Two-photon absorption and nonlinear refraction of AlN from ultraviolet to near-infrared

We report a detailed characterization of the wavelength dependency of two-photon absorption coefficient β and Kerr nonlinearity n2 in aluminum nitride (AlN) single crystal over a spectral range from 350 to 1030 nm (0.19 < Ephoto/Eg < 0.57). Femtosecond Z-scan results show that the dispersion of β compares favorably with the theoretical prediction at wavelengths shorter than 400 nm (half of bandgap Eg). However, the nonlinear refractive index n2 demonstrated a substantial deviation from theoretical calculations in the 350–450 nm regime, which is correlated with the sub-bandgap absorption band observed in AlN single crystal. The effects of trapped carrier-induced absorption and refraction were determined by femtosecond pump–probe measurements, which could be attributed to the transition from valence band maximum to mid-gap defect states in AlN. Our results provide guidance for developing AlN-based nonlinear integrated photonic devices in the UV–visible spectral range.