Two-photon absorption inside β-BBO crystal during UV nonlinear optical conversion Sheng Wu 1 , Geoffrey A. Blake 1 Sunny Sun, Henry Yu 2 1 Division of Geological and Planetary Sciences, California Institute of Technology, CA 91125 2 CASIX Inc., 21828 #D, Lassen St. Chatsworth, CA 91131 ABSTRACT We measured the two-photon absorption (TPA) cross sections inside β-BBO crystal during UV harmonic generation. We found that the 2-photon absorption is dominating the absorption effect inside the BBO crystal during UV harmonic generation. Both 2 UV photons and 1 UV photon + 1 fundamental photon absorption cross sections are significant. Possible explanations are presented, and compared with other nonlinear optical crystals. Thermal profiles inside the crystals as a result of the strong absorption processes are discussed through a computer program that simulates the heat dissipation process. We conclude that TPA is the significant factor in high power scaling of UV harmonic generation inside nonlinear optical crystals. Key words: two-photon absorption (TPA), BBO, thermal profiles 1. INTRODUCTION High power deep UV sources based on all solid state lasers are desirable in many industrial and scientific fields, e.g., UV lithography, via hole drilling, UV photolysis 1 , LIDAR and many more, for the compact, efficient and cost-effective merits of such systems. With the recent progresses in two fronts --- diode laser pumping and novel nonlinear optical (NLO) crystals (BBO, LBO, CBO, CLBO, etc.), hope is high to achieve high power deep UV through power scaling of such devices. But, as the input power in the visible increases, significant thermal distortion 2,3 starts to appear --- the conversion efficiency drops, the beam quality degrades, and the stability deteriorates. To overcome such thermal distortions, we have to first understand the thermal sources and profiles behind the nonlinear optical processes. 2. TPA IN NONLINEAR OPTICAL CRYSTALS 2.1 Currently available TPA data in NLO crystals Although strong thermal distortions have been observed in BBO, CLBO, not enough quantitative data is available to describe the absorption and thermal profile properties of these two relatively new NLO crystals. There are limited absorption data on these crystals --- particularly TPA cross sections of 2 UV photons, and of 1 UV photon + 1 fundamental visible photon at important wavelengths. Here, we measure TPA cross section at two different wavelengths for β-BBO crystal and demonstrate that TPA cross sections are indeed very important in creating the temperature profile inside the NLO crystals, and they are severely limiting the conversion efficiency at high average power. Table 1 summarizes the available TPA and temperature bandwidth data of the new NLO crystals (BBO, LBO, CLBO and CBO).