“Spectral studies of the lateral and vertical distributions of evanescent waves around the image plane of our lens show that we have achieved an imaging resolution of 1 µm, about one-14th of the working wavelength,” explains Ramamoorthy Ramesh, a materials scientist with the Department of Energy (DOE)’s Lawrence Berkeley National Laboratory’s Materials Sciences Division. Ramesh led research to develop a superlens made of perovskite oxide—which offers improvement over the standard metamaterial in terms of performance and fabrication. The superlens functions at the mid-infrared spectral region important for biomedical applications.
Conventional lenses—which create images by capturing light waves emitted by an illuminated object and then bending these light waves into focus—can resolve images no smaller than about half the wavelength of the illuminating (incident) light. Superlenses capture the evanescent light waves, which carry detailed information about features that are significantly smaller. Because evanescent waves dissipate after traveling a short distance, conventional lenses seldom see them. And while a superlens made of a metamaterial focuses propagating waves and reconstructs evanescent waves arising from the illuminated objects in the same plane, the perovskite-based superlens ignores propagating waves and only reconstructs evanescent fields. “These fields generate the sub-wavelength images that we study with near-field infrared microscopy,” says Susanne Kehr, lead author of a paper describing the work.1
|The perovskite superlens consists of bismuth ferrite (BiFeO3, dark green) and strontium titanate (SrTiO3, light green and gray) layers. Imaged objects are strontium ruthenate patterns (SrRuO3, orange) on a SrTiO3 substrate. An IR free-electron laser light source generates evanescent waves (red); the near-field probe is shown in blue. An externally applied electric field could make the superlens tunable—enabling sharper imaging and activation of specific portions of the lens. (Image courtesy Susanne Kehr et al.)|
This research represents the first application of perovskite to superlensing, and perovskites hold a number of advantages over metamaterials for superlensing, say Kehr and metamaterials expert Yongmin Liu. The perovskites they used, bismuth ferrite and strontium titanate, feature a low rate of photon absorption, and can be grown as epitaxial multilayers whose highly crystalline quality reduces interface roughness—which means reduced photon loss. This combination of low absorption and low scattering loss significantly improves resolution. Ramesh and his co-authors say that the multiferroic bismuth ferrite layer should make their superlens tunable through the application of an external electric field. This tunability could be used to change the superlensing wavelength or sharpen the final image—or more importantly, to turn the superlensing effect on and off, which would make it possible to activate and deactivate certain local areas of the lens.
“Most biomolecules have specific absorption and radiation features in this range that depend on their chemical composition and therefore yield a fingerprint in the spectra,” says Liu. “However, compared with optical wavelengths, there are significant limitations in the basic components available today for biophotonic delivery in the mid-infrared. Our superlens has the potentials to eliminate these limitations.”
1. S.C. Kehr et al., Nature Comm. 2, 249 (2011).