Optical focusing into scattering media via iterative time reversal guided by absorption nonlinearity.

Guide star-assisted time-reversal enables deep optical focusing in scattering media such as biological tissue, overcoming the diffusion limit of light. However, in practice, guide stars formed using contrast agents or ultrasonic modulation naturally occupy regions much larger than the optical resolution, thus limiting their effectiveness. We propose and experimentally demonstrate a time-reversal focusing mechanism in scattering media that achieves focusing to a single speckle grain without the need for point-like guide stars. We exploit optical absorption nonlinearity as a light-induced perturbation to create virtual guide stars. The back-scattered fields in response to high- and low-intensity illuminations are subtracted after scaling to synthesize the field from the dominant speckle grains in the medium. We use this time-reversed field as the incident illumination in subsequent iterations. The focusing is achieved despite the extended layer of nonlinear absorber via a positive feedback loop that favors higher-intensity speckle grains, converging to a single virtual guide star at the optical resolution. We also demonstrate that by adding a phase ramp in each iteration, this focus can be gradually steered beyond the memory effect range.