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±3 cm/GW at 355 nm and third-order nonlinear refractive indices of-1.91±0.38 x 10;cm;/W at 355 nm,1.79±0.36×10;cm;/Wat 532 nm and 1.61±0.32× 10;cm;/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.