Ultrasensitive Terahertz Metamaterial Sensor Based on Planar Metal–Insulator–Metal Structure

Due to the abundant spectral characteristic of biological molecules in the terahertz (THz) region, THz metamaterials are widely used in the biosensing field. The THz technology as combined with metamaterials offers label-free and nondestructive capabilities. However, the sensitivity of THz sensors is often limited by the poor spatial overlap between the analyte and the enhanced electromagnetic field. For the conventional metamaterial sensors, the analyte is typically absorbed onto the sensor surface, while the induced hot spots are confined within the insulating layer, resulting in relatively low sensitivity. In this study, we introduce a novel and ultrasensitive metamaterial sensor consisting of a planar metal-insulator–metal (p-MIM) structure with a 1-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula>m air gap between two metal stripes. Unlike conventional vertical MIM (v-MIM) metamaterial, the p-MIM metamaterial generates highly localized enhanced fields within the air gap, enabling direct interaction with the analyte. The proposed metamaterial sensor achieves a high sensitivity of 157 GHz/RIU with an analyte thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1~\mu $ </tex-math></inline-formula>m, outperforming the v-MIM metamaterials by a factor of 2.27. In addition, the metamaterial sample is fabricated using a maskless laser direct write lithography system, and its resonant spectra are measured by THz time-domain spectroscopy (THz-TDS). The enhanced sensing performance of the proposed p-MIM metamaterial sensor is experimentally demonstrated by detecting photoresist with varying thicknesses. Moreover, the proposed metamaterial sensor is also used to detect bovine serum albumin (BSA), whose limit of detection (LOD) is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.08~\mu $ </tex-math></inline-formula>g/mL. This work inspires more innovations to facilitate the development of biosensing devices.