Berkeley Lab researchers make first perovskite-based superlens for the infrared

Berkeley, CA--Researchers at the Lawrence Berkeley National Laboratory have fabricated superlenses from perovskite oxides, which are simpler and easier to fabricate than metamaterials. The lenses work in the mid-IR range and are potentially valuable for highly sensitive biomedical detection and imaging. It is also possible that the superlensing effect can be selectively turned on and off, which would open the door to ultradense data writing and storage.

Metamaterials, which are made of intricate arrays of subwavelength cells, can capture evanescent fields as well as freely propagating light waves, enabling imaging at resolutions beyond the diffraction limit. But metamaterials are difficult to make and tend to absorb a relatively high percentage of photons that would otherwise be available for imaging.

In contrast, the perovskite-based superlens does not focus propagating waves, but instead reconstructs evanescent fields only. This still allows superresolution, because evanescent waves carry information on details lost by conventional (nonmetamaterial) lenses that only focus freely propagating light waves.

Fourteenth-wave resolution

"We have demonstrated a superlens for electric evanescent fields with low absorption losses using perovskites in the mid-IR regime," says Ramamoorthy Ramesh, a materials scientist with Berkeley Lab's Materials Sciences Division, who led this research. "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 one micrometer, about one-fourteenth of the working wavelength."

Ramesh is the senior author of a paper in Nature Communications titled "Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling."

Susanne Kehr and co-researcher Yongmin Liu say that perovskites hold a number of advantages over metamaterials for superlensing. The perovskites they used to make their superlens, 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 so there are few photons lost to scattering. This combination of low absorption and scattering losses significantly improves the imaging resolution of the superlens.

"In addition, perovskites display a wide range of fascinating properties, such as ferroelectricity and piezoelectricity, superconductivity, and enormous magnetoresistance that might inspire new functionalities of perovskite-based superlenses, such as nonvolatile memory, microsensors, and microactuators, as well as applications in nanoelectronics," says Liu. "Bismuth ferrite, in particular, is multiferroic, meaning it simultaneously displays both ferroelectric and ferromagnetic properties, and therefore is a good candidate to allow for electric and magnetic tunability."

Unique microscopy approach

Combining near-field IR microscopy with a tunable free-electron laser allowed a first-of-its-kind highly detailed study of the spatial and spectral near-field responses of the superlens. This study led to the observation of an enhanced coupling between the illuminated objects--rectangles of strontium ruthenate on a strontium titanate substrate--and a near-field scattering probe--a metal-coated atomic-force microscope tip with a typical radius of 50 nm.

In their Nature Communications paper, 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, but even more importantly, might be used to turn the superlensing effect on and off.

"The ability to switch superlensing on and off for a certain wavelength with an external electric field would make it possible to activate and deactivate certain local areas of the lens," Kehr says. "This is the concept of data storage, with writing by electric fields and optical readouts."