Simplified expression for estimating the nonlinear refractive index of typical optical coating materials.

A semiempirical equation is derived that allows estimating the electronic contribution to the nonlinear refractive index of an optical material from knowledge of the linear one. The approach is restricted to practically loss-free media at low frequencies. In contrast to other approaches, no explicit data on the optical gap or Abbe number are required. A comparison with experimental data from typical oxide and fluoride materials reveals an excellent agreement between experiment and theoretical estimation.