An international team of researchers has developed a silicon-based metasurface that transmits light in a narrow range of wavelengths irrespective of a source’s polarization. The material can be used to produce optical chips for screening devices that evaluate antimicrobial activity, as well as saliva-based allergen and glucose sensors, and polarizing filters for computer vision. The results of the study are published in ACS Photonics.

_{A schematic of the developed metasurface and its application in optical screening of saliva. Illustration by Aleksandra Kutuzova}

Light is a directed oscillation of electric and magnetic fields that is naturally depolarized, causing it to be emitted in any random direction, which poses a challenge for the advancement of such nanophotonic devices as optical detectors, sensors, and filters. 

In this type of devices, light is typically controlled through resonant effects, with one of the major ones being electromagnetically induced transparency. This is a phenomenon in which a material turns transparent to polarized light in a given frequency range. This occurs due to the interaction of two different-Q factor (quality factor) resonances in antiphase modes. While a high-Q resonator efficiently stores energy, exhibits high frequency selectivity and lower radiation speed, low-Q resonators demonstrate energy losses into the environment and a wider frequency bandwidth. 

Meantime, high-Q resonators have a narrow transparency window and consequently require higher-Q electromagnetically induced transparency, which can be achieved by referring to the physics of symmetry-protected bound states in the continuum (BICs). In this case, light is trapped inside the metasurface due to its unique geometry. 

Modern devices based on the effect of electromagnetically induced transparency, due to their complex design, can recognize only one of the perpendicular directions of linearly-polarized light and therefore transmit only a part of the energy. As a result, their efficiency decreases by half. 

Scientists from ITMO’s Faculty of Physics and Zhejiang University proposed and experimentally tested a new design of a high-Q silicon metasurface that relies on two BICs – high-Q resonances in perpendicular linear polarizations. Thanks to its structure, the metasurface can transmit light regardless of a source’s polarization in a narrow range of wavelengths – 60 nm. However, as noted by the developers, the transparency window of the surface can be adjusted by tens of nanometers. 

“Our team was the first to create a metasurface that supports electromagnetically induced transparency and is polarization-independent.  We increase the quality factor of the effect by using two BICs and select the shape and size of metasurface components in order to achieve the transparency windows in two polarizations. Then, we optimize the system and combine these windows at one frequency to obtain a structure insensitive to polarization, while ensuring the stability of the effect at different geometric parameters. We also confirmed the efficiency of our approach through a series of experiments,” explains Aleksandra Kutuzova, the first author of the paper and a PhD student at ITMO’s Faculty of Physics.

A polarization-independent optical component may potentially be used to develop optical chips for screening devices that examine the antimicrobial activity of various compounds against bacteria. Its lack of sensitivity makes it applicable for use in medical research with various optical sources, for example, LEDs and lasers. Additionally, the material can enhance the sensibility of optical sensors that are used to detect small concentrations of allergens or measure glucose levels not in blood, but in saliva. It will make the testing process simpler and more affordable and convenient for patients. And last but not least, the metasurface can serve as a polarizing filter for improving the quality of computer vision images. 

“In this research, we studied the linear polarization of light that caused the metasurface to become transparent. For our next step, we’ll focus on circular polarization, the electric vector of which moves in circles, rather than along a straight line. We want to build a structure that is sensitive to various circular polarizations: for instance, one that reflects and one that transmits light at a specific frequency. The new technology can be used to optically detect organic chiral molecules that mirror one another. One of such molecules, left-handed, can be a medication, and the other, right-handed, – a poison. Two types of chiral molecules interact differently with circular polarizations,” says Mikhail Rybin, a team lead and a senior researcher at ITMO’s Faculty of Physics. 

The research is supported by the Russian Science Foundation (grant No. 24-72-10038), the National Natural Science Foundation of China (grants U2341225 and 62375242), and the Science and Technology Program of Sichuan Province (grant 2025YFHZ0297).