Efficient isotropic magnetic resonators

There is currently growing interest in small magnetic resonant structures. This interest has been spurred by the demonstration that specific subwavelength structures can exhibit effective magnetic permeability which can be tuned to negative values over a finite frequency range, and by the utilization of these structures to produce a left-handed medium ~LHM! in the microwave regime. Thus LHM has negative permittivity « and permeability m, which lead to reversed electromagnetic properties. Thus far, LHM have been fabricated from periodic arrays of thin wires ~to produce a plasmonic medium with « ,0!, combined with periodic arrays of split-ring resonators ~SRRs! to produce m,0. The SRRs are planar structures that must be arranged in an infinite three-dimensional ~3D! lattice to create a LHM. The lattice unit cell consists of three orthogonal SRRs, one along each plane of symmetry. Such an infinite lattice produces an isotropic magnetic medium, although each SRR is highly anisotropic. By anisotropic, it is meant that the response of the system depends on the direction of illumination. For example, since an isolated SRR mainly reacts to magnetic flux through its structure, it only resonates for specific illumination directions. The objective of this letter is to propose and study alternative magnetic resonant building blocks with a higher degree of isotropy. The system we consider is depicted in Fig. 1. It is composed of two intersecting SRRs normal to each other, which form a crossed SRR ~CSRR!. Each SRR is made of two aluminum ~Al! strips deposited on the inner and outer faces of low permittivity foam ~Rohacell HF 51, «51.07!. The foam width is w54 mm, the inner radius 15 mm and the outer radius 20 mm ~the separation between inner and outer Al strips is t55 mm!; it is machined from a foam plate to obtain two open rings that are then fitted into each other. The 4 mm wide Al strips are cut from 50 mm thick self-adhesive Al foil. Note that the gaps at the extremities of the two CSRRs are located at the same poles of the spherical structure ~Fig. 1!. The width of these gaps is 6 mm. For our discussion we will designate by SRRxz , respectively, SRRyz the resonator in the Oxz , respectively, Oyz