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. Here we propose an ultra-sensitive terahertz sensing platform based on the Rayleigh anomaly in hyperbolic metamaterial gratings, which are composed of periodic slits patterned in paired metal-dielectric multilayers. Simulation results show that, under normal incidence with TM 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 (or 9.06×104 nm/RIU, and extremely large figure of merit 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 Rayleigh anomaly 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 Rayleigh anomaly 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.