Abstract
A planar metallic metasurface formed of spiral elements is shown to support an isotropic backward wave over a narrow band of microwave frequencies. The magnetic field of this left-handed mode is mapped experimentally using a near-field scanning technique, allowing the anti-parallel group and phase velocities to be directly visualised. The corresponding dispersion relation and isofrequency contours are obtained through Fourier transformation of the field images.
Highlights
In 2000, Pendry and colleagues proposed a composite metamaterial designed to exhibit simultaneously negative effective permittivity and permeability[4]
The propagating modes supported by the metasurface can be manipulated by varying the geometry of the subwavelength elements in much the same way that bulk modes are controlled in metamaterials, and we describe their phase velocity in terms of a mode index
Modes guided by a metasurface can often be understood in terms of transmission line theory, where a backward wave medium is described as a resonant left-handed transmission line (LHTL)[13] composed of series capacitors loaded with inductors in parallel
Summary
A planar metallic metasurface formed of spiral elements is shown to support an isotropic backward wave over a narrow band of microwave frequencies. Modes guided by a metasurface can often be understood in terms of transmission line theory, where a backward wave medium is described as a resonant left-handed transmission line (LHTL)[13] composed of series capacitors loaded with inductors in parallel This has led to a number of studies on chains of split ring resonators acting as LHTLs14–16, where each element of the array can carry a circulating current that couples to its neighbours via a magnetic field. Other studies have reported similar effects, all incorporating current-loop elements in a planar arrangement[20,21,22,23,24] Such negative mode index behaviour can be explained in terms of an array of coupled magnetic and/or electric dipoles. A backward wave will manifest itself as a region of positive gradient (positive group velocity) for negative wavevectors (negative phase velocity), or vice-versa within the first Brillouin zone
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