Broadband second-harmonic generation in an <i>x</i>-cut lithium tetraborate whispering gallery mode resonator

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 ( to ) in an 488nm-cut LB4 WGM resonator. We also demonstrate this experimentally at three different pump wavelengths (, , and ). We find a reasonable conversion efficiency of for the generation of UV light at . This is possible due to the high-quality factors on the order of and the use of the so-called selective coupling method.