Spectrally and temporally resolved Faraday rotation is shown to be a sensitive tool to study ultrafast carrier kinetics in direct band gap semiconductors. The transient spectra in the model materials GaAs and InP directly monitor the femtosecond energy relaxation of nonequilibrium carriers for excitation densities as low as ${10}^{16}{\mathrm{cm}}^{-3}$. Especially, Faraday rotation spectra are found to more accurately follow the time-dependent carrier distribution functions than conventional differential transmission experiments. Moreover, significant Faraday rotation signals are identified for probe photon energies as far as $\sim 100\mathrm{meV}$ below the band gap. As a result, ultrafast energy relaxation and spin dynamics is also accessible with a nonresonant detection scheme.

• Received 27 May 2007

DOI:https://doi.org/10.1103/PhysRevB.76.073201