Self-assembled spontaneous structures induced by a pulsed laser on a surface of azobenzene polymer film

Abstract

The paper presents the self-assembled regularly patterned unique structures induced by the irradiation of pulsed laser beam on a surface of azobenzene polymer film. Self-assembled patterns depend on the polarization of laser beam. Linearly polarized beam gave rise to the ripple structures whose grating vector is perpendicular to the polarization of laser beam. Pitch of the ripple structures (Λ) was related to the wavelength (λ) of irradiated laser beam; Λ=λ∕n. Λ=λ∕1.05 for λ=266 and 355nm and Λ=λ∕1.3 for λ=420–540nm were obtained. These results implied that the ripple structures were produced by the interference of the incident beam with the optical wave scattered in a surface (waveguide mode light). The difference of n value is explained by the wavelength dispersion of refractive indices in the azobenzene polymeric matrix. Circularly polarized beam gave rise to the concentric wavelike structure. Pitch of wavelike structure was analyzed using two dimensional Fourier transform imaging techniques. Pitch of wavelike structure increases with increasing the number of laser shots. Furthermore, with increasing irradiation time, well-defined honeycomblike structure was also developed.