Designable Non-linear Optics of Light-Responsive, Spiroypyran-Functionalized Hydrogels

We demonstrate the potential of spiropyran-functionalized, light-responsive hydrogels to be a promising new platform for nonlinear optical materials. Nonlinear optical behavior in these materials arises from the local swelling/deswelling and resulting refractive index changes that occur due to photoisomerization between the hydrophilic ring-open merocyanine isomer and the hydrophobic ring-closed spiropyran isomer. These materials show an unprecedented combination of reversibility, soft easily processed material, low laser powers, and long-range interactions. These long-range interactions are especially promising for potential applications in all-optical computing or sensing. We show that the long-range interactions can be explained by thermal swelling or deswelling of the gel. Furthermore, the direction and magnitude of a hydrogel’s thermal response can be controlled by changing the gel composition, which allows the behavior of single and multiple beams in these gel materials to be rationally designed over a potentially vast design space. This tunable diversity of behaviors suggests the enormous potential for photoresponsive hydrogels as extremely versatile, rationally designable, soft, nonlinear optical materials.