A Novel Plasmonic Nanoantenna-Based Sensor for Illicit Materials and Drugs Detection

Terahertz (THz) spectroscopy and imaging are of significant interest in molecular detection and identification. Illicit drugs are characterized by a different absorption spectrum, with sharp absorption peaks located in the THz frequency range. In this article, the design, simulation, and fabrication of two novel plasmonic nanoantennas with resonance frequencies over the 1–10-THz range for illicit materials and drugs detection are proposed. The first nanoantenna, called the butterfly nanoantenna, is composed of two symmetrical butterfly shaped gold arms with a length of $46.48~\mu $ m, a width of $22.40~\mu $ m, and a thickness of 200 nm, separated by a gap of 20 nm. The two-layer substrate consists of 3- $\mu $ m silicon and 50-nm gold. The results of the simulation, performed by the 3-D CST Studio Suite 2023, show three different resonance frequencies at 2.1, 3.6, and 5.1 THz, respectively, with a −10-dB ${S}_{{11}}$ lower than −18 dB and a directivity of up to 11.04 dBi. Starting from the single butterfly nanoantenna design, a novel geometrical nanoantenna, called the shamrock nanoantenna, is composed of three symmetrical arms, each with a length of $23.14~\mu $ m, a width of $22.40~\mu $ m, and a thickness of 200 nm, separated by a central gap of 20 nm is introduced. The simulation results show two different resonance frequencies at 1.9 and 3.3 THz, with −10-dB ${S}_{{11}}$ lower than −26 and −20 dB, respectively. A sensitivity of 1500 GHz/RIU at 1.9 THz, 2500 GHz/RIU at 2.1 THz, 6000 GHz/RIU at 3.3 THz, 10000 GHz/RIU at 3.6 THz, and 11500 GHz/RIU at 5.1 THz is theoretically observed for the detection of illicit materials and drugs with a thickness of $5~\mu $ m of analyte. The multiband sensor based on the proposed plasmonic nanoantennas with an enhancement factor (EF) of $6.09\times {10}^{{5}}$ shows a very broad observation range providing a nondestructive method.