2D and 3D buried gratings made from azobenzene-polymer multilayer films

Abstract

Surface relief gratings (SRG) made from azobenzene polymer films by holographic exposure with actinic light show remarkable density modifications in addition to the surface relief. The origin of the huge material transport is attributed to cooperative phenomena associated with the light induced trans-cis and cis-trans isomerization of azobenzene moieties and the subsequent changing of viscoelastic properties during illumination. In case of polydisperse red 1 methacrylate (pDR1m) films and using particular illumination conditions the amplitude of the density grating (DG) can be maximized whereas the amplitude of surface undulations keeps small. The capability of DG formation makes it possible to induce grating formation in the azobenzene polymer film through a thick polymer cover layer which is not affected by the actinic light. Using PMMA as cover layer the grating is located at the PMMA - pDR1m interface (interface grating) while the sample surface stayed almost flat. This concept can be used to prepare 3D mesoscopic crystals by stacking several PMMA/pDR1m bi-layers on top of each other. The interface grating is created in each bi-layer before it becomes covered by the next bi-layer. Patterning of the upper bi-layer takes place after careful positioning of the writing position with respect to the underlying one. These 3D multilayer gratings can be used as dispersive elements for optical light. Their structural performance can be probed by means of light diffraction similar to the x-ray Laue experiment.