A strongly birefringent sulfate crystal with a wide second-harmonic generation phase-matching range

Abstract

Birefringence is pivotal for the improvement of the conversion efficiency of nonlinear optical (NLO) materials via phase matching (PM). The engineering of proper birefringence for short-wavelength ultraviolet (UV) NLO crystals, particularly for those based on homoleptic tetrahedral moieties, such as sulfates, remains challenging, mainly because of the conflicting correlation of energy bandgap and birefringence. We herein synthesized a short-wavelength UV NLO sulfate, Li₂SO₄·C₂H₅NO₂ (LSG), composed of homoleptic [SO₄]²⁻ tetrahedra and π-conjugated zwitterionic glycine. To our surprise, both experimental and theoretical studies indicate that LSG is strongly birefringent, with an experimental birefringence of 0.144 at 550 nm. Benefiting from its satisfactory birefringence, LSG exhibited desirable PM behavior in the UV regime with a type-I PM wavelength of 262 nm. Powder second harmonic generation (SHG) measurements revealed that LSG can achieve PM under 1064 and 532 nm with an efficiency that is 0.7 and 0.3 times that of KDP and β-BBO, respectively, underscoring its potential application in generating UV laser by a direct SHG process. LSG simultaneously maintained a wide UV transparency window, with an optical bandgap of 5.20 eV and a short UV cutoff edge of 220 nm, as well as high thermal stability that is comparable to benchmark KDP. Structure-performance relationship investigation clearly indicated that the π-conjugated glycine component is mainly responsible for these crucial properties.