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Phase imaging during tapping mode atomic force microscopy (TMAFM) has revealed that an optical near field caused a change in the viscoelastic property on the surface of an urethane-urea copolymer film containing donor-acceptor substituted azobenzenes. Monolayers of polystyrene microspheres with 100 nm diameter and 19 nm diameter were fabricated on the surface of the copolymer film and exposed to a 488 nm wavelength laser beam coincident with the absorption band of the azobenzene derivatives. After removal of the monolayer, the phase image of the film's surface was obtained by TMAFM. The phase shift of a cantilever oscillation (the shift was induced by a tip-sample interaction) indicated that the area affected by the optical near field of the microsphere became relatively softer (the phase shift was smaller) and the vicinal area became harder (the phase shift was larger). These results suggested that the optical near field produced a change in the density on the surface of the copolymer in nanometric dimensions. The copolymer was capable of transcribing the optical near field within the resolution of 20 nm on the basis of the viscoelastic feature.

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An indented nanostructure was formed on the surface of azobenzene-functionalized polyurethane thin films by the induced migration of polymer chains using the optical near field generated around polystyrene (PS) spheres. The PS spheres, which had a diameter ranging from 98 to 990 nm, were arranged on the surface of a photoresponsive polymer film using a self-organization process, and the films were then exposed to a linearly polarized Ar+ laser at 488 nm. A hexagonal array of indentations reflecting the original geometry and arrangement of the spheres was observed on the polyurethane surface by atomic force microscopy. The indented structure that was formed had a resolution of ∼95 nm. This structure was stable at room temperature and could be erased by heating the films above their glass transition temperature (T g). The modification depth and diameter of the dents formed depended on the size of the original PS spheres. In particular, deeper dents were formed by using PS spheres with a diameter of over 505 nm, due to the optical lens effect they created, and the irradiation onto spheres of greater than 505 nm diameter with linearly polarized light induced an anisotropic modification on the surface of the polymer films. The thickness of the polymer film significantly affected the modification depth of the dent. We prepared three different kinds of photoresponsive polyurethane, each containing a slightly different azobenzene moiety. Although these polymers all had virtually the same molecular weight, T g, and absorptivity at the wavelength of the irradiated light, the modified depth of the dent observed for each of the films was very different. Furthermore, a dependence between the irradiated light intensity and the size of dent achievable was observed, which was due to the difference in the trans−cis−trans isomerization behavior of the azobenzene moiety.

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