Metalens-based design reduces footprint and makes optical traps handy for precision sensors and measurements – ScienceDaily


Researchers have developed tiny optical tweezers based on chips that allow nanoparticles to float optically in a vacuum. Optical tweezers – which use a tightly focused laser beam to hold living cells, nanoparticles, and other objects – can be used for a wide variety of precision measurements and sensor applications. However, these optical traps are typically made with bulky optical components.

“By using an ultra-thin metal, we have reduced the diameter of the focusing lens from about 25 mm to about 0.4 mm,” said the head of the research team Tongcang Li from Purdue University. “The chip-based design can be used to create an integrated and flexible optical system for studying shallow forces by capturing an object less than 1 micrometer from a surface. It could also be useful for trapping cold atoms in a vacuum to study quantum processes. “

In optics, the journal for high-impact research of the Optica Publishing Group, researchers from Purdue University and Pennsylvania State University report the first realization of on-chip optical levitation in a vacuum with ultra-thin metals. Doing this in a vacuum helps improve the sensitivity of the system.

“Optical floating particles can be used to create accelerometers and gyroscopes that could potentially be used in navigation,” said Li. “Scientists also use optical floating particles to search for dark matter and dark energy, and briefly use gravity To study distances, which will deepen our understanding of nature. “

On the way to a portable trap

This new research builds on previous work in which researchers used optical levitation in a vacuum to create the fastest man-made rotor and most sensitive torque detector ever reported.

“The next step was to make optical levitation technology more practical by minimizing the system enough to be portable,” said Li. “We started by reducing the focus lens size by using a metalens, sort of flat lens that uses nanostructures to focus the light. “

In the new work, the researchers designed a metal that consists of thousands of silicon nanopillars. The metalens were about 50 times smaller in diameter than the conventional objective lenses they had previously used.

“Other research groups recently demonstrated metal-based optical trapping in liquids,” said Kunhong Shen, the lead author of the paper. “Although optical trapping in a vacuum helps to minimize noise from liquid or air, it is also much more difficult.”

Float with a flat lens

To test their new optical design, the researchers aimed an intense laser beam at the metals to generate trapping forces. Then they sprayed a dilute nanoparticle solution into the capture area. When a nanoparticle is caught, it appears as a bright spot that can be observed with a camera. Photon detectors measured the movement of the nanoparticles in real time.

They showed that the metalens a nanoparticle in a vacuum at a pressure of. can float 2 ×10-4 Torr – about 1/4,000,000 atmospheric pressure – with no need for feedback stabilization. They were also able to transfer a floating nanoparticle between two separate optical traps.

“Our metalens is a nanostructured layer with a thickness of only 500 nm and a large numerical aperture of around 0.9. It performs similarly to a traditional bulky lens, ”said research team leader Xingjie Ni of Pennsylvania State University. “The metalens is completely vacuum compatible. And even more interesting, we can flexibly design them for additional functions, for example filtering out low-frequency components from the focusing light, which is beneficial for the optical levitation of nanoparticles. “

Researchers are now working to improve the tiny levitation devices by increasing the metals’ transmission and focusing efficiency. They also want to make the diameter of the metals even smaller to make optical levitation more practical for real world applications.

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