Third-order nonlinearities of Bi12GeO20 crystal measured by nanosecond radiation

Abstract

Negative nonlinear refraction is observed in the photorefractive crystal Bi12GeO20 (BGO) using 7-ns, 532-nm pulsed radiation. The nonlinear refractive index of BGO is measured to be γ=−3.8×10−13cm2W−1 using the z-scan technique. The self-defocusing of nanosecond pulses is attributed to the appearance of free carriers generated from defects, donors, and interband transitions. The nonlinear losses caused by two-photon absorption and self-scattering from dynamic grating are discussed. The nonlinear absorption coefficient is measured to be β=3.8×10−8cmW−1. Moreover, a variation of γ and β with intensity is observed. The comparison with self-focusing and two-photon absorption previously measured using 55-ps pulses is discussed.