Measuring Tiny Forces with Light

Estimated reading time: 6 minutes

But even at the lowest laser powers they have used so far – just millionths of a watt – the light still contains an enormous number of photons. Someday, Shaw says, he hopes to develop a force measurement device capable of single-photon detection. The reason is that integers don’t have uncertainty; if you count individual photons, and you know how much force each photon produces, then you can calculate the force.

“It’s potentially the most precise measurement of force, if we can count them accurately,” Shaw says.

Electrostatic Force Balance

An early prototype of the Electrostatic Force Balance (EFB). The gold-colored cylinder (center) is the capacitor. Applying a force to the balance causes the rectangular flexure (center, right of capacitor) to move. The inner electrode of the capacitor (copper-colored cylinder) is attached to the flexure.

The proposed scheme would require measuring mere zeptonewtons of force (10 -21) which translates to 100 million photons per second – “about as much as you can count,” Shaw says. However, he explains, they are not nearly there yet. And it’s going to take a while.

For one thing, they have to figure out how to cool the single-photon force sensors down to just fractions of a degree above absolute zero, which requires a cryostat. But, as they learned the hard way, a typical cryostat creates too many vibrations for such precise measurements – a factor of 10,000 more than they could accept.

While they prepare to test their prototype in a new, less shaky cryostat design they’ve turned the vibration issue into a potential solution for a different problem.

“We were able to use our force sensors as accelerometers, which let us measure how much vibration the cryostats are really creating,” Shaw says. “It’s a way of testing for vibration in situ, in a place that’s typically very difficult to get to.”

The Electrostatic Force Balance

Finally, the team is working on experiments that use the forces created by higher laser powers – potentially as high as tens of kilowatts, like those used for industrial applications such as welding and cutting metals.

The experiment, which is currently designed for a 1-watt laser, uses a tabletop device called the electrostatic force balance (EFB). Like its chip-sized cousin, the EFB relies on a highly reflective mirror and a laser to create a force to be measured. But instead of using an interferometer, the EFB measures electrostatic force generated by a capacitor, whose plates are two concentric cylinders. In vacuum, the researchers reflect a 1-watt laser off the mirror, then measure that force electronically using the capacitor.

As with the smaller sensor, the laser light used in these measurements is not lost: It bounces off the mirror and – in theory – could be used directly in a laser machining process on a factory floor.

Even for these high-power laser beams, the forces created are “really, really small,” Shaw says. “We’re talking nanonewtons for a 1-watt laser beam. If you pull two atoms apart, it would break a couple of nanonewtons (10 -9).”

Shaw says it’s exciting to be able to use essentially one physics principle for accurate measurements of force, mass, and laser power across such a large range, from milligram-scale objects to atomic interactions. “Because this is still in the basic research phase, there’s a little room to develop new methods and think about things in a different way,” Shaw says.

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