  Excitons and free charge carriers with large wave vectors form a nonradiative
reservoir, which can strongly affect properties of bright excitons due to the
exciton-exciton and exciton-carrier interactions. In the present work, the
dynamics of quasiparticles in the reservoir at different areal densities is
experimentally studied in a GaAs/AlGaAs quantum well using a time-resolved
reflectance spectroscopy of nonradiative broadening of exciton resonances. The
population of the reservoir is controlled either by the excitation power or by
the temperature of the structure under study. The dynamics is quantitatively
analyzed in the framework of the model developed earlier [Kurdyubov et al.,
Phys. Rev. B {\bf 104}, 035414 (2021)]. The model considers several dynamic
processes, such as scattering of photoexcited excitons into the reservoir,
dissociation of excitons into free charge carriers and the reverse process,
carrier-induced exciton scattering into the light cone depleting the reservoir,
thermally activated nonradiative losses of charge carriers. We have found that
competition of these processes leads to highly nonlinear dynamics of reservoir
excitons, although their density is far below the exciton Mott transition
densities. Characteristic rates of the processes and their dependencies on the
excitation power and the temperature are determined.
