Review on the terahertz metasensor: from featureless refractive index sensing to molecular identification

The terahertz (THz) wave is at the intersection between photonics and electronics in the electromagnetic spectrum. Since the vibration mode of many biomedical molecules and the weak interaction mode inside the molecules fall in the THz regime, utilizing THz radiation as a signal source to operate substance information sensing has its unique advantages. Recently, the metamaterial sensor (metasensor) has greatly enhanced the interaction between signal and substances and spectral selectivity on the subwavelength scale. However, most past review articles have demonstrated the THz metasensor in terms of their structures, applications, or materials. Until recently, with the rapid development of metasensing technologies, the molecular information has paid much more attention to the platform of THz metasensors. In this review, we comprehensively introduce the THz metasensor for detecting not only the featureless refractive index but also the vibrational/chiral molecular information of analytes. The objectives of this review are to improve metasensing specificity either by chemical material-assisted analyte capture or by physical molecular information. Later, to boost THz absorption features in a certain frequency, the resonant responses of metasensors can be tuned to the molecular vibrational modes of target molecules, while frequency multiplexing techniques are reviewed to enhance broadband THz spectroscopic fingerprints. The chiral metasensors are also summarized to specific identification chiral molecules. Finally, the potential prospects of next generation THz metasensors are discussed. Compared to featureless refractive index metasensing, the specific metasensor platforms accelerated by material modification and molecular information will lead to greater impact in the advancement of trace detection of conformational dynamics of biomolecules in practical applications.