At present, the key factor for the maximum load capacity in the operation of a large solid laser facility is the damage threshold of optical elements. For example, the National Ignition Facility in the USA has an Nd:glass laser delivering pulses of energy up to 1.8 MJ carried by 192 beams and peak power of 500 TW of ultraviolet light with wavelength of 355 nm for performing ignition target experiments. Such high-energy UV beams are accompanied by obvious nonlinear effects and, thus, might damage the lenses when focusing and limit the capability of a high-power laser facility.

In order to solve the existing problems as noted, in this Letter, we propose a new way to focus third-harmonic wave without a lens in a high-energy level. By prefocusing the fundamental and second-harmonic waves before KDP/DKDP crystals, third-harmonic wave could focus automatically without a lens. At the same time, we propose noncollinear type-I noncritical phase-matching for small angular sensitivity, which provides possibility to the thought of pre-focusing. The principle and the experimental setup for KDP crystal are presented. The experimental data agrees reasonably well with theoretical calculation for KDP crystal.

J. Chen, Y. Zheng, N. An, and X. Chen, Noncollinear third-harmonic generation with large angular acceptance by noncritical phase matching in KDP crystal, Opt. Lett. 40(19), 4484 (2015).