Dichroism

This article shows absorption dependent on polarization. For components with reflectivity dependent on wavelength see dichroic mirror . For multi-colored glass based on a different effect, see dichroic glass .

Copper (II) acetate monohydrate, dichroic

In physics, dichroism (from the Greek word dichroos for “two-colored”) is the property of certain materials to absorb light to different degrees depending on their polarization .

With dichroic materials there is an optical axis so that when looking through a polarization filter in one polarization direction, due to the different absorption, a different color can be seen than in the other (ordinary and extraordinary ray). Mixed colors are obtained for intermediate polarization angles , which is why dichroism is also known as pleochroism , especially in mineralogy ( old Gr . Πλέον pléon 'more' and χρῶμα chroma 'color' or χρώσ chros 'coloration', i.e. "multicolor").

When two optical axes occur, there are three main axes of refraction, the absorption is different in three directions of polarization, and one speaks of trichroism . The multicolor expresses itself

• in a different color depth , example: the change from darker colors to paler colors in some tourmalines , or
• in a complete color change, example: synthetic alexandrite , which without a polarization filter has a color change yellow-green - violet - red-brown.

The dichroism also affects the reflective behavior of the materials.

Furthermore, there are X-ray spectroscopic effects that are based on the coupling of photons in the X-ray range to certain electron orbitals and are summarized under the term X-ray dichroism .

Dichroism is related to birefringence , in which the real part of the complex refractive index depends on the polarization. The imaginary part is the absorption coefficient , and its dependence on polarization is what causes dichroism.

Another related effect is the alexandrite effect , in which the absorption does not depend on the polarization but on the wavelength of the light.

description

Some materials (mainly crystals ) have one or more excellent optical axes.

With optically uniaxial materials, incident light is split into two partial beams depending on its polarization (always based on the vector of the electric field strength): the ordinary and the extraordinary beam . Does the material show different absorption behavior with respect to this axis, i.e. That is, if the ordinary ray is absorbed more or less than the extraordinary ray, one speaks of a dichroic crystal. With a correspondingly thick crystal, one of the two partial beams is therefore absorbed (down to below a threshold value) and only the other is transmitted .

The effect is strongly wavelength-specific and occurs only in a narrow spectral range, i.e. In other words, at a different wavelength of light the absorption effect cannot occur (one then speaks of birefringence ) or it can even be reversed. Typically, dichroic crystals are birefringent and birefringent dichroic; Exceptions exist if very specific boundary conditions exist (e.g. restrictions of the spectral range). If you look at "normal", i. H. unpolarized, white light of the entire visible spectrum, the polarization-dependent absorption of dichroic materials leads to the weakening of certain spectral ranges, which can be perceived as a change in the light color .

The dichroism becomes particularly clear when one shines linearly polarized light onto an optically uniaxial crystal with two resonance or natural frequencies (extreme colors) in the visible spectral range and observes the light passing through. If you now change the direction of polarization, the extreme colors become visible when the polarization is perpendicular or parallel to the optical axis of the crystal. For a polarization in between, mixed colors of these two colors occur, which is why mineralogy often speaks of pleochroism in general . In terms of actual observation, this choice of term is justified.

Optically multiaxial crystals have a more complex absorption behavior:

• optically biaxial crystals produce two extraordinary rays, they show trichroism ; a single crystal can have at most two optical axes.
• analogously, (poly) crystals with more than two optical axes show pleochroism with many colors; Such crystals can only be formed by cementing together many single crystals ( polycrystalline material).

Degree of dichroism

The degree of dichroism is determined by the ratio of the difference between the absorption coefficients for the parallel or perpendicular polarization ( or ) to their sum: ${\displaystyle D}$ ${\displaystyle K_{\parallel }}$ ${\displaystyle K_{\perp }}$

${\displaystyle D={\frac {K_{\parallel }-K_{\perp }}{K_{\parallel }+K_{\perp }}}}$

Linear and circular dichroism

Circular dichroism: right and left circularly polarized light is influenced differently in a layer that contains one enantiomer of an optically active chiral substance.

