A theoretical framework for determining biaxial anisotropy suitable for
reducing the incident angle dependence of a polarization grating is
proposed. The principal refractive indices are uniquely given by
quantifying the biaxial anisotropy through the
N
z
coefficient. The numerical analysis
based on the finite-difference time-domain method reveals that the
incident angle dependences of the diffraction efficiency and the
polarization ellipticity of the diffracted light are significantly
reduced when
N
z
=
0.5
. This result is expected to
contribute to the design of polarization diffractive optical elements
and the development of anisotropic materials and structures.