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Photoacoustically guided wavefront shaping for enhanced optical focusing in scattering media


Non-invasively focusing light into strongly scattering media, such as biological tissue, is highly desirable but challenging. Recently, ultrasonically guided wavefront-shaping technologies have been developed to address this limitation. So far, the focusing resolution of most implementations has been limited by acoustic diffraction. Here, we introduce nonlinear photoacoustically guided wavefront shaping (PAWS), which achieves optical diffraction-limited focusing in scattering media. We develop an efficient dual-pulse excitation approach to generate strong nonlinear photoacoustic signals based on the Grueneisen relaxation effect. These nonlinear photoacoustic signals are used as feedback to guide iterative wavefront optimization. As a result, light is effectively focused to a single optical speckle grain on the scale of 5–7 μm, which is 10 times smaller than the acoustic focus, with an enhancement factor of 6,000 in peak fluence. This technology has the potential to benefit many applications that require a highly confined strong optical focus in tissue.

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Figure 1: Principles.
Figure 2: Experimental set-up and dual-stage optimization.
Figure 3: Experimental results of stage 1: using a linear photoacoustic signal as feedback for wavefront shaping (linear PAWS).
Figure 4: Experimental results of stage 2: using a nonlinear photoacoustic signal as feedback for wavefront shaping (nonlinear PAWS).
Figure 5: Visualization of single speckle grain focusing using nonlinear PAWS.


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The authors thank K. Maslov for manufacturing the acoustic lens, C. Ma for assistance with acoustic focus calibration, T.-W. Wong for help with preparing the supplementary cartoons and J. Ballard for editing the manuscript. This work was sponsored in part by National Institutes of Health grants DP1 EB016986 (NIH Director's Pioneer Award) and R01 CA186567 (NIH Director's Transformative Research Award) as well as National Academies Keck Futures Initiative grant IS 13.

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Authors and Affiliations



P.L., J.W.T. and L.V.W. initiated the project. P.L. implemented the photoacoustically guided wavefront-shaping system. L.W. initiated the principle of dual-pulse photoacoustic nonlinearity based on the Grueneisen relaxation effect. J.W.T. wrote code for the experiment and simulations. P.L., J.W.T. and L.W. designed and ran the experiment, and prepared the manuscript. L.V.W. provided overall supervision. All authors were involved in analysis of the results and revision of the manuscript.

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Correspondence to Lihong V. Wang.

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Competing interests

P.L., J.W.T. and L.W. declare no competing financial interests. L.V.W. has financial interests in Microphotoacoustics, Inc. and Endra, Inc., which, however, did not support this work.

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Lai, P., Wang, L., Tay, J. et al. Photoacoustically guided wavefront shaping for enhanced optical focusing in scattering media. Nature Photon 9, 126–132 (2015).

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