Exciton–biexciton-plasma crossover and formation of optical gain in quantum wires

T-shaped GaAs quantum-wire lasers were measured to study origin of laser gain and the exciton Mott transition in one dimension. Origin of laser gain was ascribed not to free excitons, or localized excitons, but to biexciton–exciton population inversion near the threshold, or to the electron–hole (e–h) plasma at a high-density regime well above the threshold. Systematic micro-PL study revealed that dominant PL evolves with e–h density from a sharp free-exciton peak, via a biexciton peak, to an e–h plasma PL band. These demonstrated important roles of biexcitons in the Mott transition and in the initial formation of optical gain. Comparison with microscopic theories raised a question for arguments based on large renormalized band-edge shift before the Mott transition.