Optical parametric oscillators without phase matching

Abstract

We show that optical parametric oscillators (OPO's) based on intersubband transitions in quantum wells operate in a new and interesting regime where the gain, loss, and momentum mismatch are all of the same order of magnitude. We find that such OPO's can oscillate without phase matching. The threshold gain does not increase monotonically with idler loss, as one would expect intuitively, but experiences a minimum. This minimum comes about because of a relaxation of the momentum selection rule or phase-matching condition. We derive these results by analyzing an OPO without making any of the usual assumptions on the relative magnitudes of the gain, loss, and momentum mismatch. We also consider in detail the design of a particular quantum-well structure, the asymmetric double quantum well, for realizing such an OPO experimentally.