Effect of homogeneous broadening of optical gain on lasing spectra in self-assembled ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dot lasers

This paper reports the effect of homogeneous broadening of the optical gain on lasing spectra in self-assembled ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dot lasers. We measured the current-output power characteristics and light-emission spectra for columnar-shaped self-assembled ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum-dot lasers that show lasing from the ground state. While lasing occurred with a narrow line including a few longitudinal modes at room temperature, spectra at 80 K showed broad-band lasing emission over a range of 50–60 meV. Based on a coupled set of rate equations taking into account both the size distribution of quantum dots and a series of longitudinal cavity modes, we could clearly explain the measured temperature dependence of lasing spectra in terms of homogeneous broadening of optical gain of a single quantum dot. Dots with different energies start lasing independently at 80 K due to their spatial localization as well as a δ-function-like gain, while at room temperature the dot ensemble contributes to a narrow-line lasing collectively via the homogeneous broadening of optical gain. The magnitude of the homogeneous broadening was found to reach 16 to 19 meV at room temperature, which corresponds to the polarization dephasing of 70 to 80 fs. We present a model on the dephasing process. This paper is a demonstration of the effect of homogeneous broadening on quantum-dot laser characteristics.