Dichroism

As dichroism ( two-tone, derived from the Greek word you roos for " two-colored" ) the property of certain materials in optics referred to absorb varying degrees of light depending on the polarization and also affects the reflection behavior of the materials.

Furthermore, there are X-ray spectroscopic effects, which are based on the coupling of (X-ray ) photons at specific electron orbitals and are summarized under the term X-ray dichroism.

Description

Some materials (mostly crystals) have one or more excellent optical axes. For optically uniaxial materials incident light is split into two beams, depending on its polarization (always based on the vector of the electric field strength): the ordinary and the extraordinary ray. Indicates the material different absorption behavior with respect to these axes, that is, the ordinary stronger or weaker absorbed than the extraordinary ray, one speaks of a dichroic crystal. With a correspondingly thick crystal therefore one of the two partial beams is (completely) absorbed and only the other transmitted. The effect is strongly wavelength -specific and occurs only in a narrow spectral range, ie, at a different wavelength of light, the effect of absorption can not occur ( this is called birefringence ) or even be reversed. Usually dichroic birefringent crystals and birefringent bodies are dichroic. Exceptions exist in the presence of very specific constraints ( such as constraints of the spectral range ). Looking at "normal", i.e., non-polarized white light of the entire visible spectrum, the polarization-dependent absorption of dichroic materials results in attenuation of certain spectral ranges. This change is then perceived as a change in light color. Is particularly clear dichroism when linearly polarized light is irradiated onto an optically uniaxial crystal with the two resonant or natural frequencies (extreme colors ) in the visible spectral range and considered the incident light is. Then changing the polarization direction, the extreme colors will be visible if the polarization is perpendicular or parallel to the optical axis of the crystal. For a polarization intervene mixing colors on these two colors, and therefore is often commonly spoken in the mineralogy of pleochroism. With regard to the actual observation, this choice of terminology is warranted.

A more complex absorption behavior is present in multi-axis optical crystals, wherein a single crystal can have at most two optical axes and more can come only through another kitten of many single crystals materialize ( polycrystalline material ). Optically biaxial crystals produce two extraordinary rays, exhibit the trichroism ( three colors ). Analog multi-axis show the polycrystals pleochroism ( multi-color ) with many colors.

Degree of dichroism

The degree of dichroism is determined by the ratio of the difference of the absorption coefficients for the parallel and perpendicular polarization (respectively).

Linear and circular dichroism

The dichroism of the polarization of the incident light will be differences in the nature.

There are the linear dichroism of refers to the phenomenon that when linearly polarized light as a function of wavelength, either the extraordinary beam is more strongly absorbed than the ordinary, or vice versa. This effect was first found in the early 19th century in single crystals of the gemstone tourmaline.

There is also analogous to the circular birefringence also the effect of the circular dichroism (also called circular dichroism ) that describes the different absorption behavior of right - and left-handed polarized radiation in an optically active material. This effect was first described in 1896 by Aimé Auguste Cotton ( 1869-1951 ), see Cotton effect.

Linear and circular, magnetic dichroism

Analogously to the magneto-optical effects of birefringence can also dichroism certain materials - that is, the change in the intensity or the polarization state of light as it passes through the material - can be influenced by means of magnetic fields (magnetic induced dichroism ). Here, between the linear magnetic dichroism (rarely also of magnetic linear dichroism, Eng. Magnetic linear dichroism MLD ). and the circular magnetic dichroism distinguished.

The magnetic circular dichroism (also magnetic circular dichroism or circular magnetic dichroism called, Eng. Magnetic circular dichroism, MCD) occurs in magnetic or magnetized materials as a result of the different spin population of certain orbitals. Circular polarized light preferentially displaced electrons in certain orbitals, which, due to the spin -orbit coupling different population density.

In this case, the magnetization is parallel to the propagation direction of the light that is circularly polarized. A distinction is made between a polar and a longitudinal geometry. In polar geometry, magnetization is perpendicular to the surface, wherein the longitudinal magnetization parallel to the surface in the plane of incidence. Here, the different absorption for the two directions of polarization is utilized. This is proportional to the imaginary part of the refractive index. The measured effect thus corresponds to:

Both forms of magnetic dichroism occur both in the visible spectrum as well as in X-rays on ( X-ray dichroism ). Often, you will therefore designations specifically for the X-ray dichroism: X -ray magnetic circular dichroism ( XMCD, dt, circular, magnetic X-ray dichroism ', also magnetic x -ray circular dichroism, MXCD called ) and the less intense X -ray magnetic linear dichroism ( XMLD, dt, linear, magnetic X-ray dichroism '). It is particularly interesting MCD soft X-ray (English (soft) X -ray magnetic circular dichroism, (S ) X- MCD), where the unoccupied valence electronic structure can be measured spin resolved.

Application and materials

Application, see dichroic materials, for example, as a dichroic polarizer in the visible region of the electromagnetic spectrum. Here simple wire grid polarizers can not be used, because the wavelength becomes smaller, the required grid spacing decreases, which is only difficult to achieve even in the near- infrared. In the visible region structures are needed in the order of molecules. The U.S. physicist Edwin Herbert Land (1909-1991) succeeded in 1932 for the first time the production of dichroic films. As he turned to the elongated hydrocarbon molecules in polyvinyl alcohol by heating and stretching of the material accordingly. Such polarization films ( Polaroid filter or Polaroid film called ) are very often used and are comparatively cheap. They can be made relatively large surface area and achieve a degree of polarization greater than 99 %. The quality (e.g. in terms of the achievable polarization or transmittance ) is below that of other polarizers. Furthermore, they show disadvantages in applications with high light outputs. The polarization is achieved by absorption as described in the material, this leads to heating, and may have adverse effects on the properties of the polarizer, or even destroy it, in extreme cases.

There are also a plurality of body materials which exhibit dichroic behavior. Thus, embedded in cellulose needles from a sulfuric acid Jodchinin ( herapathite ) and a dichroic polarizer ( polarizing film ) can be used. In the same way also dichroic dyes come in plastic films for use. The required uniform alignment of the dye molecules is achieved for example by electric or magnetic fields.

In mineralogy, the dichroism is used in the characterization of minerals ( see also pleochroism - multi-color ). In addition a so-called Dichroscope used. A typical dichroic material tourmalines, such as the green Tumalin ( verdelite ). It is almost completely absorbed in the transmission of the natural light through a 1 mm thick plate of verdelite the ordinary ray, however, only the extraordinary ray is weakened

In analytical chemistry of the dichroism for structure analysis of optically active chiral molecules can be used (see also circular dichroism ).