Nonlinear optical behaviour

Nonlinear optical (nlo) effects arise from the nonlinear polarization of a material on application of an external field, which may be an externally applied potential, the electric field of a light beam or a combination of these. The effect is described by the equation 

From this it can be seen that terms involving several different frequencies are involved, i.e. the application of an externally applied field to a material will cause the reradiation of light of frequencies 0 (DC field), co (fundamental) and 2co, 3ft . (harmonics). 

Second-order nlo effects arise when the value of (or p) is non-zero, and since the effect is proportional to the square of the electric field, it is necessary for the material to be non-centrosymmetric in a similar manner to that described earlier for ferro-, pyro- and piezoelectric films. 

In general, the largest second-order effects are obtained from organic materials that incorporate conjugated electron-donor and electron-acceptor groups into the molecule. A simple example is provided by 4-nitroaniline (58), with P = 3.5 x esu. Higher P values are obtained for more extended conjugated systems such as LB merocyanines (59), with P = 2.42 X 10 esu, and hemicyanines (60), with P = 2.9 x 10 esu. 

There are many reports in the literature involving second-order nlo effects in polymeric LB systems. One approach uses a polymeric host and 

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The chemistry of metal complexes featuring alkyne and alkynyl (acetylide) ligands has been an area of immense interest for decades. Even the simplest examples of these, the mononuclear metal acetylide complexes L MC=CR, are now so numerous and the extent of their reaction chemistry is so diverse as to defy efforts at a comprehensive review. " The utility of these complexes is well documented. Some metal alkynyl complexes have been used as intermediates in preparative organic chemistry and together with derived polymeric materials, many have useful physical properties including liquid crystallinity and nonlinear optical behaviour. The structural properties of the M—C=C moiety have been used in the construction of remarkable supramolecular architectures based upon squares, boxes, and other geometries. ... 

A characteristic feature of nonlinear science generally, and of nonlinear optics in particular, is the common necessity of having to make simplifications, and then approximations in order to solve the equations of even the simplified models. These considerations apply a fortiori to the study of fluctuation phenomena in nonlinear systems, and thus account for the increasing role being played by analog and digital simulations, which enable the behaviour of the model systems to be investigated in considerable detail. 

In this chapter, we discuss the application of simulation techniques to the study of fluctuational escape and related phenomena in nonlinear optical systems that is, situations where a large deviation of the system from an equilibrium state occurs under the influence of relatively weak noise present in the system. We will be interested primarily in the analysis of situations where large deviations lead to new nontrivial behaviour or to a transition to a different state. The topics to be discussed have been selected mainly for their own intrinsic scientific interest, but also in order to provide an indication of the power and utility of the simulation approach as a means of focusing on, and... 

Finally, as for the linear optical properties, alternative approaches are being developed to calculate the nonlinear optical response of nanostructured materials. They are most of the time based on the numerical resolution of tlte equations governing the electromagnetic behaviour of a finite set of nanoparticles in a given spatial arrangement [61]. 

Okada and coworkers investigated the nonlinear optical response of silver triangular nanoprisms by pump-probe femtosecond spectroscopy [197]. They reported a different x/ value at the In-plane dipole and quadrupole plasmon resonances, which they showed to correspond to the difference in local field enhancement factors. In both cases, the spectral dispersion of the nonlinear susceptibility exhibits similar behaviour, that is negative and positive values at the low-energy and high-energy sides of the plasmon band, respectively. 

A three dimensional extended dipole model which takes account of the interactions and alignment of molecules with carbazolyl chromophores in monolayer assemblies makes a useful contribution to the detailed understanding of the behaviour of layered structures . A number of other interesting papers involve a nonlinear optical study of Frenkel excitons in LB films where there are J aggregates of pseudoisocyanine iodide , an investigation of protonation equilibria and spectral... 

A number of potential applications of conducting polymers are expected because they are organic compounds, have conjugated structure and many subordinating properties and functions, such as electronic conducting properties, electrochemically active properties (electrochemical doping-undoping), electrochromic behaviours, nonlinear optical properties, etc. 

In the present volume we discuss a relatively new and rapidly developing branch of the field, namely nonlinear optical effects in liquid crystals. Optical studies have always played a significant role in liquid crystal science. Research of optical nonlinearities in liquid crystals began at the end of the sixties. Since then it became a powerful tool in the investigation of symmetry properties, interfacial phenomena or dynamic behaviour. Furthermore, several new aspects of nonlinear processes were demonstrated and studied extensively in liquid crystals. The subject covered in this book is therefore of importance both for liquid crystal research and for nonlinear optics itself. 

We will now study this nonlinear behaviour and the applications of nonlinear optics in more detail. Several examples shall illustrate the subject [523-534]. 

Chapter 2 focuses on an important property of macromolecules, i.e., nonlinear optical (NLO) behaviour, with promising applications in development of faster and highly performing communication devices such as computers and fiber optic telecommunications. An introduction to the basic physical principles that undergo... 

Optically active polymers play a very important role in our modem society. The specialities of optically active polymers are known with their various characteristics as occurred naturally in mimicry. The present review describes the monomers and synthesis of optically active polymers from its helicity, internal compounds nature, dendronization, copolymerization, side chromophoric groups, chiral, metal complex and stereo-specific behaviour. The various properties like nonlinear optical properties of azo-polymers, thermal analysis, chiroptical properties, vapochromic behaviour, absorption and emission properties, thermosensitivity, chiral separation, fabrication and photochromic property are explained in detail. This review is expected to be interesting and useful to the researchers and industry personnel who are actively engaged in research on optically active polymers for versatile applications. 

Polydiacetylenes are particularly interesting for their optical and electrical properties. For example, large third-order nonlinear optical effects can be produced. Extensive electronic conjugation is required for such behaviour and polydiacetylenes produce the largest values yet observed for example, the solid state polymer of (20) exhibits a value of esu, and the LB polymer of (10) with n = 16,m = S exhibits a non-resonant value of 4 x 10 esu. Insulating properties are... 

In the hybrid materials based on the SiC nanoparticles and host polymer matrixes, the origin of the EO behaviour is intimately connected to the hyperpolarizabilities intrinsically involved in the SiC and depends on the interactions at the host-guest interfaces. The intrinsic effect originates from the nanocrystallite bulk in agreement with the EO behavior of 3C-SiC thin films (Vonsovici et al. 2000). The effect of the surrounding polymer on the nanocrystal nonlinear optical behavior was evaluated by numerical methods. In this case the molecular dynamic technique was first used to build the relevant architectures, which combine SiC nanocrystals and the polymers. In a second step, the EO parameters were computed and exhaustive comparison with experimental results was achieved and underlines the strength of the developed theoretical and numerical approaches. 

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