Negative exciton mass in ZnCdSe/ZnSSe superlattices observed by excitonic polariton Raman scattering

Resonant Raman scattering of excitonic polaritons at LO phonons in ZnCdSe/ZnSSe superlattices at room temperature reveals strong spatial dispersion effects near the transverse excitonic resonance frequency ωT. For incident photon energies below ωT, the LO phonon anti-Stokes frequency shifts exceed the Stokes shifts, while the inverse relation holds above ωT. All experimental results can be well described in the excitonic polariton formalism in terms of exciton branches with positive and negative excitonic masses above and below ωT, respectively, and an additional damping parameter. (author)