Physicists achieve superlensing without a superlens

Physicists at the University of Sydney have found a way to boost the limits of microscopy with minimal losses, in a milestone that has been published in the journal Nature Communications.

There are physical limits to how closely we can examine an object using traditional optical methods. This is known as the ‘diffraction limit’ and is determined by the fact that light manifests as a wave. It means a focused image can never be smaller than half the wavelength of light used to observe an object.

Attempts to break this limit with so-called ‘superlenses’ have all resulted in extreme visual losses, making the lenses opaque. The Sydney physicists have now managed to achieve superlensing with minimal losses — breaking through the diffraction limit by a factor of nearly four times — by removing the superlens altogether.

“We have now developed a practical way to implement superlensing without a superlens,” said lead author Dr Alessandro Tuniz.

“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 superlenses using novel materials. However, most materials absorb too much light to make the superlens useful.

“We overcome this by performing the superlens operation as a post-processing step on a computer, after the measurement itself,” Tuniz said. “This produces a ‘truthful’ image of the object through the selective amplification of evanescent, or vanishing, light waves.

Superlensing attempts have also previously tried to home in closely on the high-resolution information; this 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,” said Associate Professor Boris Kuhlmey, a co-author on the study.

The research was done using light at terahertz frequency at millimetre wavelength, in the region of the spectrum between visible and microwave. According to Kuhlmey, “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.”

The researchers said their work should allow scientists to further improve super-resolution microscopy, and could advance imaging in fields as varied as cancer diagnostics, medical imaging, or archaeology and forensics.

“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,” Kuhlmey said.

“And the method could even be used to reveal hidden layers in artwork, perhaps proving useful in uncovering art forgery or hidden works.”

Tuniz concluded, “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.”

Image caption: Dr Alessandro Tuniz and Associate Professor Boris Kuhlmey in their laboratory. Image credit: Stefanie Zingsheim.