Broadband second-harmonic generation in an x-cut lithium tetraborate whispering gallery mode resonator

Abstract

Lithium tetraborate (LB4) is a nonlinear crystal that has very low optical absorption in the visible and ultraviolet range. Hence, it is a very interesting material for nonlinear conversion experiments, particularly in whispering gallery mode (WGM) resonators. Here, we show theoretically that second-harmonic generation is possible over a wide range of pump wavelengths (488nm to 3320nm) in an x-cut LB4 WGM resonator. We also demonstrate this experimentally at three different pump wavelengths (1560nm, 795nm, and 517nm). We find a reasonable conversion efficiency of 0.2%/mW for the generation of UV light at 258.5nm. This is possible due to the high-quality factors on the order of 5×10⁷ and the use of the so-called selective coupling method.