Nonlinear optics molecular origins

However, its was found possible to infer all four microscopic tensor coefficients from macroscopic crystalline values and this impossibility could be related to the molecular unit anisotropy. It can be shown that the molecular unit anisotropy imposes structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from crystal symmetry. Such additional relations appear for crystal point group 2,ra and 3. For the monoclinic point group 2, this relation has been tested in the case of MAP crystals, and excellent agreement has been found, triten taking into account crystal structure data (24), and nonlinear optical measurements on single crystal (19). This approach has been extended to the electrooptic tensor (4) and should lead to similar relations, trtten the electrooptic effect is primarily of electronic origin. 

In this paper, an overview of the origin of second-order nonlinear optical processes in molecular and thin film materials is presented. The tutorial begins with a discussion of the basic physical description of second-order nonlinear optical processes. Simple models are used to describe molecular responses and propagation characteristics of polarization and field components. A brief discussion of quantum mechanical approaches is followed by a discussion of the 2-level model and some structure property relationships are illustrated. The relationships between microscopic and macroscopic nonlinearities in crystals, polymers, and molecular assemblies are discussed. Finally, several of the more common experimental methods for determining nonlinear optical coefficients are reviewed. 

The tutorial begins with a description of the basic concepts of nonlinear optics and presents illustrations from simple models to account for the origin of the effects. The microscopic or molecular origin of these effects is then discussed in more detail. Following this, the relationship between molecular responses and the effects observed in bulk materials are presented and finally some of the experimental methods used to characterize these effects are described. 

We have shown the molecular orbital theory origin of structure - function relationships for electronic hyperpolarizability. Yet, much of the common language of nonlinear optics is phrased in terms of anharmonic oscillators. How are the molecular orbital and oscillator models reconciled with one another The potential energy function of a spring maps the distortion energy as a function of its displacement. A connection can indeed be drawn between the molecular orbitals of a molecule and its corresponding effective oscillator . 

Optical nonlinearities can be explained by considering the interaction of strong electric fields with matter. If the fields have optical frequencies, the phenomena resulting from the nonlinear interactions are called nonlinear optical phenomena. Most texts on nonlinear optics (e.g., Refs. 22-25) begin the discussion of this area from considerations of macroscopic relations between the vector quantities P (the polarization vector), D (the displacement vector), and E (the electric field vector). Chemists, however, consider the molecular origin of physical phenomena, so the description of NLO phenomena that follows starts from consideration of the behavior of a single molecule in a strong electric field. 

Simpson GJ (2004) Molecular origin of the remarkable chiral sensitivity of second-order nonlinear optics. Chem Phys Chem 5 1301-1310... 

So, optimizing the nonlinearity of a material begins at the molecular structural level, which requires a detailed understanding of the origin of an induced nonlinear polarization and its relationship to the molecular electronic structure. For this reason, much attention has been paid to the theoretical calculations of the nonlinear optical response by ab initio and semi-empirical methods, providing the chemist with the information that indicates which synthetic strategy should be followed [16-24],... 

The nonlinear optical properties of rotaxanes and catenanes were studied mainly by three techniques the optical second and third harmonic generation and the electro-optic Kerr effect. As already mentioned, the harmonic generation techniques give the fast, electronic in origin, molecular and bulk hyperpolarizabili-ties, whereas the electro-optic methods are sensitive to all effects which induce optical birefringence, such as e.g. the rotation of molecules. Therefore the last technique is very useful to study the rotational mobility of molecules and/or their parts. 

The tensors and 7 constitute the molecular origin of the second-and third-order nonlinear optical phenomena such as electro-optic Pock-els effect (EOPE), optical rectification (OR), third harmonic generation (THG), electric field induced second harmonic generation (EFI-SHG), intensity dependent refractive index (IDRI), optical Kerr effect (OKE), electric field induced optical rectification (EFI-OR). To save space we do not indicate the full expressions for and 7 related to the different second and third order processes but we introduce the notations —(Ajy,ui,cj2) and 7(—a , o i,W2,W3), where the frequency relations to be used for the various non-linear optical processes which can be obtained in the case of both static and oscillating monochromatic fields are reported in Table 1.7. 

The nonlinear optical properties of polydiacetylenes are subject of an increasing interest in last years (1-4). This is due to the fact that polydiacetylenes are a one-dimensionnal system of higly polarizable conjugated tt elections The polarization depends strongly on electron delocalization length Moreover, the electronic origin of hyperpolarizability implies short response times On the other hand, the wave dispersed measurements of molecular hyperpolarizabilities yield information about forbidden electronic transitions In fact the bulk polarization can be developped in external electric field power series ... 

The prediction [19] that a low power optical field can induce appreciable director reorientation just above the dc field induced Freedericksz transition has been verified experimentally [20,21] concurrently with experimental and theoretical work on optical reorientation [22-24]. Since then, it has become one of the most intensively studied nonlinear optical effects in liquid crystals [3]. The phenomenon originates from the tendency of the director to align parallel to the electric field of light due to the anisotropic molecular polarizability. The free energy density arising from the interaction of a plane electromagnetic wave and the liquid... 

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