Dynamic underwater gradient-index lens formed by acoustic-vortex-induced cavitation

We investigated an acoustic-vortex-based dynamic lens capable of manipulating light underwater by generating pressure-tunable annular cavitation structures. High-speed imaging revealed the presence of stable bubble rings whose mean thickness increased from ~2.4 to 3.39 mm as the input pressure increased from 2.6 to 16.3 MPa. Optical measurements confirmed that higher pressures shortened the effective focal length of the lens, consistent with a larger refractive-index gradient. Ray-tracing simulations using a gradient-index (GRIN) lens model derived from the bubble geometry quantitatively reproduced the observed focusing behavior. These findings clarify the generation mechanism of such lenses and demonstrate their potential as a dynamically reconfigurable optical element for use in underwater applications.