Exciton-polariton reflectance in CuI single crystals

We have measured the normal reflection spectrum of (110) cleaved faces of single crystals of CuI in the spectral range of the Z12(3058 meV) and Z3(3699 meV) exciton lines at the temperature of 95 K. We have fitted the experimental data by computing a non local dielectric function and taken into account various additional boundary conditions (ABC) as well as the dead layer model in a semi-infinite crystal. A quantitative agreement is obtained assuming the following values for the background dielectric constant ε∞=7.2 and for the effective masses of the excitons M(Z12)=1.8 m0, M(Z3)=0.5 m0. These results do not depend on the choice of a particular ABC. The longitudinal-transverse exciton splitting energies deduced from the fit are: ωLT (Z12)=5.9 meV and ωLT (Z3)=2 meV.