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 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 THz time-domain spectroscopy, 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 micrometer-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 THz sensing and paving the way for compact, flexible, and high-performance THz photonic platforms.