Nonlinear optics, explanation

A novel second-order nonlinear optical medium which should offer considerable fabrication flexibility has been described. The physics of alignment of the highly nonlinearly polarizable moiety was discussed. However, observation of complex dynamical and thermal behavior indicates that an important role is played by the polymer liquid crystalline host. Additional properties of modified members of this family of lc polymers were consequently investigated. The explanations of guest alignment stabilization and thermal dependence of the alignability remain unresolved issues. 

Similar conclusions concerning the relation between the relative molecular orientation and the first hyperpolarizability have been drawn by Hamada [13] in the case of the MNA dimer as well as by Okuno et al. [53] in a study on linear and parallel dimers of 4-dimethylamino-4 -carboxyazobenzene and 4-dimethyl-amino-2 -nitro-4 -carboxyazobenzene. The impetus was the desire for an explanation for the large second-order nonlinear optical responses of cone-shaped azobenzene dendrimers. Okono et al. also assessed the adequacy of three classical electrostatic models (Section IV). 

Bisquaric acid was also studied by 13C CPMAS NMR between 123 and 523 K, with powdered crystals.173 This material has also potential for nonlinear optical and dielectric applications. The low-temperature spectra resolve three peaks instead of four in SQA. This compound has no dipole moment, and no phase transition was detected in the studied temperature range, although the lineshape suggests the occurrence of a phase transition below 373 K. An explanation proposed by the authors is the lack of resolution due to the accidental overlapping of the two resonances of the C OH and C — O carbons participating to the HBs, an interpretation also supported by GIAO ab initio chemical shift calculations. 

In the introduction to this chapter we gave an intuitive explanation of the origin of nonlinear optical effects and stressed the key role played by high power lasers and coherent light beams. These two concepts are defined here. We will describe one specific characteristic of laser light, namely the absorption saturation, and finally we will discuss susceptibility and frequency conversion of light. 

For every nonlinear optical effect, one would expect that there is a measurement technique to characterize it. Not all of NLO effects, however, are subject to measurements that are convenient or informative for comparative purposes or device applications. This section highlights a few of the more common test methods for NLO organic molecules and polymers, and provides references for more detailed explanations of these techniques. Significant omissions here are techniques based on the linear and quadratic electrooptic effect, which are discussed in the article Electrooptical Applications. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

The optical bleaching by stored electrons is the basis for the occurrence of strong optical nonlinearity observed in Q-particles [64]. The physical reason for this optical bleaching is still not discussed conclusively in literature. The most obvious explanation comes from a state filling model. The stored electrons occupy the lowest electronic levels in the conduction band and, consequently, the optical transition has to occur to higher electronic levels (i.e., at shorter wavelength). This effect is known in solid-state semiconductor physics as the Burstein shift [65]. Other theoretical models describe the optical bleaching as a consequence of the polarization of the exciton in the electric field of the stored electron, which is then... 

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