Extreme Terahertz Field Enhancement for Picogram Molecular Detection

Abstract

Nanogap structures capable of localizing electromagnetic fields in the subwavelength scale have attracted attention as a key platform for enhancing optical and terahertz interactions. In this thesis, I performed a comprehensive study on the fabrication, characterization, and application of various metal– insulator–metal based nanogap structures for THz molecular sensing and nanoscale spectroscopy. I investigated the structures of the bowtie nanogap and the active zerogap device, both of which are expected to exhibit a resonance peak and strong field enhancements near 1 THz. To enable large-area sample fabrication, I implemented bowtie nanogaps through an additional photolithography step using a positive-relief pattern. A tunable nanogap was also created by implementing a zerogap structure on a flexible substrate, allowing for variation in the nanogap width. These structures were designed for the detection of low-concentration molecules. To evaluate the performance of these structures, various customized measurement equipments were established, including an AFM, a confocal Raman and photoluminescence spectroscopy measurement system, and a terahertz time-domain spectroscopy system. Finite element method simulation results of the bowtie nanogap arrays predicted a high field enhancement of 13,000 near 1 THz. This structure enables picogram-level glucose detection as confirmed by terahertz time-domain spectroscopy. The active zerogap structure field enhancement was experimentally verified by its gap width tunability through AFM, optical curvature measurements, and terahertz time-domain spectroscopy. I predicted the conductivity in the active zerogap structure through finite element method simulations. These results demonstrate that an integrated approach combining nanogap structure engineering and spectroscopic application offers new possibilities and a physical foundation for high-sensitivity molecular sensing in the THz regime.