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We observe photocurrents induced in single-layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence. The photocurrent flows along the sample edges and forms a vortex. Its winding direction reverses by switching the light helicity from left to right handed. We demonstrate that the photocurrent stems from the sample edges, which reduce the spatial symmetry and result in an asymmetric scattering of carriers driven by the radiation electric field. The developed theory based on Boltzmann's kinetic equation is in a good agreement with the experiment. We show that the edge photocurrents can be applied for determination of the conductivity type and the momentum scattering time of the charge carriers in the graphene edge vicinity.

A periodically driven system with spatial asymmetry can exhibit a directed motion facilitated by thermal or quantum fluctuations. This so-called ratchet effect has fascinating ramifications in engineering and natural sciences. Graphene is nominally a symmetric system. Driven by a periodic electric field, no directed electric current should flow. However, if the graphene has lost its spatial symmetry due to its substrate or adatoms, an electronic ratchet motion can arise. We report an experimental demonstration of such an electronic ratchet in graphene layers, proving the underlying spatial asymmetry. The orbital asymmetry of the Dirac fermions is induced by an in-plane magnetic field, whereas the periodic driving comes from terahertz radiation. The resulting magnetic quantum ratchet transforms the a.c. power into a d.c. current, extracting work from the out-of-equilibrium electrons driven by undirected periodic forces. The observation of ratchet transport in this purest possible two-dimensional system indicates that the orbital effects may appear and be substantial in other two-dimensional crystals such as boron nitride, molybdenum dichalcogenides and related heterostructures. The measurable orbital effects in the presence of an in-plane magnetic field provide strong evidence for the existence of structure inversion asymmetry in graphene.

Photogalvanic effects are observed and investigated in wurtzite ͑0001͒-oriented GaN/AlGaN lowdimensional structures excited by terahertz radiation. The structures are shown to represent linear quantum ratchets. Experimental and theoretical analysis exhibits that the observed photocurrents are related to the lack of an inversion center in the GaN-based heterojunctions.

We study terahertz radiation induced ratchet currents in low dimensional semiconductor structures with a superimposed one-dimensional lateral periodic potential. The periodic potential is produced by etching a grating into the sample surface or depositing metal stripes periodically on the sample top. Microscopically, the photocurrent generation is based on the combined action of the lateral periodic potential, verified by transport measurements, and the in-plane modulated pumping caused by the lateral superlattice. We show that a substantial part of the total current is caused by the polarization-independent Seebeck ratchet effect. In addition, polarizationdependent photocurrents occur, which we interpret in terms of their underlying microscopical mechanisms. As a result, the class of ratchet systems needs to be extended by linear and circular ratchets, sensitive to linear and circular polarizations of the driving electromagnetic force.

Terahertz light helicity sensitive photoresponse in GaAs/AlGaAs high electron mobility transistors. The helicity dependent detection mechanism is interpreted as an interference of plasma oscillations in the channel of the field-effect-transistors (generalized Dyakonov-Shur model). The observed helicity dependent photoresponse is by several orders of magnitude higher than any earlier reported one. Also, linear polarization sensitive photoresponse was registered by the same transistors. The results provide the basis for a new sensitive, all-electric, room-temperature, and fast (better than 1 ns) characterisation of all polarization parameters (Stokes parameters) of terahertz radiation. It paves the way towards terahertz ellipsometry and polarization sensitive imaging based on plasma effects in field-effect-transistors

We report on the observation of the circular photogalvanic effect in Si-metal-oxide-semiconductor fieldeffect transistors with inversion channel excited by terahertz radiation. We demonstrate that in spite of the fact that the photocurrent is caused by transfer of the photon angular momentum to free carriers, it is not due to spin orientation but has a pure orbital origin. It results from the quantum interference of different pathways contributing to the free-carrier absorption of monochromatic radiation. DOI: 10.1103/PhysRevB.79.121302 PACS number͑s͒: 78.40.Fy, 72.40.ϩw, 73.40.Qv, 78.20.Ϫe The spin of electrons and holes in solid-state systems is an intensively studied quantum mechanical property showing a large variety of interesting physical phenomena. One of the most frequently used and powerful methods of generation and investigation of spin polarization is optical orientation with circularly polarized light. 1 Besides purely optical phenomena such as circularly polarized photoluminescence, the optical generation of an unbalanced spin distribution in a semiconductor may lead to helicity-dependent photocurrents, e.g., circular photogalvanic effect ͑CPGE͒. 2-4 CPGE current is excited only by light of nonzero helicity and reverses its direction upon switching the sign of circular polarization. So far, the CPGE has only been detected in materials with strong spin-orbit coupling and described by microscopic mechanisms based on spin-related processes. [2][3][4] Here we report on the observation of the CPGE caused by absorption of terahertz radiation in Si-metal-oxidesemiconductor field-effect transistors ͑Si-MOSFETs͒. The experimental demonstration of the existence of a helicitysensitive photocurrent in Si-based structures is of particular interest. Silicon is characterized by a vanishingly small spinorbit coupling which makes spin-related mechanisms of the CPGE ineffective and, therefore, cannot account for the observed helicity-dependent photocurrent. Thus, an access in explaining the CPGE is required, involving mechanisms of pure orbital ͑spin-unrelated͒ origin. Here, we show that the CPGE in our structures is due to quantum interference of different pathways contributing to monochromatic radiation absorption. This effect has been predicted theoretically 5 ͑see also Refs. 6 and 7͒ but not observed so far. Quantum interference plays an important role in various transport and optical phenomena. It has also been shown that it can induce photocurrents. Here, however, in contrast to the well-known photocurrents caused by quantum interference of one-and two-photon absorption processes in two color light, 8-11 the photocurrent is due to quantum interference in the elementary one-photon absorption process.We study n-type MOSFETs prepared on miscut Si surfaces to reduce spacial symmetry and enable photocurrents at normal incidence. The surfaces of our samples are tilted by the angle = 9.7°͑sample 1͒ or = 10.7°͑sample 2͒ from the ͑001͒ plane around x ʈ ͓110͔. We note, that the point group describing the miscut trans...

We demonstrate the injection of pure valley-orbit currents in multivalley semiconductors and present the phenomenological theory of this effect. We studied photoinduced transport in (111)-oriented silicon metaloxide-semiconductor field effect transistors at room temperature. By shining circularly polarized light on exact oriented structures with six equivalent valleys, nonzero electron fluxes within each valley are generated, which compensate each other and do not yield a net electric current. By disturbing the balance between the valley fluxes, we demonstrate that the pure valley-orbit currents can be converted into a measurable electric current.