Measurements of frequency upconversion and picosecond excitation-correlation luminescence spectra in GaAs quantum wells and determination of time constants describing exciton dynamics

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Abstract
We study the picosecond dynamics of free carriers and excitons in high‐quality GaAs quantum wells at 8 K using frequency upconversion (UC) and nonlinear picosecond excitation‐correlation (PEC) luminescence spectroscopy under nonresonant excitation conditions with carrier densities of a few 1010 cm−2. We analyze the measurements using a theoretical model of the coupled free electron–hole pair and exciton dynamics, incorporating important density‐dependent nonlinear effects, caused by exciton collisions and fermion exclusion. The time‐resolved UC photoluminescence (PL) spectra are used to deduce time constants related to free electron–hole (e–h)‐pair transformation into excitons and exciton transfer from large K to small K (k is the exciton wave vector). It is shown that unambiguous determination of exciton radiative and nonradiative decay times are possible when both UC and PEC PL spectra are considered simultaneously. We carry out a detailed line‐shape analysis of the picosecond UC and PEC PL spectra. We find that while collision broadening is mainly responsible for the observed nonlinear excitonic PEC signal, the PEC PL spectra can be fully explained only when additional nonlinear effects, such as density‐dependent radiative lifetime and fermion exclusion, are included in the analysis. From a comparison of the time evolution of both the PEC and UC PL spectra with theory, the time constants introduced in the model to describe free carrier density decay, transfer of excitons with K≠0 to K≊0 states, and their radiative and nonradiative decay are estimated to be about 50, 15, 60, and 1800 ps respectively.