Voigt effect

The Voigt effect is a magneto optical phenomenon which rotates and elliptizes linearly polarised light sent into an optically active medium. The effect is named after the German scientist Woldemar Voigt who discovered it in vapors. Unlike many other magneto optical effects such as the Kerr or Faraday effect which are linearly proportional to the magnetization (or to the applied magnetic field for a non magnetized material), the Voigt effect is proportional to the square of the magnetization (or square of the magnetic field) and can be seen experimentally at normal incidence. There are also other denominations for this effect, used interchangeably in the modern scientific literature: the Cotton–Mouton effect (in reference to French scientists Aimé Cotton and Henri Mouton who discovered the same effect in liquids a few years later) and magnetic linear birefringence, with the latter reflecting the physical meaning of the effect. For an electromagnetic incident wave linearly polarized and an in plane polarized sample , the expression of the rotation in reflection geometry is is:

and in the transmission geometry:

where is the difference of refraction indices depending on the Voigt parameter (same as for the Kerr effect), the material refraction indices and the parameter responsible of the Voigt effect and so proportional to the or in the case of a paramagnetic material. Detailed calculation and an illustration are given in sections below.

As with the other magneto optical effects, the theory is developed in a standard way with the use of an effective dielectric tensor from which one calculates systems eigenvalues and eigenvectors. As usual, from this tensor, magneto optical phenomena are described mainly by the off diagonal elements. Here, one considers an incident polarisation propagating in the z direction: the electric field and a homogenously in plane magnetized sample where is counted from the [100] crystallographic direction. The aim is to calculate where is the rotation of polarization due to the coupling of the light with the magnetization. Let us notice that is experimentally a small quantity of the order of mrad. is the reduced magnetization vector defined by , the magnetization at saturation. We emphasized with the fact that it is because the light propagation vector is perpendicular to the magnetization plane that it is possible to see the Voigt effect.