Molecular crystals, nonlinear optical

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

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... 

Molecular Optics Nonlinear Optical Properties of Organic and Polymeric Crystals... 

Except through the study of linear and nonlinear optical properties of molecular crystals, methods to determine the nature of / require evaluation of appropriate characteristics of... 

However, its was found possible to infer all four microscopic tensor coefficients from macroscopic crystalline values and this impossibility could be related to the molecular unit anisotropy. It can be shown that the molecular unit anisotropy imposes structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from crystal symmetry. Such additional relations appear for crystal point group 2,ra and 3. For the monoclinic point group 2, this relation has been tested in the case of MAP crystals, and excellent agreement has been found, triten taking into account crystal structure data (24), and nonlinear optical measurements on single crystal (19). This approach has been extended to the electrooptic tensor (4) and should lead to similar relations, trtten the electrooptic effect is primarily of electronic origin. 

Molecularlv Doped Thermotropic Liquid Crystalline Polymer. The idea of the nonlinear optical medium which is the subject of this paper results from a synthesis of the ideas of the discussion above and a few concepts from nonlinear optical molecular and crystal physics. As discusssed several places in this volume, it is known that certain classes of molecules exhibit tremendously enhanced second-order... 

A simple calculation for urea by Spackman is instructive. Urea crystallizes in an acentric space group (it is a well-known nonlinear optical material), in which the symmetry axes of the molecules coincide with the two-fold axes of the space group. All molecules are lined up parallel to the tetragonal c axis. If the electric field is given by E, and the principal element of the diagonalized molecular polarizability tensor along the c axis by oc , the induced moment along the polar c axis is... 

In this paper, an overview of the origin of second-order nonlinear optical processes in molecular and thin film materials is presented. The tutorial begins with a discussion of the basic physical description of second-order nonlinear optical processes. Simple models are used to describe molecular responses and propagation characteristics of polarization and field components. A brief discussion of quantum mechanical approaches is followed by a discussion of the 2-level model and some structure property relationships are illustrated. The relationships between microscopic and macroscopic nonlinearities in crystals, polymers, and molecular assemblies are discussed. Finally, several of the more common experimental methods for determining nonlinear optical coefficients are reviewed. 

Several organometallic moieties serving as acceptors and donors have been evaluated. Molecular crystals of stilbenes have been shown to be unusually active in second harmonic generation. The linear and nonlinear optical properties of 3-methyl-4-methoxy-4 -nitro-stilbene (MMONS) single crystals will be presented. 

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)... 

Another method is to use the molecules with relatively small ground state dipole moment (2). Weak dipole-dipole interaction can lead to non-centrosymmetric crystallization. However, it may decrease polarizability of molecules and result in low optical nonlinearities of molecular crystals. 

In this paper, we make use of molecular modelling techniques, particularly the AMI semiempirical molecular orbital method, to study the intermolecular interactions that are important for determining the manner in which crystal formation takes place. We are particularly inter ested in compounds that can potentially exhibit nonlinear optical properties. The calculational techniques are directed towards providing insight into the manner in which the desired nonlinear optical properties can be op timized in the macromolecular crystal state.(1)... 

Polymers and an Unusual Molecular Crystal with Nonlinear Optical Properties... 

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