Fürst, J.U.J.U.FürstBuse, K.K.BuseBreunig, I.I.BreunigBecker, P.P.BeckerLiebertz, J.J.LiebertzBohatý, L.L.Bohatý2022-03-122022-03-122015https://publica.fraunhofer.de/handle/publica/38902410.1117/12.2079829For second-order nonlinear-optical processes in the ultraviolet, appropriate materials with a sufficiently large band-gap typically exhibit smaller nonlinear coefficients than materials with comparably smaller band-gap. Whispering gallery resonators, with their outstanding quality factors, provide field enhancement and can compensate for these small coefficients. We report on the successful fabrication of a whispering gallery resonator made of lithium tetraborate, a suitable material for ultraviolet applications with a small nonlinear coefficient of d<inf>31</inf> = 0.073 pm/V. Quality factors of the order of 108 are observed from the ultraviolet to the near-infrared spectrum. The inferred absorption coefficients of lithium tetraborate are below 0.2 m-1 in the visible and near-infrared. Continuous-wave second harmonic generation from 490 nm light to 245 nm is observed with conversion efficiencies up to 2.2 %.enconference paper