A silica nanoparticle levitated in an optical trap could be used as a tiny pressure sensor that detects collisions with individual atoms and molecules – according to researchers in the US. Their proof-of-principle demonstration originated in the team’s work in developing novel neutrino and dark-matter detectors. It could lead to the development of a background pressure sensor to work alongside other optomechanical sensors.
In recent years, physicists have proposed that hypothetical particles such as dark matter could be detected when they occasionally scatter from nanometre-sized objects held in optical traps. Alternatively, if a nano-object contains a radioactive substance, the kinetics of the decay products could be studied by observing the object’s recoil. To make such detections however, physicists must understand the nature of background collisions between nanoparticles and residual gases in their vacuum chamber.
“Usually when an atom collides with an atom it just disappears because the impact is so large that you won’t be able to keep it in the trap,” explains Yu-Han Tseng of Yale University in Connecticut. He adds, “This is the dominant mechanism explaining why experiments by people working with atom tweezers have a limited lifetime – there will be gas colliding with your atoms.”
Simple harmonic motion
In the new research, Tseng and colleagues in the group of David Moore at Yale and a physicist at Lawrence Berkeley National Laboratory trapped a silica nanosphere of radius 50 nm using a focused laser beam within a vacuum chamber. At the focus of the trap, the particle naturally underwent simple harmonic motion. The researchers then injected small amounts of different gases into the chamber. The atoms or molecules collided randomly with the sphere, causing relatively small motional disturbances to the much larger sphere.
If the sphere had not been trapped, these collisions would have caused it to move randomly around the chamber – a familiar phenomenon called Brownian motion. Under these circumstances, it would have been impossible to track the particle. However, with the particle trapped, collisions resulted in a perturbations to the particle’s harmonic motion without dislodging it.
By measuring these perturbations, the team was able to detect the collision impulses on the particle, and from this to infer the gas pressure: “I think what’s quite unique is that, even with a macroscopic object, we can do a very precise measurement,” says Tseng.
Pressure rise
The researchers tested three different gases – krypton, xenon and sulphur hexafluoride. In each case, they started the experiment with the sphere at the highest vacuum they could achieve and gradually injected the gas, increasing the pressure until it reached around 1.5 nbar. This is well below atmospheric pressure which is about 1 bar.
They monitored the motion of the nanosphere in the direction of the trapping laser using the phase of the backscattered laser light. For each gas, they took data at seven different pressures, detecting the rate and size of individual impulses. They found that the rate increased with pressure and was broadly consistent with what would be expected from the kinetic theory of gases.
Their work shows that individual collisions can be resolved by monitoring a macroscopic system, and the researchers believe that this could be potentially be applicable in a variety of experiments – as well, potentially, as a future pressure standard. “We’d be delighted if someone were to take this further and turn it into a workable pressure sensor that you could integrate into another system,” says Tseng’s Yale colleague Clarke Hardy. The researchers’ own interests for the technology lie elsewhere, however: “The main goal of our research is to dope the nanospheres with radioactive isotopes that will undergo decays and allow us to reconstruct the momentum of emitted neutrinos,” says Hardy.
The research is described in Physical Review Letters.
Optical tweezers think big
Maxime Perdriat of Switzerland’s ETH Zurich is the lead author of a paper published earlier this year in Physical Review A. He and his colleagues have also detected single-atom impulses on a nanosphere trapped in an optical tweezer. In the Swiss work, the researchers compared non-thermal impulses – which occurred when a supersonic beam of xenon atoms was fired at the sphere – with thermal impulses. The latter were produced by blocking the beam’s path, so the xenon simply flowed into the chamber and increased the background pressure. The goal was to distinguish the signal from an atomic beam from that of background noise.
“I think David Moore’s work is an amazing result,” Perdriat says; “Contrary to what we have done he’s able to measure the distinction between when you can resolve single atoms and when you have too many atoms to resolve, and this has very important implications for dark matter, for sterile neutrinos…I think it’s a very elegant and beautiful paper.” He believes both works point towards similar future applications: “I think both works are really looking at the same long term applications in fundamental physics and particle physics in the end,” he says.
