Hydrogel-based THz wave absorber

Extensive research has been conducted on graphene-based terahertz (THz) wave absorbers, fundamental components in optical systems. A significant focus of these studies has been on enhancing the adjustability of these absorbers. This work explores the use of a hydrogel spacer as a novel approach. The thickness of the hydrogel spacer is directly influenced by the ambient moisture, introducing an additional degree of freedom in tuning the system, beyond conventional methods. An analytical model based on passive circuit elements is developed to describe this mechanism, and full-wave simulations are presented in the results section to validate the concept. The discussion highlights that while most tuning methods primarily affect the absorber’s response to incident radiation, the proposed method uniquely alters the impedance of the spacer itself. This feature complements existing tuning techniques and enables new possibilities in the design of adjustable terahertz absorbers. Additionally, the obtained results from both the circuit model and full-wave simulations, verify the possibility of sweeping the entire THz gap via changing humidity. According to the simulation results, the proposed absorber can show up to eight absorption peaks over 80%. Such a simple and efficient wave absorber in a controlled environment can play a basic building block for optical sensors.