Carrier-density-dependent increase and suppression of optical gain in T-shaped quantum-wire lasers

Carrier-density dependence of the optical-gain spectra were studied for an undoped T-shaped quantum-wire (T wire) laser by optical pumping at 4 K. The gain appeared at a low excitation power of 3 mW corresponding to the carrier density of about 3 × 10⁵ cm^-1 in the T wire. As the carrier density was increased, the peak gain increased, but it reached a maximum value of about 1 × 10⁴ cm^-1 at the carrier density of 1 × 10⁶ cm^-1 and then decreased hereafter. This peculiar gain evolution dependent on the carrier density was not explained by a simple free-electron model, which suggests significant contributions of the enhanced Coulomb interactions to the optical gain in the T-wire laser.