Have you ever stood under a dome and whispered, only to hear the echo of your voice come back much louder? Researchers at NIST used a similar principle to improve the atomic force microscope (AFM), allowing them to measure rapid changes in microscopic material more accurately than ever before.
An AFM works by using a minuscule sharp probe. The instrument detects deflections in the probe, often using a piezoelectric transducer or a laser sensor. By moving the probe against a surface and measuring the transducer’s output, the microscope can form a profile of the surface. The NIST team used a laser traveling through a circular waveguide tuned to a specific frequency. The waveguide is extremely close (150 nm) to a very tiny probe weighing about a trillionth of a gram. When the probe moves a very little bit, it causes the waveguide’s characteristics to change to a much larger degree and a photodetector monitoring the laser light passing through the resonator can pick this up.
The probe tip on an AFM — known as the cantilever — is a specialized nanostructure. The tip of the probe is usually only a few atoms across. Obviously, the smaller and lighter the cantilever, the more responsive the instrument will be, but usually the harder it is to read its output. The deflections may come from physical contact or the probe tapping against the surface. AFMs can also measure atomic forces. As the probe approaches a surface, atomic-scale attraction pulls it closer. But as it gets too close, repulsive forces push it away. These are the deflections the AFM uses to map the surface under the probe.