The optical laser, which will celebrate its 60th anniversary next year, has led to a host of technology applications that have changed our lives and researchers hope that its mechanical analogue – the phonon laser – will one day be just as important. A team at the University of Rochester and the Rochester Institute of Technology, both in New York, has now succeeded in making a phonon laser based on nanospheres of glass suspended in vacuum using an optical tweezer (or dipole trap). The device, which works in the mesoscopic mass range for the first time, might not only be used to help solve fundamental problems in quantum mechanics, it might also find use in precision metrology applications.
Researchers have been working on the phonon laser – a coherent beam amplifier for sound rather than light – for the last decade. In such a device, phonons (which are the smallest discrete unit of vibrations of a material’s crystal lattice) are amplified to generate a highly coherent beam of sound in the same way that an optical laser produces a highly coherent beam of light.
Expanding on the optical tweezer
A team led by Nick Vamivakas has now made the levitated optomechanical analogue to the optical laser by expanding on the optical tweezer. This optical dipole trap, as it is also known, was originally invented by American physicist Arthur Ashkin, who was recently awarded a share of the Nobel prize in physics. It relies on a highly focused laser beam to provide an attractive or repulsive force to physically hold and move micron-sized objects in the trajectory of the beam.
The new phonon laser, whose frequency can be tuned, is based on the centre-of-mass oscillation of silicon nanospheres, which is comprised of phonons, and Vamivakas and colleagues’ experimental apparatus consists of a free-space optical dipole trap in which they suspend the nanospheres in a vacuum chamber. The researchers then employ a feedback technique based on light scattering from the nanospheres. “By measuring the scattered light, we are then able to alter the way the beads oscillate and increase the output of energy as measured in phonons,” says Vamivakas.
“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.”
Controlling the population of steady-state, coherent, phonons
“This technique allows us to modulate the optical potential created by the laser beam that holds the nanospheres in the trap in just the right way to produce the phonon laser,” explains Vamivakas. “The feedback signals then control the centre-of-mass dynamics of the sphere.
“One signal provides nonlinear parametric cooling of centre-of-mass phonons, while the other induces linear amplification of centre-of-mass phonons,” he says. “This allows us to control the population of steady-state, coherent, phonons – into the quantum regime, in principle.”
Mesoscopic mass regime
The new device operates in the mesoscopic mass regime – that is around 1 x 10-18 kg. This makes it different to previously demonstrated phonon lasers that worked on the microscale (1 x 10-9 kg) and atomic scale (1 x 10-25 kg).
“There was a large mass regime in between these two scales, and this is the range in which our device works,” Vamivakas tells Physics World. “It is also unique in that it makes use of a levitated object. With the exception of single trapped atoms, all other phonon lasers to date have been demonstrated in mechanically clamped or tethered platforms in which the laser is attached to a substrate.
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“Using a levitated object provides a high degree of mechanical isolation not possible in these other set ups.”
Technique is readily extendable to other materials
The new laser might help advance precision metrology, he adds. What is more, the technique employed in this study, which is published in Nature Photonics 10.1038/s41566-019-0395-5, is insensitive to the structural details of the particle suspended in the optical dipole trap. This means that it could be readily extended to other materials – for example, single electrons, levitated droplets or even biological organisms.
The team, which also includes researchers from the Los Alamos National Laboratory, says that it is now busy exploring the connections between the optical laser and its phonon cousin. “We are also looking at the ways in which our laser could enhance precision measurements in levitated optomechanical systems,” reveals Vamivakas.
