Understanding the ultra-low intersubband saturation intensity in InGaAs-AlAsSb quantum wells

We report a novel intersubband material system based on strained InGaAs-AlAs-AlAsSb quantum wells that exhibits a very low, 3 fJ/μm2, saturation intensity and about 2-ps carrier-relaxation time at 1.68 μm. We performed a density matrix calculation to estimate the saturation intensity by simulating the pulsed excitation conditions of the experiment. These studies indicate slow dephasing time and reduced inhomogeneity as the possible mechanisms for the observed large nonlinearity. A detailed dephasing time calculation shows that rather than the reduction in the electron effective mass in these strained quantum wells (QWs), the effect of enhanced concentration of doped carriers in these QWs with reduced inhomogeneity could be the origin of slow dephasing and the observed large nonlinearity.