Nonlinear optics experimental studies

During the evolution of the study of organic molecules for nonlinear optics, experimental observation has enabled certain structure/NLO property relationships to be developed, from which useful insights may be gained. Computational investigations using quantum theory have also af-... 

In this paper we will briefly review the findings of an earlier study of the properties of quasicrystals using SHG and present further experimental data to probe their structure and nonlinear optical properties. Finally, a model describing the behavior of quasicrystals in an external electric field is developed. 

In the following sections we will first in Section 2 briefly discuss the necessary background to understand optical activity effects in linear and nonlinear optics and to illustrate the similarities and differences between both types. In Section 3 we present a more thorough analysis of nonlinear optical effects in second-harmonic generation, both from a theoretical and an experimental point of view. Section 4 deals with experimental examples that illustrate the usefulness of nonlinear optical activity in the study of chiral thin films and surfaces. Finally, in Section 5 we give an overview of the role of chirality in the field of second-order nonlinear optics and show that chiral molecules can be useful for applications in this field. 

The experimental geometry to study nonlinear optical activity in SHG from... 

In this section, the experimental techniques described in the previous section are applied to the study of thin Langmuir-Blodgett films of chiral molecules and polymers. We will show in detail how the second-order susceptibility of chiral thin films can be analyzed and discuss the influence of chirality on the nonlinear optical response of these films. 

Binaphthyl molecules (chiral 2,2/-dihydroxy-l,T-binaphthyl) have been studied by Hicks et al., who reported the first experimental observation of CD-SHG on surfaces.4,29,43 In this section, we investigate the nonlinear optical properties of a chiral 1,1-binaphthyl-based helical polymer. 

From the form of the polarization it is clear that in order to observe any nonlinear optical effect, the input beams must not be copropagating. Furthermore, nonlinear optical effects through the tensor y eee requires two different input frequencies (otherwise, the tensor components would vanish because of permutation symmetry in the last two indices, i.e., ytfl eee = Xijy ) For example, sum-frequency generation in isotropic solutions of chiral molecules through the tensor y1 1 1 has been experimentally observed, and the technique has been proposed as a new tool to study chiral molecules in solution.59,61 From an NLO applications point of view, however, this effect is probably not very useful because recent results suggest that the response is actually very low.62... 

One result of studying nonlinear optical phenomena is, for instance, the determination of this susceptibility tensor, which supplies information about the anharmonicity of the potential between atoms in a crystal lattice. A simple electrodynamic model which relates the anharmonic motion of the bond charge to the higher-order nonlinear susceptibilities has been proposed by Levine The application of his theory to calculations of the nonlinearities in a-quarz yields excellent agreement with experimental data. 

Hyperpolarizabilities of molecules are an active field of study, because they determine a variety of important nonlinear optical effects. However, the deduction of hyperpolarizabilities from these nonlinear optical experiments is a very complicated business. The experiments are carried out at finite frequencies, so some extrapolation is required to obtain an estimate of the static (zero frequency) value. Also, there is a contribution to the hyperpolarizability from molecular vibrations, so even after extrapolation to zero frequency the experimental result may not correspond to what is desired. It is therefore very useful to study atoms, both theoretically and experimentally, since this vibrational contribution is necessarily absent. Calculations... 

Polysilane high polymers possessing fully saturated all-silicon backbone have attracted remarkable attention recently because of their unique optoelectronic properties and their importance in possible applications as photoresists, photoconductors, polymerization initiators, nonlinear optical materials etc. A number of review articles have been published on this topic4-9. The studies in this field have stimulated both experimental and theoretical chemists to elaborate on understanding the excited state nature of polysilanes and oligosilanes and of their mechanistic photochemistry. 

The magnetic oxide compound CUB2O4 hass attracted much attention because of its nature, such as successive phase transitions, the soliton lattice, etc. The properties have been studied by several experimental methods, involving the magnetic susceptibility [1], the specific heat [2], ESR [3, 4],, uSR [5], neutron diffraction [6, 7], nonlinear optics [8] etc. In these investigations, it has been found that the material undergoes... 

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