Abstract
Lithium tetraborate (LB4) is a nonlinear crystal which 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 (488 nm to 3320 nm) in an x-cut LB4 WGM resonator. We also demonstrate this experimentally at three different pump wavelengths (1560 nm, 795 nm and 517 nm). We find a reasonable conversion efficiency of 0.2 %/mW for the generation of UV light at 258.5 nm. 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.