Second-order intersubband nonlinear optical susceptibilities of asymmetric quantum well structures.

Nonlinear optical properties of quantum wells (QW’s) and superlattices (SL’s) have recently become an object of intense studies 1,2. Quantum confinement of carriers leads to existence of strong resonances in the absorption spectra attributed to both conduction-to-valence band 3 and intersubband 4 transition. That, in turn, leads to large optical nonlinearities. Third order nonlinearity in symmetrical QW’s and SL’s have been studied by numerous authors 5-8. More recently, calculations of second order nonlinear coefficients of asymmetric QW structures were made for interband 9,10 and intersubband 10,11 transitions. Second -order nonlinear properties based on interband processes in various asymmetric QW structures were evaluated in Ref 10 for wide range of materials and QW geometries. It was snown that although both second harmonic generation (SHG) and linear electro-optic (LEO) coefficient are large (on the order of 10−10m /V) they are at least an order of magnitude smaller than what could be expected from a two-level asymmetric system with comparable transition strength. The reason for that is compensation of second-order susceptibilities associated with various ground and excited states and having opposite signs.