Creating and measuring minute forces is an area often underserved by the metrology community. There are very few references for forces on the order of tiny fractions of a newton – from micronewtons to femtonewtons, at the level of atomic interaction forces.
One device being developed at NIST could fill this gap in a chip-sized package. The sensor takes advantage of a useful property of light: photons have no mass, but they do have momentum, which allows them to apply a small force – called photon pressure – to matter.
Made of fused quartz, the NIST sensor consists of a small cantilever less than 1 cm in length. To manipulate the cantilever, researchers fit it with a mirrored surface that can reflect micro-to milliwatt-power light shining on it from an optical fiber. When this light hits the mirror, it transfers its momentum to the cantilever, which begins to vibrate. The higher the power, the more photons there are, and the larger the force that’s generated. A built-in interferometer act as a motion sensor, which gives a value for the probe’s stiffness.
NIST's Physical Measurement Lab is developing a method to make ultra-precise measurements of small forces and masses using a device smaller than a penny. This animation demonstrates the principles behind this new force-measurement system, which uses a miniature cantilever or "diving board" that vibrates in response to laser light. Video transcript: The device consists of a small cantilever less than 1cm in length. Attached to the cantilever is a small mirror. Laser light directed onto the mirror causes the device to vibrate, much like a tuning fork. Next to the mirror is an optical cavity of changing length that serves as an interferometer. Depending on the length of the cavity, the light trapped inside will constructively or destructively interfere. By measuring how light intensity in the cavity changes, researchers can determine how far the cantilever has moved. This enables them to calculate the amount of force exerted by the light source. Animation: Sean Kelley/NIST PML
Since the cantilever’s resonant frequency changes almost instantly if a mass is placed on it, the mechanism could function as a sensitive balance for objects of a milligram or less. Applications include measurements of substances that are extremely valuable or dangerous. For example, such a device could be used potentially as a field-portable, disposable tool for measuring samples of hazardous materials.
Variations on this design are being used to explore improvements to calibrations of atomic force microscopes (AFM). AFMs operate in limited spaces due to the large amount of acoustic and thermal insulation they require. This means the tip and specimen interact in a tight space within vacuum and isolation chambers – hardly an easy spot to cram calibration equipment. As a result, calibration entails removing the tip and checking it outside the instrument, a process that not only can skew AFM results but also requires calibration equipment that few people outside national metrology institutes possess. With its small size, the new chip-based sensor could work as a frequency reference for AFM probe tips that fits into the same confined space as the AFM probe tip and specimen.
Another potential use for this technology is improving laser power measurements. NIST researchers have already used radiation pressure to significantly reduce, by a factor of 50, the size and weight of NIST equipment for measuring the highest laser powers: The resulting standard reference instrument is now about the size of a shoe box. Preliminary work on a new prototype shows another reduction by a factor of 20 is possible (see photo). A near-chip-sized laser power meter would allow manufacturers to embed high-accuracy power measurements into their additive manufacturing and laser welding machines, for real-time measurements of manufactured parts.
Someday, NIST researchers hope to develop a force measurement device capable of single-photon detection. The proposed scheme would require measuring mere zeptonewtons of force (10-21), which translates to 100 million photons per second.* Difficulties moving forward include finding a way to cool the single-photon force sensors down to just fractions of a degree above absolute zero; a typical cryostat creates far too many vibrations for such precise measurements.
*100 million photons per second hitting a detector once each would impart zeptonewtons (10-21) of force. However, the proposed detector would measure the force of those 100 million photons sequentially, reflecting back and forth between a pair of detector mirrors multiple times, so the actual forces measured by the detector would be in the attonewton (10-18) range.