Investigation of symmetries of second-order nonlinear susceptibility tensor of GaSe crystals in THz domain

► Even under normal incidence, THz output power vs. pump polarization exhibits oscillations. ► Kleinman's symmetry condition (d16=d21) - violated, due to ionic nonlinear polarization. ► Spatial symmetry (d21=-d22) - violated, due to deviation of GaSe lattice from hexagonal symmetry. ► Deduced value of d16/d21 varies much more than that of d22/d21 from crystal to crystal. ► d16/d21 & d22/d21 deduced do not significantly modify THz polarization vs. pump polarization.