Single-, two-, and multi-photon driven molecular motion and nanopatterning in azo-polymer films

Abstract

We review our work on nanopatterning in azo-polymer films by single, two- and multi-phopton driven molecular motion in solid bulk polymer. It is now known that light induced molecular movement occurs below the polymer glass transition temperature by chromophores photoisomerization via either linear or nonlinear absorption, and in this paper we will show that nanoscale polymer movement is induced by a tightly focused laser beam in an azo-polymer film just at the diffraction limit of light. The deformation pattern which is produced by photoisomerization of the azo dye is strongly dependent on the incident laser polarization and the longitudinal focus position of the laser beam along the optical axis. The anisotropic nanofluidity of the polymer film and the optical gradient force played important roles in the light induced polymer movement. We explored the limits of the size of the photo-induced deformation, and we found that the deformation depends on the laser intensity and the exposure time. The smallest deformation size achieved was 200 nm in full width of half maximum; a value which is nearly equal to the size of the diffraction limited laser spot. Furthermore, a nano protrusion was optically induced on the surface of the films, beyond the limit of light diffraction, by metal tip enhanced near-filed illumination. A silver coated tip was located inside the diffraction limited spot of a focused laser beam (460 nm), and an enhanced near-field, with 30 nm light spot, was generated in the vicinity of the tip due to localized surface plasmons. The incident light intensity was carefully regulated to induce surface nanodeformation by such a near-field spot. A nano protrusion with 47 nm full width of half maximum and 7 nm height was induced. The protrusion occurs because the film is attracted towards the tip end during irradiation. At the top of the protrusion, an anisotropic nanomovement of the polymer occurs in a direction nearly parallel to the polarization of the incident light, and suggests the existence at the tip end of not only a longitudinal, i.e., along the tip long axis, but also a lateral component of the electric field of light. The azo-polymer film helps map the electric field in the close vicinity of the tip. We also report on two-photon patterning of the films. Exposure of azo polymer films, which absorb in the visible range (λ_max = 480 nm), to intense 920 nm irradiation leads to polarization dependent patterning which are associated with polymer nanomovement caused by photoselective two-photon cis ↔ trans isomerization, while irradiation at 780 nm induces multi-photon bleaching of the azo chromophore. These wavelengths hit bleaching and isomerization pathways in the chromophore, respectively.