First-Principles Evaluation of the Alkali and/or Alkaline Earth Beryllium Borates in Deep Ultraviolet Nonlinear Optical Applications

The prospects of all known noncentrosymmetric alkali and/or alkaline earth beryllium borates for nonlinear optical (NLO) generation in the deep-ultraviolet (DUV) spectral region are evaluated by first-principles studies. It is shown that all these crystals possess large enough energy band gaps and relatively strong NLO responses that satisfy the requirement for DUV second-harmonic generation (SHG). However, the practical DUV SHG capabilities of these crystals are mostly limited by the phase-matching condition, as determined not only by the optical birefringence but also by the refractive-index dispersion that is explicitly emphasized for the first time in this work. Therefore, although the K2Be2BO3F family and NaBeB3O6 both have a large birefringence, Delta n approximate to 0.08, at a wavelength of 400 nm, the DUV SHG lasers can only be produced in the former but cannot be fulfilled in the latter due to its large refractive-index dispersion in the DUV region. The crystal structural features for large birefringence and the electronic structure origins for different refractive-index dispersion behaviors are discussed. The elucidation of the structure-property relationship presents a useful guide to the exploration practically applied for DUV SHG.