Sharp Trapped-Mode Resonances in Planar Metamaterials with a Broken Structural Symmetry

Abstract

We report that resonant response with a very high quality factor can be achieved in a planar metamaterial by introducing symmetry breaking in the shape of its structural elements, which enables excitation of dark modes, i.e. modes that are weakly coupled to free space. The exotic and often dramatic physics predicted for metamaterials is underpinned by the resonant nature of their response and therefore achieving resonances with high quality factors is essential in order to make metamaterials' performance efficient. However, resonance quality factors (that is the resonant frequency over width of the resonance) demonstrated by conventional metamaterials are often limited to rather small values. This comes from the fact that resonating structural elements of metamaterials are strongly coupled to free-space and therefore suffer significant losses due to radiation. Furthermore, conventional metamaterials are often composed of subwavelength particles that are simply unable to provide large-volume confinement of electromagnetic field necessary to support high-Q resonances. As recent theoretical analysis showed, high-Q resonances involving dark (or closed) modes are nevertheless possible in metamaterials if certain small asymmetries are introduced in the shape of their structural elements [7].In this Letter we report the observation of exceptionally narrow resonant responses in transmission and reflection of planar metamaterial achieved through introducing asymmetry into its structural elements. The appearance of narrow resonances is attributed to the excitation of otherwise forbidden anti-symmetric modes, that are weakly coupled to free-space ("dark modes").Metamaterials that were used in our experiments consisted of identical sub-wavelength metallic "inclusions" structured in the form of asymmetrically split rings (ASR), which were arranged in a periodic array and placed on a thin dielectric substrate (see Fig. 1). ASRpatterns were etched from 35 µm copper cladding covering IS620 PCB substrate of 1.5 mm thickness. Each copper split ring had the radius of 6 mm and width of 0.8 mm and occupied a square translation cell of 15×15 mm (see Fig. 1). Such periodic structure does not diffract normal incident electromagnetic radiation for frequencies lower than 20 GHz. The overall size of the samples used were approximately 220 × 220 mm. Transmission and reflection of a single sheet of this meta-material were measured in an anechoic chamber under normal incidence conditions using broadband horn antennas. We studied structures with two different types of asymmetry designated as type A and B in Fig. 1. The rings of type A had two equal splits dividing them into pairs of arcs of different length corresponding to 140 and 160 deg (see Fig. 1A). The rings of type B were split along their diameter into two equal parts but had splits of different length corresponding to 10 and 30 deg (see Fig. 1B).

Spectrally selective chiral silicon metasurfaces based on infrared Fano resonances

“…One promising approach to decreasing radiative losses while maintaining finite coupling to free-space radiation is to utilize the phenomenon of Fano interference 35 originally introduced in atomic physics to describe asymmetrically shaped ionization spectral lines of atoms. More recently, the concept of Fano resonances was introduced to the field of photonics and metamaterials 23,28,[36][37][38][39] in which a photonic structure possesses two resonances generally classified as 'bright' (that is, spectrally broad and strongly coupled to incident light) and 'dark' (spectrally sharp, with negligible radiative loss). The weak nearfield coupling between the bright and dark resonances leads to coupling of the incident light to the dark resonance that maintains its low radiative loss, thereby remaining high-Q.…”

Numerical investigation of Fano resonances in metamaterials with electric asymmetry

“…The asymmetry comes from the close coexistence of resonant transmission and resonant reflection. Previous work has shown that a certain small structural (geometrical) asymmetry has to be introduced to the shape of their conducting elements to excite such resonances [2,3]. In our prior work we have shown that similar effects can be achieved without breaking the geometrical symmetry of the resonator but instead by introducing an electrical asymmetry through a combination of substrates with different dielectric properties [4].…”

Controllable enhanced dragging of light in ultradispersive media

“…The physics of coherent effects in -type three-level atoms is related to the excitation of the maximum coherence between two ground states (in alkali atoms, these are the hyperfine levels) under the condition when a special coherent state, the so-called dark state, is formed. EIT has been achieved in atomic [5,6,8] and molecular [34,35] gases, BECs [13], solid-state systems [36][37][38], metamaterials [39][40][41], and even the mechanical effect of light [42].…”