In 1961, Ugo Fano discovered a new type of resonance in the study of the autoionizing states of atoms [1]. Unlike the symmetric lineshape of the Lorentzian resonance, the Fano resonance exhibits a distinctly asymmetric lineshape, which results from the destructive interference of a narrow discrete resonance with a broad continuum of states. The Fano interference is a universal phenomenon because the manifestation of configuration interference does not depend on the matter [2], which has been realized in many systems, such as photonic crystals [3], plasmonic nanostructures [4–9], metamaterials [10–13], plasmonic metamaterials [14–16], etc. Fano resonance may be used for the design of spin filters [17], switches [18], chemical or biological sensors [19–26], etc.

Active metamaterials are promising for the multifunctional systems with tunable, switchable, and non-linear functionalities [27]. For example, to conquer the bottleneck of Fano resonance sensing that the high Q factor Fano resonance is accompanied with an extremely small resonance intensity [28, 29], gain-assisted active spoof plasmonic Fano resonance is proposed to enhance both the Q factor and resonance intensity simultaneously [30]. Active Fano resonance switches were also demonstrated. The on–off switching of the Fano resonance of a plasmonic cluster by its incorporation into a polarization rotating liquid crystal device was demonstrated in a voltage-dependent manner [31]. Electrically controlled damping of Fano resonance in a graphene-nanoantenna hybrid device was observed [32]. Active photoswitching of sharp Fano resonances in silicon-implanted terahertz asymmetric metallic split-ring resonator structure was demonstrated, where the strength of the Fano resonance is modulated by changing the optical pump powers [33]. By changing the pH value of the solution environment, the active plasmonic Fano resonance switching is enabled by varying the refractive index of a layer of polyaniline between the Au nanosphere and the Au nanoplate [34]. The Fano resonance generated in Si nanosphere dimers on a VO₂ layer can be actively tuned by utilizing the phase transition of VO₂ with temperature [35]. However, there are still great challenges that are difficult to overcome, such as slow switching speed, high operation voltage, low contrast of modulation, etc. Furthermore, it is impossible to achieve two-way or multiway Fano switches based on the discussed mechanisms.

Here, we investigated an electrically two-way Fano resonance switch in the plasmonic metamaterials, which is composed of two concentric spoof localized surface plasmon (LSP) resonators. It has been demonstrated that there would be two different Fano resonance modes when adjusting the slit in the inner spoof LSP resonator. By loading a Schottky barrier diode (SBD) across the slit in the inner LSP resonator, both Fano resonance modes can be simultaneously controlled (OFF/ON) when the SBD diode is forward biased or reverse biased. Hence, a two-way Fano resonance switch can be realized. Both simulated and measured results agree well at microwave frequencies.

To understand the physical mechanism of the two-way Fano resonance switch, we first investigate the Fano resonances of two spoof plasmonic Fano resonance Structures A and B. The three-dimensional (3D) schematic of Structure A is illustrated in Figure 1A, which contains two vertically stacked layers. The top layer is the spoof plasmonic Fano resonator, and the bottom layer is a microstrip feeding line. The top view and side view of Structure A are shown in Figure 1B, where the length l of the spoof plasmonic Fano resonator is 52 mm. The width w of the microstrip line is 1.3 mm, and the thickness h of the substrate and the thickness t_m of the copper are 0.5 and 0.018 mm, respectively. As demonstrated in Figure 1C, the spoof plasmonic Fano resonator is composed of two concentric spoof LSP resonators. The outer and inner corrugated metallic rings are printed on the dielectric substrate (F4B), whose relative dielectric permittivity is 2.65, and loss tangent is 0.002. The radiuses r₁ and r₂ of the inner and outer corrugated rings are 1.5 and 10.5 mm, respectively. Both the lengths l₁ and l₂ are 2.5 mm, and the groove depths l₃ and l₄ are 5 and 7.5 mm, respectively.

The transmission coefficients (S₂₁) are plotted in Figure 2a, which are obtained by using the commercial software HFSS. The driven modal solver is used. The wave port and the radiation boundary condition are adopted. The minimum mesh size is 0.06 mm. The spoof LSP modes with Lorentzian lineshapes of outer and inner corrugated rings are marked as O₁, O₂, O₃, and I₁ modes. The corresponding resonant frequencies are 1.0, 2.02, 2.78, and 2.52 GHz, respectively. Figure 2b illustrates the 2D E_z-field distributions on the xoy plane 2 mm above the spoof LSP resonators, where it can be seen that the spoof LSP modes O₁-O₃ are dipolar mode (n = 1), quadrupolar mode (n = 2), and hexapolar mode (n = 3) for the outer corrugate ring. The spoof LSP mode I₁ is the dipolar mode (n = 1). For structure A, the resonant peaks are denoted by A₁, A₂, A₃, and A₄. The corresponding resonant frequencies are 1.03, 1.98, 2.44, and 2.65 GHz, respectively. It can be seen that there exists an asymmetric Fano lineshape between A₂ and A₄, which stems from the destructive interference of the narrow discrete spoof LSP mode I₁ with the broad continuum state between modes O₂ and O₃ of the outer corrugated ring. Figure 2c illustrates the 2D E_z-field distributions on the xoy plane 2 mm above Structure A. It can be clearly observed that the modes A₁, A₂, and A₄ are corresponding to the spoof LSPs modes O₁, O₂, and O₃, respectively, while the resonant peak A₃ corresponds to the spoof LSPs mode I₁.

