Voigt effect
A magneto-optical phenomenon which rotates and elliptizes linearly polarised light sent into an optically active medium.
Voigt effect: magnetic twist and squeeze of light
When linearly polarized light passes through a material placed in a magnetic field perpendicular to the light's direction, two things happen simultaneously: the polarization plane rotates, and the light becomes elliptical rather than remaining linear. This is the Voigt effect, a magneto-optical phenomenon distinct from the better-known Faraday rotation, which only rotates without elliptizing.
The mechanism sits in how the magnetic field splits energy levels within the medium's atoms or ions, creating differential absorption and phase shift for light polarized in different directions. When the field points sideways relative to the beam, it treats horizontal and vertical polarization components asymmetrically. One component may be absorbed or delayed more than the other, converting pure linear polarization into an ellipse while rotating the major axis.
This effect appears prominently in magneto-optical sensors and in spectroscopy of paramagnetic materials. In high-field applications, such as astrophysical observations of neutron stars or laboratory plasma diagnostics, the Voigt effect can dominate or interfere with Faraday measurements. Rare-earth ions doped into crystals or glasses often show strong Voigt responses, making them candidates for advanced optical isolators and tunable filters.
Why it matters in practice
The Voigt effect couples both the amplitude and phase response to the applied field, whereas Faraday rotation depends only on phase. This makes Voigt-active materials sensitive detectors of field strength and direction, but also means their optical output cannot be cleaned back to linear polarization by simple rotation of an analyzer. Compensating the ellipticity requires additional optical elements or electronic correction.
The term honors Woldemar Voigt, who developed the theoretical framework for such anisotropic optical effects in the late 19th century. Modern interest has grown with magnetic recording, magneto-optical imaging in medicine, and quantum optics, where precise control of polarization under magnetic tuning is essential. The effect's strength depends on the host material, the dopant ion, the field strength, and the wavelength of light; practical devices typically operate in the visible to near-infrared range.