Researchers at the National Institute of Standards and Technology (NIST) have redesigned the atomic force microscope (AFM). AFMs, premier tools in nanotechnology, use a small probe, or tip, to map the submicroscopic hills and valleys it comes in contact with.
Although the AFM has already revolutionized the understanding of nanostructures, scientists are now eager to study nanoscale phenomena, such as the folding of proteins or the diffusion of heat, which happen too quickly and generate changes too small to be accurately measured by existing versions of the microscope.
By fabricating an extremely lightweight AFM probe and combining it with a nanoscale device that converts minuscule deflections of the probe into large changes of an optical signal inside a waveguide, the researchers have broken new ground: Their redesigned AFM measures rapid changes in structure with high precision. This lets the new AFM measure time-varying nanoscale processes that may change as quickly as 10 billionths of a second.
The new AFM features two key elements. First, the researchers shrunk and slimmed down the AFM’s probe—a small cantilever that acts like a spring—deflecting and vibrating when it touches the sample. The new probe, which was made by NIST, weighs a mere trillionth of a gram. The minuscule mass lets the probe respond more quickly to tiny forces or displacements.
The researchers integrated the cantilever with a miniature disk resonator that acts like an optical version of a whispering gallery. Just as a whispering gallery lets certain sound frequencies travel freely around a dome, the resonator lets certain frequencies of light to circulate around the disk.
The AFM cantilever and the disk are separated by only 150 nm. That’s close enough that tiny motions of the cantilever change the resonant frequencies in the disk, in effect transforming the AFM probe’s small mechanical motions into large changes in optical signal.