Sub-Wavelength Microwave Resonators Exhibiting Strong Coupling to Radiation Modes

We demonstrate a sub-wavelength spherical resonator at microwave frequencies designed to mimic the electromagnetic behavior of a negative permittivity sphere. The structure, which has a radius of ~lambda/12 (where lambda is the resonant wavelength), consists of an axially symmetric array of non-interconnected conducting elements forming a resonant spherical object. The structure exhibits many of the properties inherent to negative permittivity spherical resonators, the most notable being a very strong coupling to radiation modes despite being much smaller than the wavelength. This characteristic is quantified by the radiation Q-factor, which was observed to be near 1.5x the theoretical limit in some of the measured samples, matching the performance achievable in a negative permittivity sphere of comparable electrical size. These resonators may find application in the design of electrically small antennas, as well as in the experimental "simulation," at microwave frequencies, of nanophotonic device concepts based upon localized plasmon resonances in metal nanoparticles.