Observation of Two-Photon Absorption and Nonlinear Refraction in AlN

Two-photon absorption (TPA) and nonlinear refraction in AlN single crystals are revealed with the single beam Z-scan technique. A large TPA coefficient of 13\pm3 cm/GW at 355 nm and third-order nonlinear refractive indices of -1.91\pm0.38 \times 10^-13 cm^2/W at 355 nm, 1.79\pm0.36 \times 10^-13 cm^2/W at 532 nm and 1.61\pm0.32 \times 10^-12 cm^2/W at 1064 nm are derived. It is inferred that the TPA-generated free carriers are responsible for the large negative nonlinear refractive index at 355 nm, while the oxygen impurity induced bandgap narrowing may account for the large third-order nonlinear refractive indices at 532 nm and 1064 nm. The large nonlinear optical responses of AlN promise wide applications in the deep ultraviolet nonlinear optical devices.