Photon management in dynamic complex scattering media

We can see through a glass of water, so why can't we see through fog? After all, fog is nothing more than small droplets of water hanging in the air. Unlike a glass of water where light is only reflected from the outer edges, each droplet of fog has its own edge that reflects some light. As there are a colossal number of these droplets, so there are a colossal number of reflections. This is why we can't see through fog: the transmitted light becomes horribly scrambled as it reflects these innumerable times. The huddled bunch of light rays that make up the image of a distant object has little chance of remaining intact after encountering fog! This phenomenon occurs not just in fog, but also in a wide range of materials, including biological tissue, colloidal suspensions like milk, frosted glass, and even as light propagates through optical fibres. These are all "complex media" that are opaque because they muddle up the distribution of light. Such complex media are crucially different from something like black paint where light is absorbed, turned into heat, and lost forever. So, a natural question arises: if we can't see through complex media because they scramble light, can we unscramble what comes out? The answer to this question is yes, provided we can measure everything about the light wave emerging from the material, for a known set of different input waves. The state-of-the-art in this research field can undo the light scrambling due to a complex static medium. That is, in the special case where the configuration of the material is frozen in time, there are recently developed techniques for inverting the effect of the material and reproducing the light wave that entered it, before it became muddled. These advances already promise an enormous leap forward: from non-invasive biomedical microscopy (literally "seeing into a human body") to robust optical communications in highly scattering environments. So, we might be able to see through a cloud of fog using current knowledge, provided the water droplets don't move! We can see the obvious downside of what has been done so far is its restriction to static situations: in real life things do move. Our research will develop techniques that begin to surmount this obstruction, enabling the inversion of propagation through complex dynamic media. We have very recent preliminary work that shows it is possible to theoretically and experimentally find special light waves that avoid regions where a complex medium is most rapidly changing - and instead take the most slowly changing pathway through the medium. We have shown how this can be achieved without any knowledge of where these faster or slower moving regions are. We have also shown that - in principle - combinations of these special light waves could be unscrambled using existing approaches - transmitting images or data through only the most slowly moving parts of the medium. In our proposal, we aim to fully develop this new methodology, discover its advantages and limitations, and, with support from our industrial partners, optimize it for applications. As the research progresses, we shall also push into the regime of ultra-fast dynamic media (so-called time-varying metamaterials), where these wave front shaping ideas may have important applications in, for example, communications, but as yet have barely been considered.