Metamaterial-Based Terahertz Absorbers for Refractive Index Sensing: Types, Mechanism, and Applications

Metamaterials are artificial structures intentionally designed to possess unique properties that are not naturally occurring in nature. Among their diverse applications, metamaterial absorbers offer advantages over conventional absorbers by effectively attenuating electromagnetic wave energy at different resonance frequencies. Terahertz (THz) metamaterial absorbers operate in the THz spectrum. They find applications in sensing and imaging. A review is presented in this paper on THz metamaterial absorbers mainly focusing on their application in refractive index sensing. The different types of resonances have been identified along with the resonance mechanism behind refractive index sensing. When the refractive index of the encompassing medium changes, the resonance frequency shifts, thereby enabling its sensing. Refractive index sensing finds application in the detection of gases, liquids, biomedical cells, etc. State-of-the-art designs have been explored in terms of absorption characteristics, resonance frequencies, bandwidth, sensitivity, and sensing applications. Graphene-based absorbers have also been studied highlighting their unique properties and reported in this review This paper will inspire future research on THz metamaterial absorbers for sensing and detecting applications.