Exploiting localized surface plasmon resonances in subwavelength spiral disks for THz thin film sensing

Abstract

In this paper, we studied the sensing performance of metasurfaces comprised by spiraldisk-shaped metallic elements patterned on polypropylene substrates, which exhibited localized surface plasmon resonances in the low-frequency region of the terahertz (THz) spectrum (0.2-0.5 THz). Optimal designs of spiral disks with C-shaped resonators placed near the disks were determined and fabricated. The experimentally measured transmittance spectra of the samples coated with very thin photoresistive layers (d ~ 10-4-10-3 λ) showed good agreement with the simulations. The resonance frequency shift Δf increases with increasing d, while saturating near d = 50 μm. The narrow-band magnetic dark modes excited on symmetrical spiral disks with a 90° Cresonator demonstrated very high figure of merit (FOM) values reaching 1670 (RIUmm)-1 at 0.3 μm thick analyte. The hybrid high order resonances excited on asymmetrical densely packed spiral disks showed about two times larger FOM values (up to 2950 (RIUmm)-1) compared to symmetrical distantly spaced spirals that resembled the best FOM results found in the literature for metasurfaces fabricated with a similar technique. The demonstrated high sensing performance of spiral disks is evaluated to be promising for bio-sensing applications in the THz range.