Calculation of a nonlinear refractive index making allowance for higher induced multipoles

A correction to the nonlinear refractive index is calculated for a crystal with a cubic lattice. Two cases are considered: when an external field is directed along an edge of the lattice cube, and when it is directed along a face diagonal. Estimates show that the correction may be comparable with the nonlinear component of the refractive index resulting from other effects. Attention is drawn to the fact that the steady-state value of the correction should be established simultaneously with the appearance of electronic polarization.