| Ultrafast mid-infrared spectroscopic sources and techniques were developed to study dense excess carrier induced nonlinear recombination dynamics and absorptive nonlinearities within a new family of narrow-bandgap semiconductor materials.;The semiconductor materials consisted of multiple quantum wells in which each well was comprised of a Type II broken-gap superlattice. A typical structure, grown by molecular beam epitaxy on an undoped GaSb substrate, contained 225 A quantum wells, each formed by a Type II GaInSb/InAs superlattice separated by AlGaSb barriers. The materials/structures have effective bandgaps which can range between 2 and 5 ;The KTP based OPO was designed and constructed to allow tuning between 1.04 and 1.2 ;The pump-probe experiments consisted of optically injecting a large excess carrier concentration using pulses from the Ti:Sapphire laser and then probing near the band edge of the material using the synchronous MWIR output pulses from the OPO. By monitoring the variations in absorption, due primarily to bandfilling, as functions of optical delay between the pump and probe and for different concentrations of carriers, a direct measurement of the carrier recombination dynamics was obtained. This included both Shockley-Read-Hall and Auger recombination coefficients. Additionally, the interband absorption coefficient and free carrier absorption cross-sections were determined. Bandfilling and free carrier absorption were determined to be the dominant nonlinear optical absorptive mechanisms, with bandfilling being predominant. The carrier recombination dynamics indicated Auger to be the primary recombination mechanism for carrier densities sufficient to produce optical gain. |
