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In this paper, we present experimental results of our investigations on deformed GaAs-AlGaAs microlasers emitting around /spl lambda/=10 /spl mu/m. These quantum cascade lasers exhibit interesting features regarding the threshold current densities, optical output, and far-field pattern. A slight aberration from a circular cross section decreases the threshold current density for microlasers (which have a radius of 50 /spl mu/m). For larger deformations /spl epsiv/, the threshold starts to increase because of the increasing mirror losses. For smaller microlasers (radii between 22 and 34 /spl mu/m), the threshold current density increases already for slight deformations due to the increase of the mirror losses. The experimental results can be fitted very well with the mirror losses as a fitting parameter using a well-known and simple model. Threshold currents as low as 170 mA are measured for a cylindrical microlaser with a radius of 22 /spl mu/m. The peak optical output is increasing quasi-exponentially with rising deformation. Lasing emission from slightly concave resonator shapes is detected. The bow-tie mode and other modes-different from Whispering-gallery modes-are responsible for highly directed emission along the diagonal axis and along the short axis, respectively, of the microlasers. Single-mode emission with a side-mode suppression ratio larger than 25 dB is shown over the entire drive current range for a highly deformed microlaser. The laser line can be temperature tuned with /spl Delta//spl nu//T=-0.027 cm/sup -1//K. A dual mode switching depending on the drive current with a mode spacing of /spl Delta//spl nu/=8.1 cm/sup -1/ between 999 and 1007.1 cm/sup -1/ is observed for a less deformed microlaser.