Researchers have created tiny chip-based optical tweezers that can be used to optically levitate nanoparticles in a vacuum. Optical tweezers – which use a tightly focused laser beam to contain living cells, nanoparticles, and other objects – can be used for a variety of precision measurements and sensing applications. However, these optical traps are generally made with bulky optical components.
“By using an ultra-thin metal, we reduced the diameter of the focusing lens from around 25mm to around 0.4mm,” said research team leader Tongcang Li at Purdue University. “The chip-based design can be used to create an integrated and flexible optical system to study near-surface forces by trapping an object within 1 micron of a surface. It could also be useful for trapping cold atoms. in a vacuum to study quantum processes. “
In Optical, the Optica Publishing Group Journal for High Impact Research, Purdue University and Pennsylvania State University Researchers Report the First Achievement of On-Chip Optical Levitation in Vacuum with Ultrathin Metal . Performing this feat in a vacuum improves the sensitivity of the system.
“The optically levitating particles can be used to create accelerometers and gyroscopes that could potentially be used in navigation,” Li said. “Scientists are also using optically levitating particles to search for dark matter and dark energy and to study gravity at short distances, which will deepen our understanding of nature. “
Towards a portable trap
This new research stems from previous work in which researchers used optical levitation in a vacuum to create the fastest man-made rotor and the most sensitive torque detector ever reported.
“As a next step, we wanted to make the optical levitation technology more convenient by minimizing the system enough to make it portable,” Li said. “We started by reducing the size of the focusing lens using a metalens, a type of flat lens that uses nanostructures to focus light. “
In the new work, the researchers designed a metal made up of thousands of silicon nanopillars. The diameter of the metals was about 50 times smaller than that of the conventional objective they used before.
“Other research groups have recently demonstrated optical metal trapping in liquids,” said Kunhong Shen, the first author of this work. “While optical vacuum trapping helps minimize liquid or air noise, it is also much more difficult to do.”
Levitate with a flat lens
To test their new optical design, the researchers guided an intense laser beam onto the metals to generate trapping forces. They then sprayed a dilute nanoparticle solution into the trapping area. When a nanoparticle is trapped, it appears as a point of light that can be observed with a camera. Photon detectors measured the movement of the nanoparticle in real time.
They showed that the metalenes could levitate a nanoparticle in a vacuum at a pressure of 2 ×ten-4 Torr – approximately 1 / 4,000,000 atmospheric pressure – without the need for feedback stabilization. They were also able to transfer a levitating nanoparticle between two separate optical traps.
“Our metals are a layer of nanostructure only 500nm thick and with a large numerical aperture of about 0.9. It offers performance similar to that of a conventional bulky lens,” said the head of the Xingjie Ni research team at Pennsylvania State University. “Metals are fully compatible with vacuum. And more interestingly, we can flexibly design it to perform additional functions, for example, filtering out the low spatial frequency components of focusing light, which has been shown to be beneficial for optical levitation of nanoparticles. “
Researchers are now working to improve the tiny levitation devices by increasing the efficiency of transmitting and focusing metals. They also want to further reduce the diameter of metals to make optical levitation more convenient for real world applications.
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