Theory of transient excitonic optical nonlinearities in semiconductor quantum-well structures

TLDR

The study presents theoretical predictions of how an exciton gas and an electron‑hole plasma influence excitonic optical absorption in two‑dimensional semiconductors, benchmarking these against recent absorption‑saturation experiments in single‑ and multiple‑quantum‑well structures. A simple low‑density, near‑band‑edge theoretical framework is developed to describe the nonlinear optical response of these microstructures. The model shows that Coulomb screening is weak while phase‑space filling and exchange dominate, explains why cold excitons more effectively saturate absorption than hot carriers, and reproduces experimental data without adjustable parameters.

Abstract

We present theoretical results for the effects of an exciton gas and an electron-hole plasma on the excitonic optical absorption in a two-dimensional semiconductor and compare these with recent experimental results on absorption saturation in single- and multiple-quantum-well structures. A simple theoretical description of the nonlinear optical properties of these microstructures is developed for the case of low-density optical excitation near and above the band edge. We argue that the effects of Coulomb screening of excitons by the plasma are relatively weak in these structures but that the consequences of phase-space filling and exchange are significant in each case. We are able to explain the recent unexpected experimental result that ``cold'' excitons are more effective than ``hot'' carriers in saturating the excitonic absorption. Good agreement with the experimental data is obtained without adjustable parameters.