Refractive index of GaP and its pressure dependence

The refractive index n of GaP and its dispersion have been measured at pressures P from 0 to 13 GPa. The pressures were applied with a diamond anvil cell and measured with the ruby fluorescence technique. Within the P range mentioned, n was shown to decrease sublinearly with pressure. The data were fitted with a model which includes the lowest direct energy gap $E_0$, the corresponding bound exciton, and the $E_1$ gap as the main dispersion mechanisms. As fitting parameters only the strengths of the $E_0$ transition and its excitonic component were used.