Excitons, biexcitons, and electron-hole plasma in a narrow 2.8-nm ${\mathbf{G}\mathbf{a}\mathbf{A}\mathbf{s}/\mathbf{A}\mathbf{l}}_{\mathit{x}}{\mathbf{Ga}}_{1-\mathit{x}}\mathbf{As}$ quantum well

We report the use of imaging spectroscopy to study the transition from a biexciton to a low-density electron-hole plasma inside single quantum dots in a 2.8-nm ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum well. Using a 250-nm resolution solid immersion microscope to reduce the spatial components of inhomogeneous broadening, we not only enable studies of single excitons and biexcitons confined in quantum dots, but also probe electron-hole plasmas bound in the dots at carrier densities about an order of magnitude lower than what was previously reported. The transition from exciton to plasma is found to be a direct result of random capture of excitons in dots, and the observed band-gap renormalization of the plasma agrees with existing theory.