3-D Spiraling Self-Trapped Light Beams in Photochemical Systems

Abstract

A pair of visible laser beams self-trap and spiral about each other as they propagate through polymer gels undergoing two different photochemical reactions. When launched into gels that undergo photopolymerization of methacrylate substituents or photo-oxidation of iodide anion, two non-coplanar (skewed) Gaussian beams collide and spiral about each other as they advance through the evolving medium. In the absence of chemical reactions, the linearly polarized beams broaden naturally and propagate along their original, straight-pathed trajectories. By contrast, refractive index gradients generated by the photochemical reactions elicit self-trapping and introduce an attractive interaction between the self-trapped beams. The self-trapped beams spiral about each other when this mutual attraction perfectly counterbalances their original tendency to diverge away from each other. These findings show that the photochemically mediated interactions of incident optical fields within the gel medium impart a helical trajectory and angular velocity to the self-trapped beam pair.

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

“…While thermal and poroelastic interactions lead to repulsion between beams, we anticipated that attractive interactions may occur for short separation distances where there is an overlap in the optical field of the beams, especially in cases where thermal effects are minimal. Attractive interactions between beams are of great interest, as they can lead to the emergence of particle-like behavior, such as beam spiraling 2 and merging. 1 Attractive interactions were not observed previously in the case of two 6 mW beams separated by 25 μm in spiropyranfunctionalized p(AAm-co-AAc) hydrogels, 7 likely due to the large thermal inhibition that results from doubling the incident beam power in a small area.…”

“…A rich variety of optical phenomena, such as self-trapping, spiraling, and spontaneous pattern formation, − occur in materials that have a refractive index ( n ) that is dependent on the intensity of incident light. These phenomena are enabled by the feedback between light intensity, a resulting local change in the refractive index (Δ n ), and the effects of Δ n on the propagation of light, as described by the so-called nonlinear Schrödinger equation…”