Measuring Tiny Forces With Light

Prototype for the chip-sized small-force meter. The cantilever is near the intersection of the two rectangles in the center of the glass cylinder (Source: NIST)

Prototype for the chip-sized small-force meter. The cantilever is near the intersection of the two rectangles in the center of the glass cylinder (Source: NIST)

Photons have no mass, but they do have momentum. And that allows researchers to do counter­intuitive things with photons, such as using light to push matter around. Recently, a group of scientists led by chemist Gordon Shaw at NIST’s Physical Measure­ment Laboratory PML has been taking advantage of this property to develop devices that can create and measure minute forces, an area tradi­tionally underserved by the metro­logy community. “There are very few references for these small forces,” Shaw says. “This is a way to try and get at those.”

The experiments the group is working on can measure forces that are tiny fractions of a newton – from micro­newtons all the way down to 15 femto­newtons at the level of atomic inter­actions. A pico­newton “will stretch a DNA molecule out,” Shaw says. The PML team is currently developing two types of force-measure­ment devices that use laser light to reliably create small forces. The first is a chip-sized sensor that can use micro- to milliwatt- power light. The second is a tabletop con­traption designed for laser light of about 1 watt, but which could poten­tially be developed for light of tens of kilowatts of power.

The smaller of the two types of force meter being developed by the team is a chip-sized sensor made of fused quartz. It consists of a small canti­lever less than 1 cm in length. The bigger the force, the more the can­tilever moves. A built-in inter­ferometer acts as a motion sensor. Physically pushing the canti­lever is one way to apply a force for measurement. But researchers also need to gauge the sensi­tivity of their sensor. And the best way to measure sensi­tivity is to apply a well-known force to the cantilever and see how the inter­ferometer interprets it.

To manipulate the canti­lever with light, they fit it with a highly reflec­tive, gold-coated surface that can reflect light shining on it from an optical fiber. When this light hits the gold surface, it transfers its momentum to the canti­lever, which begins to vibrate. “If you think of a tuning fork, you can hit it and it will ring at a particular frequency or a particular tone. This does the same thing,” Shaw explains. They found that if you reflect laser light off the surface, there’s a relatively straight­forward way to calculate what the force should be based on the laser power. The higher the power, the more photons there are, and the larger the force that’s generated.

Further­more, since the canti­lever’s resonant frequency changes almost instantly if a mass is placed on it, the mechanism could also be used as a very sensi­tive balance – parti­cularly for objects that are extremely valuable or dangerous. For example, jewelers could use it as a less expensive alter­native for weighing and pricing gemstones. It could even be used poten­tially as a field-portable, dis­posable tool for measuring samples of hazardous materials.

Varia­tions on this design could also be used to improve calibrations of atomic force micro­scopes and even to measure laser power. Unlike the current standard method of measuring laser power with a cryo­genic radio­meter a chip-based laser power meter like this can be used at room tempe­rature and in real time. 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. (Source: NIST)

Reference: J. Melcher et al.: A self-calibrating optomechanical force sensor with femtonewton resolution, Appl. Phys. Lett. 105, 233109; DOI: 10.1063/1.4903801

Link: Small Mass and Small Force Metrology, National Institute of Standards and Technology NIST, Boulder, USA



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