Two-photon absorption and nonresonant nonlinear index of refraction in the intersubband transitions in the quantum wells

A simple theory for the two-photon intersubband absorption and, associated with it, nonlinear refraction in multiple quantum wells (QWs) is developed. Using it, the third-order nonlinear susceptibility is evaluated for QWs of different shapes. It is shown that χ(3) of as much as 10−7 esu can be achieved below the two-photon resonance where the absorption is small. Thus, it is preferable to use multiple QWs in nonlinear devices at frequencies less than half of the intersubband frequency.