Increasing the resolution of microscopes, by breaking the diffraction limit, can be categorized into near‐field and far‐field operations. The near‐field approaches, such as superlenses, hyperlenses, microsphere-assisted imaging, and near‐field scanning optical microscopy, use evanescent waves containing high spatial frequencies to resolve small objects, but the imaging process needs to be performed very close to the sample. Far-field options, such as emission depletion microscopy (STED), photo‐activated localization microscopy (PALM), and stochastic optical reconstruction microscopy (STORM) are able to provide the resolution at tens of nanometers by selectively activating or deactivating fluorophores, but the pre‐processing and labeling of samples with specific dyes restrict these techniques to biological specimens. Recently, a non‐invasive universal imaging technique, called super‐oscillatory lens (SOL) optical microscopy, was introduced, but this also faces challenges such as a small field of view, the need for complicated nano‐lithographic fabrication of sub‐wavelength features, and the short working distance.
Now, Minghui Hong and colleagues from the National University of Singapore and the Agency for Science Technology and Research (A*STAR), Singapore, report super-critical-lens (SCL) optical label‐free microscopy (see Figure), which clearly distinguishes a pattern with a feature size of 65 nm in air and with a 55 μm working distance. The imaging process is purely physical and captured in real time, does not require any pre‐processing of the samples or mathematical post‐processing of the imaging results. SCL microscopy is able to map the horizontal details of a 3D object through one-time scanning, something that is impossible with other planar lenses and it challenges the received wisdom that the sub‐wavelength imaging requires sub‐wavelength features of the lens. The researchers believe that the method could open the way to wide availability of high‐performance and cost‐effective nanoimaging and nano‐fabrication technologies.