Third-order optical nonlinearity processes

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... 

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

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. 

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. 

Lee, M., Kim, T.S., Choi, Y.S. Third-order optical nonlinearities of sol-gel-processed Au-SiO2 thin films in the surface plasmon absorption region. J. Non-Cryst. Solids 211, 143-149 (1997)... 

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... 

Polycondensation and imidization of w,w -diaminobenzophenone and pyromellitic dianhydride under microwave radiation was also carried out. The product polyimide was obtained in a two-step process. It is claimed that this product of microwave radiation polymerization compares favorably with a product of conventional thermal polymerization, because it exhibits third-order nonlinear optical coefficient of 1.642 x 10 esu and response time of 24 ps. The third-order optical nonlinearity of this polymer is dependent on the chain length and the molecular structure. 

Nonlinear optical properties of PTs which exhibit ultrafast responses and large nonlinearities attributed to one-dimensionality and delocalization of n-electrons along the polymer chains are also described [403,404]. Poly(4,4 -dipentoxy-2,2 -bithiophene) and poly(4,4 -dipentoxy-2,2 5, 2"-terthiophene) show a fast and high third-order nonlinearity [405]. Third-order nonlinearities depend on the nature of the polymer backbone and only slightly on the substituents [406], The optical transparency and the third-order optical nonlinearities can be tailored in random copolymers of 3-methylthiophene and methyl methacrylate [407]. A solution-processable thiophene copolymer with a side... 

From the view point of processing, hybrid materials fabrication is classified as mixing technique or impregnation process. The densification of the dye by making hybrid materials may increase of the third-order optical nonlinearity, as well as the mechanical and thermal strength. 

In 2006, Bernard et al. also examined centrosymmetric boron cluster-containing molecules. Although these molecules did not show second-order nonlinear optical effects because of their cen-trosymmetry, they are interesting candidates for third-order TPA nonlinear processes. Centrosymmetric materials are more suitable for TPA applications in the visible range than noncentrosymmetric ones, because of their lower coloration (Wang et al., 2001). The presence of donor substituents at each arm of tripodal systems increases the TPA properties (Brunei et al., 2001). Bernard et al. desaibed, for the first time, branched octupolar systems with three dodecaborate units (Figure 13.16). 

Kurihara, K., Tomaru, S., Mori, Y., Hikita, M., and Kaino, T., Third-order optical nonlinearities of a processible main chain polymer with symmetrically substituted tris-azo dyes, Appl. Phys. 

Frequency degenerate 2PA is a third order, y° nonlinear optical process whereby two photons of equal energy are simultaneously absorbed to raise a system into an excited state of energy equal to that of the sum of the two photons. The propagation... 

Through cascade second-order effects, the second-order optical nonlinearities result on a third-order optical effect in a multistep or cascade process. This process is a due to the existence of microscopic electric fields that are generated by second-order nonlinear aligning of molecular dipoles. 

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. 

Third-order nonlinear properties are also called ur-wave mixing (FWM), because they are related to the interaction of four electromagnetic waves. Although the third-order susceptibility is usually smaller than a high-intensity pump laser can render the third-order signal sufficiently strong to be observable. In the third-order optical process, the polarization can be written as described in Equation 3.131 ... 

Frequency-dependent polarizability a and second hyperpolarizability y corresponding to various third-order nonlinear optical processes have been... 

The term nonlinear optical property refers to an optical property, which can be modified by exposing the material to intense light irradiation. In this section, we focus on the cascaded first- (/ 1 ) and third-order ( / ) susceptibilities describing nonlinear absorption (ESA and 2PA) and nonlinear refraction (n2) processes. Z-scan, pump-probe, and two-photon upconverted fluorescence techniques are among the most used experimental methods for determining optical nonlinearities. 

Hales JM, Zheng S, Barlow S, Marder SR, Perry JW (2006) Bisdioxaborine polymethines with large third-order nonlinearities for all-optical signal processing. J Am Chem Soc 128 11362-11363... 

The third order nonlinear susceptibility is an important optical property of materials because of its contributions to numerous nonlinear optical processes. (1)(2) With the growing interest in all-op ical signal processing it has been proposed (3) recently that x ( 1, 2 3) and especially the degenerate third order nonlinear susceptibility x 3 (-w, to, 00) [defined as x (< >)], be utilized through its contributions to the changes in dielectric constant e with optical field strength E ... 

Up to now, many conjugated polymers have been found to possess large and very rapid third-order nonlinear optical response, which originates from the one-dimensionally delocalized ji-conjugation system along the polymer chain. Their application to the all optical signal processing devices has been expected. 

Aromatic poly(benzothiazole)s are thermally and thermooxidatively stable and have outstanding chemical resistance and third-order nonlinear optical susceptibility. Aromatic poly(benzothiazole)s can be spun into highly-oriented ultrahigh strength and ultrahigh modulus fibers. However, this type of polymer is insoluble in most organic solvents. Therefore, hexafluoroisopropylidene units are introduced in the polymer backbone to obtain soluble or processable aromatic poly(benzothiazole)s. 

Optical lithography, in compound semiconductor processing, 22 193 Optically active citronellol, 24 506 Optically transparent porous gel-silica, 23 75, 76 Optical materials nonlinear, 17 442-460 second-order nonlinear, 17 444—453 third-order nonlinear, 17 453-457 Optical memory, photochromic material application, 6 602 Optical microscopy, 16 467-487 history of, 16 467-469 in kinetic studies, 14 622 liquid immersion, 15 186 Optical mode density, 14 849, 850-852 Optical multichannel analyzers (OMAs), 23 143... 

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