A silicon coating made of precisely designed silicon pillars has been developed for glass lenses to counteract the effects of light dispersion.  

Laser pulses lasting less than a trillionth of a second enable scientists to observe chemical reactions in real time, image biological samples, build nanostructures, and send long-distance, high-bit-rate optical communications.

Ultra-short laser pulses applied to the visible spectrum must overcome a basic difficulty, that is, red light passes through transparent materials such as glass faster than blue light. When ultra-short laser pulses pass through a glass lens, the closely packed light wavelengths will separate and affect the usefulness of the beam.

The solution to this problem involves additional components that increase the size and volume of the optical device.

Now, researchers at Harvard University's John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Graz University of Technology in Austria have developed a silicon coating that, when applied to the surface of glass lenses, can counteract the dispersion effect.

The research was published in Nature Communications.

"Our flexible method can be quickly implemented in traditional optics and optical devices, and is applicable to different spectral regions and applications," said Federico Capasso, SEAS Professor of Applied Physics and Vinton Hayes Senior Researcher in Electrical Engineering, and senior author Federico Capasso Research.

The ultra-thin coating uses precisely designed silicon pillars to briefly capture and maintain the red light before re-emission.

"Our coating counteracts the dispersion effect of transparent materials, acting as a speed bump for red light and averaging the speed of light per wavelength," said Marcus Ossiander, a postdoctoral researcher at SEAS and the first author of the paper.

Researchers are said to have tested the coating by shortening the laser pulse to several trillionths of a second. Nano-pillar silicon coatings are made using the same commercial lithography tools as industrial semiconductors. These coatings can be easily applied to existing optical components quickly and expand the applicability of femtosecond laser pulses.

“Now, anyone can buy lenses, apply coatings and use lenses without worrying about dispersion,” O’Xiande said in a statement. "This method can be the basis for a series of anti-dispersion or non-dispersion optics."

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