Optical properties of gallium selenide under high pressure

Compressibility, optical-absorption, Raman-scattering, and refractive-index measurements for GaSe are reported at 300 K and pressures up to 8 GPa. A model which separates intra- and interlayer contributions along the c axis is used, with adequate deformation potentials, to quantitatively reproduce the compressibility along the c axis, the refractive-index variation, and the shift of both direct and indirect gaps under pressure. The shape of the absorption edges are calculated by the Elliott-Toyozawa formalism at all pressures, between 1.6 and 2.4 eV, with only one constant, and two pressure-dependent parameters: the exciton Rydberg and the matrix element for the direct transition. The decrease of the former, together with the measured shift of the TO and LO modes are interpreted by a large increase of the longitudinal effective charge under pressure which can be assigned to either intralayer-to-interlayer charge transfer, or to an increase of the total ionicity, or both.