Relative humidity refers to the amount of water vapor in the air, which has a wide-ranging influence on human production and daily life. For example, in the industrial sector, relative humidity significantly affects automotive manufacturing. In the healthcare field, humidity sensors that can respond quickly are used in micro-medical systems to diagnose lung diseases. In daily life, different humidity levels can affect respiratory and skin health. Therefore, the study on humidity sensors has attracted widespread attention, especially high-precision humidity sensors that have broad applications. However, the existing available air humidity sensors have drawbacks such as poor electromagnetic interference resistance and large size. Therefore, we propose a new terahertz metamaterial humidity sensor that can achieve high-precision, high-sensitivity, and real-time air humidity monitoring and can be widely used in various fields such as industry, healthcare, and daily life.

In this study, we use a combination of finite element analysis and experimental verification to investigate the feasibility of metamaterial humidity sensors. Firstly, we design a metamaterial made of stainless steel and perform electromagnetic field simulation calculations by using CST Studio Suite simulation software. We optimize the structure of the stainless-steel plate through simulation and obtain the optimal structural parameters. We also simulate and verify that the metamaterial sensor is highly sensitive to air humidity. Furthermore, we use laser micro-drilling technology to fabricate the metamaterial sensor and perform humidity sensing experiments to evaluate its humidity sensitivity performance. In the experiment, we use a homemade humidity control device to enable changes in different humidity environments. The device uses two adjustable power air pumps to blow dry and humid air, and by adjusting the power of the two air pumps, the ratio of dry to humid air can be adjusted, achieving a humidity variation between 4% and 76.1%. Finally, we compare the simulation results with the experimental results to verify the rationality and correctness of the metamaterial humidity sensor.

We use a stainless-steel dumbbell-shaped metamaterial sensor for humidity detection and propose the application of silk fibroin solution on the surface of the metamaterial sensor (Fig. 1). The simulation results show that with an increase in humidity, the transmission peak exhibits significant redshift, indicating that the sensor is highly sensitive to air humidity. Additionally, the peak frequency of the resonance peak exhibits an excellent linear relationship with relative humidity, and the frequency shift is directly proportional to the change in relative humidity (Fig. 5). The calculated figure of merit (Q_FOM) value of the sensor is 3.8, and the Q value is 5.1, which is considered a good level. The experimental results show that the humidity sensitivity of the sensor is 0.11 GHz/% (Fig. 8), which is higher than that of other similar research. By comparing the simulation and experimental data, it shows that they are consistent, which verifies the basic law that the transmission peak frequency is linearly related to relative humidity.

In this study, we propose a terahertz metamaterial humidity sensor for measuring air humidity in the range of 4%-76.1%. The sensor's unit cell is composed of dumbbell-shaped holes on a stainless-steel plate, and it operates in the terahertz frequency band. The simulation results show that the sensor has high refractive index sensitivity, laying the foundation for subsequent humidity sensing experiments. Additionally, silk fibroin is chosen as the humidity-sensitive material, which is sensitive to water molecules. Both simulation and experimental results show that the humidity sensitivity of the sensor is 0.20 GHz/% and 0.11 GHz/%, respectively. These values are higher than those of other reported similar metamaterial humidity sensors. Furthermore, the metamaterial humidity sensor proposed in this study is made of a single stainless-steel material and can be fabricated by using conventional laser drilling techniques, which has the advantages of simple preparation, low cost, small size, and suitability for mass production. Moreover, the sensor is passive and wireless, and it has a wide range of applications. In the future, it can be integrated with on-chip light sources and spectrometers to realize a single-chip integrated humidity sensing system with enormous development potential.