Abstract
We numerically and experimentally demonstrated double Fano resonances in a simple S-shaped plasmonic metasurface in the terahertz frequency range. Apart from the LC resonance and electric dipole (ED) resonance, two trapped modes are excited in two different types of asymmetric S-shaped structures in the frequency range 0.2–1.4 THz, which are mainly attributed to a magnetic dipole (MD) and an electric quadrupole (EQ). Thereafter, double Fano resonances (one Fano and one electromagnetically induced transparency (EIT) resonance) are achieved via the coupling of the two dark trapped modes and a broad bright ED at normal incidence of the metasurfaces. Furthermore, under oblique incidence, strong Fano responses can be observed; they are considerably enhanced in asymmetric structures, and even in a symmetric structure. The proposed S-shaped plasmonic metasurfaces are easy to fabricate and have potential applications in multi-wavelength optical switches, filters‚ and sensors.
Highlights
In the last decade, metamaterials (MMs) have been extensively studied because of their peculiar electromagnetic properties, such as negative index media, cloaking, and subwavelength resolution imaging [1,2,3,4,5], which cannot be obtained from naturally occurring materials
As the metasurfaces are illuminated by the x-polarized wave, the measured transmissions in Figure 6E show a clear Fano resonance at 0.6 THz and an electromagnetically induced transparency (EIT) at 1.0 THz in the asymmetric sample in Case I, but Case II (Figure 6F) shows a very weak Fano and no EIT, which are basically consistent with the simulation results
Apart from the electricLC resonance at 0.3 THz and electric dipole (ED) at 1.3 THz, two trapped modes at 0.6 and 1.0 THz are simultaneously excited in both the asymmetric structures, which are mainly attributed to the magnetic dipole (MD) and the electric quadrupole (EQ), respectively
Summary
Metamaterials (MMs) have been extensively studied because of their peculiar electromagnetic properties, such as negative index media, cloaking, and subwavelength resolution imaging [1,2,3,4,5], which cannot be obtained from naturally occurring materials. The fundamental characteristic of an MM is its electromagnetic resonance responses, one of which is the interesting trapped mode, which results from the anti-phased dipole resonance in an asymmetric plasmonic MM [6,7,8,9,10]. Multiple Fano resonances can be excited via multiple trapped modes in multi-meta-atom MMs [29,30,31,32,33], which could be useful in multi-wavelength optical switches, filters, Double Fano Resonances Terahertz Metasurfaces and sensors. Only one Fano resonance is observed in a Z-shaped one-meta-atom plasmonic metasurface [26], but three Fano resonances have been observed in two different-sized asymmetric double-bar structures, where two Fano resonances are attributed to two individual meta-atoms, and one to their combinations or metamolecule MM [33]. The metamolecule structure will increase the size of the devices, which may not be conducive to practical applications
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