| This thesis presents a comprehensive study of time-resolved intervalley transitions of hot electrons, as well as generation and detection of coherent acoustic phonons, in high-quality GaN single crystals, using a two-color (ultraviolet and near-infrared), femtosecond, pump-probe spectroscopy technique.; In investigation of the intervalley transitions of electrons between the conduction Г band and the L valley, we have found that the threshold for the Г ↔ L transition appears at the energy of 4.51+/-0.05 eV at room temperature and shifts to 4.57+/-0.01 eV at 100 K. At both temperatures, the measured electron Г→ L scattering time is limited by the 170-fs width of our pump pulses, while the return L → Г process is measurably slower and equals 1.02 ps. The total depopulation time of the L valley is estimated to be ∼20 ps. Our physical model, based on the three-state rate equations, fitted our experimental data very well and allowed us to calculate the optical phonon emission time of 290 fs. The deformation potential between the two valleys was also obtained from the intervalley scattering modeling and the value found to be 0.92 x 109eV/cm.; We also report on our experimental and theoretical studies of the time-resolved generation and detection of coherent acoustic phonons (CAPs). A train of ultraviolet laser pulses with energy above the GaN energy gap induced a transient electronic stress at the GaN surface, responsible for CAP generation. Subsequent CAP oscillations, propagating without any measurable intrinsic attenuation, were observed by scanning the transient differential reflectivity signal (Delta R/R) of the near-infrared, far-below-bandgap probe beam. The Delta R/R CAP oscillation amplitude was of the order of 10-5 to 10-6 and was dependent only on the pump-photon absorption coefficient spectral characteristics. The CAP oscillation frequency was dispersionless (proportional to the probe-beam wave vector), with the slope corresponding to 8002 +/- 22 m/s---the speed of sound in GaN---while the CAP signal phase was constant within the entire range of our experiments. The above experimental results are in excellent agreement with our theoretical modeling and the published literature data. |
