Order-of-magnitude estimates of the nonlinear optical susceptibility

We develop simple mathematical models that lead to order-of-magnitude predictions of the sizes of the electronic, nuclear, and electrostrictive contributions to the nonlinear optical susceptibility. We find that all three of these processes make comparable contributions to the third-order susceptibility, even though the optical-frequency linear susceptibility is dominated by the electronic response. We also find that when the mathematical expressions describing these contributions to the third-order susceptibility are written in terms of fundamental physical constants, they are identical to within numerical factors of the order of unity and are given in Gaussian units by x (3) = h 8/8m 4 e 10 = 4.25 × 10−16 cm2 s−1 V−2.