Terahertz Photogalvanics in Twisted Bilayer Graphene Close to the Second Magic Angle.

We report on the observation of photogalvanic effects in tBLG with a twist angle of 0.6°. We show that excitation of the tBLG bulk causes a photocurrent, whose sign and magnitude are controlled by the orientation of the radiation electric field and the photon helicity. The observed photocurrent provides evidence for the reduction of the point group symmetry in low twist-angle tBLG to the lowest possible one. The developed theory shows that the current is formed by asymmetric scattering in gyrotropic tBLG. We also detected the photogalvanic current formed in the vicinity of the edges. For both bulk and edge photocurrents, we demonstrate the emergence of pronounced oscillations upon variation of the gate voltage. The gate voltages associated with the oscillations correlate with peaks in resistance measurements. These are well explained by interband transitions between a multitude of isolated bands in tBLG.

Helicity-Sensitive Plasmonic Terahertz Interferometer.

Plasmonic interferometry is a rapidly growing area of research with a huge potential for applications in the terahertz frequency range. In this Letter, we explore a plasmonic interferometer based on graphene field effect transistor connected to specially designed antennas. As a key result, we observe helicity- and phase-sensitive conversion of circularly polarized radiation into dc photovoltage caused by the plasmon-interference mechanism: two plasma waves, excited at the source and drain part of the transistor, interfere inside the channel. The helicity-sensitive phase shift between these waves is achieved by using an asymmetric antenna configuration. The dc signal changes sign with inversion of the helicity. A suggested plasmonic interferometer is capable of measuring the phase difference between two arbitrary phase-shifted optical signals. The observed effect opens a wide avenue for phase-sensitive probing of plasma wave excitations in two-dimensional materials.

Observation of Terahertz-Induced Magnetooscillations in Graphene.

When high-frequency radiation is incident upon graphene subjected to a perpendicular magnetic field, graphene absorbs incident photons by allowing transitions between nearest Landau levels that follow strict selection rules dictated by angular momentum conservation. Here, we show a qualitative deviation from this behavior in high-quality graphene devices exposed to terahertz (THz) radiation. We demonstrate the emergence of a pronounced THz-driven photoresponse, which exhibits low-field magnetooscillations governed by the ratio of the frequency of the incoming radiation and the quasiclassical cyclotron frequency. We analyze the modifications of generated photovoltage with the radiation frequency and carrier density and demonstrate that the observed photoresponse shares a common origin with microwave-induced resistance oscillations discovered in GaAs-based heterostructures; however, in graphene it appears at much higher frequencies and persists above liquid nitrogen temperatures. Our observations expand the family of radiation-driven phenomena in graphene, paving the way for future studies of nonequilibrium electron transport.

Spin Hall photoconductance in a three-dimensional topological insulator at room temperature.

Three-dimensional topological insulators are a class of Dirac materials, wherein strong spin-orbit coupling leads to two-dimensional surface states. The latter feature spin-momentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi2Te2Se platelets for perpendicular incidence of light. The same edges exhibit also a finite bias-dependent Kerr angle, indicative of spin accumulation induced by a transversal spin Hall effect in the bulk states of the Bi2Te2Se platelets. A symmetry analysis shows that the helical photoconductance is distinct to common longitudinal photoconductance and photocurrent phenomena, but consistent with optically injected spins being transported in the side facets of the platelets.

Universal ultrafast detector for short optical pulses based on graphene.

Graphene has unique optical and electronic properties that make it attractive as an active material for broadband ultrafast detection. We present here a graphene-based detector that shows 40-picosecond electrical rise time over a spectral range that spans nearly three orders of magnitude, from the visible to the far-infrared. The detector employs a large area graphene active region with interdigitated electrodes that are connected to a log-periodic antenna to improve the long-wavelength collection efficiency, and a silicon carbide substrate that is transparent throughout the visible regime. The detector exhibits a noise-equivalent power of approximately 100 µW·Hz(-½) and is characterized at wavelengths from 780 nm to 500 µm.

Terahertz polarization conversion with quartz waveplate sets.

We present the results of calculation and experimental testing of an achromatic polarization converter and a composite terahertz waveplate (WP), which are represented by sets of plane-parallel birefringent plates with in-plane birefringence axis. The calculations took into account the effect of interference, which was especially prominent when plates were separated by an air gap. The possibility of development of a spectrum analyzer design based on a set of WPs is also discussed.