Optical near field induced change in viscoelasticity on an azobenzene-containing polymer surface

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.