Influence of Electron Density and Trion Formation on the Phase-Coherent Photorefractive Effect in Znse Quantum Wells

We investigate the influence of electron density and trion formation on the phase-coherent photorefractive effect in ZnSe single quantum wells by laser energy and temperature dependent degenerate four-wave-mixing experiments using 90-fs pulses. Two different structures with the same quantum well width and confinement energy but different barriers adjacent to the GaAs substrate are studied in order to compare the formation of a photorefractive electron density grating at specific excitation conditions. At temperatures below 35 K and laser excitation energy close to the exciton energy the formation of trions significantly suppresses the generation of an electron density grating. At lower excitation energies increasing space-charge fields reduce the trion binding energy which leads to an enhanced thermal ionization of trions resulting in a strong phase-coherent photorefractive effect. Due to the thermal dissociation of trions at temperatures exceeding 45 K a significant photorefractive effect exists even at exciton resonant excitation. The experimentally observed signal traces obtained at different excitation conditions are in good agreement with model calculations that are based on the optical Bloch equations, including inhomogeneous broadening at strong space-charge fields.