Optical rectification of ultra-short laser pulses is an attractive technique for efficient generation of terahertz pulses. An analytical expression of the optical-to-terahertz conversion efficiency in optical rectification by the use of the method of tilted-pulse-front pumping has been derived and used to simulate the maximum optical-to-terahertz conversion efficiency. The variations of the conversion efficiency of optical-to-terahertz with the length of crystal, the intensity and the pulse duration of the pumping ultra-short laser have been investigated, both in numerical and analytical solutions. In addition, the absorption of terahertz wave in LiNbO3 has also been taken into account. The results show that, the optical-to-terahertz conversion efficiency increases with the intensity of pump pulse and the length of crystal, whereas decreases with the increasing of the pulse duration for the case of the relatively large pulses duration.
In direct-drive laser fusion, the sufficient uniformity of focal spot for realizing high efficient compression and central ignition is required. However, the laser beams are difficult to achieve sufficient uniform for compressing the shell symmetrically inward. We proposed a novel scheme to achieve controllable focal length based on electro-optic effect. The electro-optic crystal was placed in the front of the laser fusion system and applied the electro field with approximate spherical distribution. Since the wavefront of laser beam is transformed through the electro-optic crystal, the focal spot of the transformed laser beam would be changed on the target. Theoretical analysis and numerical simulation have been made, and the results show that the proposed scheme could achieve enough controllable focal spot on the target.