Energy relaxation at THz frequencies in Al_xGa_1-xAs heterostructures

We report 4.2 K studies of the dependence of the in-plane, DC conductivity of a quasi 2D electron gas on the amplitude E_omega of applied fields with frequencies from 0.25 THz to 3.5 THz. We analyse the dependence of sigma _DC on E_omega assuming that electron-optical phonon scattering dominates energy relaxation, that the absorbed power has a Drude form and that the electron distribution is thermal. This simple analysis is self-consistent: Arrhenius plots of the estimated energy loss rate have a slope near -h omega _LO/k₈, for all frequencies, as expected for energy loss by optical phonon emission. We find that the effective energy relaxation time tau _epsilon varies with the frequency of the applied field, from tau _epsilon approximately 4 ps at 0.34 THz to tau _epsilon approximately 0.3 ps at 3.45 THz. This may indicate a frequency-dependent form for the hot-phonon distribution.