Driving force for formation of a surface relief grating on an azobenzene-containing polymer

The driving force for formation of a surface relief grating on an azobenzene-containing polymer is theoretically analyzed and experimentally confirmed by polarization analyses. We formulate the driving force based on the Coulomb interaction between an optical electric field and its induced polarization charges. The formula derived includes not only the optically induced gradient force but also the force due to the spatial modulation of the refractive index. The latter stems from the reorientation of the azobenzene molecules in the polymer through the cyclic trans-cis photoisomerization caused by the Ar+ laser used to construct the interference pattern.