Crystal optical properties of incommensurate phases in the plane-wave modulation region

Crystal optical properties of anisotropic optical materials of which the dielectric tensor is spatially modulated with a sinusoidal wave form are studied in the framework of the Jones calculus. Propagation of polarized light along the directions parallel to and far from the optical axes is considered. Polarization of the normal waves of the medium and the Jones matrix of a finite modulated crystal are derived, enabling us to ascertain the parameters of the apparent macroscopic optical activity. The developed model should describe the optical effects in a plane-wave region of incommensurate phases with the average inversion symmetry, occurring in the A(2)BX(4) family crystals. The boundary conditions for the phase of the modulation wave, which play a key role in crystal optics of incommensurate phases, are discussed. The model predicts a relatively small optical activity id the birefringent crystal sections and negligible or zero effect in the optical axis directions. The conclusions agree well, at least, with the non-contradictory experimental results on optical rotatory power of the A(2)BX(4) crystals. A comparison with the results derived earlier for the square modulation wave proves that the main conclusions of the model do not depend on the exact modulation shape.