Spin photocurrents generated by homogeneous optical excitation with circularly polarized radiation in quantum wells (QWs) are reviewed. The absorption of circularly polarized light results in optical spin orientation due to the transfer of the angular momentum of photons to electrons of a two-dimensional electron gas. It is shown that in QWs belonging to one of the gyrotropic crystal classes a non-equilibrium spin polarization of uniformly distributed electrons causes a directed motion of electrons in the plane of the QW. A characteristic feature of this electric current, which occurs in unbiased samples, is that it reverses its direction upon changing the radiation helicity from left-handed to right-handed and vice versa. Two microscopic mechanisms are responsible for the occurrence of an electric current linked to a uniform spin polarization in a QW: the spin polarization-induced circular photogalvanic effect and the spin-galvanic effect. In both effects the current flow is driven by an asymmetric distribution of spin-polarized carriers in k-space of systems with lifted spin degeneracy due to k-linear terms in the Hamiltonian. Spin photocurrents provide methods to investigate spin relaxation and to reach a conclusion as regards the inplane symmetry of QWs. The effect can also be utilized to develop fast detectors for determining the degree of circular polarization of a radiation beam. Furthermore, spin photocurrents under infrared excitation were used to demonstrate and investigate monopolar spin orientation of free carriers. Contents 1. Introduction 936 2. Homogeneous spin orientation-induced photocurrents 938 2.1. Removal of spin degeneracy 938 2.2. The circular photogalvanic effect 942 2.3. The spin-galvanic effect 948 2.4. The spin orientation-induced circular photogalvanic effect versus the spingalvanic effect 952

There is much recent interest in exploiting the spin of conduction electrons in semiconductor heterostructures together with their charge to realize new device concepts. Electrical currents are usually generated by electric or magnetic fields, or by gradients of, for example, carrier concentration or temperature. The electron spin in a spin-polarized electron gas can, in principle, also drive an electrical current, even at room temperature, if some general symmetry requirements are met. Here we demonstrate such a 'spin-galvanic' effect in semiconductor heterostructures, induced by a non-equilibrium, but uniform population of electron spins. The microscopic origin for this effect is that the two electronic sub-bands for spin-up and spin-down electrons are shifted in momentum space and, although the electron distribution in each sub-band is symmetric, there is an inherent asymmetry in the spin-flip scattering events between the two sub-bands. The resulting current flow has been detected by applying a magnetic field to rotate an optically oriented non-equilibrium spin polarization in the direction of the sample plane. In contrast to previous experiments, where spin-polarized currents were driven by electric fields in semiconductor, we have here the complementary situation where electron spins drive a current without the need of an external electric field.

a b s t r a c tThe nonlinear optical and optoelectronic properties of graphene with the emphasis on the processes of harmonic generation, frequency mixing, photon drag and photogalvanic effects as well as generation of photocurrents due to coherent interference effects, are reviewed. The article presents the state-of-the-art of this subject, including both recent advances and well-established results. Various physical mechanisms controlling transport are described in depth including phenomenological description based on symmetry arguments, models visualizing physics of nonlinear responses, and microscopic theory of individual effects.

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A nonequilibrium population of spin-up and spin-down states in quantum well structures has been achieved applying circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicular to the direction of light propagation. Because of the spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left handed. A microscopic model is presented which describes the origin of the photon helicity driven current. The model suggests that the system behaves as a battery which generates a spin polarized current. DOI: 10.1103/PhysRevLett.86.4358 PACS numbers: 73.50.Mx, 68.65.-k, 73.50.Pz, 78.30.Fs The spin of electrons and holes in solid state systems is an intensively studied quantum mechanical property as it is the crucial ingredient for spintronics [1,2] and several schemes of quantum computation [3][4][5]. Among others, current investigations involve the spin lifetime in semiconductor devices [6][7][8] as well as the injection of spin polarized electrons (or holes) from semimagnetic semiconductor materials into semiconductors [9][10][11] or from ferromagnetic into nonmagnetic metals [12,13].It is well known that spin polarized electrons can be generated by circularly polarized light [14,15] and, vice versa, that the recombination of spin polarized charged carriers results in the emission of circularly polarized light [10,11,14]. However, little is known about spin dependent photocurrents when a semiconductor is irradiated by circularly polarized light [15,16]. Helicity dependent photocurrents in semiconductors have been observed in bulk Te utilizing the peculiarities of the valence band structure ("camel back") at the first Brillouin zone boundary and in bulk GaAs subjected to an external magnetic field [15]. A first indication of such a photon helicity dependent photocurrent in semiconductor heterojunctions was found in recent far infrared experiments on p-type GaAs͞AlGaAs heterojunctions containing a two-dimensional hole gas [17]. This preliminary experiment was discussed in phenomenological terms and lacked the microscopic connection to the carriers' spin.The experiments on quantum wells (QWs) described below uncover a novel property of an unbalanced spin polarization: its ability to generate a directed current where the current's direction depends solely on the predominant spin orientation. This effect may be illustrated as an electron analog of mechanical systems where a rotational motion ("spin") is transmitted into a linear one ("current") like a rotating wheel on a hard surface. Below we point out that spin injection into quantum wells of zinc-blende-type material leads always to an electric current in the plane of the quantum well. The reduced dimensionality of quantum wells lowers the crystallographic symmetry and introduces k-linear terms in the Hamiltonian. These k-linear terms lift the spin degenerate of energy bands in k-space which, in...