Voigt effect

In magneto-optics, the Voigt effect describes the birefringence in a transparent, gaseous medium, with a constant magnetic field perpendicular to the direction of propagation of the light . It is stronger than similar effects, such as the Cotton-Mouton effect (for molecules or liquids) and the Majorana effect (for colloidal solutions). Together, the three effects form the magnetic analogy of the Kerr effect .

The Voigt effect was named after the German physicist Woldemar Voigt , who first described it in 1898

The Voigt effect is frequently in the literature both as a magnetic linear birefringence (engl. Magnetic linear birefringence , MLB) and as a linear magnetic dichroism designated (MLD). The effect is related on the one hand to the real part (for MLB) and on the other hand to the imaginary part (for MLD) of the change in the complex refractive index.

description

Light or an electromagnetic wave is a transverse wave made up of coupled electric and magnetic fields , the oscillation planes of which are perpendicular to one another and to the direction of propagation.

The Voigt effect occurs when this polarized electromagnetic wave propagates perpendicular to the magnetic field lines. A polarized electromagnetic wave can also be described in the form of two mutually perpendicular linearly polarized waves, one speaks of perpendicular and parallel polarized components; this usually refers to the electric field vector E of the wave. The magnetic field has the effect that the part of the electric field vector that oscillates parallel to the magnetic field propagates at a different phase velocity than the perpendicularly oscillating electric field vector. The polarization state of the wave changes due to the different phase velocities, so a linearly polarized wave generally becomes an elliptically polarized wave.

This phenomenon can be described using two indices of refraction and is called birefringence . With normal birefringence, this is caused by an anisotropic structure of the irradiated material. The Voigt effect is different: Here the birefringence is caused by a magnetic field and the change in the refractive index n is proportional to the square of the magnetic flux density B :

${\displaystyle \Delta n\propto B^{2}}$

The following applies to optically isotropic materials:

${\displaystyle \Delta n=n_{\perp }-n_{\|}}$

The change in polarization is still dependent on other variables:

${\displaystyle \delta ={\frac {q^{4}N\lambda ^{3}H^{2}l}{4\pi c^{6}n_{0}\left(m_{v}^{*}\right)^{3}}}}$

• ${\displaystyle \delta }$ ... phase shift of the wave
• ${\displaystyle e}$... elemental charge
• ${\displaystyle N}$ ... charge carrier concentration
• ${\displaystyle \lambda }$... wavelength
• ${\displaystyle B}$ ... magnetic flux density
• ${\displaystyle l}$ ... range in the material
• ${\displaystyle c}$... the speed of light ?
• ${\displaystyle n_{0}}$... (complex) refractive index of the material at the respective wavelength without a magnetic field
• ${\displaystyle (m_{v}^{*})^{3}}$ ... effective Voigt mass (has a strong crystallographic dependence)

application

The Voigt effect is used in so-called Voigt filters , a type of atomic line filter . The Voigt effect turns a gas cell into a λ / 2 retardation plate .

See also

• Verdet constant
• Magneto-optic Kerr effect

Individual evidence

1. ↑ Woldemar Voigt: Birefringence of sodium vapor in the magnetic field in the direction normal to the lines of force . In: Nachr. Kgl. Ges. Wiss. Goettingen . 1898, p. 355-359 (Birefringence of sodium vapor in a magnetic field along a direction perpendicular to the lines of force).
2. ↑ Woldemar Voigt: The fundamental physical properties of the crystals in elementary representation . BiblioBazaar, LLC, 2008, ISBN 978-0-554-79425-9 ( limited preview in Google Book Search).
3. ↑ AK Zvezdin, Anatolii Konstantinovich Zvezdin, Vi︠a︡cheslav Alekseevich Kotov, VA Kotov: Modern magneto-optics and magnetooptical materials . CRC Press, 1997, ISBN 0-7503-0362-X , pp. 36 .
4. ↑ Victor Antonov, Bruce Harmon, Alexander Yaresko: Electronic structure and magneto-optical properties of solids . Springer, 2004, ISBN 1-4020-1905-X , pp. 56 .
5. ↑ Štefan Višňovský: Optics in magnetic multilayers and nanostructures . CRC Press, 2006, ISBN 0-8493-3686-4 , pp. 27 .
6. ^ ED Palik: Anisotropic, free carrier voigt effect in n-type germanium . In: Journal of Physics and Chemistry of Solids . tape 25 , no. 7 June 1964, p. 767-771 , doi : 10.1016 / 0022-3697 (64) 90189-1 .