Optical focusing into scattering media via iterative time reversal guided by absorption nonlinearity
Abstract
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.
Copyright and License (English)
© The Author(s) 2025. This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Acknowledgement (English)
The authors would like to thank Yan Liu, Yousuf Aborahama, Yide Zhang, Zhe He, and Geng Ku for their valuable discussions and assistance.
Funding (English)
This work was supported in part by National Institutes of Health grant R35 CA220436 (Outstanding Investigator Award to L.V.W.), and by grant number 2024-337784 from the Chan Zuckerberg Initiative DAF, an advised fund of the Silicon Valley Community Foundation, also awarded to L.V.W.
Data Availability (English)
The experimental data generated in this study are provided in the Supplementary Information or available from the corresponding author upon request.
Code Availability (English)
The code implemented in this study is available from the authors upon request.
Conflict of Interest
L.V.W. has a financial interest in Microphotoacoustics, Inc., CalPACT, LLC., and Union Photoacoustic Technologies, Ltd., which, however, did not support this work. The other authors declare no competing interests.
Supplemental Material
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Additional details
- PMID
- 40841371
- PMCID
- PMC12370915
- National Institutes of Health
- R35 CA220436
- Chan Zuckerberg Initiative (United States)
- 2024-337784
- Silicon Valley Community Foundation
- Accepted
-
2025-08-06
- Caltech groups
- Division of Engineering and Applied Science (EAS)
- Publication Status
- Published