  Whispering gallery modes in GaAs disk resonators reach half a million of
optical quality factor. These high Qs remain still well below the ultimate
design limit set by bending losses. Here we investigate the origin of residual
optical dissipation in these devices. A Transmission Electron Microscope
analysis is combined with an improved Volume Current Method to precisely
quantify optical scattering losses by roughness and waviness of the structures,
and gauge their importance relative to intrinsic material and radiation losses.
The analysis also provides a qualitative description of the surface
reconstruction layer, whose optical absorption is then revealed by comparing
spectroscopy experiments in air and in different liquids. Other linear and
nonlinear optical loss channels in the disks are evaluated likewise. Routes are
given to further improve the performances of these miniature GaAs cavities.
