Optical properties parametric processes

A few further general remarks are in order at this stage. One is to note the fact that the sum over intermediate molecular states, as in Eqs. (76) and (79), in principle applies not only to electronic but also to vibrational levels. Although this issue initially received most attention in connection with molecular hyperpolarizabilities [44], it applies equally to other optical response tensors. The vibrational contributions, which were previously largely overlooked, have now been extensively studied and shown to be important in many applications [45,46]. Second, the polarizabilities associated with nonlinear parametric processes may in most circumstances be regarded as properties of the ground-state molecule, since it is the molecular ground state that usually constitutes the... 

We have heretofore had ample discussion of linear optical properties of ZnO and related materials. In this section, the nonlinear processes in ZnO are discussed, a topic that has been investigated in some detail. The research on nonlinear optical properties of semiconductors is motivated by electro-optic devices that can be used in telecommunications and optical computing as efficient harmonic generators, optical mixers, and tunable parametric oscillators, among others. The nonlinear optical properties such as second harmonic generation (SHG), that is, (2(0i, 2(02), and the sum frequency generation (SFG), that is, (materials characterization, particularly surfaces, because the second-order susceptibility coefficient is very sensitive to the change in symmetry (178,179). The crystal should be... 

One property that makes octopoles interesting is the lack of dipole moment in the ground state. This should increase the probability of non-centrosymmetric crystallization and prevent detrimental dipolar (aggregate) interaction. In addition, the ratio of off-diagonal versus diagonal P tensor components is higher than that for traditional dipolar systems, which opens up the possibility of less stringent polarization schemes for parametric and electro-optic processes [95]. Furthermore, it has been shown that the efficiency-transparency trade-off favors octopolar molecules over traditional dipolar molecules [96]. 

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