Hyperpolarizability materials

Note that to generate a large SHG, the material has to have laige values of the hyperpolarizabilities p and y. The THG needs a large y. In both cases, a strong laser electric field is a must. The SHG and THG therefore require support from the theoretical side we are looking for high hyperpolarizability materials and quantum mechanical calculations before an expensive... 

The susceptibility tensors give the correct relationship for the macroscopic material. For individual molecules, the polarizability a, hyperpolarizability P, and second hyperpolarizability y, can be defined they are also tensor quantities. The susceptibility tensors are weighted averages of the molecular values, where the weight accounts for molecular orientation. The obvious correspondence is correct, meaning that is a linear combination of a values, is a linear combination of P values, and so on. 

The polarizability expresses the capacity of a system to be deformed under the action of electric field it is the first-order response. The hyperpolarizabilities govern the non linear processes which appear with the strong fields. These properties of materials perturb the propagation of the light crossing them thus some new phenomenons (like second harmonic and sum frequency generation) appear, which present a growing interest in instrumentation with the lasers development. The necessity of prediction of these observables requires our attention. 

If neither cOyis nor cOsfg is in resonance with an electric dipole transition in the material and only electric dipole transitions are considered, the hyperpolarizability. 

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... 

Numerous compounds of the types (L)AuC=CR and Q+[Au(C=C-R)2] were investigated for their properties as NLO materials. Some of the examples were found to have the largest cubic optical non-linearity for monomeric organometallics.103 Examples are given in Scheme 18. For all of these compounds, the quadratic/cubic hyperpolarizability (linear optical and quadratic NLO response) have been determined. The studies were complemented by cyclovoltammetric measurements.58,59,103,112-117... 

The large molecular hyperpolarizability of the merocyanine chromophore (4,5) and the highly polar environment of the quasicrystals has prompted studies of the second order nonlinear optical properties of these materials (6). 

The nonlinear optical and dielectric properties of polymers find increasing use in devices, such as cladding and coatings for optical fibres, piezoelectric and optical fibre sensors, frequency doublers, and thin films for integrated optics applications. It is therefore important to understand the dielectric, optical and mechanical response of polymeric materials to optimize their usage. The parameters that are important to evaluate these properties of polymers are their dipole moment polarizability a, hyperpolarizabilities 0... 

The ethynyl-linked complexes 105 were prepared and explored as potential building blocks for nonlinear optical (NLO) materials.129 Spectroscopic and cyclic voltammetry data indicate a small but real interaction between the ferrocenyl donor group and the borabenzene unit, increasing in the order RuHyper-Rayleigh scattering revealed small values for the first hyperpolarizability / , which increases in the same order. 

The non-linear optical properties of a material are evaluated by measuring (using techniques from the field of physics) its molecular hyperpolarizability coefficient (3. 

Sum Over States [3] (SOS) approaches constitute one of the most commonly used class of methodologies for theoretical estimation of hyperpolarizabilities. The strength of this approach is related to the fact that for many compounds of interest, only a few excited states make a major contribution. The simplest scheme, proposed by Oudar and Chemla [4—5] to analyze variations of p among push-pull conjugated materials, restricts the summation to a unique excited state. In this resulting two-state approximation (TS A), the static longitudinal electronic first hyperpolarizability, Pl, is given by ... 

Third harmonic generation is used to study the purely electronic molecular second hyperpolarizability of centrosymmetric materials no other mechanism but the nonresonant electron cloud distortion can respond rapidly... 

Of the many studies of applications of L-B films, we shall mention at last the development of non-linear optical materials. A molecule such as 4-nitro-aniline has a very large hyperpolarizability (section 7.2) and can be used in principle for frequency conversion in solution if a strong electric field is used to provide an orientation of the dipoles. There is no need for such an external field if similar molecules are used in an L-B film, and hyperpolarizability values far in excess of those of commonly used inorganic crystals have been obtained. Figure 8.24 shows two examples of the molecules used in... 

The design of molecules, supermolecules and materials presenting NLO activity involves molecular and supramolecular engineering. At the molecular level, a high polarizability, leading to large quadratic ft and cubic y hyperpolarizability coeffi-... 

Methyldithiepin 19 undergoes photoinitiated 1,3-proton shift, leading to a change in hyperpolarizability, which is of great importance for organic electronic materials (Equation 5) <2001JOC1894>. 

Sulfur heterocycles, including those with more than two sulfur atoms, are used for optical materials <2000JPP2002040201>. The molecular third-order optical nonlinearity 7R (Second hyperpolarizability or nonlinear refractive index) was measured for pentathiepinethiafulvalene <1999PCA6930>. 

This paper is a tutorial overview of the techniques used to characterize the nonlinear optical properties of bulk materials and molecules. Methods that are commonly used for characterization of second- and third-order nonlinear optical properties are covered. Several techniques are described briefly and then followed by a more detailed discussion of the determination of molecular hyperpolarizabilities using third harmonic generation. 

As mentioned above, the powder SHG method is a useful technique for the screening of second-order nonlinear materials. However, because of the sensitivity of the SHG coefficients of crystalline materials to the orientational aspects of the molecular packing and because the measurement is performed on an essentially random distribution of microcrystalline particles, the powder SHG method is not generally useful for obtaining information about molecular hyperpolarizabilities. 

Characterization of Molecular Hyperpolarizabilities Using Third Harmonic Generation. Third harmonic generation (THG) is the generation of light at frequency 3co by the nonlinear interaction of a material and a fundamental laser field at frequency co. The process involves the third-order susceptibility x 3K-3 , , ) where —3 represents an output photon at 3 and the three s stand for the three input photons at . Since x(3) is a fourth (even) rank tensor property it can be nonzero for all material symmetry classes including isotropic media. This is easy to see since the components of x(3) transform like products of four spatial coordinates, e.g. x4 or x2y2. There are 21 components that are even under an inversion operation and thus can be nonzero in an isotropic medium. Since some of the terms are interrelated there are only four independent terms for the isotropic case. 

The development of design guidelines for molecules with large second hyperpolarizability, 7, is more difficult because of uncertainty in whether few or many state models are appropriate [24-28]. Some effects, such as saturation of 7 with chain length, can be addressed with one-electron hamiltonians, but more reliable many-electron calculations (already available for (3) are just beginning to access large 7 materials [24,35-38]. Theoretical and experimental work in this area should hold some interesting surprises. 

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