Birefringent Dispersion Optimization to Achieve Superior Nonlinear Optical Phase Matching in Deeper Solar-Blind UV Band from KH 2 PO 4 to BeH 3 PO 5 .

Nonlinear-optical (NLO) crystals require birefringent phase matching (BPM), particularly in the solar-blind ultraviolet (UV) (200-280 nm) and deep-UV (100-200 nm) regions. Achieving BPM requires optimization of optical dispersion along with having large birefringence. This requirement is especially critical for structures with low optical anisotropy, including classical phosphate UV-NLO crystals like KH 2 PO 4 (KDP). However, there is a scarcity of in-depth theoretical analysis and general design strategies based on structural chemistry to optimize dispersion. This study presents findings from a simplified dielectric model that uncover two vital factors to micro-optimize transparent optical dispersion: effective mass (m*) of excited states and effective number (N*) of photo-responsive states. Smoothing of dispersion occurs as m* increases and N* decreases. First-principles analysis of deep-UV KBe 2 BO 3 F 2 -family structures is used to confirm the conciseness and validity of the model. It further proposes substituting K + with Be 2+ to decrease N* and increase m* while enlarging bandgap. This will lead to improved dispersion and an overall enhancement of KDP's BPM capability. The existing BeH 3 PO 5 (BDP) is predicted to improve the shortest BPM wavelength for second-harmonic generation, from 251 nm in KDP to 201 nm in BDP. BDP's extension into the broader UV solar-blind waveband fully supports the proposed optimization strategy.