Harvard silicon coating offsets light dispersion

2021-11-25 10:41:06 By : Ms. Jeff Yang

The nanostructured surface compresses laser pulses in traditional optical devices.

Surface effects: Wavelength control A project from the metamaterial developer Federico Capasso of the Harvard SEAS laboratory can help solve the unwanted light dispersion problem in the optical system.

This may be a particular benefit for applications using femtosecond laser pulses, where the fundamental difference between the speed of red light passing through the glass and the speed of blue light passing through the same glass can lengthen the pulse and cause problems for imaging and communication. .

As published in Nature Communications, this new research has developed a silicon coating that can be applied to the surface of glass lenses and counteract dispersion effects.

"Our flexible method can be quickly implemented in traditional optics and optical devices, and is applicable to different spectral regions and applications," Federico Capasso said.

The coating consists of silicon nanopillars approximately 600 nanometers high and 160 nanometers wide, effectively acting as a waveguide array on the surface of the glass lens.

The interaction between the column and the electric field distribution of the incident light can counteract the time broadening of the pulse passing through the glass that normally occurs, substantially delaying the red wavelength and allowing the blue wavelength to catch up.

The project said in its published paper: "We customized the dispersion of silicon nanopillar arrays so that they use slow light effects to reshape pulses in time during transmission and act as ultrashort laser pulse compressors."

One result of this method is that it can be applied to two different scenarios. Nano coatings can be applied to thin substrates to intentionally shorten elongated pulses, or to thick optical elements to compensate for their group delay dispersion, making the coating the basis of a series of anti-dispersion or non-dispersion optical elements. this project.

The research continues to apply patterned nano-surfaces to light manipulation and removal of unwanted wavelength effects, which has potential value in a variety of applications. Other examples include a project at Columbia University in 2018 that developed a lens that reduces dispersion and focuses a wide range of wavelengths into the same focal point.

Engineering of complex pulse shapes

"Our coating counteracts the dispersion effect of transparent materials, acts as a speed bump for red light, and averages the speed of light per wavelength," said Marcus Ossiander, a postdoctoral researcher at Harvard SEAS.

In experiments, the coating was proven to be capable of broadband pulse compression in the visible to near-infrared spectrum, which makes it important in many potential applications. Nanopillar coatings are made using commercial photolithography tools, so they are easy to apply to existing optical components.

According to the Harvard Oceanographic Institute, future work may see stacks or more complex structures, extending the technology to design complex pulse shapes to coherently control chemical reactions and quantum systems or optimize nonlinear processes.

“Now anyone can buy lenses, apply coatings and use lenses without worrying about dispersion,” Ossiander said.