Terahertz biosensing based on bi-layer metamaterial absorbers toward ultra-high sensitivity and simple fabrication

Metamaterial absorbers have proven their ability to sense in the terahertz domain. However, the sensitivity is always limited by the poor spatial overlap between the analyte and the localized enhanced electromagnetic field. Here, we try to tackle this challenge by utilizing an absorber with a bilayer cross-shaped plate-hole structure to ingeniously excite hot-spots covering the analyte. As a result, the sensitivity is significantly improved, theoretically about 7 and 18 times higher than that of the conventional cross-shaped absorber and its complementary cross-shaped absorber, respectively. We then experimentally demonstrate its ability to quantitatively detect biotin with a sensitivity of 153 GHz/μM, higher than that of previously reported biotin sensors. Additionally, the polarization-independent nanostructure decreases the design and fabrication complexity and maintains high reflection at a wide range of incident angles over ±50°. These findings open up opportunities for metamaterial absorbers to realize ultrasensitive biosensing in the fingerprint region of the terahertz regime.