Coherent intersubband oscillations in modulation-doped quantum wells

Summary form only given. We report on the observation of coherent intersubband beats in time-domain femtosecond experiments on a high-mobility two-dimensional electron gas. The sample is a 400-/spl Aring/ one-sided modulation-doped GaAs-AlGaAs single quantum-well grown by molecular beam epitaxy. The top Al/sub 0.2/Ga/sub 0.8/As layer contains Si donors which are separated from the GaAs quantum well by a thick undoped space layer. The areal density of the electron gas at the interface is /spl rho/ /spl ap/ 2 /spl times/ 10/sup 11/ cm/sup -2/ and the low-temperature mobility is 1.2 /spl times/ 10/sup 6/ cm/sup 2//Vs. After removal of the substrate, pump-probe experiments were performed at /spl sim/ 7 K in the transmission geometry using a modelocked Ti-sapphire laser. The central wavelength of the pulses was tuned to resonate with excitons associated with heavy-hole states at /spl sim/ 800 nm.