![]() ![]() While reversible self-trapping of a single relatively low-power beam (25 µW) due to iodide photooxidation has been reported in a gel for two cycles spanning hundreds of seconds, the persistence of triiodide within the sample prevented further cycling ( 20). Moreover, Δ n in these materials can only be generated at large powers (several watts) or in the presence of an external electric field ( 2, 3, 6 – 10). Extensive studies of self-trapped beams in reversibly responsive media such as photorefractive crystals ( 16), liquid crystals ( 17), atomic vapor ( 18), and Kerr media ( 19) have focused predominantly on steady-state self-trapping conditions due to their generally fast dynamics. Advances, however, have been limited by the need for robust photoresponsive materials with switchable-rather than irreversible-changes in refractive index, Δ n, which would generate the rapidly reconfigurable self-trapped beams and transient waveguide architectures necessary for repeated dynamic interactions. Importantly, they form the basis of the next-generation light-guiding-light approach to optical signal processing, which envisions a circuitry-free, reconfigurable, and multilayered photonics technology powered by the dynamic interactions of self-trapped beams ( 13, 14). Because they travel without changing shape, self-trapped beams hold potential for optical interconnects ( 11), applications in image transmission ( 12), rerouting light ( 13, 14), and logic gates for computing ( 15). These nonlinear waves propagate without diverging through self-inscribed waveguides and exhibit intriguingly particlelike interactions such as collisions ( 5), fusion and birth ( 6), annihilation ( 7), and spiraling ( 8), typically in the short range (where there is significant overlap in their optical fields) and in rare cases, over long distances (where overlap is negligible and beams are remote) ( 9, 10). Self-trapped light beams and spatial solitons emerge in a rich variety of photoresponsive materials that display intensity-dependent changes in refractive index ( 1 – 4). 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. 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. ![]() 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. 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. We find that repeatedly switchable self-trapped visible laser beams, which exhibit strong pairwise interactions, can be generated in a photoresponsive hydrogel. Progress, however, has been limited by the need for reversibly responsive materials that host such nonlinear optical waves. Next-generation photonics envisions circuitry-free, rapidly reconfigurable systems powered by solitonic beams of self-trapped light and their particlelike interactions. ![]()
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