Time-reversal of light could improve focusing and imaging in scattering media

University of Twente (Enschede, the Netherlands), FOM Institute of Atomic and Molecular Physics (Amsterdam, the Netherlands), and Langevin Institute at École Supérieure de Physique et de Chimie de la Ville de Paris (Paris, France) researchers are developing new techniques to improve imaging and focusing in scattering media. A wave (sound or light) emanating from a point source in a scattering medium will spread and develop a speckle pattern due to interference of different scattering pathways. The time reverse—a speckled wave converging toward a point and becoming a single bright spot—is also possible due to reciprocity. In ultrasound such a wave can be created by electronically recording the outgoing wave and playing it backward, a method that is not possible in optics. However, for light it is possible to directly create a wave that converges to a focus by shaping the incident wavefront using spatial light modulators. In this case, feedback from a detector at the target point is used to shape the wavefront properly.

In the past few months, the researchers say that groups worldwide have demonstrated many of the extraordinary possibilities of wavefront-shaped light, including focusing and imaging in and through turbid media, pulse compression, and spectral selection. Remarkably, both in ultrasound and light the focus obtained through a scattering medium can in some situations be much smaller than a focus made without scattering, an effect that is being exploited for microscopy. Contact Allard P. Mosk at [email protected].