Optical Loss in Microdisk Lasers With Dense Quantum Dot Arrays

We discuss the origin of optical losses in microdisk lasers with a dense array of InGaAs quantum dots in the active region. In particular, we study the effect of microlaser diameter <inline-formula> <tex-math notation="LaTeX">$\mathbf {D}$ </tex-math></inline-formula> variation from 15 to <inline-formula> <tex-math notation="LaTeX">$200~\mu \text{m}$ </tex-math></inline-formula> on optical losses of different nature. A strong dependence of the lasing wavelength on the diameter is observed: the blue-shift with decreasing disk size implies an increase in optical losses, although in the case of an ideal cylinder, a noticeable optical loss should appear only at diameters comparable to the wavelength of light. A comparison of the spectral characteristics of microlasers with those of broad-area stripe lasers, for which optical loss can be easily found, gives a tool to evaluate optical loss in microdisk lasers, which was found to be unexpectedly high. It changes <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {\propto }\mathbf {D}^{\mathbf {-1}}$ </tex-math></inline-formula> from <inline-formula> <tex-math notation="LaTeX">$\sim 100$ </tex-math></inline-formula> cm−1 in the smallest microlasers to ~ 5 cm−1 in the largest ones. Several possible physical mechanisms of the appearance of optical losses in microlasers are considered, such as radiative loss due to the curvature of the cylindrical cavity, free carrier absorption, light scattering due to roughness of the side walls, and absorption of light in the near-surface region. The latter type of optical loss was found to be the dominant one and can explain the experimental results once the absorbing layer with a thickness of <inline-formula> <tex-math notation="LaTeX">$2~\mu \text{m}$ </tex-math></inline-formula> was suggested. Using the Gaussian approximation for Using the Gaussian approximation for the gain spectrum, the wavelength-loss relationship was simulated and a good agreement with the experimental dependence was found. The variation of the experimental results on optical loss for nominally identical microlasers was attributed to the variation of the scattering loss. The same reason can explain the scatter of the slope efficiency, which varies from <inline-formula> <tex-math notation="LaTeX">$\sim ~0.03$ </tex-math></inline-formula> to 0.25 W/A being governed by the ratio of the scattering loss to the surface absorption loss.