Laser Beam Sneaks Through Opaque Media

Researchers 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 from scientists at Yale Univer­sity and the Missouri Uni­versity of Science & Technology has potential appli­cations for deep-tissue imaging and opto­genetics, in which light is used to probe and manipulate cells in living tissue.

By shaping its spatial wavefront, a laser beam can propagate through a strongly scattering medium without lateral diffusion. In addition, the backscattering of light is suppressed. (Source: Yale U.)

“Typically, an optical beam propa­gating through a diffusive medium such as fog will spread laterally, but we have discovered that a special preparation of the laser beam can transmit all incoming light without lateral spread,” said principal inves­tigator Hui Cao. 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. “Our method works for any opaque medium that does not absorb light,” Cao said. “Enhancing optical energy in opaque scat­tering media is extremely important in opto­genetics and deep-tissue imaging,” his colleague Hasan Yilmaz said. „Currently, pene­tration depth to probe and stimulate or image neurons inside the brain tissue is limited due to multiple-scat­tering.“ (Source: Yale U.)

Reference: H. Yilmaz et al.: Transverse localization of transmission eigenchannels, Nat. Phot., online 4 March 2019; DOI: 10.1038/s41566-019-0367-9

Link: Hui Cao Research Lab., Dept. of Applied Physics, Yale University, New Haven, USA

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