Fermi-Edge Singularities in ${\mathrm{Al}}_{\mathit{x}}{\mathrm{Ga}}_{1-\mathit{x}}\mathrm{As}$ Quantum Wells: Extrinsic versus Many-Body Scattering Processes

A Fano resonance mechanism is evidenced to control the formation of optical Fermi-edge singularities in multisubband systems such as remotely doped ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ heterostructures. Using Fano parameters, we probe the physical nature of the interaction between Fermi sea electrons and empty conduction subbands. We show that processes of extrinsic origin like alloy disorder prevail easily at 2D over multiple diffusions from charged valence holes expected by many-body scenarios.