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March 30th, 2011

Perovskite-based tunable superlens developed

Published on March 30th, 2011 | By: Eileen De Guire

Experimental setup shows an IR free-electron laser light source and perovskite superlens consisting of 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. Credit: Kehr, et. al.

The DOE’s Lawrence Berkeley National Lab and Ramamoorthy Ramesh are in the news again. A new release from the Berkeley Lab announces a novel mode of fabricating a superlens for the infrared spectrum using, for the first time, perovskite-based oxides.

Ramesh is the leader of this research and senior author of a recent Nature Communications paper titled, “Near-field examination of perovskite-based superlenses and superlens-enhanced probe-object coupling” (doi:10.1038/ncomms1249).

The innovation is an alternative to super-resolution imaging approaches that are based on metamaterials. In brief, metamaterials are tough to make and absorb a lot of precious light energy.

According to the release, “The perovskite-based oxides on the other hand are simpler and easier to fabricate and are ideal for capturing light in the mid-infrared range. This opens the door to highly sensitive biomedical detection and imaging. It is also possible that the superlensing effect can be selectively turned on/off, which would open the door to high dense data writing and storage.”

The group says it able to achieve an imaging resolution of λ/14 at the superlensing wavelength.

One of the biggest challenges for the researchers according to the report was finding the right combination of perovskites that would make an effective superlens. What they landed on was a layer of bismuth ferrite and a layer of strontium titanate with thicknesses of 200 and 400 nanometers, respectively. These thin-films were grown by pulsed-laser deposition.

This atomic-force microscopy image shows the subwavelength strontium ruthenate rectangles that were imaged with perovskite-based superlens using incident IR light of 14.6 micrometer wavelengths. Image from Kehr, et. al

AFM image of subwavelength strontium ruthenate rectangles imaged with perovskite-based superlens using incident IR light of 14.6 micrometer wavelengths. Credit: Kehr, et. al.

In the lab’s release, Susanne Kehr, formerly with Ramesh’s Berkeley research group and now with the University of St. Andrews (Scotland) and Yongmin Liu, a metamaterials expert at Berkeley’s NSF Nanoscale Science and Engineering Center, provide additional information about the advantages of perovskites. “The bismuth ferrite and strontium titanate material 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,” they say. “This combination of low absorption and scattering losses significantly improves the imaging resolution of the superlens.”

This research was carried out by an international collaboration of scientist, including ACerS member Lane Martin at the University of Illinois, Champaign-Urbana.

Because of thickness and related wavelength issues, these investigators sought and found a way to gain detailed control over the superlens, selective tuning sections or toggling the effect via an external electric field.

“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 read-outs.”

Liu says that the mid-infrared spectral region at which their superlens functions is prized for biomedical applications. “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.”

Liu suggests there the world is full of opportunities for these materials, saying, “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.”

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