Polarization optically induced

The first and third order terms in odd powers of the applied electric field are present for all materials. In the second order term, a polarization is induced proportional to the square of the applied electric field, and the. nonlinear second order optical susceptibility must, therefore, vanish in crystals that possess a center of symmetry. In addition to the noncentrosymmetric structure, efficient second harmonic generation requires crystals to possess propagation directions where the crystal birefringence cancels the natural dispersion leading to phase matching. 

The optical transition moments for vibrational or electronic transitions between defect states have specific orientations with respect to the defect coordinates. The absorption strength of polarized light for each of the differently oriented centers is proportional to the square of the component of the transition moment that is along the polarization direction. Hence, a stress-induced redistribution of the defects among their different orientations will be detected as an anisotropy in the polarized optical absorption. A convenient measure of the anisotropy is the dichroic ratio, defined as... 

Figure 37 Schematic representation of the optically induced polarization in optically linear materials...

In the last few years, several workers have analyzed charge density distribution in molecular crystals with non-linear optical (NLO) properties [71-74]. The NLO response can, in principle, be explained by an anharmonic distortion of the electron density distribution due to the electric field of an applied optical pulse. The polarization P induced in a molecule is... 

The extensive jt-delocalized system of metal-dithiolene complexes is also responsible for the nonlinear optical properties (NLO) which have been recently reviewed . The interaction of radiation with the matter induces an instantaneous displacement (polarization Pq = /X = aE, where a is the linear polarizability) of the electronic density away from the nucleus at small field (linear optics). At high fields (laser light) the polarizability of the molecule can be driven beyond the linear regime and a nonlinear polarization is induced (NLO) = aE + fiE" + y E + and for the bulk material... 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

In polyurethane 33, the azobenzene moieties are separated from each other along the polymer backbone by isophorone units and have the free volume necessary for isomerization and molecular reorientation. This polymer s azobenzene moieties can be photoisomerized readily in mixed LBK films, as demonstrated by measuring the optically induced birefringence that originates from the photoreorientation of the chromophore upon polarized irradiation see Section 6.5.2). 

Another mechanism responsible for the optically induced anisotropy is angular redistribution (AR) of molecules. This mechanism has been widely developed to explain photoinduced birefringence and dichroism. In most experimental cases, there is evidence of some rotation of molecules during the photoisomerization cycle (see Reference 2, for example). This rotation results in AR, because the molecules remain longer in states with lower excitation probability, and so more molecules are accumulated perpendicular to the pump polarization. The AR process is initiated by the AHB, and these two processes should be studied simultaneously in the framework of general... 

Stracke, A., Bayer, A., Zimmermaim, S., Wendorff, J. H., Wirges, W., Bauer-Gogonea, S., Bauer, S., and Gerhard-Multhaupt, R. Relaxation behaviour of electrically induced polar orientation and of optically induced non-polar orientation in an azo-chromophore side group polymer. J. Phys. D-Appl. Phys. 1999, 32, pp. 2996-3003. 

Figure 2.1. Polarization (P) induced in a molecule s electron cloud by an incident optical electric field E. Scattering may be in various directions, but 90° and 180° are shown.

Figure 1.4 Optically induced interparticie energy surface on the (y, z) plane of Figure 1.3. The scale is in /ATts c units. Field polarization directed along the x-axis, wavevector on thez-axis. Copyright (2008) by the American Physical Society.

Figure 1.5 Optically induced potential energy landscapes for four identical interacting particles as a function of the vector positions of one of them, when the other three are at (0, 0, 0), (0, 0, 3), and (0, 0, 6). Scale in lla k lAits c units. Polarization and wavevector as in Figure 1.3. Black circular shapes represent local divergences in energy shift in the proximity of the fixed particles. Copyright (2008) by the American Physical Society.

For a nonresonant process, light is not absorbed by the sample. The optical nonlinearity basically originates from the anharmonicity of the electronic system. Traditionally, the nonresonant optical nonlinearity of a material is described via the perturbational approach. In this approach, the polarization, Pd, induced in a medium by an external electromagnetic field, E, can be written as... 

Microscopy. The polarized optical micrographs of thin films of HX-205 and F-185 neat resins were obtained using a Zeiss ultraphot microscope equipped with a polarizer and an analyzer. Thin films, approximately 100 microns thick, were prepared by thln-sectlonlng the resin sheet with a razor blade at room temperature. The domains were observable because of light scattering as a result of refractive Index mismatch between the rubber domain and the epoxy matrix, as well as to stress-induced birefringence produced by the thermal stress Imposed on the domains. 

The second-order property on which we focus in this work is optical rotation, i.e., the rotation of plane-polarized light induced by non-racemic sample of a chiral... 

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