Second-order nonlinear optical molecules

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. 

SF radiation may be accompanied with inelastic nonlinear scattering, although the scattering cross section is extremely small, typically in the order of 10 cm" per molecule. Such an inelastic scattering effect in the second-order nonlinear optical... 

Second-Order Nonlinear Optical Processes in Molecules and Solids... 

The EO effect is a second-order nonlinear optical (NLO) effect. Only non-centrosymmetrical materials exhibit second-order NLO effects. This non-centrosymmetry is a condition, both at the macroscopic level of the bulk arrangement of the material and at the microscopic level of the individual molecule. All electro-optic modulators that are presently used by telecom operators are ferro-electric inorganic crystals. The optical nonlinearity in these materials is to a large fraction caused by the nuclear displacement in the applied electric field, and to a smaller fraction by the movement of the electrons. This limits the bandwidth of the modulator. The nonlinear response of organic materials is purely electronic and, therefore, inherently faster. 

However, non-centrosymmetry does not automatically imply a dipolar molecule, or, more generally, vectorial properties. Also molecules without a dipole moment can exhibit second-order nonlinear optical properties. Tetrahedral molecules, such as CC14, and trigonal molecules, such as BC13, also lack centrosymmetry. However, they cannot be oriented in an electric field, due to the absence of a dipole moment. Therefore, they can simply not be measured by EFISHG. Also ionic species cannot be measured, since these migrate, rather than rotate, under the influence of an applied field. 

Reviewed in Zyss J, Chemla DS (1987) Quadratic nonlinear optics and optimization of the second-order nonlinear optical response of molecular crystals. In Chemla DS, Zyss J (eds) Nonlinear and optical properties of organic molecules and crystals. Academic Press, Orlando, p 23... 

The area of molecular nonlinear optics has been rejuvenated by efforts to investigate three-dimensional multipolar systems, functionalized polymers as optoelectronic materials, near infrared optical parametric oscillators and related aspects.71 There have been some advances in chromophore design for second-order nonlinear optical materials 72 these include onedimensional CT molecules, octopolar compounds and organometallics. Some of the polydiacetylenes and poly(/>-phenylenevinylene)s appear to possess the required properties for use as third-order nonlinear optical materials for photonic switching.73... 

This has always been held to be true. However, in a limited number of examples, centric molecules do give rise to second-order nonlinear optical effects in the bulk. This is attributed to solid-state effects. See, for example. Ref (12). 

Lindsay, G. A. and Singer, K. D. (eds) (1995). Polymers for Second-Order Nonlinear Optics. ACS Symposium Series. American Chemical Society, Washington, DC Liptay, W. (1969). Angew. Chem. 81, 195 Angew. Chem. Int. Ed. Engl. 8, 177 Liptay, W. (1974). Excited States, Vol. 1. Dipole Moments and Polarizabilities of Molecules in Excited Electronic States (ed. E. C. Lim). Academic Press, New York, p. 129... 

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