Second harmonic generation manifestation

We have shown in this paper the relationships between the fundamental electrical parameters, such as the dipole moment, polarizability and hyperpolarizability, and the conformations of flexible polymers which are manifested in a number of their electrooptic and dielectric properties. These include the Kerr effect, dielectric polarization and saturation, electric field induced light scattering and second harmonic generation. Our experimental and theoretical studies of the Kerr effect show that it is very useful for the characterization of polymer microstructure. Our theoretical studies of the NLDE, EFLS and EFSHG also show that these effects are potentially useful, but there are very few experimental results reported in the literature with which to test the calculations. More experimental studies are needed to further our understanding of the nonlinear electrooptic and dielectric properties of flexible polymers. 

It may not be a coincidence that nitroanilines have been a useful class of compounds for second harmonic generation (12-13). They naturally associate into polar arrays, which are manifested in their crystal structures. The important question is whether or not these polar arrays are likely to bias the resulting crystal structures to be acentric. Although we do not have large enough data sets to make a statistically convincing case, from a set of 32 primary and secondary nitroanilines and analogs, about 40% of their crystal structures are acentric. These compounds are all capable of... 

The dielectric tensor describes the linear response of a material to an electric field. In many experiments, and particularly in optical rheometry, anisotropy in is the object of measurement. This anisotropy is manifested as birefringence and dichroism, two quantities that will be discussed in detail in Chapter 2. The nonlinear terms are responsible for such effects as second harmonic generation, electro-optic activity, and frequency tripling. These phenomena occur when certain criteria are met in the material properties, and at high values of field strength. 

The catalytic activity of the VO(stearate)2-L/o tartrate system with the suggested presence of a dinuclear oxidovanadium(IV) tartrato complex leads to an enanti-oselective oxidative coupUng polymerization of 2,3-dihy-droxynaphthalene [13]. Very recently, the second harmonic generation and spin-dimer behavior were observed for oxovanadium(IV) tartrates [14], In addition, a study appeared on an interaction of tartratovanadates with chiral Fe(II) and Ni(II) tris(2,2 -bipyridine) complexes manifesting itself with the solid state by packing into homochiral layers in the crystal structure [15]. 

One aspect of Eq. (90) deserving comment is its amenability for the identification of resonances. Three-photon resonances are manifest in the first and second terms, through the appearance of the factor (Euo — 3to — zTu) two-photon resonances (Euo — 2to — iTu) are featured in the second and fourth, and single-photon resonances (Euo — to — iTu) are seen in each of the first six. Since exploitation of the latter kind of resonance is in practice usually avoided because of the competing linear absorption with which it is associated, it is the two- and three- photon resonances that are of the most interest. Under suitable conditions, third-harmonic generation in either of those cases is driven largely by just two of the contributions to Eq. (90). Other contributions, signifying... 

Further features are evident when the relative magnitudes of the dipole difference d and the transition dipole p ° are considered. One immediately striking feature is the observation that the second, fourth, sixth, and eighth terms all disappear if d = 0, leaving only terms associated with virtual excitation routes. [Note that no such routes were manifest in the second-harmonic result. If d = 0 then the entire expression Eq. (85) becomes zero—any process involving an odd number of photons has to entail at least one 00 or uu segment in the interaction sequence.] In the third-harmonic case, in particular, both terms associated with two-photon resonances disappear—in other words, there can be no two-photon resonance enhancement of third-harmonic generation under such circumstances. If, however, d p °, then the even terms of Eq. (90)... 

X and x are the second and third-order nonlinear optical susceptibilities. Two important manifestations of optical nonlinearities are harmonic generation and refractive fndex modulation by electric and optical fields. Their origin can conveniently be explained by considering a plane wave propagation through the nonlinear medium. The polarization is then given by... 

Pyroelectrics. In a crystal belonging to polar classes there is only one polar axis with a symmetry of the polar vector. These crystals are also piezoelectric, but, in addihon, manifest spontaneous polarization and all other polar properties. Such crystals are called pyroelectrics. An example is tourmaline having symmetry and shown in Fig. 13.1c. Pyroelectric crystals are also used in techniques as piezoelectrics and also as detectors of infrared light or a heat how. There are many organic pyroelectric crystals, e.g., p-nitroaniline, one of the best generators of the optical second harmonic. 

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