Molecular alignment nonlinear materials

Utilization of the second-order nonlinearity of a material requires that the nonlinear molecules or side-groups be arranged in a non-centro-symmetric configuration. In the previous section it was pointed out that the more commonly used polymer deposition techniques resulted in an isotropic, amorphous film, and would thus show no second-order nonlinear properties. However, controlled deposition and/or subsequent processing can be employed to create a degree of molecular alignment. 

In terms of beam delivery, the DLW method is based on optical microscopy, confocal microscopy [4,6,13] and laser tweezers [14] (for reviews on laser tweezers see [ 15,16]). These techniques allow for a high spatial 3D resolution of a tightly focused laser beam with optical exposure of micrometric-sized volumes via linear and nonlinear absorption. In addition, mechanical and thermal forces can be exerted upon objects as small as 10 nm molecular dipolar alignment can be controlled by polarization of light in volumes of with submicrometric cross-sections. This circumstance widens the field of applications for laser nano- and microfabrication in liquid and solid materials [17-22]. 

Since the dipoles of chromophore molecules are randomly distributed in an inert organic matrix in amorphous PR materials, the material is centrosymmet-ric and no second-order optical nonlinearity can be observed. However, in the presence of a dc external field, the dipole molecules tend to be aligned along the direction of the field and the bulk properties become asymmetric. Under the assumption that the interaction between the molecular dipoles is negligible compared to the interaction between the dipoles and the external poling field (oriented gas model), the linear anisotropy induced by the external field along Z axis at weak poling field limit (pE/ksT <[Pg.276]

However such benefits are obtained at expenses of some additional fabrication procedures. After deposition, organic materials are centrosymmetric on a macroscopic scale and they can not be endowed of second order nonlinear properties. Poling, i.e. the orientation of the microscopic molecular dipoles, is required in order to break this symmetry. One of the major challenges concerns the effective translation of high molecular nonlinearities (/ry3), where /r is the chromophore dipole moment and p is the first molecular hyperpolarizability, into large macroscopic EO activities rss) in poled polymers with high alignment temporal stability [15,16]. 

The EFISH method (Singer and Garito, 1981) permitted for the first time the establishment of a correlation between molecular structure of organic chromophores and the first hyperpolarizability p. In this method an electric field is applied to a solution of the nonlinear optical materials, resulting in an alignment of the dipoles. A direct determination of P with the EFISH method is not possible the third-order polarization y is measured, the dipole moment p. must be known, and with these values the hyperpolarizability p can be calculated. The EFISH technique is not readily applied to salts as the solutions conduct electricity. 

Our interest in bowlic liquid crystals has arisen from the proposal that bowl shaped molecules may exhibit polar (noncentrosymmetric) organization in the liquid crystalline phases [4, 8, 9]. Indeed bowlic liquid crystals are natural noncentrosymmetric building blocks since a head-to-tail organization maximizes the interactions between bowlic cores. New methodologies for the creation of noncentrosymmetric structures in molecular solids and liquids are critical to the development of new materials with ferroelectric and second order nonlinear optical (NLO) properties [14, 15]. Liquid crystalline methods are particularly attractive since liquid crystalline materials are easily deposited for device construction and are readily aligned. 

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