Abstract:
We propose an experiment to use the magneto-optical Faraday effect to probe the dynamic Hall conductivity of spin liquid candidates. Theory predicts that an external magnetic field will generate an internal gauge field. If the source of conductivity is in spinons with a Fermi surface, a finite Faraday rotation angle is expected. We predict the angle to scale as the square of the frequency rather than display the standard cyclotron resonance pattern. Furthermore, the Faraday effect should be able to distinguish the ground state of the spin liquid, as we predict no rotation for massless Dirac spinons. We give a semiquantitative estimate for the magnitude of the effect and find that it should be experimentally feasible to detect in both $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and, if the spinons form a Fermi surface, Herbertsmithite. We also comment on the magneto-optical Kerr effect and show that the imaginary part of the Kerr angle may be measurable.

Abstract:
We propose a terahertz radiation source based on the excitation of plasma resonances in graphene structures by means of mixing two NIR laser signals with a THz difference frequency. The process is the photo-thermo-electric effect which has recently been demonstrated to be operative at THz frequencies in graphene. An antenna couples the THz radiation out of the sub-wavelength graphene element and into the far field. The emission is monochromatic with a bandwidth determined by that of the NIR laser sources. The output power of the device as a function of the emitter frequency is estimated at tens of microWatts.

Abstract:
Fundamental topological phenomena in condensed matter physics are associated with a quantized electromagnetic response in units of fundamental constants. Recently, it has been predicted theoretically that the time-reversal invariant topological insulator in three dimensions exhibits a topological magnetoelectric effect quantized in units of the fine structure constant $\alpha=e^2/\hbar c$. In this work, we propose an optical experiment to directly measure this topological quantization phenomenon, independent of material details. Our proposal also provides a way to measure the half-quantized Hall conductances on the two surfaces of the topological insulator independently of each other.

Abstract:
While evidence of a topologically nontrivial surface state has been identified in surface-sensitive measurements of Bi2Se3, a significant experimental concern is that no signatures have been observed in bulk transport. In a search for such states, nominally undoped single crystals of Bi2Se3 with carrier densities approaching 10^16 cm^-3 and very high mobilities exceeding 2 m^2 V^-1 s^-1 have been studied. A comprehensive analysis of Shubnikov de Haas oscillations, Hall effect, and optical reflectivity indicates that the measured electrical transport can be attributed solely to bulk states, even at 50 mK at low Landau level filling factor, and in the quantum limit. The absence of a significant surface contribution to bulk conduction demonstrates that even in very clean samples, the surface mobility is lower than that of the bulk, despite its topological protection.

Abstract:
A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of free carriers in the pyrochlore Eu2Ir2O7 match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-insulator transition at T_N = 110 K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase, and which decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below T_N is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity v_F=4x10^7 cm/s, a logarithmic renormalization scale Lambda_L=600 K, and require a Fermi temperature of T_F=100 K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.

Abstract:
We report gated terahertz Faraday angle measurements on epitaxial Bi2Se3 thin films capped with In2Se3. A plateau is observed in the real part of the Faraday angle at an onset gate voltage corresponding to no band bending at the surface which persists into accumulation. The plateau is two orders of magnitude flatter than the step size expected from a single Landau Level in the low frequency limit, quantized in units of the fine structure constant. At 8 T, the plateau extends over a range of gate voltage that spans an electron density greater than 14 times the quantum flux density. Both the imaginary part of the Faraday angle and transmission measurements indicate dissipative off-axis and longitudinal conductivity channels associated with the plateau.

Abstract:
Sub-wavelength graphene structures support localized plasmonic resonances in the terahertz and mid-infrared spectral regimes. The strong field confinement at the resonant frequency is predicted to significantly enhance the light-graphene interaction, which could enable nonlinear optics at low intensity in atomically thin, sub-wavelength devices. To date, the nonlinear response of graphene plasmons and their energy loss dynamics have not been experimentally studied. We measure and theoretically model the terahertz nonlinear response and energy relaxation dynamics of plasmons in graphene nanoribbons. We employ a THz pump-THz probe technique at the plasmon frequency and observe a strong saturation of plasmon absorption followed by a 10 ps relaxation time. The observed nonlinearity is enhanced by two orders of magnitude compared to unpatterned graphene with no plasmon resonance. We further present a thermal model for the nonlinear plasmonic absorption that supports the experimental results.

Abstract:
Various bandstructure engineering methods have been studied to improve the performance of graphitic transparent conductors; however none demonstrated an increase of optical transmittance in the visible range. Here we measure in situ optical transmittance spectra and electrical transport properties of ultrathin-graphite (3-60 graphene layers) simultaneously via electrochemical lithiation/delithiation. Upon intercalation we observe an increase of both optical transmittance (up to twofold) and electrical conductivity (up to two orders of magnitude), strikingly different from other materials. Transmission as high as 91.7% with a sheet resistance of 3.0 {\Omega} per square is achieved for 19-layer LiC6, which corresponds to a figure of merit {\sigma}_dc/{\sigma}_opt = 1400, significantly higher than any other continuous transparent electrodes. The unconventional modification of ultrathin-graphite optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques enable the investigation of other aspects of intercalation in nanostructures.

Abstract:
Gated terahertz cyclotron resonance measurements on epitaxial Bi2Se3 thin films capped with In2Se3 enable the first spectroscopic characterization of a single topological interface state from the vicinity of the Dirac point to above the conduction band edge. A precipitous drop in the scattering rate with Fermi energy is observed that is interpreted as the surface state decoupling from bulk states and evidence of a shift of the Dirac point towards mid-gap. Near the Dirac point, potential fluctuations of 50 meV are deduced from an observed loss of differential optical spectral weight near the Dirac point. Potential fluctuations are reduced by a factor of two at higher surface Fermi levels in the vicinity of the conduction band edge inferred from the width of the scattering rate step. The passivated topological interface state attains a high mobility of 3500 cm2/Vsec near the Dirac point.

Abstract:
This paper comments on the recently reported quantum oscillations in underdoped YBCO crystals. The implications to other experiments and some hurdles to validating the proposed interpretation are discussed.