Refractive index changes, electro-optic properties

Ferroelectric lithium niobate (LiNbOs) has been of considerable interest because of its nonlinear optical properties. Conversion of infrared into visible radiation in LiNb03 crystals has been observed (Midwinter and Warner, 1967 Arutyunyan and Mkrtchyan, 1975). Electro-optic coefficients of LiNbOs have been determined for a wide range of frequencies ranging from the visible (Smakula and Claspy, 1967) to the millimeter-wave portion of the spectrum (Vinogradov et al., 1970). Other nonlinear optical properties such as photovoltaic effects (Kratzig and Kurz, 1977) and optically induced refractive index changes (Ashkin et al., 1966 Chen, 1969) have also been observed. 

Electro-optic Properties 9.7.1 Refractive Index Changes... 

Electro-optic effects describe a change in optical properties arising from an applied electric field. This can include changes in colour or optical absorption, characteristic of electrochromic materials (Section 9.6), or a change in refractive index. In perovskites, it is this latter effect that is important. These crystals have been used in electro-optic devices to modulate the phase, the amplitude or the polarisation of a light beam traversing the medium and so function as shutters and other components in optical/electronic circuits. 

In general, the distortions on the electronic wave function of liquid crystal molecules caused by an applied field do not cause appreciable change to its contribution to the refractive indices (see Chapter 10). However, the orientation of the molecules can be dramatically altered by the apphed field. This process alters the overall optical properties of the medium and is the principal mechanism used in liquid-crystal-based electro-optical devices. As noted in Section 6.2.2, the electrically induced orientational refractive index changes could be Pockel or Kerr effect. In this and the next sections, we shall focus on nematic liquid crystals in which the director axis reorientation is a Kerr-like effect that is, the process is quadratic in the applied field. 

Electro-optic effects refer to the changes in the refractive index of a material induced by the application of an external electric field, which modulates their optical properties [61, 62], Application of an applied external field induces in an optically isotropic material, like liquids, isotropic thin films, an optical birefringence. The size of this effect is represented by a coefficient B, called Kerr constant. The electric field induced refractive index difference is given by... 

The Surface Plasmon Resonance technique has proved to be very versatile, finding a number of applications where small changes in the properties of a material characterise a physical change in the vicinity of the measurement [2]. As a technique for investigating the linear and non-linear bulk optical properties of novel materials in thin film form, the SPR method is both accurate and sensitive [3]. For non-linear organic materials, particularly in LB film form, information on film thickness, refractive index and electro-optic coefficient may be straightforwardly obtained by computer analysis, even for a molecular monolayer with thiclmess of the order of only 3 nm or less. 

Below, taking an electric field as a typical example of an external field, we will discuss the electro-optic effects that result from the effect of this field on the optical properties. Electro-optic effects are effects such as changes to the deformation and rotation of the index ellipsoid caused by the action of an electric field (E). When the field is applied, the change in the refractive index tensor A(l/np, which is an optical parameter, can be expressed as follows by expanding the electric field terms ... 

Noncentrosymmetric crystals show other properties in addition to frequency conversion, for instance the linear electro-optic or Pockels effect the linear change of the refractive index induced by an applied DC electric field. Furthermore, the point groups and allow for the existence of a permanent electric dipole moment. Indeed, crystals... 

Structure is unwound and h points in a fixed direction, as in Figure 1.12d, then p will point in one direction. Clearly, this and other director axis reorientation processes are accompanied by considerable change in the optical refractive index and other properties of the system, and they can be utilized in practical electro- and opto-optical modulation devices. A detailed discussion of smectic liquid crystals is given in Chapter 4. 

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