Threading light through dynamic complex media
The scattering of light impacts sensing and communication technologies throughout the electromagnetic spectrum. Overcoming the effects of time-varying scattering media is particularly challenging. In this article we introduce a new way to control the propagation of light through dynamic complex media. Our strategy is based on the observation that many dynamic scattering systems exhibit a range of decorrelation times -- meaning that over a given timescale, some parts of the medium may essentially remain static. We experimentally demonstrate a suite of new techniques to identify and guide light through these networks of static channels -- threading optical fields around multiple dynamic pockets hidden at unknown locations inside opaque media. We first show how a single stable light field propagating through a partially dynamic medium can be found by optimising the wavefront of the incident field. Next, we demonstrate how this procedure can be accelerated by 2 orders of magnitude using a physically realised form of adjoint gradient descent optimisation. Finally, we describe how the search for stable light modes can be posed as an eigenvalue problem: we introduce a new optical matrix operator, the time-averaged transmission matrix, and show how it reveals a basis of fluctuation-eigenchannels that can be used for stable beam shaping through time-varying media. These methods rely only on external camera measurements recording scattered light, require no prior knowledge about the medium, and are independent of the rate at which dynamic regions move. Our work has potential future applications to a wide variety of technologies reliant on general wave phenomena subject to dynamic conditions, from optics to acoustics.