Characterizations of nonlinear optical properties on GaN crystals in polar, nonpolar, and semipolar orientations

We report the basic nonlinear optical properties, namely, two-photon absorption coefficient (beta), three-photon absorption coefficient (gamma), and Kerr nonlinear refractive index (n(kerr)), of GaN crystals in polar c-plane, nonpolar m-plane, and semipolar (2021) plane orientations. A typical Z-scan technique was used for the measurement with a femtosecond Ti:S laser from wavelengths of 724 nm to 840 nm. For the two-photon absorption coefficient (beta), similar values were obtained for polar, nonpolar, and semipolar samples, which are characterized to be similar to 0.90 cm/GW at 724nm and similar to 0.65 cm/GW at 730 nm for all the three samples. For the Kerr nonlinear refractive index (n(kerr)), self-focusing features were observed in this work, which is different from previous reports where self-defocusing features were observed on GaN in the visible and near-UV spectral regions. At 724 nm, nkerr was measured to be similar to 2.5 0 x 10 (-14) cm(2)/W for all three samples. Three-photon absorption coefficients (gamma) were also determined, which were found to be consistent with previous reports. This study provides valuable information on the basic nonlinear optical properties of III-nitride semiconductors, which are vital for a wide range of applications such as integrated photonics and quantum photonics.