The Studies Of High Energy Nanosecond Terahertz Parametric Sources

Terahertz wave is the electromagnetic wave whose frequency is in the range from 0.1 THz to 10 THz and lies between the microwave and the infrared wave. The terahertz technology has significant scientific values and comprehensive application potentiality in the fields of materials science, biological medicine, information communication, aeronautics and astronautics and national safety.There are two kinds of methods for THz generation, electronic ones and optical ones. The optical parametric method which belongs to the optical methods is a very important one for the THz generation. The terahertz parametric sources, which are based on the stimulated polariton scattering in the nonlinear media, have the merits of narrow linewidth, good coherence, room temperature operation and tunable output. However, the THz-wave energy is relatively low.In this thesis, we investigate the causes for the low THz-wave energy output by the terahertz parametric oscillator, demonstrate a new scheme, which combines a terahertz parametric oscillator and a Stokes seed injected terahertz parametric generator, to obtain high energy THz wave, and made a research on the tuning characteristics of Stokes light in the terahertz parametric oscillator.The main contents of this thesis are as follows:1. We conduct experimental research on the terahertz parametric oscillator (TPO) based on MgO:LiNbO3 crystal. The factors preventing the TPOs from generating high pulse energy THz wave are summarized. First, the incident pump pulse intensity is restricted by the relatively low damage threshold of the MgO:LiNbO3 crystal. Second, when a Stokes pulse is built up, the pumping pulse is significantly depleted. That means, there is a time interval between the peaks of the pump and Stokes pulses. While the THz-wave generation is related to the product of the pump intensity and the Stokes light intensity, this incomplete temporal overlap between the pump and Stokes pulses impairs the parametric process. Third, due to the limitation of the resonant cavity for the Stokes wave and the small phase matching angle between the pumping and the Stokes beams, the pumping beam size cannot be very large. In addition, the Stokes beam size in the cavity is less than the pump beam size. The relatively small pumping beam size and the incomplete spatial overlap between the pumping and Stokes beams constrict the effective interaction volume of the three mixing waves.2. Based on the above analysis, a new scheme, which combines a terahertz parametric oscillator and a Stokes seed injected terahertz parametric generator (spi-TPG), is demonstrated. High-energy terahertz pulses in duration of nanoseconds are generated by increasing the pumping pulse energy and beam size. The duty of the TPO here is to generate Stokes pulses. They are used as the seed of the surface-emitted spi-TPG. The time delay between the pump and Stokes pulses is adjusted to guarantee they have good temporal overlap. The pump pulses have large pulse energies and a large beam size. The Stokes beam is enlarged to make its size be larger than the pump beam size to give a large effective interaction volume. The results show that there is a great improvement in the THz output energy.3. The tuning Stokes laser characteristics based on the stimulated polariton scattering in MgO:LiNbO3 crystal are investigated. By varying the phase matching angle between the pump and Stokes beams, the tuning Stokes laser is obtained. When the KTP crystal is put in the Stokes oscillator, a tuning green laser is achieved.