Electro-optic effects applications

This electro-optical effect, commonly observed as transient changes in optical birefringence of a solution following application, removal, or reversal of a biasing electric field E(t), has been used extensively as a probe of dynamics of blopolymer solutions, notably by O Konski, and is a valuable tool because it gives information different in form, but related to, results from conventional dielectric relaxation measurements. The state of the subject to 1975 has been comprehensively presented in two review volumes edited by O Konski (25). The discussion here is confined to an outline of a response theory treatment, to be published in more detail elsewhere, of the quadratic effect. The results are more general than earlier ones obtained from rotational diffusion models and should be a useful basis for further theoretical and experimental developments. 

Despite these shortcomings it will become clear that in the one-dimensional NLO-phores treated in this section, which display a wide range of seemingly disparate chemical structures, the crude model works surprisingly well. Thus, as a consequence of the validity of the two-state model, their second-order polarizabilities in principle reduce to p-nitroaniline . The reader may even gain the impression that the efforts to improve on the hyperpolarizabilities of even the simplest and most easily accessible -n systems (like p-nitroaniline) have been futile. It is true that an efficiency-transparency trade-off exists At a given wavelength of absorption (related to A ) a maximum value for the second-order molecular polarizability per volume element exists which is not tremendously different from that of very basic unoptimized rr systems. However, for applications like the electro-optical effect, a bathochromic shift of the UV-visible absorption is tolerable so that to strive for maximum hyperpolarizabilities is a viable quest. Furthermore, molecular structures with the same intrinsic second-order polarizabilities may differ substantially in their chemical stabilities and their abilities to be incorporated into ordered bulk structures. 

Patel J.S., Lee Sin-Don and Goodby J. W. Physical Properties of Smectic Liquid Crystals and Novel Electro-optic Effects. Spatial Light Modulator Technology, Materials, Devices, and Applications (Uzi Efron, Marcel Dekker, New York, 1995). 

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... 

Kikuchi and coworkers found the rapid electro-optical effect in polymer-stabilized blue phases and presented the major potential of such phases in application to display devices and optical modulation devices [45,46]. Tra-... 

Isotropic media can be made birefringent by application of an electric field. This phenomenon is an electro-optic effect.5 There are in fact several electro-optic effects the Pockels effect, the electro-optic Kerr effect, the Stark effect in atoms and molecules, the Franz-Keldysh effect in semiconductors, etc. (see Table 4.6). We will limit our discussion in this section to the Pockels effect and the electro-optic Kerr effect. 

Figure Bl.4.6. Left an experimental optical THz pump-probe set-up using sub-picosecond THz pulse generation and detection by the electro-optic effect. Right the application of such pulses to the relaxation of optically excited TBNC in toluene. The THz electric field used for these experiments is shown in the upper-right inset. Three exponential decay terms, of order 2, 50 and 700 ps, are required to fit the observed temporal relaxation of the solvent [51].

For CW applications of optical-heterodyne conversion, two laser fields are applied to the optoelectronic material. The non-linear nature of the electro-optic effect strongly suppresses continuous emission relative to ultrashort pulse excitation, and so most of the CW research carried out to date has used photoconductive antennae. The CW mixing process is characterized by the average drift velocity vand carrier lifetime Tq of the mixing material, typically... 

A further electro-optical effect which is studied for applications in display is electrochromism. Electrochromism is characterized as a reversible colour change induced in some organic and inorganic materials by an applied electric field or current. According to Chang [263], the physical mechanisms responsible for these phenomena are electronic and electrochemical in nature. A schematic electrochromic reaction can be formulated as follows ... 

Electro-optic (EO) phenomena are related to the interaction of an electric field with an optical process. The classical electro-optic effects, the Pockels and the Kerr effect, discovered in 1893 and 1875 with quartz and carbon disulfide, respectively, refer to the induction of birefringence in certain materials under the influence of an external electric field. Application of an electric field to the sample causes a change in the refractive index. In the case of the Pockels effect. An is linearly proportional to E, the strength of the applied electric field [see Eq. (3-1)]. Hence, it is also called the linear electro-optic effect In contrast. An is proportional to E in the case of the Kerr effect [see Eq. (3-2)]. 

Since 1990, self-assembly techniques involving covalent linkages and layer-by-layer fabrication were reported to yield robust noncentric super lattices highly suited for applications such as SHG and electro-optic effects. " Spontaneous and sequential adsorption of appropriately derivatized adsorbates onto substrates in a self-limiting fashion can yield thin films with uniform polar orders in individual layers. This represents an attractive approach to the construction of intrinsically acentric chromophoric super lattices. Li et reported synthetic routes to the... 

Ceramic PLZT has a number of structures, depending upon composition, and can show both the Pockels (linear) electro-optic effect in the ferroelectric rhombohedral and tetragonal phases and the Kerr (quadratic) effect in the cubic paraelectric state. Because of the ceramic nature of the material, the non-cubic phases show no birefringence in the as-prepared state and must be poled to become useful electro-optically (Section 6.4.1). PMN-PT and PZN-PT are relaxor ferroelectrics. These have an isotropic structure in the absence of an electric field, but this is easily altered in an applied electric field to give a birefringent electro-optic material. All of these phases, with optimised compositions, have much higher electro-optic coefficients than LiNb03 and are actively studied for device application. 

We have prepared multi-component photorefractive polymers for optical data storage applications (8). This photorefractivity is a combination of two functionalities electro-optic effect and photoconductivity. We have achieved holographic diffraction efficiencies up to >30% in our composites. 

The second application uses the converse flexoelectric effect, i.e. a field-induced splay-bend distortion, to generate a fast, symmetric and thresholdless linear electro-optic effect in a cholesteric liquid crystal. 

The linear electro-optic effect in a cholesteric, i.e. a hard-twisted chiral nematic (the helical pitch must be less than the wavelength of visible light) was a very original proposal for using the flexoelectric effect in a new display, shutter or modulator device. The patent application by R.B. Meyer and J.S. Patel dates from 1987 and was granted in 1990. The physics was developed in a series of papers by these authors and later elaborated by others.It is now commonly called the flexoelectro-optic effect. 

In order to understand better the application potential of this electro-optic effect and its fundamental differences from nematic electro-optics based on dielectric anisotropy, and in particular to appreciate the many remaining challenges in its practical application in a device, we will give a derivation of the dynamical properties. The formahsm is slightly different from that used by some authors " and more similar to other descriptions. In the presence of an electric field E the free energy density is written, if we neglect the influence fi om the surfaces,... 

It was recently shown that doping poly(pPIN) with iodine produces a semi-conductive material, with conductivities of about 8 X 10 S cm typical of non-conjugated doped polymers [99]. This material also showed a large quadratic electro-optic effect which would make it useful for non-linear optics applications [100]. 

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