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Detecting nanometric displacements with optical ruler metrology

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Science  24 May 2019:
Vol. 364, Issue 6442, pp. 771-775
DOI: 10.1126/science.aaw7840

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Superoscillatory displacement metrology

The resolving power of light (or other waveforms, for that matter) is typically limited to about half the wavelength. However, multiple interference of waves gives rise to subwavelength “hotspots” in the phase owing to what is termed superoscillation of the wave field. Yuan and Zheludev used a specially designed metasurface to interfere laser light (wavelength λ = 800 nm) and created a superoscillatory ruler comprising these hotspots. They demonstrated the ability to measure displacements of around λ/800 while operating at a wavelength of 800 nm. They then showed theoretically that resolving powers of around λ/4000, i.e., atomic-scale displacements, may be possible. The technique should prove useful to metrology applications requiring precision measurements.

Science, this issue p. 771

Abstract

We introduce the optical ruler, an electromagnetic analog of a physical ruler, for nanoscale displacement metrology. The optical ruler is a complex electromagnetic field in which singularities serve as the marks on the scale. It is created by the diffraction of light on a metasurface, with singularity marks then revealed by high-magnification interferometric observation. Using a Pancharatnam-Berry phase metasurface, we demonstrate a displacement resolving power of better than 1 nanometer (λ/800, where λ is the wavelength of light) at a wavelength of 800 nanometers. We argue that a resolving power of ~λ/4000, the typical size of an atom, may be achievable. An optical ruler with dimensions of only a few tens of micrometers offers applications in nanometrology, nanomonitoring, and nanofabrication, particularly in the demanding and confined environment of future smart manufacturing tools.

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