Sub-wavelength grating structures made in an isotropic medium induce form birefringence effects with the induced optical axis parallel to the grating vector. The phase shift between the two orthogonal electric field components exiting this grating varies linearly with the thickness of the grating. When a grating with sub-wavelength period is formed on a uniaxial birefringent material with the grating vector aligned parallel to is natural optical axis, the total effect is enhanced birefringence for the material. Smaller thickness of the material is required to achieve the same phase shift. If the natural optical axis and the induced optical axis lie within the surface plane and have an angle between them, the phase shift varies nonlinearly with the thickness of the grating.