Relaxation of quasi-two-dimensional electrons in a quantizing magnetic field probed by time-resolved cyclotron resonance

We have measured the picosecond time-resolved cyclotron resonance of photogenerated transient carriers in undoped ${\mathrm{I}\mathrm{n}\mathrm{S}\mathrm{b}/\mathrm{A}\mathrm{l}}_{0.09}{\mathrm{In}}_{0.91}\mathrm{Sb}$ quantum wells by two-color pump-probe spectroscopy in a magnetic field. The strong conduction-band nonparabolicity of InSb causes the average cyclotron mass of the electrons, which we monitor directly in time, to decrease as the electrons relax towards the band edge. In addition, the nonparabolicity results in multiple resonances due to the strongly energy-dependent mass and g factor, allowing us to determine the time evolution of the Fermi-Dirac distribution function for the excited carriers in quantizing magnetic fields.