Looking and listening through complex media

Lead Research Organisation: University of Exeter
Department Name: 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.

Planned Impact

This proposal aims to show that the combination of wavefront shaping and single photon detection can enable previously unthought forms of imaging in complex media. The development of fast, reliable and easy to use cameras able to image around a corner, track objects moving behind scattering media, and listen from a remote location, have obvious implications in security and monitoring. The approaches we are proposing are paradigm-changing approaches to some very old and very hard problems and promise to deliver both active and fully passive methods for imaging behind walls or through thick scattering media. This will therefore have a profound impact on the scientific community working in these fields.

Maybe even more importantly, the technology we will develop to enable such end goals will have an impact of their own on both industry and society.

The UK is home to world-leading research on SPAD technology and SPAD arrays. QuantIC, the EPSRC Quantum Technologies Hub for Quantum Imaging has placed SPAD technology and its applications at the forefront of its activities, thus leading to the engagement of a number of companies, ranging from small spin-offs and SMEs to multinationals. Examples of companies directly engaged in the development, use and applications of SPAD array technology in collaborations with the research team are Photon Force, DSTL, Horiba, Thales, Leonardo, ST Microelectonics, and JCCBowers. Other companies such as IDQuantique and Andor have expressed explicit interest to investigate future developments. This places the UK therefore not only at the forefront of research in this field but also as a driving force in the move towards commercial exploitation.

Moreover, vision is the most direct way in which humans explore the world around them. Therefore, innovation in imaging technology is easily understood by the public. Imaging technologies are thus a great gateway for science popularization and to introduce the public to otherwise complex scientific results. In particular, the difficulty of seeing in complex environments (fog, behind a wall etc.) are experienced by everyone on a daily basis, the outcomes of this project will allow us to engage the public with the research in cutting-edge optics.

Publications

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Description We have preliminary data showing that the relative motion of multiple objects hidden behind a scattering layer can be tracked by measuring the speckle autocorrelation.
Exploitation Route Once the measurement will be completed and the technique fully developed, this will open a new route to non-invasively track the motion of fluorescently labelled cells inside opaque tissues. Tracking large object (human sized) through thicked media (walls) will require more work.
Sectors Aerospace, Defence and Marine,Pharmaceuticals and Medical Biotechnology