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## 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

D variation from 15 to 200 μm 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 broadarea

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 D−1 from 100 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 2 μm 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 0.03 to 0.25 W/A being governed by the

ratio of the scattering loss to the surface absorption loss.