  Photogenerated excitonic ensembles confined in coupled GaAs quantum wells are
probed by a complementary approach of emission spectroscopy and resonant
inelastic light scattering. Lateral electrostatic trap geometries are used to
create dense systems of spatially indirect excitons and excess holes with
similar densities in the order of 10$^{11}$ cm$^{-2}$. Inelastic light
scattering spectra reveal a very sharp low-lying collective mode that is
identified at an energy of 0.44 meV and a FWHM of only ~50 $\mu$eV. This mode
is interpreted as a plasmon excitation of the excess hole system coupled to the
photogenerated indirect excitons. The emission energy of the indirect excitons
shifts under the application of a perpendicular applied electric field with the
quantum-confined Stark effect unperturbed from the presence of free charge
carriers. Our results illustrate the potential of studying low-lying collective
excitations in photogenerated exciton systems to explore the many-body phase
diagram, related phase transitions, and interaction physics.
