PICOSECOND TIME-RESOLVED FAR-INFRARED EXPERIMENTS ON CARRIERS AND EXCITONS IN GAAS-ALGAAS MULTIPLE-QUANTUM WELLS

We have investigated the far-infrared (FIR) optical properties and pop ulation relaxation dynamics of carriers and excitons in 100-Angstrom G aAs-AlGaAs multiple quantum wells by visible-FIR pump-probe spectrosco py. Our experimental setup is capable of resolving transient changes o f the complete complex amplitude transmission coefficient Delta t/t be tween 0.3 and 2.3 THz with high sensitivity (\Delta t/t\ greater than or equal to 5 X 10(4)) and picosecond time Using picosecond laser pump ing pulses to create excitons, we measured the exciton lifetime of 250 ps by monitoring the resulting transient 1s-2p exciton absorption (2. 0 THz) of a picosecond FIR probing resolution. When free carriers were excited above the quantum-well band edge, we observed not only the de cay of the free-carrier population but also the growth of the 1s excit on population between 10 and 100 ps. The latter effect is attributed t o the relaxation of free carriers into the lowest (1s) exciton level.