Dichroic

A dichroic material is one in which two opposite polarizations of light travel at different speeds. A thin slab of it can be used to change the polarization of light. If the two polarizations are right and left circular, it rotates linearly polarized light (see circular dichroism); if they are horizontal and vertical, it converts diagonally polarized light to circularly polarized or flips it to the other diagonal.

A slab that delays one polarization by 1/2 wave with respect to the other is a half-wave plate. A circular one rotates polarization by 90 degrees; a linear one flips one diagonal to the other diagonal and exchanges left and right circularly polarized light.

A slab that delays one polarization by 1/4 wave is a quarter-wave plate. A circular one rotates by 45 degrees; a linear one converts diagonal light to circular light and circular light to the other diagonal.

Two pieces of dichroic material with different axis directions can be made into a Wollaston prism. This takes light of two different polarizations and sends them in different directions. With a quarter-wave plate switch, this is used as the detector in quantum cryptography.