Magneto-optic effect
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A magneto-optic effect is any one of a number of phenomena in which an electromagnetic wave interacts with a magnetic field, or with matter under the influence of a magnetic field. The most important magneto-optic effect is the Faraday effect, in which the plane of polarization is rotated under the influence of a magnetic field parallel to the direction of propagation. This effect may be used to modulate a lightwave, and is used to make Faraday rotators and optical isolators.
In particular, in a magneto-optic material the presence of a magnetic field (either externally applied or because the material itself is ferromagnetic) can cause a change in the permeability tensor ε of the material. The ε becomes anisotropic, a 3×3 matrix, with complex off-diagonal components, depending of course on the frequency ω of incident light—such a permeability is sometimes called gyromagnetic. If the absorption losses can be neglected, ε is a Hermitian matrix. The resulting principal axes become complex as well, corresponding to circularly-polarized light where left- and right-circular polarizations can travel at different speeds (analogous to birefringence). This leads to the Faraday effect, breaks time reversal symmetry (at least locally), and allows devices such as optical isolators to be constructed.
See also
Source: from Federal Standard 1037C and from MIL-STD-188
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