Whispering gallery modes in bulk cylindrical gallium arsenide and gallium phosphide samples have been examined both in darkness and under white light at 50 K. In both samples we observed change in permittivity under light and dark conditions. This results from a change in the polarization state of the semiconductor, which is consistent with a free electron-hole creation/recombination process. The permittivity of the semiconductor is modified by free photocarriers in the surface layers of the sample which is the region sampled by whispering gallery modes.
Hartnett, J., Mouneyrac, D., Le Floch, J-M., Krupka, J., Tobar, M., & Cros, D. (2008). Modified permittivity observed in bulk gallium arsenide and gallium phosphide samples at 50 K using the whispering gallery mode method. Applied Physics Letters, 93(6), online - approx 5-20pp. https://doi.org/10.1063/1.2969905