Polarization Dependence of the Formation of Surface Relief Gratings in Azobenzene-Containing Molecular Glasses

Abstract

This paper presents a comprehensive study of the formation of surface relief gratings in a series of photoresponsive molecular glasses. Holographic experiments were performed on films of the azobenzene-containing molecular glasses. Seven relevant polarization configurations of the writing beams were systematically applied, and simultaneously the diffraction efficiency was monitored during the process of inscription. The temporal evolution of the diffraction efficiency can be precisely simulated with a model which takes both the surface relief and the phase grating in the volume into account. From the measured diffraction efficiencies, the modulation heights can be directly calculated and they were independently confirmed by atomic force microscopy. We found that all experimental results can be explained with the gradient force model, and we suggest that the grating heights generated with different writing polarizations can be ascribed to the varying strengths of the gradient force. For materials with different substituents at the azobenzene chromophore, the optical susceptibility at the writing laser wavelength and, therefore, the gradient force varies. By applying the most efficient polarization configuration in combination with the best material, we were able to reach modulation heights of up to 600 nm, which is a factor of 2 higher than modulations usually reported for azobenzene-containing polymers.

Synthesis and Structure–Property Relations of a Series of Photochromic Molecular Glasses for Controlled and Efficient Formation of Surface Relief Nanostructures

“…Other polarization configurations yield the same trend, although not always as pronounced as with the þþ458 scheme. [29] A clear correlation with the maximum SRG height in respect to the glass transition could not be established. For example, the glass transition temperatures of compounds 4a, 4b, 4c differ by only 7 8C (4a: 92 8C; 4b: 99 8C; 4c: 93 8C) but the achievable SRGs differ substantially.…”

“…[31] The refractive index n and the absorption constant k were measured with an ellipsometer and from these values the susceptibility was calculated. [29] With respect to optical susceptibility there are two exceptions, 4c and, less pronounced, 4e. Whereas 4c always features very low SRGs and long writing times, 4e develops comparatively high SRGs with fast build-up.…”

“…A detailed study of the influence of the polarization settings has been published elsewhere. [29] The configuration AE458 generates mainly a polarization grating with a small admixture of an intensity grating. Since polarization gratings are connected with a homogeneous light intensity, a light-induced plastification is expected to occur throughout the sample film.…”

“…Since after some seconds of irradiation the SRG always gives rise to a much higher DE than the orientation grating of the chromophores in the bulk, [29] we can calculate the height d of the SRG directly from the measured DE. To this end the theory of Magnusson and Gaylord, [30] which is usually applied to thin volume gratings, is modified to account for SRGs.…”

Refractive-index determination of solids from first- and second-order critical diffraction angles of periodic surface patterns

“…Holographic exposure of many azobenzene derivatives gives rise to photo-induced mass transport over macroscopic distances, which results in the formation of surface relief gratings (SRGs). 6,7 However, the applicability of our method is not limited to photo-active systems because it only requires the periodic pattern. We demonstrate this for a transparent colorless polydimethylsiloxane (PDMS) working stamp.…”