Terahertz Sensing Based on Metasurfaces

Abstract

The terahertz (THz) band has very attractive characteristics for sensing and biosensing applications, due to some interesting features such as being a non‐ionizing radiation, very sensitive to weak interactions, thus, complementing typical spectroscopy systems in the infrared. However, a fundamental drawback is its relatively long wavelength (10–1000 µm) which makes it blind to small features, hindering seriously both thin‐film and biological sensing. Recently, new ways to overcome this limitation have become possible thanks to the advent of metasurfaces. These artificial structures are planar screens usually made of periodic metallic resonators and whose electromagnetic response can be controlled at will by design. This design freedom allows metasurfaces to surpass the restrictions of classical THz spectroscopy, by creating fine details comparable to the size of the thin films or microorganisms under test. The strong field concentration near these small metasurface details at resonance makes them highly sensitive to tiny variations in the nearby environment, allowing for an enhanced detection more accurate than classical THz spectroscopy. The main advances in THz metasurface sensors from a historical as well as application‐oriented perspective are summarized. The focus is put mainly on thin‐film and biological sensors, with an aim to cover the most recent advances in the topic.

Exploiting Localized Surface Plasmon Resonances in Subwavelength Spiral Disks for THz Thin Film Sensing

“…The highest FOM ≈ 2950 [RIU·mm] -1 is reached at the smallest analyte thickness (d = 0.3 µm) that is ≈ 77 % bigger than in case of distantly spaced disks. This result is comparable with the best FOM value (3450 [RIU·mm] -1 ) obtained at the same analyte thickness for a labyrinth metasurface absorber implemented with a similar fabrication techniques [16,35].…”

“…According to [16], the frequency sensitivity of the sensor is defined as the ratio of the resonance frequency variation to the product of the analyte thickness and refractive index na: S = Δf/(d· na), with the units of GHz·[RIU·mm] -1 . The dependencies S(d) plotted in Figure 5 In sensing applications, the full width at the half minimum (FWHM) in the units of frequency is another important characteristic related with the Q-factor of the resonance.…”

“…This explains high prospects of using plasmon resonances in the THz range for sensing and spectroscopy. It is worth noting, however, that the THz frequencies (ωTHz ~ 10 12 Hz) are several orders of magnitude lower than the plasma frequency of free electrons in metal (ωp ~ 10 16 Hz) and their collision frequency (ωc ~ 10 14 Hz [11]), and, therefore, 2 of 11 plasmon resonances formally cannot be excited in this spectral region. To overcome these limitations, Pendry and his colleagues suggested making periodic corrugation of flat surface [12] that leads to forming bound surface states with high confinement of electromagnetic field, so called "spoof surface plasmon resonances".…”

Materials for Terahertz Optical Science and Technology

“…They facilitate enhanced confinement of THz fields in a small mode volume, thereby boosting their applications as ultrasensitive sensors, nonlinear devices, resonant modulators and phase shifters. Besides modulation, the metamaterial structures enable enhanced sensitivity of the structures for thin-film sensing, biomolecule sensing and cancer/tumor cell detection at THz frequencies (see the review by M. Beruete and I. Jáuregui-López [15] ). In the MEMS structures, balance between the restoring forces of bimorph cantilevers and the external forces dictate the active reconfiguration, whereas in VO 2 , change in its conductive properties during the metal-toinsulator transition is used to modulate the THz resonances in metamaterials.…”

“…Interfacing the graphene layer with metamaterials also enables an efficient modulation of resonant THz waves through voltage control (see the work on graphene metasurfaces from X. Chen et al [14] ). Besides modulation, the metamaterial structures enable enhanced sensitivity of the structures for thin-film sensing, biomolecule sensing and cancer/tumor cell detection at THz frequencies (see the review by M. Beruete and I. Jáuregui-López [15] ). The strong THz resonances with the combination of dielectric engineering in metamaterials enable sensitive molecule-specific detection capabilities by enhancing the resonant vibrational/ absorption peaks of the target bio/chemical molecules or any material systems, as reported in M. Seo and H.-R. Park.…”