Wavefront shaping techniques are being actively developed to achieve optical focusing through and inside opaque scattering media. These techniques promise to revolutionize biophotonics by enabling deep-tissue non-invasive optical imaging, optogenetics, optical tweezing, and light-based therapy. Among the existing wavefront shaping techniques, optical time-reversal-based techniques determine the optimum wavefront globally based on the principle of time reversal, without the need to perform time-consuming iterations to optimize each mode in sequence. In all previous optical time-reversal-based wavefront shaping experiments, Nyquist sampling criterion was followed so that the scattered light field was well-sampled during wavefront measurement and wavefront reconstruction. In this work, we overturn this conventional practice by demonstrating that a high-quality optical focus can still be achieved even when the scattered light field is under-sampled. Even more strikingly, we show both theoretically and experimentally that the focus achieved by the under-sampling scheme can be one order of magnitude brighter than that achieved by the well-sampling schemes used in previous works, where 3×3 to 5×5 pixels sampled one speckle grain on average. Moreover, since neighboring pixels were uncorrelated in feedback-based wavefront shaping, introducing the concept of sub-Nyquist sampling in time-reversal-based wavefront shaping makes the optimal phase maps obtained using these two different methods consistent. We anticipate that this newly explored under-sampling scheme will transform the understanding of optical time reversal and boost the performance of optical imaging, manipulation, and communication through opaque scattering media.
Yuecheng Shen, Yan Liu, Cheng Ma, and Lihong Wang, "Applying sub-Nyquist sampling in optical time-reversal-based wavefront shaping to boost targeted light transport through opaque scattering media (Conference Presentation)," Proc. SPIE 10502, Adaptive Optics and Wavefront Control for Biological Systems IV, 105020Z (Presented at SPIE BiOS: January 29, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2287064.5752159295001.
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