Nonlinear optical materials second-order effects

Nonlinear Optical Applications. Second-order nonlinear (NLO) materials based on ElAPs have shown promise for use in the photonics industry extensive research has been conducted in this area over the past several decades (356,357). Devices based on poly(diacetylene) have been used to demonstrate all-optical switching at 1.6 tm (358). The NLO process occurs when an electromagnetic field interacts with a medium. When the medium is subjected to an electric field E(0) and an optical field E(co), the nonlinear effect arises from field-induced... 

By comparison with an octadecyloxy stilbazium iodide monolayer (x<2> = 0.51 x 10"6 esu) [15], the value of the effective second-order susceptibility x r at 45° incidence was estimated to be 1.0 x 10"7 esu. The value is fairly large, in comparison with those of the conventional nonlinear optical materials (e.g. LiNbOs). Other pyrazine derivatives (C120PPy and C12SPPy) also gave thick noncentrosymmetric LB films with fairly large second-order nonlinearity by the alternating deposition with arachidic acid. The estimated Xeff values of the pyrazine LB films are listed in Table 3. 

Electrooptic displays, liquid crystal polymers in, 75 110 Electrooptic effect, 74 675 Electrooptic modulation, second-order nonlinear optical materials for, 77 444... 

Two of the most important nonlinear optical (NLO) processess, electro-optic switching and second harmonic generation, are second order effects. As such, they occur in materials consisting of noncentrosymmetrically arranged molecular subunits whose polarizability contains a second order dependence on electric fields. Excluding the special cases of noncentrosymmetric but nonpolar crystals, which would be nearly impossible to design from first principles, the rational fabrication of an optimal material would result from the simultaneous maximization of the molecular second order coefficients (first hyperpolarizabilities, p) and the polar order parameters of the assembly of subunits. (1)... 

In this review, the focus is on electro-optic materials. Such materials are members of the more general class of second-order nonlinear optical materials, which also includes materials used for second harmonic generation (frequency doubling). The term second-order derives from the fact that the magnitude of these effects is defined by the second term of the power series expansion of optical polarization as a function of applied electric fields. The power series expansion of polarization with electric field can be expressed either in terms of molecular polarization (p Eq. 1) or macroscopic polarization (P Eq. 2)... 

The device performance of any application in the real world has to be guaranteed for a certain time without or only minor degradation. Some 10,000 hours of operation at temperatures up to 80 °C under illumination are required. Research about the lifetime of organic nonlinear optical material has only started in the past years as the chances for applications based on the second-order susceptibilities x have improved. Nevertheless, only little is known about the longtime effects in nonlinear materials under real world conditions. 

Cascading. In most cases, the distinction between second- and third-order nonlinearities is evident from the different phenomena each produce. That distinction blurs, however, when one considers the cascading of second-order effects to produce third-order nonlinear phenomena (51). In a cascaded process, the nonlinear optical field generated as a second-order response at one place combines anew with the incident field in a subsequent second-order process. Figure 2 shows a schematic of this effect at the molecular level where second-order effects in noncentrosymmetric molecules combine to yield a third-order response that may be difficult to separate from a pure third-order process. This form of cascading is complicated by the near-field relationships that appear in the interaction between molecules, but analysis of cascaded phenomena is of interest, because it provides a way to explore local fields and the correlations between orientations of dipoles in a centros5nnmetric material (52). 

Combination with Static Fieids. A common technique, useful for optoelectronic devices, is to combine a monochromatic optical field with a DC or quasistatic field. This combination can lead to refractive index and absorption changes (linear or quadratic electrooptic effects and electroabsorption), or to electric-field induced second-harmonic generation (EFISH or DC-SHG, 2o) = co + co + 0) in a quasi-third-order process. In EFISH, the DC field orients the molecular dipole moments to enable or enhance the second-harmonic response of the material to the applied laser frequency. The combination of a DC field component with a single optical field is referred to as the linear electrooptic (Pockels) effect (co = co -I- 0), or the quadratic electrooptic (Kerr) effect ( = -I- 0 -I- 0). EFISH is discussed in this article, however, for the important role that it has played in the characterization of nonlinear optical materials for other applications. 

Di Bella, S. Fragala, I. Ratner, M. A. Marks, T. J. Chromophore environmental effects in saltlike nonlinear optical materials. A computational study of architecture/anion second-order response relationships in high-p stilhazolium self-assembled films. Chem. Mater. 1 5, 7,400-404. 

The summation runs over repeated indices, /r, is the i-th component of the induced electric dipole moment and , are components of the applied electro-magnetic field. The coefficients aij, Pijic and Yijki are components of the linear polarizability, the first hyperpolarizability, and the second hyperpolarizability tensor, respectively. The first term on the right hand side of eq. (12) describes the linear response of the incident electric field, whereas the other terms describe the nonhnear response. The ft tensor is responsible for second order nonlinear optical effects such as second harmonic generation (SHG, frequency AotAAin, frequency mixing, optical rectification and the electro-optic effect. The ft tensor vanishes in a centrosymmetric envirorunent, so that most second-order nonlinear optical materials that have been studied so far consists of non-centrosyrmnetric, one-dimensional charge-transfer molecules. At the macroscopic level, observation of the nonlinear optical susceptibility requires that the molecular non-symmetry is preserved over the physical dimensions of the bulk stmcture. 

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