Optical nonlinearity, third-order

Nonlinear refraction phenomena, involving high iatensity femtosecond pulses of light traveling in a rod of Tfsapphire, represent one of the most important commercial exploitations of third-order optical nonlinearity. This is the realization of mode-locking ia femtosecond Tfsapphire lasers (qv). High intensity femtosecond pulses are focused on an output port by the third-order Kerr effect while the lower intensity continuous wave (CW) beam remains unfocused and thus is not effectively coupled out of the laser. 

In an effort to identify materials appropriate for the appHcation of third-order optical nonlinearity, several figures of merit (EOM) have been defined (1—r5,r51—r53). Parallel all-optical (Kerr effect) switching and processing involve the focusing of many images onto a nonlinear slab where the transmissive... 

Winter, C. S. Oliver, S. N. Rush, J. D. Hill, C. A. S. Underhill, A. E. Third-order Optical Nonlinearities in Metal Dithiolate Complexes. In Organic Molecules for Nonlinear Optics and Photonics Messier, J., Kajzar, F., Prasad, P., Eds. NATO ASI Series E, 194 Kluwer Boston, 1991 pp 383-390. 

Nonintuitive Light Propagation Effects In Third-Order Experiments. One of the first tasks for a chemist desiring to quantify second- and third-order optical nonlinear polarizability is to gain an appreciation of the quantitative manifestations of macroscopic optical nonlinearity. As will be shown this has been a problem as well for established workers in the field. We will present pictures which hopefully will make these situations more physically obvious. 

Here, we demonstrate that oriented PAV films with well-developed -conjugated system can be fabricated through the regulation of orientation of precursor polymer chains by use of the Langmuir-Blodgett technique, and that large and anisotropic third-order optical nonlinearity was observed in the oriented PAV films. 

Third-order optical nonlinearity of the oriented MOPPV LB film was evaluated by the THG measurement [30]. The detailed procedure is described in the section 2.1. 

The process of THG is driven by third-order optical nonlinearity, which defines the nonlinear optical polarizability at the frequency 3[Pg.128]

A. Microscopic and Macroscopic Third-Order Optical Nonlinearities... 

A variety of experimental techniques have been used to obtain information about the third-order optical nonlinearities and optical power limiting behavior of materials. This section includes descriptions of those techniques that have been used or have potential use with organometallics. For an excellent source of information about other techniques, the interested reader is directed to Ref. 6. 

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>. 

Second-order optical nonlinearities result from introduction of a cubic term in the potential function for the electron, and third-order optical nonlinearities result from introduction of a quartic term (Figure 18). Two important points relate to the symmetry of this perturbation. First, while negative and positive p both give rise to the same potential and therefore the same physical effects (the only difference being the orientation of the coordinate system), a negative y will lead to a different electron potential than will a positive y. Second, the quartic perturbation has mirror symmetry with respect to a distortion coordinate as a result, both centrosymmetric and noncentrosymmetric materials will exhibit third-order optical nonlinearities. If we reconsider equation 23 for the expansion of polarization of a molecule as a function of electric field and assume that the even-order terms are zero (i.e., that the molecule is centrosymmetric), we see that polarization at a given point in space is ... 

The two important consequences of the third-order optical nonlinearities represented by x are third-harmonic generation and intensity dependence of the refractive index. Third-harmonic generation (THG) describes the process in which an incident photon field of frequency (oj) generates, through nonlinear polarization in the medium, a coherent optical field at 3a>. Through x interaction, the refractive index of the nonlinear medium is given as n = nQ+n I where n describes intensity dependence of the refractive index ana I is the instantaneous intensity of the laser pulse. There is no symmetry restriction on the third-order processes which can occur in all media including air. 

In Equation 6, n (a>.) is the intensity independent refractive index at frequency u).,.0 Tlie sum in Equation 5 is over all the sites (n) the bracket, < >, represents an orientational averaging over angles 0 and . Unlike for the second-order effect, this orientational average for the third-order coefficient is nonzero even for an isotropic medium because it is a fourth rank tensor. Therefore, the first step to enhance third order optical nonlinearities in organic bulk systems is to use molecular structures with large Y. For this reason, a sound theoretical understanding of microscopic nonlinearities is of paramount importance. 

Structural Requirements for Third-Order Optical Nonlinearity... 

Although one loosely uses a x value for a material, in reality there are a number of relevant parameters which describe the third order optical nonlinearities. These parameters are ... 

