A simple empirical relation is found to exist between the linear and the third-order nonlinear optical susceptibilities. This empirical relation holds within the available experimental accuracy for gases at low pressures. The applicability of this empirical relation to crystalline solids, and its bearing on the generalized Miller's rule for these solids, are discussed.

• Received 3 February 1970

DOI:https://doi.org/10.1103/PhysRevB.2.2045