In dichroism, a distinction is made in terms of the type of polarization of the incident light:

• Linear dichroism describes the phenomenon that with linearly polarized light, depending on the wavelength, either the extraordinary ray is absorbed more strongly than the ordinary one, or vice versa. This effect was first found at the beginning of the 19th century in single crystals of the gemstone tourmaline .
• analogous to the circular birefringence , there is also the effect of circular dichroism (also circular dichroism called), the right- the different absorption behavior and left-handed polarized radiation in an optically active describes material. This effect was first described by Aimé Auguste Cotton in 1896 , cf. Cotton effect .

Application and materials

Dichroic materials are used for. B. as a dichroic polarizer in the visible range of the electromagnetic spectrum . Simple wire grid polarizers can no longer be used here, because the shorter the wavelength, the smaller the grid spacing required. This is already difficult to achieve in the near infrared range ; structures on the order of molecules are necessary in the visible range .

The US physicist Edwin Herbert Land succeeded in producing dichroic foils for the first time in 1932. To do this, he aligned the elongated hydrocarbon molecules in polyvinyl alcohol accordingly by heating and stretching the material. Such polarizing films (called Polaroid filters or films ) are used very frequently and are comparatively inexpensive. They can be produced over a comparatively large area and achieve a degree of polarization of over 99%. However, their quality (e.g. with regard to the degree of transmission or polarization that can be achieved ) is below that of other polarizers. Furthermore, they have disadvantages in applications with high light outputs . The polarization due to absorption in the material leads to heating and can have negative effects on the properties of the polarizer, and in extreme cases even destroy it.

But there are also bodies made of several materials that show dichroic behavior. For example, needles made from sulfuric acid iodinequinine ( herapathite ) can be embedded in cellulose and used as a dichroic polarizer (polarization film). Dichroic dyes are also used in plastic films in the same way . The uniform alignment of the dye molecules required for this is achieved, for example, by magnetic or electrical fields .

In analytical chemistry , circular dichroism can be used to analyze the structure of optically active chiral molecules.

For use in mineralogy, see the following chapter.

Pleochroism in Mineralogy

In mineralogy, dichroism is used to characterize minerals and is usually called pleochroism . The pleochroic properties of a mineral can be determined with the help of a dichroscope and used for determination and testing, especially with gemstones . Its effect on the depth of color is particularly important for gemstone cutters when choosing the cut in order to avoid colors that are too dark or light (pale).

Pleochroism occurs with clear, colored stones as well as with opaque ones. When natural light is transmitted through an approx. 1 mm thick plate made of green tourmaline ( Verdelite ), the ordinary beam is practically completely absorbed, while the extraordinary beam is only weakened.

Special forms of pleochroism are:

• the dichroism shows two different main colors and occurs in uniaxial crystals, i.e. H. in crystals with an optical axis .
• the trichroism shows three different main colors and occurs in biaxial crystals, i.e. H. in crystals with two optical axes.

Minerals of the cubic crystal system and amorphous substances show no pleochroism .

Depending on how the ordinary and extraordinary ray differ in color, the object can be assigned certain properties with regard to its crystal structure ; It can happen that with certain crystals, up to three different colors become visible when you turn them during the test:

Colours		optical property	Crystal structure
a color
		Isotropic	amorphous ( glass ), microcrystalline , cubic
two colors
		Anisotropic , birefringent , optically uniaxial	Trigonal , tetragonal , hexagonal
three colors in two different directions
		Anisotropic, birefringent, optically biaxial	Triclinic , monoclinic , rhombic

Theoretical foundations and reasons for whether such an effect can occur are dealt with in a sub-area of ​​theoretical crystallography .

Alexandrite , hiddenite , kunzite , ruby , sapphire and tourmaline reveal the multicolor color with the naked eye.

Further examples:

• Andalusite - yellow, olive green, red-brown to dark red
• Benitoite - colorless, purple to indigo blue or greenish blue
• Cordierite - light yellow to green, violet to blue-violet, light blue
• Malachite - almost colorless, yellowish green, deep green
• Tanzanite - purple, blue and brown or yellow

Magnetic dichroism

Analogous to the magneto-optical effects of birefringence , the dichroism of certain materials - i.e. the change in the intensity or the polarization state of the light when passing through the material - can also be influenced by magnetic fields ( magnetically induced dichroism ). A distinction is made here:

• the linear magnetic dichroism (seldom also called magnetic linear dichroism, English magnetic linear dichroism , MLD)
• the circular magnetic dichroism (also called magnetic circular dichroism or magnetic circular dichroism, English magnetic circular dichroism , MCD).

