Quasiuniaxial

Quasiuniaxial is a descriptive term used in optics and materials science to characterize anisotropic media that are close to uniaxial in symmetry but not exactly so. In an ideal uniaxial medium, the dielectric permittivity tensor has two equal principal values in the plane perpendicular to a single optic axis (ε1 = ε2 ≠ ε3). A quasiuniaxial medium exhibits only a small deviation from this condition, such as ε1 ≈ ε2 with a small difference, or a tiny nonzero off-diagonal component, so the symmetry is nearly, but not perfectly, uniaxial.

The practical consequence is a nearly circular index ellipsoid in the transverse plane, giving light propagation that is almost governed by one optic axis. The degree of quasiuniaxiality is often described by a small parameter that measures the deviation from ε1 = ε2 or from zero off-diagonal terms. As a result, refractive indices for two orthogonal polarizations are nearly identical, producing weak birefringence that may vary with wavelength, direction, or external conditions.

Quasiuniaxial behavior occurs in several contexts, including nematic liquid crystals near the boundary to a biaxial phase, strained or imperfect uniaxial crystals, and media subjected to external fields or stresses that slightly break uniaxial symmetry. It is also used in theoretical modeling to simplify analyses by treating the material as uniaxial to leading order while including higher-order corrections that account for small biaxiality.

See also: uniaxial, biaxial, optical indicatrix, liquid crystals, anisotropy, birefringence.