Editing Crystal optics (section)

==Other effects==

Certain [[nonlinear optics|nonlinear optical]] phenomena such as the [[electro-optic effect]] cause a variation of a medium's permittivity tensor when an external electric field is applied, proportional (to lowest order) to the strength of the field. This causes a rotation of the principal axes of the medium and alters the behaviour of light travelling through it; the effect can be used to produce light modulators.

In response to a [[magnetic field]], some materials can have a dielectric tensor that is complex-[[Hermitian]]; this is called a gyro-magnetic or [[magneto-optic effect]].  In this case, the [[Principal axis (crystallography)|principal axes]] are complex-valued vectors, corresponding to elliptically polarized light, and time-reversal symmetry can be broken.  This can be used to design [[optical isolator]]s, for example.

A dielectric tensor that is not Hermitian gives rise to complex eigenvalues, which corresponds to a material with gain or absorption at a particular frequency.