Tunable Long-Range Interactions between Self-Trapped Beams driven by the Thermal Response of Photoresponsive Hydrogels

Spiropyran-functionalized hydrogels are a promising platform for nonlinear optical materials. These gels show an unprecedented combination of reversibility and processability while requiring low laser power. Long-range beam interactions are especially intriguing for applications in all-optical computing or sensing. Here we show that thermal swelling or deswelling of the gel accompanies the light-driven spiropyran isomerization and that these thermal effects play a significant role in both single-beam self-trapping and long- and short-range interactions of multiple beams. The direction and magnitude of a hydrogel’s thermal response are controlled by the gel composition, which allows the behavior of single and multiple beams in these gel materials to be programmed over a vast chemical design space. We demonstrate this potential by comparing two-beam interactions in spiropyran-functionalized poly(acrylamide-co-acrylic acid) gels to those in poly(N-isopropylacrylamide-co-acrylic acid) gels, which undergo swelling or contraction with the increase in temperature, respectively. The presence of a secondary field that influences beam behavior leads to repulsive long-range interactions in the former gels, while the latter lack long-range repulsive interactions and show behaviors indicative of short-range attractive interactions, such as spiraling and beam fusion. This tunable diversity of behaviors suggests the enormous potential for hydrogels as versatile, soft, and nonlinear optical materials.