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 population relaxation dynamics of carriers and excitons in 100-Angstrom GaAs-AlGaAs multiple quantum wells by visible-FIR pump-probe spectroscopy. Our experimental setup is capable of resolving transient changes of the complete complex amplitude transmission coefficient Delta t/t between 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 pumping 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 decay of the free-carrier population but also the growth of the 1s exciton population between 10 and 100 ps. The latter effect is attributed to the relaxation of free carriers into the lowest (1s) exciton level.