Next is structure B, whose 3D schematic is illustrated in Figure 3A. The top view of Structure B is shown in Figure 3B. From Figure 3C, it can be seen that Structure B is also composed of two concentric LSP resonators. The difference with Structure A is that there is a slit in the inner corrugated ring, where the slit is cut at the position of θ = 90°.

The simulation transmission coefficients S₂₁ of Structure B, outer corrugated ring, and inner corrugated ring with a slit are given in Figure 4a. The LSP modes are different for the inner corrugated ring and the corrugated ring with a slit. Here, the spoof LSP modes of outer and inner corrugated rings are also marked as O₁, O₂, O₃, and I₁ modes. Figure 4b illustrates the corresponding 2D E_z-field distributions on the xoy plane 2 mm above the spoof LSP resonators. The resonant frequencies of the outer corrugated ring are the same, and the spoof LSP modes E_z-field distributions are also unchanged, as shown in Figure 4b. It can be seen that the resonant frequency of the inner ring with a slit has changed from 2.52 to 1.76 GHz, as the mode I₁ of the inner ring with a slit is the half-integer LSP mode (n = 0.5). For structure B, the resonant peaks are marked as B₁, B₂, B₃, and B₄. The corresponding resonant frequencies are 1.12, 1.53, 2.22, and 3.05 GHz, respectively. We can see that there appears an asymmetric Fano lineshape between B₁ and B₃, which results from the destructive interference of the narrow discrete mode I₁ with the broad continuum state between modes O₁ and O₂ of the outer corrugated ring, and there is no resonance mode between modes O₂ and O₃ of the outer corrugated ring. Figure 4c illustrates the 2D E_z-field distributions on the xoy plane 2 mm above Structure B. It can be clearly observed that the modes B₁, B₃, and B₄ are corresponding to the spoof LSPs modes O₁, O₂, and O₃, respectively, whereas the resonant peak B₂ corresponds to the mode I₁.

Considering the responses of passive Structure A and Structure B, it can be concluded that when there exists a slit in the inner corrugated ring, the Fano resonance appears between the O₁ and O₂ modes, and when there is no slit in the inner corrugated ring, the Fano resonance appears between the O₂ and O₃ modes. Hence, if a switch diode is loaded across the slit, the two Fano resonance modes could be switched. As there is no charge carrier depletion region at the junction, the SBD diode has a shorter recovery time than the PIN diode. For a small signal, the switching time of the SBD diode is only 100 ps. Here, an SBD diode (MACOM MA4E 1317) is used to switch the Fano resonance mode, whose operating bias voltage is 0.6–0.8 V. The optimum bias voltage is 0.75 V. The DC voltage source is GW Instek linear DC power supplies GPS-1850D. Figure 5a illustrates the schematic of the active two-way Fano resonance switch, where an SBD diode is mounted across the slit in the inner corrugated ring. The capacitance is used to isolate the DC signal, whereas the inductance is used to isolate the AC signal. The fabricated sample is shown in Figure 5b. In the simulation, the SBD is equivalent to an RLC series circuit, where R_s is the series resistance, L_s is the inductance arising from the metallic contacting strap, and C_j is the diode junction capacitance, and the values are 4 Ω, 0.45 nH, and 0.02 pF, respectively. The simulated transmission coefficients S₂₁ are plotted in Figure 5c. It can be seen that when the SBD diode is forward biased (ON), there is no slit in the corrugated ring. The structure is equivalent to Structure A, where the second Fano resonance appears. When the SBD diode is reverse biased (OFF), there is a slit in the corrugated ring. The structure is equivalent to Structure B, where the first Fano resonance appears. Hence, a two-way Fano resonance switch could be realized. The sample is measured by using a vector network analyzer (Agilent N5227A). The experimental results shown in Figure 5d agree well with the simulation results. When the diode is ON, the Fano resonance appears between the quadrupolar mode and hexapolar mode. When the diode is OFF, the Fano resonance appears between the dipolar mode and the quadrupolar mode. The measured transmission coefficients are almost 10 dB lower than the simulation results, which may be caused by the higher series resistance R_s and the welding quality in the measurements.

In summary, we have proposed and experimentally demonstrated an active two-way Fano resonance switch in the microwave frequency. When the SBD diode is forward biased, the second Fano resonance appears between the quadrupole mode and hexapole mode. When the SBD diode is reverse biased, the first Fano resonance appears between the dipolar mode and the quadrupole mode. The experimental results agree well with the simulated results. The Fano resonance switch has advantages such as two-way switches, fast switching speed, and low operation voltage, which could find applications in plasmonic circuits, sensors, devices, etc.

All datasets presented in this study are included in the article/supplementary material.

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.