Refractive index sensing with hollow metal–insulator–metal metasurfaces

Abstract

Refractive index sensing with metal–insulator–metal (MIM) metasurfaces featuring a continuous dielectric film between two metallic layers suffers from a low spatial overlap between high field enhancement regions and the analyte placed above. Recent studies have thus turned toward hollow MIM metasurfaces, particularly suited for fluid analytes which can infiltrate the hollow cavities. Here we describe a general procedure for reaching the optimal design in three most relevant configurations: mushroom-type structures with narrow dielectric pedestals carrying the top metallic ribbon array, hollow structures with the metallic ribbon array resting on a distant lateral support, and hollow structures in which the metallic ribbons are carried by an encapsulating layer from top. We contend that since a majority of the resonant eigenmode energy is contained within the analyte, very high refractive index sensitivities are possible for three different measurement methods: spectral, reflectance and phase interrogation. This is confirmed by numerical simulations demonstrating terahertz spectral sensitivities of above 700 GHz RIU−1 with a normalized sensitivity of around 0.6 (RIU stands for refractive index unit). Detection limits and dynamic ranges are estimated for both bulk refractive index sensing and thin film detection. Refractive index sensitivities and corresponding figure-of-merit factors are shown to reach maxima in the critical coupling regime characterized by equal radiative and non-radiative decay rates of the resonant mode which is controlled by cavity height. Since this regime is associated with a zero reflectance which prevents measurements of any signal, metasurfaces should operate close to the critical coupling point where reflected beam is still measurable. The final optimization is done by decay rate engineering in order to achieve narrower resonances and improve sensing performance.