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 (β), three-photon absorption coefficient (γ), 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 ( β), similar values were obtained for polar, nonpolar, and semipolar samples, which are characterized to be ∼0.90 cm/GW at 724 nm and ∼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, n_kerr was measured to be ∼2.5 0 × 10⁻¹⁴ cm²/W for all three samples. Three-photon absorption coefficients (γ) 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.