  Optically pumped InAs quantum dot microdisk lasers with grooves etched on
their surface by a focused ion beam is studied. It is shown that the radial
grooves, depending on their length, suppress the lasing of specific radial
modes of the microdisk. Total suppression of all radial modes except for the
fundamental radial one is also demonstrated. The comparison of laser spectra
measured at 78 K before and after ion beam etching for microdisk of 8 $\mu$m in
diameter shows a six-fold increase of mode spacing, from 2.5 nm to 15.5 nm,
without significant decrease of the dominant mode quality factor. Numerical
simulations are in good agreement with experimental results.
