  We analyze the magnitude of the radiation pressure and electrostrictive
stresses exerted by light confined inside GaAs semiconductor WGM optomechanical
disk resonators, through analytical and numerical means, and find the
electrostrictive force to be of prime importance. We investigate the geometric
and photoelastic optomechanical coupling resulting respectively from the
deformation of the disk boundary and from the strain-induced refractive index
changes in the material, for various mechanical modes of the disks.
Photoelastic optomechanical coupling is shown to be a predominant coupling
mechanism for certain disk dimensions and mechanical modes, leading to total
coupling g$_{om}$ and g$_0$ reaching respectively 3 THz/nm and 4 MHz. Finally,
we point towards ways to maximize the photoelastic coupling in GaAs disk
resonators, and we provide some upper bounds for its value in various
geometries.
