Deep-Subwavelength (~λ/160,000) Terahertz Sensing with an Ultrathin Flexible Metasurface

Terahertz (THz) sensing of deep-subwavelength dielectric films remains a formidable challenge due to the stark mismatch between the long wavelengths of terahertz radiation and the nanoscale thicknesses of the analytes. Although high-quality-factor (Q) metallic resonators are widely used to enhance light-matter interaction, their performance is fundamentally constrained by intrinsic radiative and non-radiative losses. Overcoming these limitations is crucial to achieving strong local-field enhancement and detecting ultrathin films. Here, we present a planar metasurface sensor fabricated on an ultra-low-index, flexible cyclic olefin copolymer (COC) substrate, engineered to achieve strong electromagnetic field confinement within an effective mode volume of 7.52 μm³ (approximately 10-7 (λ/n)3) at 0.94 THz. This design achieves a high Q/Veff ratio of ~1.463, effectively overcoming the limitations of conventional high-Q resonance approaches. Using conventional terahertz time-domain spectroscopy (THz-TDS), we experimentally detect an ultrathin 2 nm germanium (Ge) overlayer equivalent to λ/160,000, where λ is the resonant wavelength. To the best of our knowledge, this demonstrates the thinnest analyte layer ever detected at terahertz frequencies, achieved through exceptional sensitivity of micron-scale resonators that obviate the need for complex nanoscale fabrication. The sensor exhibits a refractive index sensitivity of 15.54 GHz/RIU for a 40 nm analyte layer, establishing a new paradigm for deep-subwavelength terahertz (THz) sensing and paving the way for compact, flexible, and high-performance THz photonic platforms.