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Ag/TiO2 Flat Lens Shows Promise For Lab-On-A-Chip Devices

EuroPhotonics
Sep 2016
AMSTERDAM — A flat optical lens could help reduce the size of computer hard drives and enable further miniaturization of microscopes and other technologies. The lens, comprising thin layers of silver and titanium dioxide, offers a larger field of view and a shorter working distance than traditional curved lenses, allowing close placement to an object of interest.

Researchers at FOM Institute AMOLF created the thin layers of silver and titanium dioxide using physical vapor deposition, and cited a major challenge to be optimizing the layer thicknesses with subnanometer precision. The researchers found that a 10-layer structure alternating between 53.2-nm-thick layers of silver and 25-nm-thick layers of titanium dioxide produced the best flat lens.

UV flat lens
Scanning electron microscope image of a cross section of a fabricated multilayer structure. Five unit cells of Ag (light) and TiO2 (dark) are clearly visible. The TiO2 layer is overcoated by Pt to improve the quality of the focused ion beam cross-section. Courtesy of Maas, et al./Optica, a publication of The Optical Society (OSA). 
The experimental lens operates in the UV spectrum due to the materials used, though the researchers believe other materials could produce a flat lens that works at other wavelengths. The researchers used the lens to produce a clear image of a 100-nm slit placed about 350 nm from the lens. The demonstration lens had lateral dimensions of 25 × 25 μ, and the researchers say that it could be enlarged to centimeters or more.

In addition to enabling lab-on-a-chip devices due to its small size and ability to focus light from a range of angles, the flat lens could be used used with optical recording techniques such as magneto-optical recording or heat-assisted magnetic recording to increase the information storage capacities of hard drives.

The researchers are also examining the possibility of tuning the optical properties of the lens using an electrical signal, which could bring promising applications in telecommunications.

“Information is transmitted through optical fibers with optical signals,” said researcher Ruben Maas. “Connecting this optical signal to an electrical signal, or imprinting an electrical signal onto an optical signal is a relatively slow and tedious process at the moment, but we envision that a flat lens could be used for electro-optical coupling by applying a voltage over the multilayer structure, which then modulates the transmission.”

They noted that lens does show significant loss of light due to reflection, but the ability to use it in ways that are impossible for traditional lenses — such as on a very small scale, where alignment is critical, or very close to the object — will override this drawback for those applications.

The research was published in Optica, a publication of The Optical Society (OSA) (doi: 10.1364/optica.3.000592).


GLOSSARY
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
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