A terahertz metasurface for efficient molecular fingerprint detection via frequency matching strategies

The metasurface-enhanced fingerprint detection, endowed with their strong local electric field capabilities, provides a powerful platform to elucidate the trace molecular vibrational characteristics in terahertz band. However, the intrinsic refractive index of the analyte can induce a redshift in the resonant frequency, complicating the alignment of the resonant peak with the molecular absorption peak. Herein, we propose a reverse frequency matching strategy that effectively reduces the refractive index sensitivity of the designed metasurface, while simultaneously augmenting the local electric field, thus enhancing light-molecular coupling. By modulating the slot width to induce polarization sensitivity, the designed metasurface enables efficient molecular fingerprint sensing at dual polarizations while maintaining a refractive index sensitivity below 60 GHz/RIU and achieving an average electric field enhancement factor |E/E0| more than 36 times. As a proof of concept, we have demonstrated the qualitative and quantitative detection of L-cystine under two different polarizations, achieving the limits of detection as low as 0.219 μg/mm2. This achievement not only offers new insights into THz trace molecular fingerprint detection but also shows great potential in fields such as biosensing and polarimetric chemistry.