Dynamics of optical nonlinearities in GaN

The dynamics of optical nonlinearities in bulk GaN crystal are investigated by a modified time-resolved pump-probe system with phase object at 532 nm under picosecond pulse excitation. The sample's optical nonlinearities can be determined by measuring the normalized transmittance with/without an aperture in the far field. The different nonlinear mechanisms in GaN are separated. Experimental results show that the nonlinear response of GaN is a combination of bound-electronic and free-carrier nonlinearities mechanisms. With the theory of two-photon-induced free-carrier nonlinearity, the related optical nonlinear parameters, including free-carrier nonlinearities and free-carrier life time which are difficult to be determined, are obtained.