Electrooptic switching, second-order effects

The photoinduced phenomenon where the change in refractive index is proportional to the second order term of applied voltage is called an electrooptical Kerr effect. Kerr cells of liquid or solid transparent media can be used together with polarizers to transmit or block light, depending on whether an electric field is applied or not. Such light switches have many uses in laser technology. 

Second-order materials which exhibit quadratic effects such as SHG or electrooptic (EO) switching (in which the refractive indices of EO materials can be modulated by application of an electric static field). The materials must be non-centrosymmetric at both microscopic and macroscopic levels. 

The second-order NLO properties are of interest for a variety of NLO processes [1-3]. One of the most relevant is the SHG, originated by the mixing of three waves two incident waves with frequency co interact with the molecule or the bulk material with NLO properties, defined by a given value of the quadratic hyperpolarizability, fi, or of the second-order electrical susceptibility, respectively, to produce a new electrical wave, named SH, of frequency 2co. Another important second-order NLO process is the electrooptic Pockels effect which requires the presence of an external d.c. electric field, E(0), in addition to the optical E co) electrical field. This effect produces a change in the refractive index of a material proportional to the applied electric field, and can be exploited in devices such as optical switches and modulators [1-3]. 

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