Linear terms in ~ in the macroscopic optical properties of zincblende type

Situations under which off-diagonal symmetric or antisymmetric terms in the diel-ectric tensor eij(w, q") of zincblende-type materials, proportional to the wavevector of the light q, may exist are reviewed and clarified. Off-diagonal linear terms in ~ " have been associated with macroscopic effects on the polarization of the light of two different kinds: optical activity (antisymmetric) and gyrotropic birefringence (symmetric). Contrary to recent reports concerning GaAs, we show that gyrotropic birefringence caused by the symmetric off-diagonal linear terms in the dielectric ten-sor does not exist in bulk crystals under equilibrium conditions. Any such reported effects hould be attributed to residual internal stress or surface ffects