Modelling third-harmonic generation of large aperture convergent beam with tiling biaxial crystals

The scheme of third-harmonic generation (THG) of large aperture convergent beam with tiling biaxial crystals is theoretically calculated and experimentally verified in this study. In this model, tiling crystals, with different cutting angles, placed at the downstream of the focus lens, are employed to generate sum frequency by collinear type-II phase-matching (PM). The changes of polarization directions are analyzed when light passes through a focusing lens. The refractive indexes in biaxial crystal, corresponding polarization directions, nonlinear coefficients and PM configuration are also described in detail, by employing the Lagrange multiplier. THG conversion efficiency was measured by using three tiling LiB3O5 (LBO) crystals all with a thickness of 8 mm, which is in reasonably good agreement with the theoretical calculation. The THG scheme of convergent beam provides a promising way to avoid damage of fused silica lens irradiated by the high-energy ultraviolet light in inertial confinement fusion.