Editing Kerr effect (section)

== Kerr electro-optic effect ==
The Kerr electro-optic effect, or DC Kerr effect, is the special case in which a slowly varying external electric field is applied by, for instance, a [[voltage]] on electrodes across the sample material. Under this influence, the sample becomes [[birefringent]], with different indices of refraction for light [[Polarization (waves)|polarized]] parallel to or perpendicular to the applied field. The difference in index of refraction, ''Δn'', is given by
:<math>\Delta n = \lambda K E^2,\ </math>
where ''λ'' is the wavelength of the light, ''K'' is the ''Kerr constant'', and ''E'' is the strength of the electric field. This difference in index of refraction causes the material to act like a [[waveplate]] when light is incident on it in a direction perpendicular to the electric field. If the material is placed between two "crossed" (perpendicular) linear [[polarizer]]s, no light will be transmitted when the electric field is turned off, while nearly all of the light will be transmitted for some optimum value of the electric field. Higher values of the Kerr constant allow complete transmission to be achieved with a smaller applied electric field.

Some [[Polar molecule|polar]] liquids, such as [[nitrotoluene]] (C<sub>7</sub>H<sub>7</sub>NO<sub>2</sub>) and [[nitrobenzene]] (C<sub>6</sub>H<sub>5</sub>NO<sub>2</sub>) exhibit very large Kerr constants. A glass cell filled with one of these liquids is called a ''Kerr cell''. These are frequently used to [[modulation|modulate]] light, since the Kerr effect responds very quickly to changes in electric field. Light can be modulated with these devices at frequencies as high as 10&nbsp;[[gigahertz|GHz]]. Because the Kerr effect is relatively weak, a typical Kerr cell may require voltages as high as 30&nbsp;[[kilovolt|kV]] to achieve complete transparency. This is in contrast to [[Pockels cell]]s, which can operate at much lower voltages. Another disadvantage of Kerr cells is that the best available material, [[nitrobenzene]], is poisonous. Some transparent crystals have also been used for Kerr modulation, although they have smaller Kerr constants.

In media that lack [[inversion symmetry]], the Kerr effect is generally masked by the much stronger [[Pockels effect]]. The Kerr effect is still present, however, and in many cases can be detected independently of Pockels effect contributions.<ref>{{cite journal |journal=Phys. Rev. A |volume=82 |issue=1 |page=013821 |date=2010 |title=Direct Kerr electro-optic effect in noncentrosymmetric materials |first1=Mike |last1=Melnichuk |first2=Lowell T. |last2=Wood |doi=10.1103/PhysRevA.82.013821|bibcode = 2010PhRvA..82a3821M }}</ref>