Excitonic polaritons in electric fields at GaAs surfaces

We report line-shape analyses of reflectance spectra in $n$-type GaAs obtained for various illumination intensities and applied electric fields. We describe the excitonic motion under an inhomogeneous surface field by using a dielectric model with field-dependent eigenenergies and damping. The excellent agreement between theoretical spectra and experimental data leads to the determination of surface-field parameters. It is found that a decreasing illumination intensity and/or an increasing external electric field enhance the surface charge and space charge, leading to an increase of the internal surface field.