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.
Highlights
Localized plasmon resonance (LPR) is the resonant oscillation of the charge density of free electrons in conducting particles of sub-wavelength size in an oscillating external field, accompanied by the enhancement of the near field at the surface of the particles [1]
We present the results of numerical simulations and the experimental testing of spiral metal disks patterned on a dielectric substrate to excite electric and magnetic multipole localized surface plasmon resonances (LSPRs)
In the beginning of our study, we searched for the optimal design of spiral disks having narrow-band resonances in the region of 0.2–0.5 THz to allow the testing of the fabricated structures with the backward wave oscillator (BWO) spectrometer
Summary
Localized plasmon resonance (LPR) is the resonant oscillation of the charge density of free electrons in conducting particles of sub-wavelength size in an oscillating external field, accompanied by the enhancement of the near field at the surface of the particles [1]. LPRs exhibit strong spatial localization and amplification of the electromagnetic field near the particle surface with the resonance frequency depending on the shape, size and conductivity of the particle and its environment. This explains the high prospects of using plasmon resonances in the THz range for sensing and spectroscopy It is worth noting, that the THz frequencies (ωTHz ~ 1012 Hz) are several orders of magnitude lower than the plasma frequency of free electrons in metal (ωp ~ 1016 Hz) and their collision frequency (ωc ~ 1014 Hz [11]) and plasmon resonances cannot formally be excited in this spectral region. Numerical simulations in the THz region for a periodic structure of azimuthally grooved golden disks on dielectric substrates [18,19,20,21] showed that high-order (multipole) electric and magnetic LSPRs can be excited under oblique illumination or when combining the grooved disk with a C-shaped resonator.
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