A scanning electron microscope image of the compressor with a 158 nm diameter nanopillar. Credit: Harvard SEAS
Today, a variety of applications use quick bursts in laser light that last less than one trillionth of a second. Scientists can now use these ultrashort laser pulses to see chemical reactions in action, image delicate biological samples and build precise nanostructures. They also allow them to send high-bitrate optical communications long distances.
However, any use of ultrashort laser pulses within the visible spectrum must overcome one fundamental problem: red light travels faster through transparent materials such as glass than blue. The ultrashort pulses of laser light pass through glass lenses, and the tightly packed wavelengths are separated, negating their usefulness.
Optics researchers have been struggling with this chromatic dispersion issue for many decades. Most solutions today involve adding components to increase the size or bulk of optical devices.
Researchers at Harvard John A. Paulson School of Engineering and Applied Sciences have created a silicon coating that can be applied to the glass lens' surface to counteract dispersion.
Nature Communications published the research.
Federico Capasso is the Robert Wallace Professor in Applied Physics, Vinton Hayes Senior Fellow in Electrical Engineering at SEAS, and the senior author of this study.
This ultra-thin coating is made up of precisely-designed silicon pillars which capture and hold briefly red light before reemitting it. This temporary hold allows slower-moving blue light catch up.
Marcus Ossiander (postdoctoral researcher at SEAS) and the first author of this paper stated that the coating "counteracts the dispersive effect of transparent materials, acting like a speed bump to red light and average out the speed of every wavelength of light," he said.
Researchers tested the coating by reducing laser pulses to a few quadrillionths. Nanopillar silicon coating was created using the same commercial tools as industrial semiconductors. This makes it possible to quickly apply these coatings on existing optical components and extend the usefulness of femtosecond pulses.
Ossiander stated that anyone can now buy a lens and apply the coating to it without worrying about dispersion. This approach could be used to create a variety of non-dispersive or anti-dispersive optics.
Continue reading Bridging optics and Electronics: A new spatial light modulator marries the optical and electronic realms
More information: Ahmed H. Dorrah et al, Structuring total angular momentum of light along the propagation direction with polarization-controlled meta-optics, Nature Communications (2021). Journal information: Nature Communications Ahmed H. Dorrah et al, Structuring total angular momentum of light along the propagation direction with polarization-controlled meta-optics,(2021). DOI: 10.1038/s41467-021-26253-4
