Spectral Dependence and Polarization Anisotropy of Optical-Transmission Modulation by Acoustoelectric Domains in CdS

The modulation by acoustoelectric domains of monochromatic light transmitted through platelets of CdS has been studied with emphasis on the spectral variation and polarization dependence of the effect near the band edge. Samples of several crystallographic orientations were used. It was found that for light propagating along the $c$ axis of the crystal, the domain-in-duced modulation was strongest with the light polarized along the drift-field direction, in agreement with previously reported work on GaSb. The spectral variation of the transmission modulation observed with polarized light propagating perpendicular to the $c$ axis was consistent with the double band edge of CdS. The modulation resembles a shift of the absorption spectrum to longer wavelengths. Recently published theoretical and experimental studies of the Franz-Keldysh effect, together with the observed spectral, polarizational, and orientational dependences of the optical modulation lead us to conclude that the modulation is produced by the Franz-Keldysh effect. Further, it is shown that the fluctuating high-frequency electric fields in the domain, rather than the "steady" field across the domain, produce the effect. Previously measured properties of the domain, such as current noise, induced birefringence, and induced light emission, corroborate this interpretation. Other proposed mechanisms are shown to be invalid.