Helmholtz resonator for Surface Enhanced Terahertz Absorption spectroscopy

Helmholtz optical cavities have recently emerged as promising platforms for enhancing light–matter interactions at subwavelength scales. There resonance have been successfully exploited in the infrared for broadband molecular detection using Surface Enhanced InfraRed Absorption (SEIRA) spectroscopy. In this work, we extend the concept to the terahertz (THz) range, introducing a Helmholtz resonator architecture tailored for Surface-Enhanced Terahertz Absorption (SETA). Such resonators are designed to operate in the over-coupled regime, where reflectivity remains non-zero, but can transition toward critical coupling upon the introduction of an absorbing analyte. This change leads to a measurable reflectivity contrast that can be exploited for molecular sensing. We present both simulated and experimental results demonstrating the interaction between the resonator and thin layers of 2,4-dinitrotoluene (2,4-DNT). Numerical results show a reflectivity contrast of up to 20% as a function of analyte thickness. Experimental measurements on fabricated samples confirm the resonance behavior and the expected contrast.