20 October 2016

The amount of light, passing through an opaque layer, can be enhanced using smart techniques. At the same time, the amount of reflected light diminishes, researchers of the University of Twente now show. It’s as if light speckles have a conversation of their own.

The UT scientists, publishing their new findings in Physical Review A, previously developed a technique for sending light through highly scattering substances like paint or biological tissue. Under normal circumstances, light barely passes because it is internally reflected many times, by the numerous nano particles of the material. Shaping the incident light in a smart way, it will find its way to the ‘exit’. This technique, called wavefront shaping, has a huge impact on optics and already was in the top 10 of most promising new developments of the American Institute of Physics.

Using the technique, a pattern of bright and less bright speckles appear. Enhancing the intensity of one single speckle has an effect on the surrounding area, research now shows: the surrounding area also has a higher intensity. At the same time, the amount of light that is reflected against the surface, diminishes. Inside the layer an energy redistribution seems to take place, as if the transmitted and reflected light speckles talk to each other.

The next interesting question is: will reflection be suppressed in a wide area? Or just in the area in which the intensity of transmitted light is higher, around that single enhanced speckle? The remarkable result is that reflection only diminishes in the area optimized by wavefront shaping. This implies a new type of correlation between transmitted and reflected light. A recent publication already predicted this correlation, but the UT scientists now experimentally prove it for the first time.

This new knowledge can lead to better energy harvesting using solar cells: with less reflection and more transmission, the efficiency can be improved. Also in the field of secure optical communication, these new results help. In medical imaging, sharper images will be possible even through opaque tissue.

The research has been done within the Complex Photonics group, part of UT’s MESA+ Institute for Nanotechnology.

The paper “Controlling the intensity of light in large areas at the interfaces of a scattering medium”, by Oluwafemi Ojambati, John Hosmer-Quint, Klaas-Jan Gorter, Allard Mosk en Willem Vos, appears in Physical Review A, one of the journals of the American Physical Society.