Polar whispering gallery modes in polystyrene microspheres under uniaxial stress

Optical whispering gallery modes (WGMs) in a microcavity optical resonator have a high Q-factor, which makes them extremely sensitive to changes in the shape and in the refractive index of the resonator. We investigated how much each of the two factors –shape deformation and refractive index change- contributes to the detuning of polar WGMs in solid polystyrene microspheres (PMs) which are put under uniaxial stress. The PMs were mounted between windows of a flow chamber and the stress on the PMs was varied by pressure on one window. The induced deformation reduces the geometrical circumference of the PM, which induces a blue shift of the WGMs. At the compressed flattened poles, however, the refractive index increases locally which shifts the WGMs to lower frequencies. How much these two effects balance each other out depends mainly on the elasto-optical constant of the microspheres. In our case, where the elasto-optical constant was 0.57 x 10-10Pa-1, we observed that the polar WGMs shift to higher frequencies with increasing pressure. This effect depended on the mode order n and azimuthal mode number m.