Ultra-sensitive terahertz sensing based on Rayleigh anomaly in hyperbolic metamaterial gratings

Terahertz sensors are promising for biomedical, environmental and security applications. Challenges in terahertz sensing have been the limited sensitivity and small figure of merit (FOM). Here we propose an ultra-sensitive terahertz sensing platform based on the Rayleigh anomaly (RA) in hyperbolic metamaterial gratings, which are composed of periodic slits patterned in paired metal-dielectric multilayers. Simulation results show that, under normal incidence with transverse-magnetic polarization and designed with proper parameters, the proposed structure has a super-high quality factor of 516, an ultra-high sensitivity of 1.56 THz RIU-1 (or 9.06x10(4)nm RIU-1), and extremely large FOM of 355 RIU-1 at frequency of 2.272 THz, corresponding to a large normalized sensitivity of 0.69 RIU-1. By comparing with other multilayered gratings, we attribute this exceptionally high sensing performance to the strong local field enhancement over an extremely large area in the sensing region at the RA frequency, which is found to be unique for hyperbolic metamaterial gratings. We further show that the resonance frequency decreases linearly with the grating period, following the RA equation, whereas the large normalized sensitivity keeps constant. We expect this work provides a novel strategy to design extremely sensitive sensors in terahertz as well as other frequency regimes.