Abstract

Absorption of light is reversible as long as phase relaxation has not yet occurred. Excitons created by a first laser pulse can then be quenched by a second phase-locked pulse when the light interferes destructively with the excitonic amplitude. The previously absorbed optical energy is then re-emitted. The left figure demonstrates this coherent control of exciton density in 12 nm GaAs quantum wells (T = 4 K) measured with transient reflectivity. Arrows at the bottom of the diagram mark the arrival of the 100 fs pump pulses created by a mode-locked Ti:sapphire laser with subsequent actively stabilized Michelson interferometer which ensures a pump pulse separation with λ/100 accuracy. Excitation with a single pulse creates a long-lived exciton population. The oscillations are caused by heavy-hole light-hole quantum beats. When the pump pulse separation is (n+l/2)λ_exciton, pulse I creates a temporary exciton population which is quenched again when pulse 2 interferes destructively with the coherent exciton amplitude from pulse I. This technique allows creation of ultrashort exciton pulses which are only limited by the laser pulse width and combines the strength of resonant with the speed of non-resonant excitation. In case of constructive interference at a separation of nλ_exciton, pulse 2 doubles the exciton amplitude, resulting in 4 times the density created by a single pulse.

© 1996 IEEE