Highly Sensitive and Tunable Absorption-Induced Transparency for Terahertz Fingerprint Sensing With Spoof Surface Plasmon Polaritons

A spoof surface plasmon polaritons (SSPPs) sensor for enhanced terahertz fingerprint detection is numerically and experimentally demonstrated. As lactose thin film is deposited from a prism surface to spoof plasmon surface, the physical principle of sensing changes from total internal reflection (TIR) to absorption-induced transparency (AIT), which is induced by the coupling between a narrowband molecule vibrational resonance of lactose and broadband spoof plasmonic resonance. The coupling strength of the coupled plasmon-lactose system increases with an increasing thickness of lactose (within the enhanced electrical field decay range). The sensitivity based on the AIT effect of fingerprint detection is four times higher than the TIR effect, where the differential reflectance $\Delta {R}$ is obtained from 22.9% to 45.9% compared to that from 15.3% to 17.3%. $8~\mu $ m lactose film was successfully detected and displayed a clear mode splitting in the experiment. It is noted that the resonant frequency can be tuned from 0.47 to 0.59 THz by easily adjusting the coupling air gap, which can compensate for random fabrication errors. The coupling between spoof plasmonic resonance and molecular vibrational modes provides a new way for studying light-matter interactions and flexible fingerprint detection of different molecules with high sensitivity in the terahertz region.