April 2, 2019

Thirty-three years ago, Arthur Ashkin showed how a very tightly focused laser beam attracts tiny particles towards it. When the laser beam moves, the particles move with it, held in the focus of the “optical tweezer” Ashkin created.

This discovery, which earned Ashkin a share of the 2018 Nobel Prize in Physics, has since been applied in a variety of ways. For example, researchers have used optical tweezers to trap and sort healthy cells from infected ones.

Now University of Rochester and Rochester Institute of Technology scientists have found another use, which could shed new light on the fundamental properties of lasers–and perhaps lead to better sensors and other devices.

In a paper in Nature Photonics, they describe trapping nanoparticle-sized silica beads with an optical tweezer in a vacuum. The oscillation of the beads is comprised of phonons–basic units of vibrational energy. In addition, the beads cause some of the laser light to scatter. By measuring the scattered light, the researchers are then able to alter the way the beads oscillate and increase the output of energy as measured in phonons.

“If we do it just right, we can cause an oscillation that starts at one amplitude, and becomes bigger and bigger, until we start to exhibit mechanical motion that is analogous to what you would see if you turned on an ordinary optical laser in our labs,” says co-author Nick Vamivakas, an associate professor of quantum optics and quantum physics at Rochester. He is also lead investigator of a $3 million, multi-university Office of Naval Research grant that is funding the research.

Because the tweezer is operating in a vacuum, “we can simulate the dynamics of an optical laser in a very controlled way,” Vamivakas says. “It will allow us to learn about lasers in a way that wouldn’t be possible otherwise.”

In addition, because the tweezer enables precise measurements of nanoscale particles, Vamivakas is hoping it can be used to test the validity of some basic theories of quantum mechanics, such as quantum wavefunction collapse.

Vamivakas says his lab is still exploring possible practical applications—such as sensors and accelerometers—that could result from the new tweezer. “It’s like when the laser was first discovered,” Vamivakas says. “Nobody knew what the laser would actually be used for. At this point, it is the demonstration that matters.”

The research is a continuation of Vamivakas’ exploration of optical tweezers that began soon after he arrived at the University of Rochester in 2011. Four years ago, for example, his lab was the first to levitate individual nanodiamonds in a vacuum.

Researchers from Northwestern, Yale, University of Maryland, Rochester Institute of Technology, and University of Washington are also collaborating as part of the $3 million grant. It was awarded as part of an ONR basic research challenge for fiscal year 2018, to use levitated optomechanics to gain a better fundamental understanding of quantum and statistical mechanics and thermodynamics, with possible applications for information processing, high resolution sensing, and improved metrology.

Collaborators on this paper included three members of the Vamivakas lab—lead author and PhD student Robert Pettit, PhD student Danika Luntz-Martin, and postdoctoral research associate Justin Schultz–as well as Mishkat Bhattacharya ‘05, an associate professor of physics at RIT; Wencaho Ge and Pardeep Kumar, postdoctoral fellows at RIT; and Levi Neukirch, a former PhD student in the Vamivakas lab, now a researcher at Los Alamos National Laboratory.