Ultrathin Dual-Band Polaritonic Metasurface Enabling Robust Far-Infrared Detection

The far-infrared (FIR) spectrum hosts rich light–matter interactions, enabling applications in medicine, energy, and sensing. However, the intrinsic scaling of the resonance wavelength with structural thickness renders FIR devices bulky, hindering miniaturization and reducing efficiency. Here, we design a hybrid metasurface by combining dual Reststrahlen band SrTiO3 with high-index Ge, utilizing surface phonon polaritons (SPhPs) to enable FIR resonances in ultrathin meta-atoms. Experiments reveal an SPhP resonance at wavelengths up to λ = 21.9 μm using meta-atoms with only h = 70 nm height (h/λ = 0.32%), markedly lower than previously reported ratios. Unlike traditional phonon-polaritonic platforms, our design features angle-insensitive resonances and achieves a high Q quality factor (Q ≈ 100). Building on these advantages, we conducted a proof-of-concept polymer detection study in which the FIR mode enabled efficient thickness sensing and material identification, with thinner metasurfaces demonstrating superior performance. Overall, this work clarifies how to reconcile long-wavelength resonances with ultrathin device geometries, providing practical guidance for the development of far-infrared technologies.