The recent development of periodically poled lithium niobate (PPLN) has led to a variety of highly efficient and compact all-solid-state nonlinear optical devices. Of particular interest are the optical parametric oscillators. We have most recently demonstrated for the first time a highly efficient broadly tunable femtosecond optical parametric oscillator (fs OPO) using PPLN. The threshold for oscillation of the PPLN fs OPO is remarkably low, on the order of 50 mW. As a result, it is now possible to use a diode-pumped and frequency-doubled Nd:YVO 4 laser, rather than an Ar-ion laser, as the pump source for the mode-locked Ti:sapphire laser for pumping the fs OPO. Such an OPO can operate from approximately 1 μm to 5.4 μm. Femtosecond sources have been used in a variety of experiments to study the dynamics of hot carriers in semiconductors. Because of the limited available wavelength range of earlier femtosecond sources, most such studies have been limited to electron dynamics in the conduction band of III–V compounds such as GaAs. With the extended tuning range of the recently developed fs OPO's, it is now possible to study a much wider variety of materials and transitions. We have recently obtained preliminary results on the ultrafast dynamics of holes in GaAs using two-wavelength tunable femtosecond spectroscopy, by pumping the valence-to-conduction band transition at approximately 800 nm and simultaneously probing the transitions between the split-off band and the perturbed heavy-hole band at approximately 3 μm. These recent results are summarized and discussed briefly.
Highly efficient, high-power, broadly tunable, cryogenically cooled and diode-pumped Yb:CaF_2