Topological Insulators Light the Way to Photonic Applications

Two members of the NTU team: Dr Harish N. S. Krishnamoorthy (left) and Dr Giorgio Adamo (right).

Scientists from Nanyang Technological University (NTU Singapore) and the University of Southampton have published a paper in Nature Communications reporting that a new class of materials, chalcogenide topological insulators, is especially suitable for creating optical devices.

It is well known that lenses can focus light into tight spots, and prisms can split white light into colours. However, conventional optical elements like lenses and prisms are constrained by many fundamental limitations. For example, conventional lenses are unable to focus light down to less than an optical wavelength (about 500 nanometres).

Over the past two decades, scientists have explored different ways of overcoming these limitations. One popular approach, called “plasmonics”, uses metallic materials to squeeze light into extremely short distances of a few nanometres.  However, plasmonic devices have the disadvantage of being extremely lossy: most of the light energy is converted into electric currents flowing inside the metals, and subsequently dissipated as heat.

Now, a team of physicists and engineers led by Associate Professor Cesare Soci of NTU’s School of Physical and Mathematical Sciences has shown that materials called chalcogenide topological insulators can efficiently confine infrared light in photonic nanostructures.

Unlike plasmonic materials, chalcogenide topological insulators are “dielectric” materials that have very high refractive index and low losses. The losses in these materials at infrared frequencies are about one hundred times lower than conventional plasmonic materials such as gold and silver, while their refractive index is about twice as high as that of competing non-plasmonic materials such as Si and Ge. This makes them highly suitable for creating compact but efficient optical devices, such as next-generation sensors.

Left: Scanning electron microscope images of the photonic nano-slits structures created by Assoc. Prof. Soci and his team.
Right: Computer simulations showing the resonating electromagnetic fields inside one of the nano-slits. Figure credit: Krishnamoorthy et. al

“Topological insulator chalcogenides, as a class of materials, have been studied for several years by materials scientists, but mainly for the purpose of transporting electric currents, like in a wire,” says Associate Professor Soci.  “Our group investigated using them for a different purpose, namely creating photonic nanostructures.  It turns out that at infrared frequencies, their properties are much better than other materials!”

The researchers used state-of-the-art nanofabrication techniques to create prototype nanostructures by carving the surfaces of thin topological insulator crystals, and examined how the nanostructures interact with light at different frequencies.  They discovered that the nanostructures scatter infrared light with unusual efficiency, aided by special currents flowing along their surfaces.

Looking ahead, the team aims to build on this discovery and find ways to exploit the unique properties of topological insulator chalcogenides for novel infrared devices.  They envision applications for telecommunications, thermal imaging, chemical sensing, quantum optics and more.

H. N. S. Krishnamoorthy, G. Adamo, J. Yin, V. Savinov, N. I. Zheludev, and C. Soci, Infrared dielectric metamaterials from high refractive index chalcogenides, Nature Communications 11, 1692 (2020)

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