Adaptive beam combining and interferometry with photorefractive quantum wells

Abstract

We present a comprehensive study of excitonic electroabsorption and two-wave mixing in photorefractive quantum wells. By combining these two measurements, we are able to determine the internal grating writing efficiency for converting an external spatial light modulation into an internal space-charge field. The internal writing efficiency at a fringe spacing Λ=40 µm is found to be a decreasing function of applied field, varying from ξ=0.4 at low fields to 0.2 at 12 kV/cm. The two-wave mixing efficiency in the quantum wells exceeds 40% and is used for adaptive beam combining and laser-based ultrasound detection. The quantum wells balance the hot-electron-induced photorefractive phase shift with excitonic spectral phase to guarantee quadrature in homodyne detection of ultrasound-induced surface displacements. The ability to tune through multiple quadratures is demonstrated here for the first time to our knowledge. We derive a noise-equivalent surface displacement of 1.7×10-6 Å (W/Hz)1/2 at a field of 12 kV/cm and a fringe spacing of Λ=40 µm. This value is within a factor of 7 of the shot-noise limit of an ideal interferometer.