An electro–optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light.
The term encompasses a number of distinct phenomena, which can be subdivided into In December 2015, two further electro-optic effects of type (b) were theoretically predicted to exist [1] but have not, as yet, been experimentally observed.
Using a less strict definition of the electro-optic effect allowing also electric fields oscillating at optical frequencies, one could also include nonlinear absorption (absorption depends on the light intensity) to category a) and the optical Kerr effect (refractive index depends on the light intensity) to category b).
This is because of competing acousto-optic deflectors, the small number of resolvable spots and the relatively high price of electro-optic crystals.
The electro-optic Pockels effect in nonlinear crystals (e.g. KDP, BSO, K*DP) can be used for electric field sensing via polarisation state modulation techniques.