We show that, for dc electric fields easily applicable to quantum wells, and for optical fields close to a transition frequency, the nonlinear susceptibilities derived from perturbation theory diverge. Alternatively, a general expression for the susceptibility based on solving the adiabatic quantum-well problem is presented, leading to a radically different behavior at large fields. Our approach is valid at low modulating rates for any system which is soluble under an applied electric field.

• Received 28 April 1993

DOI:https://doi.org/10.1103/PhysRevB.48.10950