Thermal nonlinearities in GaSe

The nonlinear light absorption at the fundamental absorption edge under laser excitation in GaSe crystals has been investiqated experimentally. It is shown that at high excitation levels, the absorption coefficient shifted towards lower energies (red shift). The nonlinear refractive index is positive at frequencies below the absorption edge and negative on the high-energy side. The laser-induced negative index change is referred to as a selfdefocusing optical nonlinearity. The positive Δn(ω) on the high-energy side to a selffocusing optical nonlinearity. The measured parameters showed that GaSe crystals exhibits strong nonlinear properties.