Quenching of excitonic quantum-well photoluminescence by intense far-infrared radiation: Free-carrier heating

Undoped GaAs/${\mathrm{Al}}_{0.3}$ ${\mathrm{Ga}}_{0.7}$As and GaAs/AlAs quantum wells are simultaneously excited by weak visible light and intense far-infrared (FIR) radiation with electric fields polarized parallel to the planes of the quantum wells. The frequency of the FIR radiation ranges from 6 to 119 ${\mathrm{cm}}^{\mathrm{-}1}$ with intensities up to 700 kW/${\mathrm{cm}}^2$. Peaks in the excitonic photoluminescence (PL) are broadened and quenched by the intense FIR radiation; the PL line shapes are consistent with the FIR radiation heating the carriers without significantly heating the lattice. Despite the excitonic nature of the PL, the power and frequency dependence of the carrier heating is consistent with free-carrier absorption of FIR radiation. Energy and momentum relaxation times for the free carriers are extracted from fits to a Drude model. We suggest that the FIR radiation heats free carriers that are not contributing to luminescence, and that these hot carriers in turn heat the luminescing excitons.