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 spiral-disk-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° C-resonator demonstrated very high figure of merit (FOM) values reaching 1670 (RIU·mm)−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 (RIU·mm)−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.

Principles of Measuring Thickness and Refractive Index of Thin Dielectric Film by Using Multiresonance Archimedean Spiral Metasurfaces with C Resonator in the Terahertz Frequency Range

“…In this article, we continue investigating the properties of spiral-based metasurfaces and their application to measuring the characteristics of analytes [35,43,44]. In [35], the authors compared a logarithmic spiral metamaterial with an azimuthally grooved metal cylinder and studied the influence of their various parameters on resonance excitation.…”

“…In [35], the authors compared a logarithmic spiral metamaterial with an azimuthally grooved metal cylinder and studied the influence of their various parameters on resonance excitation. The next article [43] was devoted to different designs of metasurfaces based on Archimedean spirals. Their disadvantages and advantages were analyzed.…”

“…The number of arms m = 12 and their width w = 8 μm are related to nonlocal effects and spectrum stability against small variations due to incidence angle error and losses in the metal. The distance to the C-shaped resonator RC = 204 μm, its angular width α = 90 deg, and its width wr = 8 μm relate to the symmetry of the structure, the position of the resonance dips, and the efficiency of excitation of higher modes [43]. To analyze the interaction of the metasurface with the analyte, we used the COMSOL Multiphysics software with the numerical scheme shown in Figure 1a.…”

“…If the polarization of the incident beam is perpendicular to the C resonator (𝐸 ⊥ ), only the fundamental electric dipole mode of the spiral structure and diffraction mode will be observed (see Figure 2). [32,43,44] The several dips in the spectra not only allow one to expand the spectral range for analysis, but also improve the method of retrieving the analyte characteristics, which will be shown below.…”

Numerical Optimization of Refractive Index Sensors Based on Diffraction Gratings with High Aspect Ratio in Terahertz Range

“…The use of SPR in the terahertz frequency range on thin metal films is limited due to the high values of the optical constants of metals [ 5 , 6 ]. To overcome this problem, different authors suggest their solutions based on 2D and 3D metamaterials, subwavelength gratings, graphene, and graphene nanotube, as well as others [ 7 , 8 , 9 ]. For example, metagratings, which can act both as couplers and sensors, attract the increased attention of researchers [ 10 , 11 , 12 ].…”