Efficient Surface Structuring of Photomechanical Solid Polymers by Near-Zero Light Absorption

Abstract

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- and nanotextured by single-step irradiation with weakly absorbed low-power red light. We report highly efficient surface structuring induced by interference patterns of two coherent nonresonant red (wavelength of 632.8 nm), less than 5 mW, laser beams operating in the near-zero absorption tail of azo-polymer films 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 that is counterintuitive and thus never reported to date. Low- and high-energy n – π* and π – π* excitations of the azo dye are equally efficient in inducing isomerization and mass motion of polymers. Our work is of central importance to photomechanics, and to surface texturing in particular, since it demonstrates that efficient material motion can be driven by weekly absorbed, i.e., low-energy excitation, low-power red light, in solid films of polymers well below the glass transition temperature, pointing to possible biological and cost-effective industrial applications.