Non Linear Optical Properties Second Order

Second order non-linear optical properties have been reported for a variety of TTF donor-acceptor compounds <02T7463> and the palladium complex 84 is a room-temperature semiconductor <02CL936>. Preparation of the zinc and cadmium compounds 85 has been reported <02CC1474> and aromatic fused TTFs such as 86 form thin films with useful electrical properties <02JAP265466>. A ferromagnetic interaction occurs in the salt of a TTF... 

Butcher and Cotter (1990), Boyd (2003) and Sutherland (2003) provide overviews of non-linear optics. Prasad and Williams (1991) have discussed the non-linear optics of molecules and polymers. Detailed accounts on this topic can be found in Bosshard et al. (1995), Kajzar and Swalen (1996) and Kuzyk and Dirk (1998). More specialised reviews are those of Yesodha et al. (2004), on non-linear polymers with azobenzene pendent groups, and Sioncke et al. (2003), who describe the second order non-linear optical properties of chiral molecules and polymers. 

Iron alkynyl complexes containing aromatic moieties are of particular interest as building blocks for molecular devices. Second-order non-linear optical properties have been investigated for various Fe(ii) and Fe(iii) complexes of the type [(dppe)Gp Fe(G=G-Ar)] where Ar = pyridine, GsH4N(GH3), or/>-G6H4X (X = NO2, GN, GF3, H, OMe, and NH2). These compounds have shown significant quadratic redox-switchable polarizabilities. Further examples of redox-active iron alkynyl synthons containing aromatic units have been reported, where the iron alkynyl unit is... 

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

A LiAl-LDH intercalated with 4-nitrohippuric acid has been shown to exhibit second harmonic generation -532 nm radiation from incident 1064 nm [142]. It was reported that the ability of the intercalated material to exhibit non-linear optical properties is due to a perpendicular monolayer packing of the acid molecules in the interlayer. Such an orientation leads to an ordered arrangement of dipoles, giving rise to a bulk dipole moment in the solid. Crystals of the pure acid exhibit no frequency-doubling characteristics due to a centrosymmetric packing in the crystal. 

One of the more interesting applications of non-linear optical effects is the generation of the second harmonic. This phenomenon results when a laser beam passes through a material having second-order NLO properties (hence, composed by non-centrosymmetric molecules) the light emitted has a frequency double that of the incident radiation (or the wavelength has been halved). 

The fundamental component (aE) is linear in E and represents the linear optical properties discussed above. The second (jfiE-E) third ( yE-E-E) and subsequent harmonic terms are nonlinear in E and give rise to NTO effects. The / and values are referred to, respectively, as the first and second hyperpolarisabilities. The second harmonic term gives rise to second harmonic generation (SHG), the third results in frequency tripling effects, and so on. Importantly, since only the time-averaged asymmetrically induced polarisation leads to second-order NLO effects, the molecule and crystal must be non-centrosymmetric, otherwise the effects will cancel one another. Third-order effects, however, may be observed in both centrosymmetric and non-centrosymmetric materials. 

Recently, a second-order nonlinear photonic crystal has been realized.38 In this nonlinear optical bandgap material, there is a periodicity in the nonlinear optical properties of the engineered material. With this definition, a periodically poled second-order nonlinear optical material could be called a nonlinear photonic crystal. However, its linear optical properties do not show a periodicity, except for the (small and useless for bireffingent phase-matching) poling-induced birefringence. Here, the material is the same in the complete structure. It is only periodically made into a non-centrosymmetric structure for second-order nonlinear and phase-matching... 

TTiis review attempts to survey second-order molecular polarizabilities of organic molecules. Emphasis will primarily be given to the underlying physical principles of non-linear optics (e.g. molecular topology), and to the establishment of structure-property relationships. We will largely remain paradigmatic and will not attempt an exhaustive compilation of data because several good review articles and books exist that incorporate tabulations of numerous... 

There are so many publications in the realm of organic molecules for non-linear optics that this review has focused on second-order polarizabilities. The translation of these properties into bulk structures could only be hinted at and devices could not be mentioned at all. We have tried to develop a formal description of tensorial properties that is consistent with the SI system and would like to suggest to groups working in the area to adopt it in order to... 

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