Optical orientation of electron spins by linearly polarized light

Abstract

Absorption of circularly polarized light in semiconductors is known to result in optical orientation of electron and hole spins. It has been shown here that in semiconductor quantum well structures spin orientation of carriers can be achieved by linearly or even unpolarized light. Moreover, the sign and magnitude of the spin orientation can be varied by rotating the polarization plane of incidence light. The effect under study is related to reduced symmetry of the quantum wells as compared to bulk materials and, microscopically, caused by zero-field spin splitting of electron and hole states. PACS numbers: 72.25.Fe, 72.25.Rb, 72.25.Dc, 78.67.De Spin-dependent phenomena in semiconductor structures are the subject of extensive ongoing research. One of the most widespread and powerful methods for creating spin polarization and investigating kinetics of spinpolarized carriers is optical orientation of electron and nuclear spins by circularly polarized light [1,2,3,4,5]. This effect can be interpreted as a transfer of the photon angular momenta to free carriers. Under interband excitation by circularly polarized light, direct optical transitions from the valence band to the conduction band can occur only if the electron angular momentum is changed by ±1. These selection rules lead to the spin orientation of photoexcited carriers, with degree and sign of spin orientation depending on the light helicity.In the present paper we show that in low-dimensional semiconductor systems spin orientation of carriers can be achieved by linearly or even unpolarized light. The effect under consideration is related to reduced symmetry of the low-dimensional structures as compared to bulk compounds and is forbidden in bulk cubic semiconductors. Microscopically, it is caused by asymmetrical photoexcitation of carriers in spin subbands followed by spin precession in an effective magnetic field induced by the Rashba or Dresselhaus spin-orbit coupling [6].The effect is most easily conceivable for direct transitions between the heavy-hole valence subband hh1 and the conduction subband e1 in quantum well (QW) structures of the C s point symmetry, e.g. in (113)-or (110)-grown QWs based on zinc-blende-lattice compounds. In such structures the spin component along the QW normal z is coupled with the in-plane electron wave vector. This leads to k-linear spin-orbit splitting of the energy spectrum as sketched in Fig. 1, where the heavyhole subband hh1 is split into two spin branches ±3/2 shifted relative to each other in the k space. Due to the selection rules the allowed optical transitions from the valence subband hh1 to the conduction subband e1 are | + 3/2 → | + 1/2 and | − 3/2 → | − 1/2 , as illustrated in Fig. 1 by dashed vertical lines. Under excitation with linearly polarized or unpolarized light the rates of both transitions coincide. In the presence of the spin splitting, the optical transitions induced by photons of the fixed energyhω occur in the opposite points of the k space for the electron spin states ±1/2. Such an asymmetry o...

Current-induced spin polarization in a two-dimensional hole gas

“…18,19,20 So far most theoretic investigations about the CISP deal with the electron SOC systems. 4,5,6,7,8,9,10,21,22,23,24,25,26 Thus the CISP in the 2DHG system as shown in Silov's experiments was also interpreted in terms of the linear-k Rashba coupling of the 2DEG systems with several parameters adjusted. 12 As we shall show later, this treatment is not appropriate for 2DHG.…”

Thermal orientation of electron spins

“…Much effort in this field is currently focused on the development of novel methods of spin orientation of free carriers ranging from the spin Hall effect [2] and spin-dependent tunneling [3] to the optical orientation by linearly polarized light [4]. Here we show that the spin orientation of free electrons can be achieved in semiconductor nanostructures by simple electron gas heating.…”

“…We assume that the spin relaxation time of carriers is much longer than the thermalization time controlled by electron-electron collisions which is in turn much longer than the momentum relaxation time τ p , and Ω k τ p ≪ 1. Then, the spin generation rate is given by [4] …”

Quantum states and linear response in dc and electromagnetic fields for the charge current and spin polarization of electrons at the Bi/Si interface with the giant spin-orbit coupling

“…[24][25][26][27] Besides the response to the dc electric field, the optical properties of SO-split band spectrum always attracted significant attention starting from the conventional semiconductor structures with big SO coupling. [28][29][30][31][32] In our previous papers we have observed an important role of SO coupling in conventional InGaAs-based semiconductor superlattices on the energy band formation 33 which directly affected both charge and spin response for the excitation by the electromagnetic radiation 34,35 and by the dc electric field. 36 It is known that the spin polarization configurations in semiconduc-tors may have a rather long relaxation time 4,37,38 which makes them as important as the conventional charge current setups for their applications in the nanoelectronics and spintronics.…”