Polarity of the Fano Resonance in the Near-Field Magnetic-Dipole Response of a Dielectric Particle Near a Conductive Surface: Polarity of the Fano Resonance in the Near-Field Magnetic-Dipole Response of a Dielectric Particle Near a Conductive Surface

Fano resonances are observed in the near-field electric-field spectra of dielectric particles sustaining magnetic dipole resonances and located in the vicinity of a conductive surface (reflector). The polarity of these Fano resonances is defined by the relative positions of the resonant particle, the observation points, and the reflector. This paper studies two cases corresponding to common near-field measurement techniques, where the electric field is detected by aperture-based probes and scattering probes. In the first case, as the particle moves away from the reflector, the polarity of the observed Fano resonance reverses every quarter-of-a-wavelength of the particle's magnetic resonance. In the second case, in addition to the periodic change of polarity, a phase shift of pi is detected between the fields detected at the opposite sides of the resonant particle. This work proves, theoretically, that the observed Fano resonance polarities arise due to the interference between the external electric-field standing wave and the resonant fields generated by the particle's magnetic dipole moment. The findings are supported by a recent terahertz experiment and the gigahertz experiment presented in this paper.