Energy relaxation of lower-dimensional hot carriers studied with picosecond photoluminescence

To study the energy relaxation of lower-dimensional hot carriers, picosecond time- and energy-resolved photoluminescence measurements have been carried out on bulk GaAs and GaAs/${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As quantum-well structures in the presence of magnetic fields up to B=20 T. For GaAs the results show that the energy-relaxation rate reduces with increasing strength of the magnetic field. This cooling behavior is adequately described by a model for energy relaxation containing the magnetic-field-dependent kinetics of the coupled carrier–nonequilibrium-LO-phonon system. For the quantum-well structures, an increasing magnetic field normal to the quasi-two-dimensional layers reduces the carrier cooling up to B=8 T, while at higher field strengths an enhancement in cooling is observed up to B=20 T. We suggest this effect to be due to a reduction in energy relaxation rate by LO-phonon emission, so that at B>8 T carrier cooling is taken over by acoustic-phonon emission, which increases with magnetic field.