Femtosecond all-optical polarization switching based on the virtual excitation of spin-polarized excitons in quantum wells

An optically addressed nonlinear polarization switch based on the near-resonant excitation of a spin-polarized population of virtual excitons is demonstrated in a multiple quantum well. Pulse-width-limited switching (∼400 fs full width at half maximum) and high-contrast performance (194:1) are achieved in a thin (40 well) sample at 100 K. Differential absorption measurements identify the dominant switching mechanisms.