Ultra-compact broadband terahertz spectroscopy sensor enabled by resonant-gradient metasurface

Research on two-dimensional designer optical structures, especially ultra-thin optical elements dubbed 'metasurfaces', focused on engineering resonances to enrich the broadband spectral information, catering to the increasing sensing and detection demands. Current designs are constrained by discrete sampling of the spectrum, limiting continuous space-to-spectrum mapping. Here we present an integrated terahertz (THz) plasmonic gradient micro-photonic approach for encoding broadband spectra to observe molecular fingerprints. This innovation utilizes smooth variations in the metasurface's unit cells to achieve an extraordinary density of resonances, exciting unique optical modes referred to as bound states in the continuum (BICs) in a non-periodic structure. The device covers a spectral width of 0.9 THz with only 19 modes in a compact 400 × 125 μm² area. We demonstrate real-time, label-free identification of multiple analytes with enhanced vibrational fingerprints without complex scanning, offering a compact size, broad capabilities, and adjustable resolution to advance portable THz spectroscopy.