Refractive index changes, electro-optic

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. 

In non-polar, isotropic crystals or in glasses, there is no crystallographic direction distinguished and the linear electro-optic effect is absent. Nevertheless a static field may change the index by displacing ions with respect to their valence electrons. In this case the lowest non-vanishing coefficients are of the quadratic form, i.e. the refractive index changes proportionally to the square of the applied field Kerr effect . 

The parameters that are measured run a wide gamut from the routine (current, potential or some electrical parameter) to the exotic (e.g., beam deflection due to refractive index changes). A hierarchical approach to discussing these variant methods has been described [52]. Thus, the methods in Table 2 fall under the categories of purely electrical (entries 1-3, 8 and 9), purely optical (entry titled photoluminescence spectroscopy and entries 12 and 13), electro-optic (electroluminescence spectroscopy) or opto-electric (entries 4-7). We can also distinguish between frequency-resolved (entries 3-7) and time-resolved (entries 10 and 14) measurements, although it must be noted that in many instances (e.g., entries 8 and 11) both steady-state and time-resolved approaches are feasible. 

The initial sensitizer anion presence makes recombination of mobile holes possible in the dark regions. Which are the compensator sites Here, there exist different explanations. One possibility is that some of the electro-optic dye molecules present in photorefractive composites to provide refractive index change may become charged positively. An alternative theory in the case of amorphous materials is that the amorphous disorder leads to defect sites forming local potential minima at which positive charge may be immobilized (Figure 5). 

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... 

Table 4.1-149 Electro-optical constants of zinc compounds. Under the influence of an electric field, the refractive index changes in accordance to the nonlinearity of the dielectric polarization (Pockels effect). Crystals with hexagonal symmetry have three electro-optical constants rsi, 733, 751 crystals with cubic symmetry have only one electro-optical constant 741...

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. 

Linear electro-optic effect (Pockels effect) refers to the modification of the refractive indices of certain NLO materials by a low-frequency or (DC) electric field. The amplitude of modulation field is assumed to be much larger than the light field Ex Ej. The resulting small refractive index change is linearly proportional to the electric field ... 

Changes in the refractive index by the electro-optic effect lead to phase encoding of the incident light distribution... 

An applied electric field can also change a material s linear susceptibility, and thus its refractive index. This effect is known as the linear electro-optic (LEO) or Pocket s effect, and it can be used to modulate light by changing the voltage applied to a second-order NLO material. The applied voltage anisotropically distorts the electron... 

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