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).

We report the first experiential observation and theoretical analysis of the new phenomenon of planar chiral circular conversion dichroism, which in some aspects resembles the Faraday effect in magnetized media, but does not require the presence of a magnetic field for its observation. It results from the interaction of an electromagnetic wave with a planar chiral structure patterned on the sub-wavelength scale, and manifests itself in asymmetric transmission of circularly polarized waves in the opposite directions through the structure and elliptically polarized eigenstates. The new effect is radically different from conventional gyrotropy of three-dimensional chiral media.Since Hetch and Barron [1] and Arnaut and Davis [2] first introduced planar chiral structures to electromagnetic research they have become the subject of intense theoretical [3,4] and experimental investigations with respect to the polarization properties of scattered fields [5,6,7]. It was understood by many that planar chirality is essentially different in symmetry from threedimensional chirality. Whereas in three-dimensional chiral structures the sense of perceived rotation remains unchanged for opposing directions of observation (think, for example, of a helix observed along its axis), planar chiral structures possess a sense of twist that is reversed when they are observed from opposite sides of the plane to which the structure belongs. Consequently, if planar chiral structures were to exhibit a polarization effect (due to this twist) for light incident normal to the plane, the sense of the effect would be reversed for light propagating in opposite directions. Such behavior has never been observed before, but if proven would be of profound benefit to the development of a new class of microwave and optical devices.In this paper we report such a polarization sensitive effect. It is a previously unknown fundamental phenomenon of electromagnetism that asymmetric materials can generate behaviors that in some ways resemble the famous non-reciprocity of the Faraday effect, which emerges when a wave propagates through a magnetized medium. However, the phenomenon reported here does not require the presence of a magnetic field and results from an electromagnetic wave's transmission through a chiral planar structure patterned on the sub-wavelength scale. Both in the Faraday effect and in that produced by planar chirality, the transmission and retardation of a circularly polarized wave are different in opposite directions. In both cases the polarization eigenstates, i.e. polarization states conserved on propagation, are elliptical (circular).There are also essential differences between the two phenomena. The asymmetry of the Faraday effect with respect to propagation in opposite directions applies to the transmission and retardation of the incident circularly polarized wave itself. The planar chirality effect leads to the (partial) conversion of the incident wave into one of opposite handedness, and it is the efficiency of this conversion that is as...

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We demonstrate a classical analog of electromagnetically induced transparency in a planar metamaterial. We show that pulses propagating through such metamaterials experience considerable delay. The thickness of the structure along the direction of wave propagation is much smaller than the wavelength, which allows successive stacking of multiple metamaterial slabs leading to increased transmission and bandwidth.