Observation of Two-Photon Absorption and Nonlinear Refraction in A1N

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 134-3 cm/CW at 355 nm and third-order nonlinear refractive indices of-1.91±0.38×10-13 cm2/W at 355nm, 1.79±0.36 × 10-13 cm2/W at 532nm and 1.61±0.32 ×10-12 cm2/W at 1064nm 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 532nm and 1064nm. The large nonlinear optical responses of AIN promise wide applications in the deep ultraviolet nonlinear optical devices.