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By Malliaras G.G.

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5)). Photorefractivity in PVK based Composites 51 equal to Tg [33]. From the above it is clear that by lowering the Tg of the composites with the use of proper plasticizer, an increase of the electrooptic coefficient up to three times can be achieved. 4 PVK:TNF:EPNA PVK:TNF:HONB (r33 - r13) (pm/V) 3 2 1 0 0 10 20 30 electric field (V/µ m) Fig. 5 : The electrooptic coefficient r33-r13 versus the electric field across the sample for the EPNA and the HONB composites, measured at a modulation frequency of 1 kHz and at room temperature.

4, the square root of the SHG signal from a PVK:TNF:EPNA sample is plotted, exhibiting the expected quadratic behaviour with electric field3 . SHG would essentially vanish as soon as the voltage was switched off. From the above it is clear that it is possible to induce a certain degree of molecular orientation at room temperature. As with SHG, no electrooptic response could be measured on the sandwiched samples at the absence of an electric field. As the electric field was switched on, the electrooptic coefficient would reach a constant value and decay back to almost zero after the field was removed, within the temporal resolution of the measurement (a few seconds).

This reduces the symmetry to C∞m and the only nonzero components of the electrooptic tensor are r33 and r13=r23=r42=r51 [8]. The most popular technique for measuring the electrooptic coefficients of photorefractive polymers is a simple ellipsometric technique, proposed independently by Teng and Man [9] and Schildkraut [10]. It offers two main advantages: First, measurements can be performed on the same sandwiched samples that are prepared for photorefractive characterization, in contrast with most other techniques that require the fabrication of waveguides.

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