CHARACTERIZATION OF SURFACE RELIEF GRATINGS PHOTOINDUCED ON CHIRAL AZO POLYMERS

It is well know that polarized light can induce birefringence and photomodulate the circular dichroism in films of azo polymers by means of the light driven orientation of the chromophores induced by the photoisomerization of azobenzene moieties[1]. Irradiation of azo polymer films with an interference pattern for a period of time longer than that required for photoinduced orientation produces a modification of the film surface. Surface Relief Gratings (SRGs) with depths of up to one micron could be obtained and more than one grating can be inscribed consecutively on the same spot on the polymer film. A strong dependence of the grating depth from the polarization of the impinging interfering beams has been established since the discovery of this phenomena. Recently it has been reported[2] that unusual superhelix-like patterns can be directly photofabricated on the surface of achiral azo polymer films by the combination of the photoinduced chiral orientation and the photoinduced SRG formation. This effect is possible only if the interfering beams are polarized in the correct way. The interpretation of this new feature is likely linked to the photomodulation of circular dichroism. In this context, we have realized and characterized by optical and AFM techniques several SRGs with secondary helicoidal structures inscribed in a series of films of chiral polymethacrylates containing azobenzene moieties. In particular, the photoinduced SRGs on poly[(S)-3-methacryloyloxy-1-(4′-cyano-4-azobenzene)pyrrolidine], exhibit a high diffraction efficiency. [1] Natansohn A., Rochon P.; Chemical Reviews 102 (2002), pag 4139-4176 [2] Kim M.-J., Kumar J., Kim D.-Y.; Advanced Materials 15 (2003), pag 2005-2008