Abstract

Excitonic optical nonlinearities in GaAs multiple quantum wells (MQWs) are well known[1] theoretically and experimentally. Phase space filling (PSF) reduces the absorption across the exciton resonance when electrons fill the lowest k-states in the conduction band, while Coulomb interactions are responsible for exciton screening and line width broadening. It has been shown previously that the PSF contribution to exciton saturation is spin de- pendent.[2] Confinement in MQWs lifts the degeneracy between the light and heavy hole bands and gives access to polarisation sensitive optical selection rules. This allows the generation of 100% spin- polarised electrons and holes by using circularly polarised light resonant with the heavy hole exciton. At room temperature the spin orientation of the electron is maintained for tens of picoseconds after the rapid ionisation of the exciton while the hole spins relax on a sub-picosecond timescale. In this work, we demonstrate the creation of spin gratings for the first time by using a polarisation grating which is formed when two beams of orthogonal linear polarisation intersect. A periodic modulation of the electric field polarisation occurs across the excitation region as shown in Figure 1(a) and provides a modulation in circular polarisation, Figure 1(b). Therefore the spin dependent PSF nonlinearity will produce a spatial modulation of the electron spin orientation within a MQW excited close to the heavy hole resonance. Monitoring the grating decay for different grating periods via a diffracted probe beam allows the determination of the electron diffusion coefficient.

© 1996 Optical Society of America