In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different () (not to be confused with dispersion), or one in which light rays having different polarizations are absorbed by different amounts.
In beam splitters
The original meaning of
dichroic, from the
Greek language dikhroos, two-coloured, refers to any optical device which can split a beam of light into two beams with differing wavelengths. Such devices include
and
dichroic filter, usually treated with
, which are designed to reflect light over a certain range of wavelengths and transmit light which is outside that range. An example is the
dichroic prism, used in some
, which uses several coatings to split light into red, green and blue components for recording on separate CCD arrays, however it is now more common to have a
Bayer filter to filter individual pixels on a single CCD array. This kind of dichroic device does not usually depend on the polarization of the light. The term
dichromatic is also used in this sense.
With polarized light
The second meaning of
dichroic refers to the property of a material, in which light in different polarization states traveling through it experiences a different absorption coefficient; this is also known as
diattenuation. When the polarization states in question are right and left-handed circular polarization, it is then known as circular dichroism (CD). Most materials exhibiting CD are
chiral,
although non-chiral materials showing CD have been recently observed.
Since the left- and right-handed circular polarizations represent two spin angular momentum (SAM) states, in this case for a photon, this dichroism can also be thought of as spin angular momentum dichroism and could be modelled using quantum mechanics.
In some ,, such as tourmaline, the strength of the dichroic effect varies strongly with the wavelength of the light, making them appear to have different colours when viewed with light having differing polarizations. This is more generally referred to as pleochroism,
In liquid crystals
Dichroism, in the second meaning above, occurs in
liquid crystals due to either the optical anisotropy of the molecular structure or the presence of impurities or the presence of
dichroic dyes. The latter is also called a
guest–host effect.
See also
