Nonlinear optical phase shift in InAs quantum dots measured by a unique two-color pump∕probe ellipsometric polarization analysis

Abstract

The nonlinear optical phase shift in self-assembled InAs quantum dots (QDs) under resonant excitation in a ground-state transition was measured by a unique two-color pump∕probe ellipsometric polarization analysis. This ellipsometric analysis makes use of the large optical birefringence of SK-QD [(SK) — Stranski-Krastanov] originating from the asymmetric structure. A phase shift of 0.5π rad was obtained at an input pump pulse energy density of 30pJ∕μm2, a detuning of 11meV, and a time delay of 20ps in a 1mm long waveguide having QDs with a peak wavelength of 1290nm, a volume density of 4×1015cm−3, and inhomogeneous broadening of 35meV. Analysis revealed that the phase shift is mainly attributed to the absorption saturation for TE-polarized light, though other mechanisms also could contribute at higher pumping. The calculation, based on the two-level approximation, revealed that the minimum energy density for π shift is 240fJ∕μm2, calculated under ideal conditions.