Excitons and polaritons in InP

We report an investigation of the reflectivity spectra of InP at normal incidence, at pumped-helium temperature, and under [100] uniaxial stress. Three transverse exciton frequencies associated to the 1s ground state and the 2s and 3s excited states have been found. We deduce an exciton binding energy ${E}_{\mathrm{ex}=5.1\ifmmode\pm\else\textpm\fi{}0.1}$ meV. From a detailed investigation of the 1s ground state in terms of the three-branch polariton dispersion curves we achieve a very satisfactory agreement between theory and experiment. Resolving the fine structure of the exciton ground state, we find (i) the exchange energy \ensuremath{\Delta}=0.04\ifmmode\pm\else\textpm\fi{}0.02 meV and (ii) the longitudinal-transverse splitting ${E}_{\mathrm{LT}=0.17\ifmmode\pm\else\textpm\fi{}0.02}$ meV. The surface dead layer which corresponds with the best experimental fit is twice the exciton Bohr radius, as expected for an intrinsic surface-exciton-free layer.