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-A 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 Δt/t between 0.3 and 2.3 THz with high sensitivity (|Δt/t| ≥ 5 × 10−4) and picosecond time resolution. 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 pulse. 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.