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

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 0.5pi rad was obtained at an input pump pulse energy density of 30 pJ/mum(2), a detuning of 11 meV, and a time delay of 20 ps in a 1 mm long waveguide having QDs with a peak wavelength of 1290 nm, a volume density of 4x10(15) cm(-3), and inhomogeneous broadening of 35 meV. 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 pi shift is 240 fJ/mum(2), calculated under ideal conditions. (C) 2004 American Institute of Physics.