Fano Resonances in Terahertz Metasurfaces: A Figure of Merit Optimization

Abstract

COMMUNICATIONlinewidth accompanied with an extremely small resonance intensity. Typically, in most of the Fano resonant plasmonic and metamaterial systems, the quality factor declines exponentially with the increase in the resonance intensity. Thus, it becomes very important to investigate the tradeoff between the quality factor and the intensity of Fano resonances. Terahertz is a perfect regime to study this tradeoff behavior due to the ease of fabrication and the precise control that could be exercised in designing metamaterial samples with extremely small variation in the geometry of the chosen meta-atoms. Terahertz split-ring resonators (SRRs) with dual split capacitive gaps that consist of two unequal metallic wires form an asymmetric resonator that have been demonstrated in the recent past to be excellent candidates in exciting the Fano resonance with ultrahigh quality factor ( Q factor). [ 2,5 ] Such a high Q factor design can overcome the radiative loss to a large extent due to the strong confi nement of photons in the resonators. [ 25 ] The Fano resonances have also been demonstrated to be potential candidates for designing ultrasensitive sensors. [ 4,26 ] Strong confi nement of energy in such systems occur due to the antiparallel oscillating currents in the metasurface array that minimizes the radiative losses if arranged in a large periodic lattice. Therefore, weak coupling of the current mode to the free space occurs at Fano resonance once the intrinsic symmetry of the unit cell is broken, which actually breaks the resonance equilibrium in the adjacent arms. Such a weak free space coupling enables long decay time and has been argued to be an excellent cavity to realize metasurfacebased fl at lasing spaser. [ 27 ] However, the ultrahigh Q factor is obtained at the expense of the Fano resonance intensity which makes it challenging to effi ciently harness this low-loss resonance feature at subwavelength scales. The high Q resonance at low intensities also presents the diffi culty in measuring the Fano resonance with low resolution and low signal-to-noise ratio systems. Therefore, it is extremely important to excite a rather high Q resonance that has strong intensity in the transmission spectra in order to exploit these resonances for several photonic applications.In this work, we address the problem of optimizing the Q factor and the resonance intensity of the Fano resonances by probing the Figure of Merit (FoM) that we defi ne here as the product of quality factor and the resonance intensity. In order to thoroughly study the factors that determine the behavior of Fano resonances, we investigated the infl uence of structural confi guration on Fano resonance with geometrically symmetric and asymmetric SRRs through detailed experiments and simulations. The asymmetry parameter in the Fano resonator is defi ned as 1 2 1 2

Active Photoswitching of Sharp Fano Resonances in THz Metadevices

“…THz probe is derived from nonlinear generation using the ZnTe crystal via optical rectification process and is detected using the electro-optic sampling technique. [5,12] When the Si-TASR MM is illuminated by the pump beam of 1 mW power (fluence = 3.5 μJ cm −2 ), strength of the observed Fano resonance weakens, as shown by the open circles in Figure 2a. During the measurements, THz beam is delayed by 12 ps with respect to the pump pulse, where the free carrier density of the photoexcited silicon is the maximum.…”

“…Figure 1b depicts the optical microscopy image of the fabricated sample with the unit cell dimensions as shown in the inset of the Figure. Asymmetry in the structure is introduced by displacing one of the two capacitive split gaps from the vertical symmetry axis by x = 15 μm within the Scattering processes occurring in light-matter interaction are important tools in investigating the resonant and non-resonant optical properties of materials. [10] In the recent past, there have been few demonstrations on tailoring the intensity and quality factors (Q) of these Fano resonances by changing the asymmetry TASR structure and the system asymmetry is quantified by the asymmetry parameter α, [12] which corresponds to the difference in the lengths of the two metallic wires that forms the TASR structure (described in the Supporting Information). Typically, the symmetric spectral features described by the Lorentz model result from overlapping of distinct resonances of different origin, whereas the asymmetric spectral features showing the Fano line shape is a consequence of the Fano type of interference effects in the system.…”

“…[11][12][13][14] Recently, the effect of conductivity on the Fano resonance was probed, where the strength of Fano resonance could be tuned by choosing different materials as the resonators in the lower asymmetry regime. [11][12][13][14] Recently, the effect of conductivity on the Fano resonance was probed, where the strength of Fano resonance could be tuned by choosing different materials as the resonators in the lower asymmetry regime.…”

Solid analyte and aqueous solutions sensing based on a flexible terahertz dual-band metamaterial absorber

“…[10][11][12][13] Recently, THz metamaterial sensors have been used in the nondestructive and label-free detection of materials. [14][15][16][17][18] However, the sensitivity of the previously reported metamaterial biosensors is relatively lower due to the radiative and nonradiative loss. Most detection techniques were carried by different shifts in resonance frequency and rarely considered the changes in amplitude resonance.…”

Displacement Current Mediated Resonances in Terahertz Metamaterials

“…[ 18 ] The most common method to increase Q-factor of MMs without changing material properties is to design asymmetric split resonators (ASRs) by breaking the symmetry of the MMs. [ 19,20 ] The asymmetric design reduces the radiation loss of the resonator and increases Q-factor up to 30. [21][22][23][24] Another method uses coupling between MMs in a super unit to excite both odd and even modes of the MMs.…”