Nonlinear intersubband absorption of a hot quasi-two-dimensional electron plasma studied by femtosecond infrared spectroscopy

The transient ($n=1\mathrm{}$) to ($n=2\mathrm{}$) intersubband absorption of a pure electron plasma in $n$-type ${\mathrm{Ga}}_{0.48}$ ${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$ ${\mathrm{In}}_{0.52}$As quantum wells is studied in femtosecond pump-probe experiments. The ultrafast dynamics of nonlinear absorption shows strong changes when tuning the midinfrared pulses over the intersubband absorption line. The nonlinear optical response is determined by both intersubband relaxation with a time constant of 1.3 ps and the intraband dynamics of ($n=1\mathrm{}$) electrons, which are monitored in an independent experiment.