Our theoretical understanding of third-order optical nonlinearity at the microscopic level is really in its infancy. Currently no theoretical method exists which can be reliably used to predict, with reasonable computational time, molecular and polymeric structures with enhanced optical nonlinearities. The two important approaches used are the derivative method and the sum-over-states method (7,24). The derivative method is based on the power expansion of the dipole moment or energy given by Equations 3 and 4. The third-order nonlinear coefficient Y is, therefore, simply given by the fourth derivative of the energy or the third derivative of the induced dipole moment with respect to the applied field. These... 

To conclude this article, it is hoped that the discussion of relevant issues and opportunities for chemists presented here will sufficiently stimulate the interest of the chemical community. Their active participation is vital for building our understanding of optical nonlinearities in molecular systems as well as for the development of useful nonlinear optical materials. It is the time now to search for new avenues other than conjugation effects to enhance third-order optical nonlinearities. Therefore, we should broaden the scope of molecular materials to incorporate inorganic and organometallic structures, especially those involving highly polarizable atoms. 

The increasing use of optical fibre in the telecommunications network will, ultimately, require all-optical signal processing to exploit the full bandwidth available. This has led to a search for materials with fast, large third order optical nonlinearities. Most of the current materials either respond in the nanosecond regime or the nonlinearity is too small (1-3). Organic materials are attractive because of their ultra-fast, broadband responses and low absorption. However the main problem in the materials studied to date, e.g. polydiacetylenes (4) and aromatic main chain polymers (5), has been the small nonlinear coefficients. 

Partially substituted derivatives of polyacetylene are synthesized via the ring-opening metathesis polymerization (ROMP) of cyclooctatetraene (COT) and its derivatives. Certain poly-COT derivatives afford soluble, highly conjugated poly acetylenes. These materials exhibit large third-order optical nonlinearities and low scattering losses. 

Table TV. Summary of Composition and Third-Order Optical Nonlinearities at 1907 nm...

It is also revealed that the intramolecular charge transfer derived from substituted donor and acceptor contributes to the increase in the third-order optical nonlinearity of dye-attached polymer systems. values larger than 10 10 esu can be achieved even in relatively short Tt-electron conjugated dye-attached polymers. 

Cyanine dyes which possess large third-order optical nonlinearities are found and they are incorporated into a polymer to fabricate a thin film for device applications. The %(3) is 2 x 10"11 esu for a polymer film with 50 wt% cyanine dye. 

Acceptor species concentrations, equations, 400-401 Acentric materials biomimetic design, 454-455 synthesis approaches, 446 Ar-(2-Acetamido-4-nitrophenyl)pyrrolidene control of crystal polymorphism with assistance of auxiliary, 480-482 packing arrangements, 480,481-482/ Acetylenes, second- and third-order optical nonlinearities, 605-606 N-Acetyltyrosine, phase-matching loci for doubling, 355,356/, t Acid dimers, orientations, 454 Active polymer waveguides, applications, 111... 

Poly(3-alkyl-a-thiophene) systems show significant third-order nonlinear susceptibilities ( ) Though, oligothiophenes have been studied for their third-order susceptibilities, accurate third-order optical nonlinearity data obtained by degenerate four-wave mixing or electric-field-induced second harmonic generation (EFISH) are difficult to attain reliably on samples with poor solubility characteristics (92MM1901). 

The coplanarity has endowed arylated TEEs with some of the highest known third-order optical nonlinearities and, in the case of acentricity, also very large second-order nonlinear optical effects. Furthermore, the strain-free planarity allows cis- and trans-arylated TEEs to interconvert upon photochemical excitation without competition from undesirable thermal isomerization. 

May JC, Lim JH, Biaggio I, Moonen NNP, Michinobu T, Diederich F (2005) Highly efficient third-order optical nonlinearities in donor-substituted cyanoethynyl-ethene molecules, Optics Lett. 30 (in press)... 

Kamada K, Ueda M, Sakaguchi T, Ohta K, Fukumi T. Femtosecond optical Kerr study of heavy atom effects in the third-order optical nonlinearity of thiophene homologues purely electronic contribution. Chem Phys Fett 1996 263 215-222. 

Some heteroleptic dithiolene complexes studied for their third-order NLO properties and involving the dmit or the mnt ligands are shown in Scheme 27 (455, 456). Only the dmit-containing complexes exhibit a large third-order optical nonlinearity, due to their larger planar conjugated system compared to the mnt analogues. 

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