Second-order non-linear optical materials

O—H bonds, hydrogen atom abstraction from, 9,127 Organic materials for second-order non-linear optics, 32,121 Organic reactivity, electron-transfer paradigm for, 35, 193 Organic reactivity, structure determination of, 35,67... 

Organic Materials for Second-Order Non-Linear Optics... 

Second-order non-linear optics continne to be an area of research becanse of its tremendous potential in the design of photon-based new materials for optical switching, data manipulation, and information processing [12]. 

Gaudry, J.B. Capes. L. Langot. P. Marcen, S. Koll-mannsberger, M. Eavastre, O.iFreysz. E. Letard. J.F. Kahn. O. Second-order non-linear optical response of metallo-organic compounds Towards switchable materials. Chem. Phys. Lett. 2000. 324. 321-329. 

Materials with appreciable values of j8 leading to frequency doubling are called second-order Non Linear Optics (NLO) materials. Such materials lack a center of symmetry since the magnitude of the induced M is different for +E and -E. Molecules of this sort typically have donor and acceptor groups on opposite sides of the molecule making it easier for electrons to move in one direction than the other. Of course, to be active, the molecule must be oriented. With polymers, this can be accomplished by poling where the material is subjected to an electric field above its glass temperature and been quenched with the field on so as to freeze the molecule in their ordered state. 

Fig. 1. Principle of an IR-VIS SF spectrum. The spatial and temporal overlap of an infrared and visible laser pulse at the interlace generates light at the smn frequency which is emitted in a defined direction. A vibrational spectrum is obtained by scanning the infrared frequency. All second-order non-linear optical effects probe specifically the molecules at interfeces, and contributions from the bulk phase are to a large extent suppressed. Note. In an electrolyte solution, the majority of material is dissolved in the bulk and the interfecial region comprises only a tiny fraction of total material of the system. (Reprinted with Permission from Ref. 63. Copyright 2007 American Chemical Society.)...

As an alternative to planar waveguiding structures we report here the fabrication of crystal cored fibres in which it is possible to maintain uniform guide dimensions over long lengths. These fibres with organic crystal core material having large second order non-linearity could be used for miniaturization of visible laser sources and realization of parametric amplifiers for optical communications. 

For an organic material to show electro-optical activity it must exhibit high second order non-linear behaviour. This is one of the terms in the equation that defines the polarisation (u) of an organic molecule as shown below, where E is the perturbing electric field. 

One requirement for second-order non-linearity in optical molecules is that they exhibit non-centrosymmetric symmetry, i.e. they must be dipolar in nature and all point in the same direction. Hence, materials suitable for electro-optical uses should have high figures for the multiplier where p is the dipole moment and P the molecular second order optical non-linearity parameter. ... 

For most materials the higher-order non-linear optical effects are small and extremely difficult to detect. So the NLO effects investigated most are the second-order effects. 

In order to obtain organic non-linear optical materials, a number of aniliniun L-tartrate salts are prepared. The structure of the complexes between L-tartaric acid and m-anisidine or p-toluidine was studied by X-ray diffraction of a single crystal. m-Anisidinium L-tartrate monohydrate shows pronounced second harmonic generation activity (which is the quadratic term of the equation describing the total polarization of a molecule), while p-toluidinium L-tartrate did not show any detectable pronounced second-harmonic generation activity156. 

---