We report, for the first time, the characterizations on optical nonlinearities of beta-phase gallium oxide (b{eta}-Ga2O3), where both (010) b{eta}-Ga2O3 and (-201) b{eta}-Ga2O3 were examined for two-photon absorption (TPA) coefficient, Kerr refractive index, and their polarization dependence. The wavelength dependence of the TPA coefficient and Kerr refractive index was estimated using a widely used analytical model. b{eta}-Ga2O3 exhibits a TPA coefficient of 1.2 cm/GW for (010) b{eta}-Ga2O3 and 0.58 cm/GW for (-201) b{eta}-Ga2O3. The Kerr refractive index is -2.14*10^(15) cm2/W for (010) b{eta}-Ga2O3 and -2.89*10^(15) cm2/W for (-201) b{eta}-Ga2O3. In addition, b{eta}-Ga2O3 shows stronger nonlinear optical anisotropy on the (-201) plane than on the (010) plane, possibly due to highly asymmetric crystal structure. Compared with that of gallium nitride (GaN), the TPA coefficient of b{eta}-Ga2O3 is 20 times smaller, and the Kerr refractive index of b{eta}-Ga2O3 is also found to be 4 to 5 times smaller. These results indicate that b{eta}-Ga2O3 has the potential for use in ultra-low loss waveguides and ultra-stable resonators and integrated photonics, especially in the UV and visible wavelength spectral range.
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