The diffraction limit enforces physical restrictions on how closely an object can be examined using traditional optical methods. Previous attempts to develop superlenses that image beyond the diffraction limit have met with extreme visual losses, to the point of making the lenses opaque. A virtual superlensing approach developed by researchers at the University of Sydney has broken through the diffraction limit by a factor of nearly four times. The researchers’ innovative approach to superlensing could improve superresolution microscopy for fields as varied as medical imaging, archaeology, and forensics. Researcher Alessandro Tuniz (left) and professor Boris Kuhlmey in their Sydney Nanoscience Hub laboratory. Courtesy of Stefanie Zingsheim. Most attempts to develop superlenses have focused on high-resolution data, which decays exponentially with distance and is quickly overwhelmed by low-resolution data, which does not decay as quickly. Moreover, to measure high-resolution data, the probe needs to near the object, which can cause distortion in the image. The researchers placed the light probe for the virtual superlens far away from the object and used the probe to collect both high- and low-resolution data. Spatial resolution was affected by a trade-off between measurement distance and signal-to-noise ratio, rather than by distance alone. By measuring farther away from the object, the researchers prevented the probe from interfering with the high-resolution data. “By moving our probe farther away we can maintain the integrity of the high-resolution information and use a post-observation technique to filter out the low-resolution data,” professor Boris Kuhlmey said. The superlens operation is performed as a post-processing step on a computer, after the measurement is taken. “This produces a ‘truthful’ image of the object through the selective amplification of evanescent, or vanishing, lightwaves,” researcher Alessandro Tuniz said. The data encoded in the evanescent waves is probed without affecting image quality by reconstructing truthful images of the near-field. Most materials used to make superlenses absorb too much light for the superlens to be useful. The virtual superlens circumvents losses by removing the need for materials. The evanescent fields are measured in air, rather than after a structured material, and the reversal of decay is achieved numerically. The researchers quantified trade-offs between noise and measurement distance and experimentally demonstrated a virtual superlens through post-processing. They reconstructed complex images with subwavelength features down to a resolution of λ/7. The images were taken from a distance, greatly reducing field perturbation by the probe. Scientists used a new superlens technique to view an object just 0.15 mm wide using a virtual post-observation technique. The object ‘THZ’ (representing the terahertz frequency of light used) is displayed with initial optical measurement (top right), after normal lensing (bottom left), and after superlensing (bottom right). Courtesy of the University of Sydney Nano Institute. The researchers worked with the terahertz frequency to develop the virtual superlens. “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,” Kuhlmey said. Although the virtual superlens technique is well suited to terahertz near-field photoconductive setups, it could be adapted for use in any near-field experiment that measures amplitude and phase, and could provide a pathway to increasing the imaging resolution of near-field setups at any frequency. “Our technique could be used at other frequency ranges,” Tuniz said. “We expect anyone performing high-resolution optical microscopy will find this technique of interest.” The researchers envision many uses for the virtual superlens. “Our method could be applied to determine moisture content in leaves with greater resolution, or be useful in advanced microfabrication techniques, such as nondestructive 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.” The capability to measure near fields without perturbing them could be particularly useful for imaging fields in structures that are sensitive to perturbations, for example, high-Q/topological resonances, photonic crystal defects, and nanoresonators. “We have now developed a practical way to implement superlensing, without a superlens,” Tuniz said. “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.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-41949-5).
