Fermi edge singularity in absorption spectra of cold neutral electron-hole system

We studied ultracold neutral electron-hole (e-h) systems in separated electron and hole layers in GaAs/AlGaAs coupled quantum well heterostructure. The long e-h lifetimes due to the layer separation allow the e-h systems to cool to low temperatures. We found a strong enhancement of the spatially indirect photoluminescence at the Fermi energy of the ultracold plasma, the evidence of the excitonic Fermi edge singularity due to the Cooper-pair-like excitons at the Fermi energy. We also found a strong enhancement of the spatially direct absorption at the Fermi energy of the ultracold plasma. The absorption was probed by photoluminescence excitation spectroscopy of the spatially indirect photoluminescence. We controlled the e-h density by the laser excitation power and, with the increasing e-h density, observed a crossover from the absorption of hydrogen-like excitons at low e-h densities to the Fermi edge singularity in absorption at high e-h densities.