Quasi-Waveguiding and Topological Charge Detection of Vortex Beams via Chlorophyll-Induced Thermal Nonlinearity

Nonlinear effects such as self-induced waveguide play important roles on propagation of vortex beams in many applications such as optical imaging and communication in biological tissues, significantly influencing the penetration depth, energy distribution, and efficiency. In this work, we investigate the propagation dynamics of vortex beams through chlorophyll solutions and analyze the concentration-dependent nonlinearity across vortex beams with varying topological charges. Notably, our study reveals that a vortex beam in chlorophyll solutions is capable of establishing a quasi-optical waveguide over a relatively long distance at specific power levels attributed to the self-defocusing nonlinearity. Most interestingly, we observe that the output profiles of the vortex beams shrink with increasing power, accompanied by slight fragmentation, before expanding into asymmetric rings with fork-like fringes above a certain power threshold. The number of “hollows” from fragmentation and the count of forked fringes both depend on the topological charge. These intriguing spatial self-phase modulation phenomena are primarily attributed to azimuthal instability and interference caused by thermal conduction and convection effects, respectively. Our theoretical model, based on the nonlocal nonlinear Schrödinger equation with thermal effects, aligns well with the experimental observations. Our findings provide valuable insights into the micro-dynamics of particles in optofluids and chlorophyll's optical properties, offering a cost-effective, simplified method for detecting the topological charge of vortex beams and developing related medical and biological applications.