Optical materials, nonlinear

Nonlinear polarization characteristics of centrosymmetric molecules modified by the introduction of substituent groups are expressed in the following manner. The dielectric polarization consists of odd-number-order terms as in Eq. (5.23). 

Substituents on an aromatic ring induce a distortion in the -electron system, mostly via mesomeric effects. These induce changes in the light electric field and, consequently, influence the dielectric polarization P. In general terms, P can be expressed by Eq. (5.24). 

Where uM is the induced dipole moment due to the mesomeric effect ( , = /iJA). The second-order molecular polarizability, P, is given by Eq. (5.25). 

As the molecular polarizabilities, A and C, are inherent properties of unsubstituted centrosymmetric molecules, the p value of the corresponding substituted derivative is 

Quantum mechanical expressions of the molecular polarizability can be derived from Eq. (1.163) which describe the hyperpolarizability of scattering phenomena, discussed in Chapter 1. The second-order molecular polarizability is given by the following equations  

8 tt Di Bella, S., Second-order nonlinear optical properties of transition metal complexes , Chem. Soc. Rev. 2001, 30, 355-366. 

Three independent values of x are required for an anisotropic crystalline solid to characterize the linear optical properties. 

Laser light, possessing intense electric fields (10 -10 V/cm), gives rise to a number of nonlinear effects. The above equation for P thus contains higher-order terms  

Desirable Intrinsic Properties of Materials for Nonlinear Applications 

The nature or source of polarization within a crystal has been used to classify nonlinear optical materials into two classes. One class comprises materials in which the polarization is a result of the relative displacement of ions, such as in ferroelectrics. The second class of materials owe their polarizability to the ease with which the valence electrons or electronic charge distribution about atoms or 

The requirement of acentricity and an examination of the atoms and molecular groups (and their disposition) have been useful in the search for new nonlinear optical materials. In addition, the Miller delta rule, which relates the magnitude of the nonlinear optical susceptibility d to the linear optical susceptibility x, is valuable in gaining information on the strength of the nonlinear effects to be expected from a given material. Miller s equation is 

Further subclassification of nonlinear optical materials can be explained by the foUowiag two equations of microscopic, ie, atomic or molecular, polarization,, and macroscopic polarization, P, as power series ia the appHed electric field, E (disregarding quadmpolar terms which are unimportant for device appHcations)  

Materials are also classified according to a particular phenomenon being considered. AppHcations exploiting off-resonance optical nonlinearities include electrooptic modulation, frequency generation, optical parametric oscillation, and optical self-focusing. AppHcations exploiting resonant optical nonlinearities include sensor protection and optical limiting, optical memory appHcations, etc. Because different appHcations have different transparency requirements, distinction between resonant and off-resonance phenomena are thus appHcation specific and somewhat arbitrary. 

Many classes of second-order material appHcations can be envisioned by noting the sinusoidal nature of electromagnetic radiation and rewriting equation 2 

Materials for Electrooptic Modulation. The fundamental phenomenon of Pockel s effect is a phase change, A( ), of a light beam in response to a low frequency electric field of voltage, V. Relevant relationships for coUinear electrical and optical field propagation are as foUows (1 6)  

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

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Most of the envisioned practical applications for nonlinear optical materials would require solid materials. Unfortunately, only gas-phase calculations have been developed to a reliable level. Most often, the relationship between gas-phase and condensed-phase behavior for a particular class of compounds is determined experimentally. Theoretical calculations for the gas phase are then scaled accordingly. 

Enclosure also changes the redox properties of a compound, its color, and other physical properties (1,2). On this basis nonlinear optical materials, luminescence markers, controlled light switches, and other high-tech devices might be designed and prepared (15,17,137). 

Fig. 1. Representative device configurations exploiting electrooptic second-order nonlinear optical materials are shown. Schematic representations are given for (a) a Mach-Zehnder interferometer, (b) a birefringent modulator, and (c) a directional coupler. In (b) the optical input to the birefringent modulator is polarized at 45 degrees and excites both transverse electric (TE) and transverse magnetic (TM) modes. The appHed voltage modulates the output polarization. Intensity modulation is achieved using polarizing components at the output.

Applications Involving Nonlinear Index Phenomena. The index of refraction, n, can be expressed for nonlinear optical materials as... 

The cadmium chalcogenide semiconductors (qv) have found numerous appHcations ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, teUuride, and oxide, are used as phosphors in luminescent screens and scintiUation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recently attracted attention as a third-order, nonlinear optical switching material (see Nonlinear optical materials). DiaLkylcadmium compounds are polymerization catalysts for production of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl, they catalyze the polymerization of ethylene and propylene. 

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