Order-of-magnitude estimates of the nonlinear optical susceptibility

We develop simple mathematical models that lead to order-of-magnitude predi ctions of the sizes of the electronic, nuclear, and electrostrictive contri butions to the nonlinear optical susceptibility. We find that all three of these processes make comparable contributions to the third-order susceptibi lity, even though the optical-frequency linear susceptibility is dominated by the electronic response. We also find that when the mathematical express ions describing these contributions to the third-order susceptibility are w ritten in terms of fundamental physical constants, they are identical to wi thin numerical factors of the order of unity and are given in Gaussian unit s by chi((3)) = (h) over bar(8)/8m(4)e(10) = 4.25 x 10(-16) cm(2) s(-1) V-2 .