Multi-band localized surface plasmon enhanced terahertz biosensor chip with electrostatic and acousto-optic active tuning

Introduction Terahertz (THz) biosensing can read biomolecular vibrations without labels, yet two practical problems limit its sensitivity: sub-wavelength biological samples interact only weakly with the radiation, and 1/f noise dominates slow readouts. Methods We propose a multi-band localized surface plasmon (LSP)-enhanced THz biosensor chip in which a gold metasurface of nine spectrally isolated cross-resonator sub-arrays sustains LSP resonances at 0.1–0.9 THz, concentrating the field in deep-sub-wavelength hot spots at the resonator arm ends and gaps. An in-plane electrostatic bias tunes any resonance by up to 7.5%, and a 60 MHz surface acoustic wave (SAW) modulates the transmitted signal so that lock-in readout can reject roughly 15 dB of low-frequency noise. Coupled-mode theory was used to compute the transmission, near-field enhancement, tuning, and detection-limit improvement; the same array supports two readout modes, broadband THz time-domain spectroscopy (THz-TDS) and direct THz-induced micro-current detection. Results The calculations give a field enhancement of 17–33×, per-band redshifts of 1.6–2.8% that vary by band and can be read as a nine-point spectral pattern against calibration spectra, and a 39.6–45.4 dB improvement in detection limit over bare THz-TDS, corresponding to a 95–186× reduction in the minimum detectable amplitude. Discussion The intended analytes are proteins, sugars, antibiotics, and bacteria, measured as dried surface films or thin (≤5 µm) aqueous layers. The layout uses standard microfabrication and no cryogenics, providing a practical route toward high-sensitivity, multi-band label-free THz biodetection.