THIRD ORDER NONLINEAR OPTICAL NLO PROPERTIES

Recent studies have shown that o-acetylide metal complexes exhibit very encouraging third-order nonlinear optical (NLO) properties [85,86]. Hence, there is considerable current interest in o-acetylide complexes and conjugated, bridging, bimetallic systems. 

THIRD ORDER NONLINEAR OPTICAL (NLO) PROPERTIES 12.3.1 The NLO Effect and Practical Requirements... 

Some quinones, having the ability to form intra- and/or intermolecular hydrogen bonds, exhibit high molecular hyperpolarizability and are third-order nonlinear optical (NLO) materials. Compound 39 has a %(3) of 5 x 10 11 esu at 1.9 pm, and is a third-order NLO material.23 The optoelectric properties of quinoid compounds correlate with their structures in crystals or on thin films.23... 

Conjugated polymers satisfy these requirements and have thus emerged as the most widely studied materials for their susceptability. Some of the examples of conjugated polymers, that have been studied for their third order NLO properties, are polydiacetylenes, poly-p-phenylenevinylenes and polythiophenes. However, CVD has only been used in the case of poly-p-phenylenevinylenes (PPV) [section 3.4], although values have not been reported. An excellent review of third order nonlinear optical properties of PPV in general, can be found in literature. Recently, McElvain et al. ° reported the values of CVD polyazomethines to be... 

The polymers having delocalized r-electron in the main chain have been expected to possess extremely large third-order optical susceptibility.However, such an extended jr-electron conjugation generally rendered the polymers insoluble and infusible as well, which has seriously limited the fabrication of practical NLO devices. Recently, it was reported that the third-order nonlinear optical properties of poly(l,6-heptadiyne)s which were environmentally stable, soluble, and processable. The third-order optical nonlinearities of poly(l,6-heptadiyne)s bearing NLO active chomophores were evaluated for the first time. The third-order nonlinear susceptibility... 

The importance of the hyperpolarizability and susceptibility values relates to the fact that, provided these values are sufficiently large, a material exposed to a high-intensity laser beam exhibits nonlinear optical (NLO) properties. Remarkably, the optical properties of the material are altered by the light itself, although neither physical nor chemical alterations remain after the light is switched off. The quahty of nonlinear optical effects is cmciaUy determined by symmetry parameters. With respect to the electric field dependence of the vector P given by Eq. (3-4), second- and third-order NLO processes may be discriminated, depending on whether or determines the process. The discrimination between second- and third-order effects stems from the fact that second-order NLO processes are forbidden in centrosymmetric materials, a restriction that does not hold for third-order NLO processes. In the case of centrosymmetric materials, x is equal to zero, and the nonhnear dependence of the vector P is solely determined by Consequently, third-order NLO processes can occur with all materials, whereas second-order optical nonlinearity requires non-centrosymmetric materials. 

As with phthalocyanines, the third-order nonlinear optical susceptibility, of porphyrins can be manipulated by chemical substitution. The third-order NLO properties of several tetraphenylporphyrin compounds were first reported by Meloney et The was measured by the degenerate four-wave mixing (DFWM) technique from... 

The nonlinear optical (NLO) susceptibilities of bioengineered aromatic polymers synthesized by enzyme-catalyzed reactions are given in Tables 2, 3, and 4. Homopolymers and copolymers are synthesized by enzyme-catalyzed reactions from aromatic monomers such as phenols and aromatic amines and their alkyl-substituted derivatives. The third-order nonlinear optical measurements are carried out in solutions at a concentration of 1 mg/mL of the solvent. Unless otherwise indicated, most of the polymers are solubilized in a solvent mixture of dimethyl formamide and methanol (DMF-MeOH) or dimethyl sulfoxide and methanol (DMSO-MeOH), both in a 4 1 ratio. These solvent mixtures are selected on the basis of their optical properties at 532 nm (where all the NLO measurements reported here are carried out), such as low noise and optical absorption, and solubility of the bioengineered polymers in the solvent system selected. To reduce light scattering, the polymer solutions are filtered to remove undissolved materials, the polymer concentrations are corrected for the final x calculations, and x values are extrapolated to the pure sample based on the concentrations of NLO materials in the solvent used. Other details of the experimental setup and calculations used to determine third-order nonlinear susceptibilities were given earlier and described in earlier publications [5,6,9,17-19]. 

TABLE 2 Third-Order Nonlinear Optical Properties of Polymers Synthesized in Monophasic Media and Other NLO Samples... 

This review covers the theoretical background and some of the practical aspects of nonlinear optics, including a description of the origins of third-order nonlinearities, systems of units that are encountered, experimental techniques that have been used or may be used to probe the third-order NLO properties of organometallic complexes, and computational methods that have or could be used to calculate third-order NLO properties. Subsequent sections collect comprehensive data of organometallic complexes in tables categorized by complex type and discussions of the results of third-order NLO measurements and calculations performed on organometallic... 

For third order nonlinear application, however, it is the concentration of the NLO dye that is of paramount importance, not the orientational aspects. The third order response is typically dependent on the extent and nature of electronic conjugation. Polydiacetylene, for example, shows very high because of the delocalized electronic structure along the n-conjugated polymer chains. The large dye molecules posses quite reasonable molecular hyperpolarizability. A high concentration of these dyes without a concomittant deterioration of linear optical properties can lead to useful bulk third order NLO coefficients. 

Several of the third-order nonlinear effects described in section 4.1.3 can be used to characterize bulk materials. Degenerate four-wave mixing (DFWM) is used for measuring third-order properties of films and solutions [43-45], and though this experiment is complex to set up and interpret, it can give valuable information on the magnitude, sign and speed of the nlo process, as well as an indication of the nature of the excitation process. Results from DFWM can be found in section 4.3. Optical Kerr effect (OKE) [46] and electrical Kerr effect (EKE) [47] measurements have also been used to characterize third-order properties of nlo polymers. It is important to note that THG, DFWM, OKE and EKE all measure different parts of the third-order susceptibility, and... 

Organic compounds with delocalized 7r-electron systems are leading candidates for nonlinear optical (NLO) materials, and interest in these materials has grown tremendously in the past decade [108-118]. Reliable structure-property relationships—where property here refers to first-order (linear) polarizability a, second-order polarizability and third-order polarizability y—are required for the rational design of optimized materials for photonic devices such as electro-optic modulators and all-optical switches. Here also, quantum-chemical calculations can contribute a great deal to the establishment of such relationships. In this section, we illustrate their usefulness in the description of the NLO response of donor-acceptor substituted polymethines, which are representative of an important class of organic NLO chromophores. We also show how much the nonlinear optical response depends on the interconnection between the geometric and electronic structures, as was the case of the properties discussed in the previous sections [ 119]. 

Polymers, because of their transparency and moderate refractive index, make ideal media for the propagation of light. These aspects of polymers are descrihed in Chapter 8.10. Under certain circumstances, rrsrrally rmder intense exposme to laser light, nonlinear optical (NLO) effects can be observed. NLO properties derive from interaction of electrons in materials with electric field of light waves. Such effects and the associated materials can be classified into second-order and third-order NLO response. These materials are of great interest becarrse of their ability to carry out fast optical switching. 

This review summarizes the current state of materials development in nonlinear optics. The relevant properties and important materials constants of current commercial, and new, promising, inorganic and organic molecular and polymeric materials with potential in second and third order NLO applications are presented. 

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