Quadratic Electro-Optic Effect in Doped Nonconjugated Conductive Polymer 1,4-Trans-Polyisoprene, an Organic Quantum Dot System

Thin, optically uniform films of 1,4-trans-polyisoprene have been prepared on quartz substrate from a toluene solution. These films have been characterized using FTIR and optical absorption spectroscopy before and after doping with iodine. The optical absorption spectrum at low doping shows two peaks: one at $\sim $ 4.2 eV due to radical cation and the other at $\sim $ 3.2 eV due to charge-transfer. Doping leads to a reduction of the intensity of =C-H bending vibration-band due to formation of radical cations upon charge-transfer. Quadratic electro-optic measurements have been made using field-induced birefringence method at 633 nm. A modulation depth of $\sim $ 0.13{\%} has been observed for an applied field of 1.1 V/$\mu $m for a 0.37 $\mu $m thick film. The modulation depth had a quadratic dependence on applied field. The Kerr coefficient as measured is exceptionally large and has been attributed to the subnanometer size metallic domains (quantum dots) formed upon doping and charge-transfer.