Scanning electron microscope image of the silicon coating, with 158nm-diameter nanopillars. Image: Harvard SEAS.
Scanning electron microscope image of the silicon coating, with 158nm-diameter nanopillars. Image: Harvard SEAS.

Quick bursts of laser light, lasting less than a trillionth of a second, are used in various applications. For example, these ultrashort laser pulses allow scientists to observe chemical reactions in real-time, image delicate biological samples, build precise nanostructures and send long-distance, high-bitrate optical communications.

But any application of ultrashort laser pulses in the visible spectrum must overcome a fundamental difficulty – red light travels faster than blue light through transparent materials like glass. So when an ultrashort laser pulse passes through a glass lens, the tightly packed wavelengths of light separate, destroying the usefulness of the beam.

This chromatic dispersion problem has plagued optical researchers for decades. Most current solutions require additional components that increase the size and bulk of optical devices.

Now, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed a silicon coating that, when applied to the surface of a glass lens, can counteract the effects of dispersion. They report this novel coating in a paper in Nature Communications.

“Our flexible approach can be rapidly implemented in conventional optics and optical setups and be adapted to different spectral regions and applications,” said Federico Capasso, professor of applied physics and senior research fellow in electrical engineering at Harvard SEAS, and senior author of the paper.

The ultra-thin coating employs precisely designed silicon pillars that briefly capture and hold red light before re-emitting it. This temporary halt allows the slower-moving blue light to catch up.

“Our coating counteracts the dispersive effects of transparent materials, acting as a speed bump for red light and averaging out the speed of each wavelength of light,” said Marcus Ossiander, a postdoctoral research fellow at Harvard SEAS and first author of the paper. The researchers tested the coating by shortening laser pulses to only a couple quadrillionths of a second.

The researchers fabricated the nanopillar silicon coating with the same commercial lithography tools used to fabricate industrial semiconductors, making it easy to quickly apply these coatings to existing optical components and expand the applicability of femtosecond laser pulses.

“Now, anyone can buy a lens, put the coating on and use the lens without worrying about dispersion,” said Ossiander. “This approach can be the basis for an array of anti- or non-dispersive optics.”

This story is adapted from material from Harvard SEAS, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.