dichroism (original) (raw)

Author: the photonics expert (RP)

Definition: wavelength-dependent transmission, alternatively polarization-dependent absorption

Alternative terms: diattenuation, pleochroism

Category: article belongs to category general optics general optics

Related: dichroic mirrorspolarizers

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DOI: 10.61835/2gi Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

The term dichroism (with the corresponding adjective dichroic) is used with two different meanings, explained in the following sections.

Wavelength-dependent Transmission

The term can be used for optical elements which somehow act on light with a substantial dependence on the optical wavelength. In particular, the transmissivity can be wavelength-dependent. The term is particularly used in conjunction with dielectric mirrors; see the article on dichroic mirrors.

Polarization- and Wavelength-dependent Absorption

Dichroism can also refer to the polarization-dependent absorption of light in certain materials — an effect also known as diattenuation when it involves linear polarization. At first, this use of the term dichroism may seem counterintuitive, since the term literally suggests a dependence on wavelength (i.e., color) rather than polarization. However, some crystalline materials — such as tourmaline, where the effect was first observed — appear in different colors depending on the polarization of the incident light. This occurs because their optical absorption, and consequently their apparent color or reflectance, depends on both polarization and wavelength. The wavelength dependence essentially results from the anisotropic band structure. Of course, there are not just two distinct colors, as the name might suggest, but rather a continuous range of color mixtures for intermediate polarization orientations.

Note that there is also the phenomenon of trichroism, observed on biaxial crystals. That means that the observed colors are mixtures of three rather than two different colors.

Polarization-dependent absorption is used for some kinds of optical polarizers, namely for sheet polarizers based on polymer films. For such applications, the films may be optimized for minimum wavelength dependence, and the term dichroism may then be regarded as somewhat inappropriate.

Linear and Circular Dichroism

Dichroism is in most cases related to linear polarization directions, but there is also circular dichroism, where the difference in optical properties occurs for different circular polarization directions.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What does the term 'dichroism' mean?

Dichroism has two common meanings. It can describe optical elements with strongly wavelength-dependent properties, like dichroic mirrors. It can also refer to the polarization-dependent absorption of a material, causing it to appear in different colors depending on the light's polarization.

Why is polarization-dependent absorption called 'dichroism'?

The term originated from observing crystals like tourmaline, which exhibit two different colors (di-chroic) when viewed with light of different linear polarizations. This happens because the material's absorption spectrum depends on the polarization.

What is the difference between linear and circular dichroism?

Linear dichroism describes different absorption for different linear polarization directions. In contrast, circular dichroism refers to the difference in absorption for left- and right-handed circularly polarized light, an effect which is observed in chiral materials.

What are some applications of dichroism?

The wavelength-dependent effect is used in dichroic mirrors for spectral filtering. The polarization-dependent absorption is exploited in optical polarizers, for example in sheet polarizers based on polymer films.

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