Looking and Listening in Complex Media

Lead Research Organisation: University of Glasgow
Department Name: School of Physics and Astronomy

Abstract

Scattering of light is what gives complex media like fog, paint, biological tissues, and most materials around us their opaque appearance. It is also the reason why we can't see inside or through many objects: even if light is not absorbed, multiple scattering scrambles any information into a uniform halo which seems to contain very little information. But in fact, elastic light scattering is a fully deterministic process, so the information about the object we would like to image is conserved. What happens is that the information is present in a scrambled form, not readily usable by us.
In recent years, the idea that this scrambled information can be used to our advantage started to make its way in to the scientific community and today wavefront shaping allows us to manipulate scattered light with an unprecedented accuracy. Proper imaging techniques that exploit wavefront shaping and the properties of scattered light to our advantage, are still few and far between, but the fundamental Physics is well established and ripe for the development of real-world applications.
We propose to apply wavefront shaping techniques to the recently developed field of time-of-flight imaging, where the arrival time of single photons is detected to reconstruct position and shape of objects we have no direct line of sight with. This is usually accomplished by sending an ultrashort laser pulse to scatter from a surface (e.g. a wall) from where it can bounce on the hidden object, scatter again toward us, and then be detected. The problem with this approach is that at each bounce the light is dispersed in all directions, so that only a very small fraction is finally available for detection. Wavefront shaping will allow us to control where light is scattered, so as to increase the signal to noise ratio by orders of magnitude with respect to current approaches. The control over the scattered light will also allow us to focus the laser light directly on the target object and raster scan over it, thus allowing us complete 3D hyperspectral imaging of an otherwise hidden object.
We will apply the same principle to the problem of "laser microphone", where the light backscattered from a vibrating surface is detected to remotely reconstruct the sound producing the vibration. Exploiting wavefront shaping we will enhance the directionality of the backscattered light, increasing the signal to noise, and thus both the maximum working distance (>100m) and the range of materials one can detect sound from.
Furthermore, we will take advantage of the complex properties of light scattering, and in particular of speckle correlations, to develop a completely passive technique (relying only on ambient illumination) to image and track objects hidden in a complex environment.

Publications

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