Non linear optical materials

If one or two laser beams are directed into NLO materials, harmonics or sum and difference frequencies are generated. Potassium di-deuterium orthophosphate, KD2PO4, is widely used to generate 2, 3,4 and 5 harmonics of 1.06 mm radiation. 

FIGURE 12.49 Crystal structure of KTi0P04. K+ = open circles. Chains of -O-Ti-O-Ti-O- are parallel to [c]. A continuous network of O-Unked TiOg octahedra and PO4 tetrahedra is present. 

Potassium titanyl phosphate, K TiO PO4 (KTP), is a much favoured material for use in optoelectronics. It is widely used for second harmonic generation of Nd lasers emitting near 1 mm, and is a superior material for many optical waveguide applications. 

K2CO3 2(NH4)2HP04 2Ti02 2KT10P04 -r 4NH3 -r 3H2O -r CO2 

The NLO coefficients are believed to arise from the alternating long and short Ti-0 linkages along the -Ti-O-Ti-0- chains. The compounds A TiO PO4, where A = K, Rb, Cs, NH4, Tl X = P or As, form an isostructural series, but only the K salts have sufficient asymmetry in their Ti-0 linkages to give rise to special NLO properties. 

It can be safely predicted that applications of liquid crystals will expand in the future to more and more sophisticated areas of electronics. Potential applications of ferroelectric liquid crystals (e.g. fast shutters, complex multiplexed displays) are particularly exciting. The only LC that can show ferroelectric property is the chiral smectic C. Viable ferroelectric displays have however not yet materialized. Antifer-roelectric phases may also have good potential in display applications. Supertwisted nematic displays of twist artgles of around 240° and materials with low viscosity which respond relatively fast, have found considerable application. Another development is the polymer dispersed liquid crystal display in which small nematic droplets ( 2 gm in diameter) are formed in a polymer matrix. Liquid crystalline elastomers with novel physical properties would have many applications. 

Non-linear second-order optical properties such as second harmonic generation (SHG) and the linear electrooptic effect are due to the non-linear susceptibility in the relation between the polarization and the applied electric field. SHG involves the 

Crystalline lithium niobate was for many years the only material that could be used in devices, an example of its use being in electro-optic modulators for interfacing with fibre optic technologies. Unfortunately, this material is not very satisfactory because it is difficult to grow, and hence expensive, and only shows modest electrooptic properties. 

As a consequence attention over the last decade has been focused on finding alternatives and the materials of choice have been organic chromophores particularly in conjunction with polymers. 

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A concise survey of where the broad domain of optical information processing had got to a few years ago is in a book issued by the European Commission (Kotte et al. 1989), while a good overview of non-linear optical materials is by Bloor (1994). 

The index of refraction in a non-linear optical material can be written ... 

A.J. Hceger, D. Moses, M. Sinclair, Semiconducting polymers fast response non-linear optical materials, Synth. Met., 1986. 15, 95. 

The benzannulation reaction of ethynylferrocene 120 with the diterpenoid chromium alkoxycarbene 119 leads to novel diterpenoid ferrocenyl quinones 121 which, due to their electron-transfer properties, are regarded as potential candidates for non-linear optical materials [71] (Scheme 52). 

A rapidly increasing number of publications on polysilanes documents current interest in these polymers (JJ. Polysilanes are potentially applicable in microlithography as high resolution UV-resists (2J, imageable etch barriers ), or contrast enhancement layers (4). They have been successfully used as precursors to Si-C fibers (5J and ceramic reinforcing agents ((L). Polysilanes have also initiated polymerization of vinyl monomers (J ). Doping of polysilanes have increased their conductivity to the level of semiconductors (8). Very recently polysilanes were used as photoconductors (9) and non-linear optical materials (10b... 

Figure 14.11. A non linear optical material formed from nanopores in an InP membrane. The picture on the left is the top view, whereas that on the right shows the cross section. [Reproduced with permission from Ref. 60. Copyright 2003 Wiley-VCH.]... 

R.W. Munn and C.N. Ironside, Principles and Applications of Non-Linear Optical Materials, Chapman Hall, Glasgow, 1993. 

Of the many studies of applications of L-B films, we shall mention at last the development of non-linear optical materials. A molecule such as 4-nitro-aniline has a very large hyperpolarizability (section 7.2) and can be used in principle for frequency conversion in solution if a strong electric field is used to provide an orientation of the dipoles. There is no need for such an external field if similar molecules are used in an L-B film, and hyperpolarizability values far in excess of those of commonly used inorganic crystals have been obtained. Figure 8.24 shows two examples of the molecules used in... 

Crosslinked LC elastomers (Figure 19d) are very promising for piezoelectric and ferroelectric applications, and also as non-linear optic materials. The first synthetic step to such materials is the preparation of usual side chain or combined LC copolymers doped with a small part of side chains containing a polymerizable >C=C< double bond at the end (Figure 23 shows a particular example of a crosslinkable LC polymer64). The copolymer can be further photocrosslinked, giving an elastic polymer film which reveals... 

Clathrates are useful for the control of the stereo- and regiospecificity of intra-cavity chemical reactions and can be used to engineer materials properties such as a polarity leading to non-linear optical materials, and ferroelastic or ferroelectric behaviour. 

Electrodes represent an unrivaled platform onto which interfacial supramolecular structures can be assembled. They can be fully characterized before assembly and offer a convenient means to both probe and control the properties of the film. The interest in this area has increased dramatically in recent years because adsorbed monolayers enable both the nature of the chemical functional groups and their topology to be controlled. This molecular-level control allows the effects of both chemical and geometric properties on electron transfer rates to be explored. Moreover, these assemblies underpin technologies ranging from electrocatalysis to redox-switchable non-linear optical materials. 

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