Siberian Federal University: Physicists Have Studied an Optical Effect that will Help Create New Sensors

Scientists from Siberian Federal University as part of an international research team have for the first time experimentally discovered a chiral Tamm plasmon polariton which is localized at the interface between a cholesteric liquid crystal and a metasurface. Based on the discovered effect, it is possible to construct a whole new range of photonics devices, in particular, bio- and temperature sensors that will allow obtaining home analysis data, as well as laser radars and microlasers with a “twisted” beam.

The conventional Tamm plasmon polariton is a light wave trapped between a thin metal film and a multilayer reflector. Such wave becomes chiral when a medium that does not have mirror symmetry is used as a reflector. For example, a cholesteric liquid crystal, consisting of oriented elongated molecules, the direction of which twists in space like a helical spiral, similar to a DNA helix. A metal mirror is of little use in this case because, when reflected from it, the wave changes the direction of polarization. For example, incident light of right-handed circular polarization is reflected with left-handed circular polarization. For this reason, the light wave is not locked, but constantly leaks through the liquid crystal. This problem can be solved by replacing the metal film with a metasurface (an artificially created array of meta-atoms) of subwavelength elements, the dimensions of which are smaller than the wavelength of light.

The Siberian researchers showed that the wavelength of a chiral Tamm plasmon polariton can be tuned by changing ambient temperature. According to scientists, this effect can form the basis of modern sensors for medical analysis. The article was published in the peer reviewed journal «Materials».

« “We have fabricated a metasurface consisting of 190 x 70 x 70 nanometre gold nanobricks placed on a 100-nm layer of silicon dioxide (SiO2) deposited on a 200-nm-thick reflective gold plate. The nanobricks were 45° relative to the orientation of the liquid crystal. The combination of such a metasurface with a cholesteric liquid crystal made it possible to lock the light with right circular polarization, while the left-hand polarized light could freely leak through the structure”, explained Rashid Bikbaev, associate professor of the Department of Electrical Technology and Electrical Engineering, researcher at the Laboratory of Nanotechnology, Spectroscopy and Quantum Chemistry of the School of Engineering Physics and Radioelectronics SibFU.

The researchers also showed how the wavelength of a chiral Tamm plasmon polariton could be changed by adjusting ambient temperature. For example, a temperature increase by only 3 degrees (from 26° to 29° C) shifts the wavelength of the localized state by more than 100 nanometres.

“In nature, chiral photonic structures can be observed on bluish fern leaves and spruce needles, in the skin of some berries and integuments of beetles and butterflies shining in the sun. At the same time, the experimental implementation of artificial analogues of such structures is laborious and time-consuming work. One needs to build a theoretical model and find appropriate materials and technologies. In the case of chiral Tamm plasmon polaritons, we tested several experimental schemes with various types of anisotropic mirrors based on nanocomposites, multilayers, and metasurfaces. And now, after five years of searching, this beautiful idea came to fruition”, said Ivan Timofeev, head of the research team from the Russian side, professor of the Department of Theoretical Physics and Wave Phenomena, head of the Laboratory of Nanotechnologies, Spectroscopy and Quantum Chemistry of the School of Engineering Physics and Radioelectronics of the SibFU.

The team also included scientists from the L. V. Kirensky Institute of Physics of Krasnoyarsk Research Centre (SB RAS) and the National Yang Ming Chiao Tung University (Taiwan).