Optical time-reversal techniques are being actively developed to focus lightthrough or inside opaque scattering media. When applied to biological tissue,these techniques promise to revolutionize biophotonics by enabling deep-tissuenon-invasive optical imaging, optogenetics, optical tweezers and photodynamictherapy. In all previous optical time-reversal experiments, the scattered lightfield was well-sampled during wavefront measurement and wavefrontreconstruction, following the Nyquist sampling criterion. Here, we overturnthis conventional practice by demonstrating that even when the scattered fieldis under-sampled, light can still be focused through or inside opaque media.Even more surprisingly, we show both theoretically and experimentally that thefocus achieved by under-sampling is usually about one order of magnitudebrighter than that achieved by conventional well-sampling conditions. Moreover,sub-Nyquist sampling improves the signal-to-noise ratio and the collectionefficiency of the scattered light. We anticipate that this newly exploredunder-sampling scheme will transform the understanding of optical time reversaland boost the performance of optical imaging, manipulation, and communicationthrough opaque scattering media.
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