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The utility of the Shear method in obtaining thin single crystal films of polydiacetylenes has been demonstrated previously. In this paper the principles of the Shear method are elucidated with analogy to the well-known L-B method. Both the methods, L-B and Shear, utilize the polar molecular interactions with the substrates as a control parameter for the molecular organization. The major difference between the two is that one (L-B) utilizes a liquid as the subphase while the other (Shear) uses solid substrates. Because of the nature of the interface the L-B approach leads to only one-dimensional organization and poor optical quality for the films while the Shear method provides 3-D organization and excellent optical quality.Thin single crystal films of a specific polydiacetylene, PTS, are prepared on oxide coated silicon substrates by the Shear method. Waveguiding through the PTS films is demonstrated for the first time. Much progress is also made in the waveguide-structural fabrication of PTS films. The results are quite encouraging in terms of device applications of polydiacetylenes as optical switches and logic gates.

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Polydiacetylenes, a class of organic polymers are important because of large nonresonant I x(3) I and subpicosecond response times. The nonresonant domain of these materials covers the wavelength from -0.8 g to -2.0 ti which is a desirable spectral window from the standpoint of telecommunication applications. The magnitude of x(3) in this wavelength range is --109 esu which is the largest nonresonant value known for any material. The absorption spectrum of these materials consists of a sharp excitonic peak at -2 eV (a -106 cm-1) followed by weak interband transitions at -2.4 eV. The excitonic state has been shown to be a combination of Frenkel and charge -transfer states.l Thus these excitons are unlike the excitons that are usually observed in inorganic semiconductors and seldom dissociate into free carriers (as evidenced by poor photoconductivity throughout the absorption range).2 Consequently the relaxation time of the excited state of these systems has been found to be as low as -1.8 ps. Away from the resonance the response is instantaneous even at -0.3 ps time scale. These are some of the unique characteristics that make polydiacetylenes attractive for many future applications in all-optical signal processing. In this paper we will discuss the crystal growth, our present understanding of the origin of nonlinearity and waveguiding aspects of these materials.