Reversible Self-Focusing in Light-Responsive Spiropyran Functionalized Gels

Self-focusing, where interactions between a light beam and its medium creates a refractive index gradient that acts as a waveguide for the beam, has been observed at low powers (nW-mW) in a variety of soft-matter systems. This intensity dependent refractive index change is often created by the irreversible photopolymerization of monomers in solution. However, a reversible system would enable potential applications in soft all-optical computing based on dynamic interactions between self-focused beams. Here we show a fully reversible soft self-focusing medium based on stimuli-responsive hydrogels (poly(acrylamide-co-acrylic acid) and poly(N-isopropylacrylamide)) doped with photoresponsive spiropyran pendant groups. In this system the intensity-dependent contraction of the gel leads to a local increase in the refractive index, resulting in self-focusing of the incident beam. In additition to being reversible this system shows long-range interactions between beams separated by over 10 beam widths. To explain these experiments we have developed a numerical model coupling the spiropyran isomerization, gel dynamics, and the propagation of light. This model allows us to design more complex beam interactions and thus to create systems where light is dynamically controlled by light.