Electro- and magneto-optical effects

Voigt or Cotton-Mouton effect Zeeman effect (transmission) Lo-Surdo-Stark effect Electric double refraction 

Magneto-optic rotation Electro-optic effects  

Kerr effect (reflection) Faraday effect (transmission) Kerr effect (quadratic) Pockets effect (linear) 

A good introduction to electro- and magneto-optical effects can be found in the book by Harvey on Coherent Light [158]. The main effects and the relationship between them are indicated in table 4.1. Many atoms are readily produced as vapour columns, using standard laboratory methods [159]. The natural mode in which to conduct experiments on unperturbed free atoms is therefore in transmission. As table 4.1 emphasises (the reason is given below), the Faraday effect contains equivalent information to the Zeeman effect in transmission. Actually, what Harvey calls the Zeeman effect in transmission is usually referred to as the inverse Zeeman effect [160], to distinguish it from the Zeeman effect observed in emission.5 

5 There is another use of the expression inverse Zeeman Effect which can be found in [162], namely to describe the magnetic moment induced in a sample by the passage of a polarised light beam through a transparent medium. However, the more common usage is the one adopted in the text. 

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The effect consisting in a change in magnetic pmneability under the influ ice of an electric field still awaits detection, although considered theoretically for paramagnetics by Van Vleck, as wdl as for diamagnetics. Non-linear Electro- and Magneto-optical Effects, Optical saturation in an electric field. When an optically isotropic dielectric is placed in a very... 

Table 4.1. Classification of electro- and magneto-optical effects...

Reorientation of director L (or the optical axis) of the macroscopic volume of a liquid crystal under the effect of a field or flow of a liquid is Ae basis of most of the Imown electro- and magneto-optical effects. The anisotropy of the electrical and magnetic properties of the medium (dielectric constant Ae, diamagnetic susceptibility A%, electrical conductivity Aa) is the direct cause of orientation. The rearrangement processes are a function of the initial mientation of the molecules of the liquid crystal and its viscoelastic properties. The change in the c tical properties as a result of reorientation is the consequence of the optical anisotropy of liquid crystals. 

A detailed survey of the known electro- and magneto-optical effects in low-molecular-weight liquid crystals of different structure is given in [3]. We will only mention the basic types of effects which will be required later in the analysis of the processes which take place in polymaic LC systems. 

The orientation of comb-shaped polymers under the effect of electric and magnetic fields, the types of observed electro- and magneto-optical effects, and the characteristic values of the orientation order parameter, dielectric, and diamagnetic anisotropy serve as the basis for drawing analogies in the orientational behavior of low-molecular-weight liquid crystals and comb-shaped LC polymers. 

L.M. Blinov, Electro-Optical and Magneto-Optical Effects in Liquid Crystals, Wiley, Chichester, 1984. 

The first studies of the effect of external influences on LC polym concerned lyotropic systems. However, studies have concentrated on thmno-tropic polymers in recent years, and the number of publications on the electro-and magneto-optical properties of these systems has increased [1, 2]. 

Electro-optic and magneto-optic phenomena contain terms of nonlinear optics effects (see Eqs. (4.32), (4.33), (4.36), and (4.39)). On the other hand, acousto-optic effects which arise from a periodical density fluctuation of the medium, analogous to the Brillouin scattering phenomenon, do not contain terms of nonlinear optics, as a general rule.5) The perturbation of light propagation by sonic waves differs from that induced by electric and magnetic fields. As the electric susceptibility, xe, is a function of the density of the medium, it will be influenced by the periodical density fluctuation induced in a medium by sound waves. 

For a quantitative evaluation of the properties of colloidal dispersions, monodispersity of particles—uniformity in size and shape—is a prerequisite. The quantum size effects are particularly studied, since they lead to interesting mechanical, chemical, electrical, optical, magnetic, electro-optical, and magneto-optical properties that are quite different from those reported for bulk materials [8,10,12,13]. 

Blinov, L.M. Electro-Optical and Magneto-Optical Properties of Liquid Crystals. Wiley, Chichester (1983) Blinov, L.M., Chigrinov, V.G. Electrooptic Effects in Liquid Crystal Materials. Springer-Verlag, New York (1993)... 

If we think of some of Faraday s most important experimental discoveries of electro-magnetic rotation, of electro-magnetic induction, of the magneto-optical effect and of diamagnetism, then his two laws of electro-chemistry stand out as his only statements of definitive quantitative relationships. It is then of great interest to understand why Faraday, in this case, chose to use quantitative methods in his experimental work. 

This book was conceived as a renewed version of the earlier published original book, Electro-Optical and Magneto-Optical Properties of Liquid Crystals (Wiley, Chichester, 1983) written by one of us (L.M. Blinov). That book was first published in Russian (Nauka, Moscow, 1978) and then was modified slightly for the English translation. Since then new information on electrooptical effects in liquid crystals has been published. Novel effects have been discovered in nematics and cholesterics (such as the supertwist effect), and new classes of liquid crystalline materials, such as ferroelectric liquid crystals, appear. Recently, polymer liquid crystals attracted much attention and new electrooptical effects, both in pure polymer mesophases and polymer dispersed liquid crystals, were studied. An important contribution was also made in the understanding of surface properties and related phenomena (surface anchoring and bistability, flexoelectricity, etc.). 

Particular nonlinear optical phenomena arise also when static electric or magnetic fields are applied. The molecular states and selection rules are thereby modified, leading, for instance, to higher-order, nonlinear-optical variants of the linear (Pockels) and quadratic (Kerr) electro-optical effect, or of the linear (Faraday) and quadratic (Cotton-Mouton) magneto-optical effect. 

The first contribution to the polarization induces a modification of the wave propagation in the material, for both its amplitude and phase, but without any frequency change. This phenomenon is known as the optical Kerr effect, by analogy with the magneto-optic and electro-optic Kerr effects where the medium refractive index varies proportionally with the square of the applied magnetic or electric static field. The second contribution corresponds to the third harmonics generation (THG). 

Measurement of Other 0[4ical Properties. Measurement of other optical properties such as strain-, electro-, magneto-, and acoustooptical effects involve specialized techniques beyond the scope of this chapter. The same is true for measurements of transmittance, refractive index profile, dispersion, and other important properties of optical communications fiber. Birefringence and strain-optical measurements will, however, be discussed in Chap. 6, in relationship to annealing and strengthening of glass. 

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