FOCUSING COHERENT Light
PIONEERING EXPERIMENT
FOCUSING COHERENT LIGHT THROUGH OPAQUE STRONGLY SCATTERING MEDIA
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Ivo M. VELLEKOOP 1, Allard P. MOSK 2, *
1
TechMed Centre, University of Twente, Enschede, The Netherlands
2
Debye Institute and Department of Physics, Utrecht University, Utrecht, The Netherlands * a. p. mosk @ uu. nl
Materials such as biological tissue, paper and paint scatter laser light into a disordered speckle pattern. In an experiment published in 2007, it was shown that this scattering process can be controlled, and that by shaping the incident wavefront one can make the scattered light form a bright focal spot. The ideas leading to this experiment and some of its implications for applications and fundamental physics are described in this article. https:// doi. org / 10.1051 / photon / 202614029
If we had served as the reviewers of our 2007 paper reporting the first focusing of light through strongly scattering materials [ 1 ], we might have concluded that it contained neither deep new theory nor a particularly challenging experiment, and that it offered no clear pathway to applications. Yet despite these apparent shortcomings, the paper was received with remarkable enthusiasm by an open and rapidly growing community. At the time, the general sentiment in the field was that randomly scattered light could not be meaningfully controlled. Lower-frequency waves such as ultrasound and radio frequency could be manipulated using time reversal, as elegantly demonstrated by the group of Mathias Fink, and adaptive optics could correct for aberrations that slightly distort an image. However, light that is scattered in free space is distributed over a very large number of modes – millions even for samples of a mm in size – and controlling all these modes was considered impractical, even though there were forwardlooking theoretical ideas such as by Isaac Freund proposing to employ correlations in scattered light. The dominant approach toward imaging applications, however, was blocking the scattered light and using only the ballistic light, i. e. that portion of the wave energy that was not scattered over large angles [ 2 ]. In the Complex Photonic Systems group of Ad Lagendijk and Willem Vos, we studied correlations in multiple scattered light with an emphasis on fundamental phenomena such as Anderson localization. The original goal of our project was to experimentally reveal the open channels of light transport, that, when the incident wavefront is tailored to them, were predicted to give rise to an enhanced total transmission [ 3 ].
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