The interaction of light with the atoms or molecules of a material is wavelength dependent. A result of this dependence is the resonant interactions related to material dispersion. Birefringence is another consequence of such resonant interaction, which is the change in refractive index with the polarization of light. The orderly arrangement of atoms in some crystals results in different resonant frequencies for different orientations of the electric vector relative to the crystalline axes. In turn, this results in different refractive indices for different polarizations. Unlike dispersion, birefringence can be avoided by using amorphous materials such as glass, or crystals that have simple symmetries, such as NaCl or GaAs. We can also “use” birefringence to modify the polarization state of light, which is a useful thing to do in many situations. The optical components that do this “trick” are called birefringent waveplates or retardation plates (or just waveplates or retarders, for short).
Waveplates
By taking just the right slice of a crystal with respect to the crystalline axes, it can be arranged so that the minimum index of refraction is exhibited for one polarization of the electric vector of a linearly polarized wave. The wave is polarized along the fast axis, since its phase velocity will be a maximum.
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