The circular magnetic dichroism occurs as a result of the different spin occupation of certain orbitals in magnetic or magnetized materials in which the magnetization is aligned parallel to the direction of propagation of the circularly polarized light. A distinction is made between:

• a polar geometry in which the magnetization is perpendicular to the surface
• a longitudinal geometry in which the magnetization is parallel to the surface in the plane of incidence.

Here, the different absorption for the two polarization directions is used, which is proportional to the imaginary part of the refractive index. The measured effect thus corresponds to:

${\displaystyle {\begin{aligned}\mathrm {Im} (n_{+}-n_{-})&=\mathrm {Im} \left({\sqrt {\varepsilon _{xx}+\mathrm {i} \,\varepsilon _{xy}}}-{\sqrt {\varepsilon _{xx}-\mathrm {i} \,\varepsilon _{xy}}}\right)\\&\approx \mathrm {Re} \left({\frac {\varepsilon _{xy}}{\sqrt {\varepsilon _{xx}}}}\right)\end{aligned}}}$

Both forms of magnetic dichroism also occur in the X-ray range (magnetic X-ray dichroism ):

• the linear magnetic X-ray dichroism (English X-ray magnetic linear dichroism , XMLD)
• the stronger circular magnetic X-ray dichroism ( X-ray magnetic circular dichroism , XMCD; also magnetic x-ray circular dichroism , MXCD).

MCD is particularly interesting in the soft X-ray magnetic circular dichroism , (S) X-MCD), where the unoccupied valence band - electronic structure can be measured with a spin resolution.

literature

• Herbert Daniel: Physics: optics, thermodynamics, quanta . Walter de Gruyter, 1998, ISBN 3-11-014630-4 , p. 192 .
• Ludwig Bergmann, Heinz Low , Clemens Schaefer: Optics: Wave and Particle Optics . Walter de Gruyter, 2004, ISBN 3-11-017081-7 , p. 557-559 .
• Walter Schumann: Precious stones and gemstones. All species and varieties in the world. 1600 unique pieces . 13th revised and expanded edition. BLV Verlags GmbH, Munich a. a. 2002, ISBN 3-405-16332-3 .

Web links

Commons : Dichroism  - collection of images, videos and audio files

• Mineral Atlas: Dichroism (Wiki)
• Prof. Leopold Rössler's jewelry dictionary - pleochroism

Individual evidence

1. ↑ ^{a b} Ludwig Bergmann, Heinz low, Clemens Schaefer: Optics: wave and particle optics . Walter de Gruyter, 2004, ISBN 3-11-017081-7 , p. 558 .
2. ^ GIA Gemological Institute of America Inc Jeniffer Stone-Sundberg: Challenges in Orienting Alexandrite: The Usambara and Other Optical Effects in Synthetic HOC-Grown Russian Alexandrite . Retrieved July 10, 2016.
3. ^ ^{A b} Herbert Daniel: Physics: optics, thermodynamics, quanta . Walter de Gruyter, 1998, ISBN 3-11-014630-4 , p. 192 .
4. ↑ ^{a b c d} Manfred von Ardenne: Effects of physics and their applications . Harri Deutsch Verlag, 2005, ISBN 3-8171-1682-9 , p. 777-778 .
5. ^ Rainer Dohlus: Photonics . Oldenbourg Wissenschaftsverlag, 2010, ISBN 978-3-486-58880-4 .
6. ↑ ^{a b} Ekbert Hering, Rolf Martin, Martin Stohrer: Physics for engineers . Springer, 2008, ISBN 978-3-540-71855-0 , pp. 584 .
7. ^ Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 194 .
8. ↑ See this website of the California Institute of Technology, Pasadena, California, USA for an illustrative picture.
9. ^ W. Roy Mason: Magnetic Linear Dichroism Spectroscopy . In: A practical guide to magnetic circular dichroism spectroscopy . Wiley-Interscience, 2007, ISBN 978-0-470-06978-3 , pp. 188 ff . ( limited preview in Google Book search).