Focusing light through living tissue

Tissues such as skin, fat or cuticle are non-transparent because inhomogeneities in the tissue scatter light. We demonstrate experimentally that light can be focused through turbid layers of living tissue, in spite of scattering. Our method is based on the fact that coherent light forms an interference pattern, even after hundreds of scattering events. By spatially shaping the wavefront of the incident laser beam, this interference pattern was modified to make the scattered light converge to a focus. In contrast to earlier experiments, where light was focused through solid objects, we focused light through living pupae of Drosophila melanogaster. We discuss a dynamic wavefront shaping algorithm that follows changes due to microscopic movements of scattering particles in real time. We relate the performance of the algorithm to the measured timescale of the changes in the speckle pattern and analyze our experiment in the light of Laser Doppler flowmetry. Applications in particle tracking, imaging, and optical manipulation are discussed.