Nanomovement of Azo Polymers Induced by Longitudinal Fields

Surface deformations were induced in azo-polymer films by a focused laser spot that has a longitudinal field (E-z). We found that the deformation patterns induced by E-z were strongly dependent on polymer film thickness. The polymer formed a dip at the center of the focused spot when the film thickness is thinner than 37 nm, while the polymer formed a protrusion when the film thickness is thicker than 37 nm. These results imply that upward and downward forces are competing inside the polymer film, and the balance between them finally decides the surface topology (dip versus protrusion) of the film. We also found that the deformation patterns were dependent on the refractive index of the material on the film. We calculated the light field distribution inside the polymer film, and by comparing the experimental results we found that both anisotropic photofluidic force and optical gradient force might play important roles in the polymer movement. In addition, we found by changing the wavelength of the irradiation light that optical gradient force exerted on not the side-chain of the azobenzene moiety but the main chain of the polymer contributes to the polymer movement.