Theoretical modeling of femtosecond pump-probe spectroscopy in GaN systems

We present theoretical simulation of the femtosecond pump-probe spectroscopy in GaN systems for photoexcitation both far below and far above the band gap. Semiconductor Bloch equations for carrier distribution and exciton polarization are solved numerically. The simulation results are compared with experiment. The experiment for both cases was performed at 10 K to study the non-equilibrium carrier dynamics in bulk GaN. For pump below the band gap, prominent AC Stark effects are observed, and the theoretical simulation gives line-shapes of the differential absorption spectra in qualitative agreement with experiment. If the carrier screening and band renormalized effects are properly scaled, then good quantitative agreement between theory and experiment can be obtained for various pump intensities and detuning energies. For pump far above band gap, the theoretical, simulation shows a fast carrier relaxation due to LO phonon emission and carrier-carrier scattering with scattering time on the order of 10-100 fs, while experimentally, we find that the hot carriers are strongly confined in a non-thermal distribution and they relaxed collectively to the band edge at a surprisingly slow rate (with relaxation time around 1 ps).