Exciton-polariton reflectance in CuI single crystals

We have measured the normal reflection spectrum of (1 (1) over bar 0) cleav ed faces of single crystals of CuI in the spectral range of the Z(12)(3058 meV) and Z(3)(3699 meV) exciton lines at the temperature of 95 K. We have f itted the experimental data by computing a non local dielectric function an d taken into account various additional boundary conditions (ABC) as well a s the dead layer model in a semi-infinite crystal. A quantitative agreement is obtained assuming the following values for the background dielectric co nstant epsilon(infinity) = 7.2 and for the effective masses of the excitons M(Z(12)) = 1.8 m(0), M(Z(3)) = 0.5 m(0). These results do not depend on th e choice of a particular ABC. The longitudinal-transverse exciton splitting energies deduced from the fit are: omega(LT) (Z(12)) = 5.9 meV and omega(L T) (Z(3)) = 2 meV. (C) 1999 Elsevier Science S.A. All rights reserved.