Efficient mass motion of solid polymers induced by red light

Mechanics enabled by photon absorption generates work from materials and is perhaps one of the most important applications of light active matter, which mediates an intensity-to-stress transduction and moves in intensity gradients. Photomechanics can be put at work in surface structuring of materials, which has applications in several areas of science and engineering including photonics and biology and medicine. We demonstrate that photosensitive materials can be unexpectedly micro- nano- textured by a single step irradiation with weakly absorbed low power red light. We report highly efficient surface structuring induced by interference patterns of two coherent non-resonant red (wavelength 632.8 nm), less than 5 mW, laser beams operating in the near-zero absorption tail of azo-polymer fims with optical densities in the 0.02-0.09 range; i.e. 80-95% light transmission. The heights of the observed structures are comparable to those obtained by resonant absorption; a feature which is counter intuitive, and thus never reported to date. Low and high energies n-π^* and π-π^*excitations of the azo dye, are equally efficient in inducing isomerization and mass motion of polymers. Our work is