Researchers from ITMO in collaboration with colleagues from Russia and China have suggested a new design for an ultrasensitive terahertz sensor that contains radiation inside the structure and amplifies its interaction with the analyzed substance. According to calculations, such a system will be capable of highly accurately observing changes in the composition of liquid media without the use of chemical reagents. In the future, the solution can be used in medical diagnostics, environmental monitoring, and industrial quality control. The results of the Russian Science Foundation-supported study are published in Nanophotonics.

^{Credit: ITMO}

Terahertz radiation is a promising tool for medical diagnostics and environmental monitoring. It is safe for humans, can penetrate opaque materials, and interacts with the oscillations of large organic molecules. However, existing terahertz sensors have significant drawbacks: low sensitivity, which prevents them from detecting small changes in substance concentration, and low selectivity, which makes it difficult for the sensor to distinguish between different types of molecules. This occurs because the wavelength of terahertz radiation is several times larger than the size of the molecules: the wave effectively “doesn’t notice” the particle, resulting in very weak interaction between the light and the substance.

Researchers from ITMO, Ioffe Institute, and Harbin Engineering University (China) have suggested a new terahertz sensor design that can “lock” light inside and thus increase its interaction with a substance. The system consists of a metasurface made of graphene strips, a microresonator, and a gold reflective layer. Together, they create the conditions within which terahertz radiation almost doesn’t reflect from the structure and its energy is effectively concentrated inside the microresonator with the studied substance.

When this structure is irradiated with a terahertz wave, it creates a special phenomenon called Fano resonance; it is characterized by an asymmetric shape of the absorption line. As a result, the system enables enhanced light-matter interaction: external radiation leakage is balanced by internal losses, reflection drops to nearly zero, and absorption reaches its maximum. Thus the light is neither reflected nor scattered; instead, it is completely absorbed by the analyte as it repeatedly passes through the sensor structure. 

Another benefit of the suggested system is the option to electrically tune it. By changing the voltage applied to the graphene metasurface, it is possible to control the resonance parameters and adapt the sensor to different conditions while maintaining high sensitivity.

In the future, the authors are planning to produce a prototype sensor and adapt the platform to detect chiral molecules – compounds that possess mirror-symmetrical forms. Even though their chemical composition is identical, these forms can interact with the body differently: one can demonstrate medicinal properties, while the other can prove ineffective or even harmful. That’s why identifying them is crucial in pharmaceutics. 

“We expect that the suggested sensor design will allow us to create a device that is several times more sensitive to changes in media composition than existing counterparts. Such a device will react to the smallest shifts in the refractive index, such as trace amounts of glucose molecules, proteins, viruses, or toxins in a liquid,” explains Mikhail Rybin, one of the paper’s authors, a senior researcher at ITMO’s Faculty of Physics. 

Among the sensor’s applications are the detection of cancer cells and viruses directly inside biopsy samples without the need to stain the biological material or conduct complex preparations; identification of trace amounts of pesticides, herbicides, or industrial pollutants in water; and quality control in the pharmaceutical and food industries when analyzing liquid media.

The project is supported by the Russian Science Foundation, the National Natural Science Foundation of China, and the Postdoctoral Fellowship Program of China.