Electromodulation of Optical Absorption

Under the conditions used for the electro-optical measurements on the various device structures that are reported in sections S and 6, we will expect to see a substantial electric field applied across at least a fraction of the polyacetylene layer, and we need to characterise the modulation of the %-n absorption edge with electric field. This electromodulation response is the Franz-Keldysh effect, and arises through modulation of the electron states near the band edge in the applied field. It is found to be very large in Shirakawa polyacetylene [54,55] and it has been pointed out that this is due to the strong non-linear electronic response that characterises the conjugated polymers [55]. In the low field limit, we expect to see a response that varies quadratically with the applied electric field, and that is proportional to the second differential of the absorption coefficient, a, 92a/aE2 [54]. 

This experiment can be performed directly by measurement of the modulation of the optical transmission with an applied electric field, Focos(cot) with lock-in detection of the optical response at 2o), twice the modulation frequency of the applied field. The two geometries that can be used are either in-plane, with contacts arranged to give a gap of typically 100 im, or the sandwich geometry, with two mi-transparent metal contacts applied to either side of a polymer film. The in-plane geometry has been used for 

One further parameter that can be measured from this type of structure through measurement of the cq acitance is the relative dielectric constant, We find a value of  

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