High Tg azopolymers for Photorefractive applications

In recent years, the development of organic materials for optoelectronics and photonics has been continuously progressing, driven by the prospect of manufacturing flexible, low cost devices with tailorable properties for specific applications. Photorefractive (PR) materials are photoconductors in which the space redistribution of charges photogenerated by a non-uniform illumination creates an electric field. In turn, such space-charge field affects the refractive index if the material exhibits non-linear optical properties and/or spontaneous (or field-induced) birefringence. The resulting hologram is then phase shifted with respect to the illumination pattern, a property unique to PR gratings and with important consequences for applications. Both low molecular weight and polymeric amorphous organic systems have been extensively studied as PR materials in the last two decades. They may be divided in two general classes, depending on their glass transition temperature (Tg) being above or below the operational temperature, usually room temperature (RT). In the second case, electric field-induced molecular reorientations are possible during the encoding process, leading to a spatially modulated birefringence and to high refractive index contrast with two (electro-optic and birefringence) contributions. When Tg is instead higher than RT, in order to obtain macroscopic second-order non-linear properties, orientational order is induced during the so-called pre-poling process, which is needed in order to break the centrosymmetry of the bulk material. The pre-poling is an electric field-induced chromophore orientation obtained at temperatures above Tg, followed by a cooling to RT with the field still on. As no birefringence contribution is present, high Tg polymers usually exhibit lower refractive index modulations. Field-induced reorientations at the molecular level are then of particular relevance in PR organic materials. In this work, we show how, even at temperatures well below Tg, molecular reorientations can be induced in polymers in order to obtain a PR response. The compounds used are multifunctional side-chain polymers exhibiting chiral, semiconducting, photochromic and non-linear optical (azoaromatic moieties) properties. While light-activated conformational mobility is a necessary requirement at such temperatures, an electric field (applied + photogenerated) is also necessary in order to encode a phase shifted hologram, via the reorientation of polar chromophores. Our data indicate that the conformational mobility of interest when considering macroscopic photorefractive index modulations is not the simple trans-cis isomerization by itself, but a chromophore reorientation process induced by the electric field and assisted by the photo-isomerization. In polymers with polar chromophores, the writing time of the refractive index modulation, which is long at lower absorption, was found to decrease by two orders of magnitude upon using illumination at a wavelength where absorption is higher. This feature is particularly interesting, since it allows the elimination of the poling step while at the same time adding a birefringence contribution to the index modulation.