Super-Resolution Technology Gets a Boost From NIST’s Nanomechanical Motion Sensor

Robert L. Stevenson, Ph.D.

Until the super-resolution explosion, detection of small movement with light was restricted to objects larger than the diffraction limit of the light, which is a few hundred nanometers. This limited the study of nanomaterials, until the invention of the atomic force microscope (AFM). AFMs provide exquisitely sensitive detection, but are generally slow. Super-resolution technology focuses on novel, usually very clever transduction principles that use light to improve images and reduce feature size.

 Figure 1 – Schematic of laser beam focused on the cantilever of a plasmonic gap resonator. When the cantilever vibrates, it changes the plasmon resonance frequency, which perturbs the reflection of the laser light focused on the top of the cantilever. The resonator can detect movements as small as 2 picometers, which is 50 times smaller than a hydrogen atom. The resonator was developed by NIST to measure the nanoscale motion of nanoparticles and single molecules. (Image reproduced courtesy of Brian J. Roxworthy, NIST.)

A research team at NIST’s Center for Nanoscale Science and Technology (Gaithersburg, Md.) developed a scalable localized gap-plasmon device capable of measuring movements as small as 2 picometers in an area 150 times smaller than the diffraction limit of their optical system. The measurements are fast, with temporal response in the microsecond range.¹

The device consists of a silicon nitride cantilever that vibrates as a result of thermal motion. Its base material is gold, which supports plasmon just outside its surface. Plasmon refers to the wave-like motion of groups of electrons traveling on the air/gold interface.

Mechanical motion of the cantilever disrupts the gap plasmon, which changes the optical properties of the cantilever. So, when a laser beam is focused on the cantilever, the reflected beam is modulated with a signal generated by the perturbation. Changes in the signal can be related to changes in the cantilever environment. This is sensed as a modulation in the intensity of the reflected laser beam, as shown in Figure 1.

Reference

1. Roxworthy, B.J. and Aksyuk, V.A. Nanomechanical motion transduction with a scalable localized gap plasmon architecture. Nat. Commun. 2016, 7, 13746; doi:10.1038/ncomms 13746.