Simplified Superlens Captures IR Light

Superlenses fabricated from perovskite oxides are simpler and easier to fabricate than metamaterials and are ideal for capturing light in the mid-infrared range, potentially opening the door to highly sensitive biomedical detection and imaging, say researchers at Lawrence Berkeley National Laboratory. The superlensing effect also may be selectively turned on and off, which would make possible highly dense data writing and storage.

Superlenses originally earned their superlative by being able to capture the evanescent light waves that blossom close to an illuminated surface and never travel far enough to be "seen" by a conventional lens. Superlenses hold enormous potential in a range of applications, depending upon the form of light they capture, but their use has been limited because most have been made from metamaterials. The unique optical properties of metamaterials, which exhibit negative refraction, arise from their structure rather than their chemical composition. However, they can be difficult to fabricate and tend to absorb a relatively high percentage of photons that otherwise would be available for imaging.

This experimental setup shows an IR free-electron laser light source and a perovskite superlens consisting of bismuth ferrite (BiFeO₃) and strontium titanate (SrTiO₃) layers. Imaged objects are strontium ruthenate patterns (orange) on a SrTiO₃ substrate. The near-field probe is shown in blue and the evanescent waves in red. (Images: Susanne Kehr et al)

Superlenses overcome the diffraction limit by capturing the evanescent light waves, which carry detailed information about features on an object that are significantly smaller than the wavelengths of incident light. Because evanescent waves dissipate or "vanish" after traveling a very short distance, conventional lenses seldom ever see them.

"We have demonstrated a superlens for electric evanescent fields with low absorption losses using perovskites in the mid-infrared regime," said Ramamoorthy Ramesh, a materials scientist 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 1 μm, about 1/14 of the working wavelength."

Ramesh and his colleagues presented their findings in a paper in the journal Nature Communications titled "Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling."

"A superlens made out of a metamaterial focuses propagating waves and reconstructs evanescent waves arising from the illuminated objects in the same plane to produce an image with subwavelength resolution," said Susanne Kehr, a former member of Ramesh's Berkeley research group and now with the University of Saint Andrews in the UK. "Our perovskite-based superlens doesn't focus propagating waves but instead reconstructs evanescent fields only. These fields generate the subwavelength images that we study with near-field infrared microscopy."

This atomic force microscopy image shows the strontium ruthenate rectangles that were imaged with a perovskite-based superlens using incident light of 14.6 μm.

Kehr and team member Yongmin Liu said that perovskites hold a number of advantages over metamaterials for superlensing. The perovskites they used to make their superlens, bismuth ferrite (BiFeO₃) and strontium titanate (SrTiO₃), 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," Liu said. "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."

This research represents the first application of perovskite materials to superlensing. One of the biggest challenges was to find the right combination of perovskites that would make an effective superlens. The perovskite thin films they fabricated were grown by pulsed-laser deposition and found to be single phase and fully epitaxial. However, this too was a challenge.

"Our superlenses consisted of a layer of bismuth ferrite and a layer of strontium titanate with thicknesses of 200 and 400 nm, respectively, which is rather thick for epitaxial growth with pulsed laser deposition," Kehr said. "At these thicknesses, accurate thickness and flat interfaces become a problem."

Near-field IR microscopy combined with a tunable free-electron laser provided 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 SrTiO₃ substrate – and a near-field scattering probe – a metal-coated atomic force microscope tip with a typical radius of 50 nm.

"At certain distances between the probe and the surface of the object, we observed a maximum number of evanescent fields," Ramesh said. "Comparisons with numerical simulations indicate that this maximum originates from an enhanced coupling between probe and object, which might be applicable for multifunctional circuits, infrared spectroscopy and thermal sensors."

In their Nature Communications paper, Ramesh and his co-authors report that the multiferroic BiFeO₃ 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 said. "This is the concept of data storage, with writing by electric fields and optical readouts."

Liu said that the mid-IR region at which their superlens functions is prized for biomedical applications.

"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," he said. "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 potential to eliminate these limitations."

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