We report experimental and theoretical studies of the excitonic optical Stark effect in GaN photoexcited below the excitonic resonances with various polarization configurations and pump detunings, using nondegenerate pump-probe spectroscopy at 10 K. We observed that the Stark effect in GaN is strongly dependent on pump and probe relative linear polarizations. We found that this dependence results from the small spin-orbit splitting in GaN and a mixing of A and B valence bands induced by a linearly polarized pump. Using two different circular polarization configurations, we also observed splitting of degenerate excitons because of different optical Stark shifts. Our experimental results are explained by a simple theoretical model.
The non-equilibrium carrier dynamics in GaN epilayer for carrier densities ranging from 4 X 1017 to 1019 cm-3 at 10 K was studied by femtosecond pump-probe transmission spectroscopy. Spectral hole burning was initially peaked roughly at the excitation energy for an estimated carrier density of 4 X 1018 cm-3 and gradually redshifted during the excitation. Because of reduced carrier-carrier and carrier-phonon scattering, a very slow energy relaxation of the hot carriers at these densities were observed. We show that the hot carriers were strongly confined in a non-thermal distribution and they relaxed collectively to the band edge.
We present theoretical simulation of the femtosecond pump- probe spectroscopy in GaN systems for photo-excitation 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).
Femtosecond pump-probe measurements were performed in GaN epilayers to study carrier dynamics in the band edge region. Excitonic absorption was found to begin saturating at a pump fluence of 20 (mu) J/cm2 which corresponds to an estimated carrier density of 1 X 1018 cm-3. At zero delay between pump and probe, induced absorption is observed below the unpumped band gap due to ultrafast bandgap renormalization. After 375 fs, a large induced transparency is observed just below the excitonic resonance which is due to a transient electron-hole plasma. After 1 ps, the absorption has partially recovered to a level associated with excitonic phase-space filling. The absorption then recovers with a characteristic time of approximately 20 ps, a value which increases with increasing excitation density.
Conference Committee Involvement (5)
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XI
22 January 2007 | San Jose, California, United States
Ultrafast Phenomena in Semiconductors and Nanostructure Materials X
23 January 2006 | San Jose, California, United States
Ultrafast Phenomena in Semiconductors and Nanostructure Materials IX
24 January 2005 | San Jose, California, United States
Ultrafast Phenomena in Semiconductors and Nanostructure Materials VIII
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