Molecular nonlinearities

We now consider this issue in a more rigorous fashion. The inference of molecular orientation can be explamed most readily from the following relation between the surface nonlinear susceptibility tensor and the molecular nonlinear polarizability... 

Another group of conjugated thiophene molecules for future appHcations are those being developed as nonlinear optical (NLO) devices (75). Replacement of benzene rings with thiophene has an enormous effect on the molecular nonlinearity of such molecules. These NLO molecules are able to switch, route, and modulate light. Technology using such materials should become available by the turn of the twenty-first century. 

Zyss J (1993) Molecular nonlinear optics materials, physics and devices. Academic Press, Boston MA... 

Zyss, J. Molecular Nonlinear Optics Materials, Physics and Devices 1994, Academic Press Boston. 

Other less definite yet important effects such as profile changes due to nonlinear refractive index alteration in spatially nonuniform high power beams must be carefully considered. As example, the use of nonidentical liquids and optical paths prior to and in, say, EFISH cells and the usual quartz calibration cells could cause potentially inaccurate x determinations. Obviously these types of considerations are important when precise experimentation to test fine models of molecular behavior are intended, but have not stood as obstacle to uncovering the important general trends in molecular nonlinearity enhancement. 

MAP has a non-centrosymmetric structure (24) P2- leading to four non-vanishing crystalline nonlinear coefficients. According to the previously defined two-level model, there are also four non-vanishing molecular nonlinearity coefficients which are linearly related to the crystalline coefficients. (3)... 

Int. J. Quantum Chem. special issue on Molecular Nonlinear Optics 43 (1) (1992) edited by M.A. Ratner and P.O. Lowdin. 

S. Kielich, Molecular Nonlinear Optics, Nauka, Moscow, 1986 (in Russian). 

X 10-12 esu Table XIV) are significantly less than 3)(to) obtained by DFWM (Table XIII). A difference of four orders of magnitude was also found with the cubic molecular nonlinearities of the mixed cobalt-iron cluster [CoFe2(/U3-S)(/U3-Se)(CO)6(77 -QH5)], a significant frequency dependence as Amax is at —550 nm, the larger value is likely to be significantly resonance enhanced. 

One purpose of this tutorial paper on optical characterization is to provide a brief introduction for chemists to the concepts and methods involved in studies of the nonlinear optical properties of molecules and materials. The intent is to familiarize chemists with the range of commonly used techniques and their physical basis. An attempt is made to provide some background on macroscopic nonlinear optics, relating to what is actually measured, and the connection to molecular nonlinear optical properties. This paper is not intended to be a detailed or comprehensive review. The reader is referred to introductory (1, 2) and advanced (3-6) texts on nonlinear optics for more detailed or complete coverage of the subject. 

Out of the large range of possible nonlinear optical effects, chemists are likely to encounter only a limited number of measurement techniques. These include both second- and third-order NLO characterization methods. A brief listing of the different types of measurements, the nonlinear susceptibility involved and the related molecular nonlinear polarizabilities is given here. 

In spite of the potential advantages, useful organic NLO materials have not yet been developed because the necessary molecular and macroscopic characteristics have only recently begun to be understood. However, because bulk NLO properties in organic materials arise directly from the constituent molecular nonlinearities, it is possible to decouple molecular and supramolecular contributions to the NLO properties. One can then semiquantitatively predict relative macroscopic nonlinearities based on theoretical analyses of the individual molecules (7). Reliable predictions of this kind are vital for the efficiency of a program aimed towards developing new organic materials with tailored NLO properties. 

The nonlinear refractive index, r, linear absorption, a, and two photon absorption coefficient, jS, of metal dithiolenes and rare earth metallocenes have been measured at 1064 nm using 100 ps and 10 ns pulses. These measurements have shown resonance enhanced molecular nonlinearities of 10M05 times those of carbon disulphide can be obtained. Figures of merit based on the nonlinear refractive index and the linear and two photon absorption coefficients are within the limits required for devices. 

The molecular nonlinearity, pp (determined by EFISH measurements in various solvents), of dithienothiophene-based chromophores is strongly influenced by solvent polarity and the acceptor strength (96CPL (255)147 99MI2227 02JOC205 02MI559 04CEJ3805). 

Cheng, L. T., Tam, W., Stevenson, S. H., Meredith, G. R., Rikken, G., Marder, S. R., Experimental investigations of organic molecular nonlinear optical polarizabilities. 1. Methods and results on benzene and stilbene derivatives. J. Phys. Chem. 1991,95, 10631-10643. 

Zyss J (ed) (1994) Molecular nonlinear optics. Academic, New York... 

The bond length alternation approach describes only the position of the molecular nonlinearities on the BLA axis, but does not give an absolute magnitude of the curves in Fig. 18. Schemes as conjugation length expansion (see below) are believed to increase the magnitude of the curves in the bond length alternation picture. 

Dick B, Stegeman G, Twieg J,Zyss J (eds) (1999) Molecular Nonlinear Optics Materials, Phenomena, Devices, Special issue of Chem Phys 245... 

As can be readily ascertained from the tables in this section, researchers have mostly employed the EFISH or HRS techniques to measure second-order nonlinearities of organometallics. Although the initial interest in this field was in metal carbonyl-based complexes, the majority of the measurements are now of ferrocenyl-based complexes or metal acetylides. One reason for this may be pragmatic despite the vast panoply of possible complex types that exists, ferrocenyl or acetylide complexes can be synthesized in high yield by well-established methodologies, both revealed large molecular nonlinearities in initial studies, and both are (comparatively) oxidatively and thermally stable, the latter an important consideration for (putative) longer-term device applications. 

R.L. Sutherland Handbook of Nonlinear Optics (Dekker, New York, 1996) J. Zyss (Ed.) Molecular Nonlinear Optics (Academic, San Diego, 1994) H.G. Kuzyk, C.W. Dirk Characterization Techniques and Tabulations for Organic Nonlinear Optical Materials (Dekker, New York, 1998) G.P. Banfi, D. Fortusini, P. Dainesi, D. Grando, S. Sottini J. Chem. Phys. 108, 4319 (1998)... 

The area of molecular nonlinear optics has been rejuvenated by efforts to investigate three-dimensional multipolar systems, functionalized polymers as optoelectronic materials, near infrared optical parametric oscillators and related aspects.71 There have been some advances in chromophore design for second-order nonlinear optical materials 72 these include onedimensional CT molecules, octopolar compounds and organometallics. Some of the polydiacetylenes and poly(/>-phenylenevinylene)s appear to possess the required properties for use as third-order nonlinear optical materials for photonic switching.73... 

For the two previous series of compounds, only values of the molecular hyperpolarizability, [S0, have been determined. To our knowledge, no bulk nonlinear susceptibility, /l2 , values have been reported for these compounds. Actually, the X-ray structures reported for some of these compounds show that they crystallize in centrosymmetrical space groups, which results in canceling the molecular nonlinearities. 

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