Cause by the diffraction limit a focused image can never be smaller than half the wavelength of light used to observe an object. Attempts to break this limit with super lenses have all hit the hurdle of extreme visual losses, making the lenses opaque. Now, physicists at the university of Sydney have shown a new pathway to achieve super­lensing with minimal losses, breaking through the diffrac­tion limit by a factor of nearly four times. The key to their success was to remove the super lens altogether.

The work should allow scientists to further improve super-resolution micro­scopy, the researchers say. It could advance imaging in fields as varied as cancer diagnostics, medical imaging, or archaeo­logy and forensics. Alessandro Tuniz from the Nano Institute said: “We have now developed a practical way to implement super­lensing, without a super lens. “To do this, we placed our light probe far away from the object and collected both high- and low-resolution information. By measuring further away, the probe doesn’t interfere with the high-resolution data, a feature of previous methods.”

Previous attempts have tried to make super lenses using novel materials. However, most materials absorb too much light to make the super lens useful. Tuniz said: “We overcome this by performing the superlens operation as a post-processing step on a computer, after the measurement itself. This produces a truthful image of the object through the selective amplifi­cation of evanescent, or vanishing, light waves. Boris Kuhlmey added: “Our method could be applied to determine moisture content in leaves with greater resolution, or be useful in advanced micro­fabrication techniques, such as non-destructive assessment of microchip integrity. And the method could even be used to reveal hidden layers in artwork, perhaps proving useful in uncovering art forgery or hidden works.”

Typically, super­lensing attempts have tried to home in closely on the high-resolution information. That is because this useful data decays exponen­tially with distance and is quickly overwhelmed by low-resolution data, which doesn’t decay so quickly. However, moving the probe so close to an object distorts the image. “By moving our probe further away we can maintain the integrity of the high-resolution infor­mation and use a post-observation technique to filter out the low-resolution data,” Kuhlmey said.

The research was done using light at terahertz frequency at millimeter wavelength. Kuhlmey said: “This is a very difficult frequency range to work with, but a very interesting one, because at this range we could obtain important infor­mation about biological samples, such as protein structure, hydra­tion dynamics, or for use in cancer imaging.” Tuniz added: “This technique is a first step in allowing high-resolution images while staying at a safe distance from the object without distorting what you see. Our technique could be used at other frequency ranges. We expect anyone performing high-reso­lution optical microscopy will find this technique of interest.” (Source: U. Sydney)

Reference: A. Tuniz & B. T. Kuhlmey: Subwavelength terahertz imaging via virtual superlensing in the radiating near field, Nat. Commun. 14, 6393 (2023); DOI: 10.1038/s41467-023-41949-5

Link: Institute of Photonics and Optical Science, School of Physics, University of Sydney, Camperdown, Australia