polarisation. For proper no Voigt rotation can be observed. is proportional to the square of the magnetization since and . Transmission geometry The calculation of the rotation of the Voigt effect in transmission is in principle equivalent to the one of the Faraday effect. In practice, this configuration is not used in general for ferromagnetic samples since the absorption length is weak in this kind of material. However, the use of transmission geometry is more common for paramagnetic liquid or cristal where the light can travel easily inside the material. The calculation for a paramagnetic material is exactly the same with respect to a ferromagnetic one, except that the magnetization is replaced by a field ( in or ). For convenience, the field will be added at the end of calculation in the magneto-optical parameters. Consider the transmitted electromagnetic waves propagating in a medium of length L. From equation (5), one obtains for and : At the position , the expression of is: where and are the eigenvectors previously calculated, and is the difference for the two refraction indices. The rotation is then calculated from the ratio , with development in first order in and second order in . This gives: Again we obtain something proportional to and , the light propagation length. Let us notice that is proportional to in the same way with respect to the geometry in reflexion for the magnetization. In order to extract the Voigt rotation, we consider , and real. Then we have to calculate the real part of (14). The resulting expression is then inserted in (8). In the approximation of no absorption, one obtains for the Voigt rotation in transmission geometry: Illustration of Voigt effect in GaMnAs As an illustration of the application of the Voigt effect, we give an example in the magnetic semiconductor where a large Voigt effect was observed. At low temperatures (in general for ) for a material with an in-plane magnetization, exhibits a biaxial anisotropy with the magnetization aligned along (or close to) <100> directions. A typical hysteresis cycle containing the Voigt effect is shown in figure 1. This cycle was obtained by sending a linearly polarized light along the [110] direction with an incident angle of approximately 3° (more details can be found in ), and measuring the rotation due to magneto-optical effects of the reflected light beam. In contrast to the common longitudinal/polar Kerr effect,