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We present a study of the terahertz (THz) generation by optical rectification in a 2 mm thick <110> ZnTe crystal. The conditions for the most efficient THz generation are described. We investigate the competition between optical rectification (OR), second harmonic generation (SHG), twophoton absorption (TPA) and free-carrier absorption in the diffraction limit, for excitation spot sizes smaller than the THz wavelength. We find that while free-carrier absorption contributes significantly to reduce the THz emission, this contribution does not depend on the excitation spot size. On the contrary, two-photon absorption induces a strong dependence of the THz emitted power for decreasing excitation sizes.

Currently, the output laser energy was enlarged mostly by using laser application technology and different pump styles were used, which made the laser volume large. In this thesis, the output laser energy was increased by improving the quality of laser crystal and using new style doped laser crystal, which can increase the capacity usage ratio of solid laser, and the beam quality was improved as well. In the paper, a new double-doped Nd:Ce:YAG laser crystal pumped by Xe-lamp and the plane-plane resonant was used as main oscillator of laser. There were two advantages using the method. Firstly, the absorption spectrum and emission band spectrum of Ce 3+ were analyzed, which can create energy transition between Ce 3+ and Nd 3+ . The Ce 3+ ion in double-doped laser crystal can absorb pump light at ultraviolet band and create stimulated emission, which located at the absorption band of Nd 3+ and made more Nd 3+ ion create energy level transition. The inverted population increased. As result the capacity usage ratio of pumped energy was increased and the output energy of laser was enlarged. Secondly for the advantage of the limitation of plane-plane resonant to laser beam was strong, the angle of divergence was small at far field and the beam quality was good. Finally the pulsed laser energy was compressed to spike pulse by using passive Q-switched, and the peak power was increased. The experiments were carried out under the condition of passive Q-switched. When the input voltage was 800V, the laser beam was obtained at the oscillator stage, whose output energy was 651.5mJ. The pulse width was 20ns. The slope efficiency was 0.49%. Angle of divergence was better than 1.2 mrad. And peak power was 32.5KW. The experiment result is accordant with theoretical analyses. This solid laser has important potential application.Key word: pulse solid laser, energy transition, large energy, high peak power, double-doped laser crystal of Nd:Ce:YAG, beam quality 1 Introductions