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A group of researchers from Yale and Missouri University of Science & Technology, have found a way to pre-treat a laser beam so that it enters opaque surfaces without dispersing - like a headlight that’s able to cut through heavy fog at full strength. The discovery has potential applications for deep-tissue imaging and optogenetics, in which light is used to probe and manipulate cells in living tissue. A study announcing the technique was published in an edition of the journal Nature Photonics.

According to Principal Investigator Hui Cao, the John C. Malone Professor of Applied Physics and of Physics at Yale, typically, an optical beam propagating through a diffusive medium such as fog will spread laterally, but the team has discovered that a special preparation of the laser beam can transmit all incoming light without lateral spread. The researchers used a spatial light modulator (SLM) and a charge-coupled device (CCD) camera to analyze an opaque material that is made of a layer of white paint. The SLM tailored the laser beam incident on the front surface of the material, and the CCD camera records intensity profiles behind it. With this information, the laser finds a “route” through the white paint.

The result is a beam that is more concentrated, with more light per volume inside and behind the opaque material. In addition to a layer of white paint, the materials in which the laser would be effective include biological tissue, fog, paper, and milk. The new method works for any opaque medium that does not absorb light, according to Cao.

The first author of the study is Yale postdoctoral research associate, Hasan Yılmaz. Additional authors are Yale postdoctoral researchers, Chia Wei Hsu and Missouri University of Science and Technology and Associate Professor, Alexey Yamilov. Enhancing optical energy in opaque scattering media is extremely important in optogenetics and deep-tissue imaging, according to Yılmaz. Currently, penetration depth to probe and stimulate or image neurons inside the brain tissue is limited due to multiple-scattering.

Financial support from the U.S. Office of Naval Research and the US-Israel Binational Science Foundation helped to fund the research.