Second harmonic generation organic materials

Lamberth, C. Murphy, D. M. Mingos, D. M. P. Second Harmonic Generation Properties of Some Coordination Compounds Based on Pentadionato and Polyene Ligands. In Organic Materials for Nonlinear Optics II Harm, R. A., Bloor, D., Eds. Royal Society of Chemistry London, 1991 pp 183-189. 

Organic Materials for Optical Second Harmonic Generation... 

A number of special optical techniques such as the study of second harmonic generation by irradiation of non-centrosymmetric systems by high intensity laser light will be discussed in relation to particular materials and problems. However, one optical technique having a general applicability, namely ellipsometry, must be discussed here. It is one of the best techniques available to determine the thickness of a thin organic film. Such determinations are important as they allow one to have an independent check on the number of layers deposited, given that the thickness of one layer has been determined by X-ray diffraction. 

Figure 2 shows the tradeoffs between crystal and polymer organic NLO materials for device applications. Although cither type of materials could in principle be used for both applications, crystals are best for second harmonic generation, and poled polymers are best for electro-optic waveguide devices such as modulators and switches. 

We have recently conducted a set of intracavity second harmonic generation experiments using the organic nonlinear material DAN (4-(N,N-dimethylamino)-3-acetamidonitrobenzcne) and an optically pumped cw Nd YAG laser (2). Figure 3 shows the experimental setup. Quasi-cw operation was achieved with crystal samples immersed in index matching fluid in an antireflection coated cuvette that was placed internal to the Nd YAG laser cavity. 

Another type of organic dye was introduced through co-condensation of TMOS and precursor 107. For a thin film (4.2 pm thick), a very high second-harmonic generation signal was obtained (d33 = 150 pm V 1, X = 1.34 pm). Interestingly, this study demonstrates the importance of the processing parameters used for the elaboration of the material... 

The work reported here has shown that inclusion complexation of organic and organometallic chromophores by thiourea, TOT and cyclodextrins can induce second harmonic generation capability in the polar crystals which result, even when the original bulk materials are themselves incapable of SHG. Structural evidence has been presented to show tht the solid state inclusion structures are acentric, and a simple electronic picture t0 the polarization response of these materials within the two-state modeP ° has been discussed. In an earlier section we remarked that of the many complexes we have made, only one has NOT been acentric. This result was not anticipated. We postulate that it is a natural tendancy in such materials, rather that an exception. If we consider a dipolar molecule in isotropic solution, we can imagine that if it were to aggregate, it would do so in a head to tail fashion in order to minimize electrostatic repulsion. The situation is illustrated in Scheme 3. The arrangement that would result is centrosymmetric. 

Apart from the tuning of the host-guest interactions, the elaboration of efficient optical devices also relies on the optimization of the matrix mechanical properties and thermal stability. This is clearly demonstrated in the development of hybrid materials for NLO, and especially second harmonic generation (SHG). These properties rely on their noncentrosymetric organization of organic chromophores in the condensed phase. Becanse incorporation in a silica... 

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