Nonlinear optics chiral molecules

Koroteev N I 1995 BioOARS—a novel nonlinear optical technique to study vibrational spectra of chiral biological molecules in solution Biospectroscopy 1 341-50... 

Given the interest and importance of chiral molecules, there has been considerable activity in investigating die corresponding chiral surfaces [, and 70]. From the point of view of perfomiing surface and interface spectroscopy with nonlinear optics, we must first examhie the nonlinear response of tlie bulk liquid. Clearly, a chiral liquid lacks inversion synnnetry. As such, it may be expected to have a strong (dipole-allowed) second-order nonlinear response. This is indeed true in the general case of SFG [71]. For SHG, however, the pemiutation synnnetry for the last two indices of the nonlinear susceptibility tensor combined with the... 

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. 

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. 

The systems discussed up to now all showed chiral susceptibilities that were of the same order of magnitude or smaller than the achiral susceptibility components. The system that we discuss in this section has chiral susceptibilities that dominate the nonlinear optical response.53 The material is a chiral helicenebisquinone derivative shown in Figure 9.22. In bulk samples, the nonracemic, but not the racemic, form of the material spontaneously organizes into long fibers clearly visible under an optical microscope. These fibers comprise columnar stacks of helicene molecules.54,55 Similar columnar stacks self-assemble in appropriate solvents, such as n-dodecane, when the concentration exceeds 1 mM. This association can be observed by a large increase in the circular dichroism (CD) of the solutions. 

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... 

Keywords chiral molecules optical activity pseudoscalars liquids second-order nonlinear optics ... 

Sum-frequency generation (SFG) at second-order and the nonlinear Raman spectroscopy BioCARS at fourth-order can also probe chiral molecules. They have no analog in linear optics. We show that both are only symmetry allowed in a fluid, if the fluid is chiral. However, in contrast to optical activity phenomena, these processes arise entirely from induced electric-dipoles (without magnetic or quadrupolar transitions) and they are not circular differential. All laser beams can be linearly polarized and no polarization modulation is required as the detection of a sum-frequency (yiz. five-wave mixing) photon is in itself a measure of the solution s chirality. Since an achiral solvent can not contribute to the signal, these techniques are sensitive, background-free probes of molecular chirality. The SFG... 

Most linear optical phenomena such as refraction, absorption and Rayleigh scattering are described by the first term in Eq. (1) where is the linear susceptibility tensor. The higher order terms and susceptibilities are responsible for nonlinear optical effects. The second-order susceptibility tensor T underlies SFG, whereas and BioCARS arises within As we are concerned with optical effects of randomly oriented molecules in fluids, we need to consider unweighted orientational averages of the susceptibility tensors in Eq. (1). We will show that the symmetries of the corresponding isotropic components and correspond to time-even pseudoscalars the hallmark of chiral observables [2]. 

This chapter is organized as follows In Section 2 we discuss the general symmetry requirements of chiroptical processes in isotropic media. In particular, we consider linear and nonlinear optical activity and we describe how frequency conversion at second-order (and at fourth-order) is specific to chiral molecules in fluids. In Section 3 we discuss our work on SFG in optically active solutions, and the computation of the SFG pseudoscalar is described in Section 4. Results from recent computations are given in Section 5. Conclusions are drawn in Section 6. 

The isotropic part of the first hypcrpolarizability, fi, may be calculated using a number of methods - ranging from a simple single-centre chiral molecular orbital approach to ab / z7/(9 alculations at varying levels of approximation (see Section 4). We have computed fi for a number of chiral molecules in order to establish the typical strength and frequency dispersion of the pseudoscalar [30, 54, 59, 71]. fi is a measure of tlie signal strength in SFG experiments and a convenient measure that facilitates direct comparison with achiral second-order nonlinear optical processes. 

Fischer and Champagne present an overview of linear and nonlinear optical properties of chiral molecules in isotropic media. The authors state the general symmetry requirements of chiroptical processes, and show that nonlinear chiral spectroscopies can arise within the electric dipole approximation. The authors describe sum-frequency-generation experiments at second order and demonstrate how nonlinear optics can be used to determine the absolute conformation of a chiral molecule in solution. This is discussed with recourse to electric-field induced... 

The functions of bioniolecules, bioactive molecules, ferroelectronic crystalline liquids, organic nonlinear optical molecules and so on often arise from their chirality. This means that the development of methods for obtaining enantiopure compounds is very important. Many methods have been reported for obtaining enantiopure compounds and these can be roughly classified into two categories, enantioseparation and asymmetric synthesis. Each category is further classified into physical, chemical, and biological methods. 

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