Opto-chemo-mechanical transduction in photoresponsive gels elicits switchable self-trapped beams with remote interactions

Significance Self-trapped light beams hold potential for optical interconnects, applications in image transmission, rerouting light, logic gates for computing and, importantly, for the next-generation light-guiding-light signal processing, which envisions a circuitry-free and reconfigurable photonics powered by the dynamic interactions of self-trapped beams. In conventional nonlinear materials, however, self-trapping suffers from either the need for large incident beam powers and loss of beam interactions at large distances, or it is slow and irreversible. We show that rapidly and repeatably switchable self-trapped laser beams with remote communication capabilities can be elicited at exceptionally small intensities in a pliant, processable hydrogel functionalized with a chromophore. The ability to generate self-trapped beams with this unique set of properties offers unprecedented opportunities to develop light-guiding-light technologies.
Next-generation photonics envisions circuitry-free, rapidly reconfigurable systems powered by solitonic beams of self-trapped light and their particlelike interactions. Progress, however, has been limited by the need for reversibly responsive materials that host such nonlinear optical waves. We find that repeatedly switchable self-trapped visible laser beams, which exhibit strong pairwise interactions, can be generated in a photoresponsive hydrogel. Through comprehensive experiments and simulations, we show that the unique nonlinear conditions arise when photoisomerization of spiropyran substituents in pH-responsive poly(acrylamide-co-acrylic acid) hydrogel transduces optical energy into mechanical deformation of the 3D cross-linked hydrogel matrix. A Gaussian beam self-traps when localized isomerization-induced contraction of the hydrogel and expulsion of water generates a transient waveguide, which entraps the optical field and suppresses divergence. The waveguide is erased and reformed within seconds when the optical field is sequentially removed and reintroduced, allowing the self-trapped beam to be rapidly and repeatedly switched on and off at remarkably low powers in the milliwatt regime. Furthermore, this opto-chemo-mechanical transduction of energy mediated by the 3D cross-linked hydrogel network facilitates pairwise interactions between self-trapped beams both in the short range where there is significant overlap of their optical fields, and even in the long range––over separation distances of up to 10 times the beam width––where such overlap is negligible.