Abstract

The resonant enhancement and the controllable optical-dipole-matrix elements of intersubband transitions in quantum wells have led to the observation of extremely large optical nonlinear coefficients^1-3. Second-harmonic nonlinear optical coefficients as high as χ^(2),(2ω, ω, ω) ≅ 7.6 × 10⁻⁷ (m/ v) have been observed,² and the third-harmonic nonlinear optical coefficient observed,³ χ⁽³⁾(3ω, ω, ω, ω) ≅ 4 × 10⁻¹⁴(m/ v)², is to our knowledge the largest reported in any material system. Despite these extremely large nonlinearities, the expectations for an efficient device have failed to materialize. In fact, the highest second-harmonic conversion efficiency obtained to date, η ≅ 3 × 10⁻⁴, is limited in part by the intersubband selection rules, the narrow epitaxial layers, and the surface limitations on the usable intensities.⁴ In this paper we demonstrate that beyond these experimental issues there are fundamental limits arising from the basic nature of the nonlinear interaction. We believe these limitations must be addressed if intersubband nonlinear phenomena are to advance to the device stage.

© 1995 Optical Society of America