Dirac point and 170 meV below the conduction band minimum) and it is stabilized in the intrinsic regime while enhancing electron mobility. This represents the first use of a charged polymer gate for modulating TI charge density. The mobility of surface state electrons is enhanced to a value as high as ~1600 cm^2/Vs at 5K.Comment: Submitted, 5 pages of text, 6 figure

Aging and reduced bulk conductance in thin films of the topological insulator Bi2_2Se3_3

We report on the effect of exposure to atmospheric conditions on the THz conductivity of thin films of the topological insulator Bi$_2$Se$_3$. We find: 1) two contributions of mobile charge carriers to the THz conductivity immediately after growth, and 2) the spectral weight of the smaller of these decays significantly over a period of several days as the film is exposed to ambient conditions, while the other remains relatively constant. We associate the former with a bulk response, and the latter with the surface. The surface response exhibits the expected robustness of the carriers from 2D topological surface states. We find no evidence for a third spectral feature derived from topologically trivial surface states.Comment: 3 pages, 3 figures. 4 pages, modified tex

Anomalous high energy dispersion in photoemission spectra from insulating cuprates

Angle resolved photoelectron spectroscopic measurements have been performed on an insulating cuprate Ca_2CuO_2Cl_2. High resolution data taken along the \Gamma to (pi,pi) cut show an additional dispersive feature that merges with the known dispersion of the lowest binding energy feature, which follows the usual strongly renormalized dispersion of ~0.35 eV. This higher energy part reveals a dispersion that is very close to the unrenormalized band predicted by band theory. A transfer of spectral weight from the low energy feature to the high energy feature is observed as the \Gamma point is approached. By comparing with theoretical calculations the high energy feature observed here demonstrates that the incoherent portion of the spectral function has significant structure in momentum space due to the presence of various energy scales.Comment: 5 pages, 3 figure