Resonantly loaded apertures for sub-diffraction near-field imaging and surface probing

It is demonstrated that electromagnetic (EM) transmission through a subwavelength or non-resonant aperture in a conductive screen can be significantly enhanced by loading it with folded metallic strips exhibiting resonant properties. When illuminated by an EM plane wave, these loaded apertures enable very tight, subwavelength, collimation of the EM power in the near-field zone. The authors propose quasi-planar resonant insertion geometries that should allow, for the first time, simultaneous subwavelength near field focusing of both electric and magnetic fields. The proposed technique for resonance transmission enhancement and near-field confinement forms a basis for a new class of microwave near-field imaging probe with subwavelength resolution capable of operating over a wide range of imaging distances (0.01-0.25λ). Measurement results demonstrate the possibility of high-contrast (more than 3 dB in amplitude and 40° in phase) near-field subwavelength imaging of active sources as well as the characterisation of planar subwavelength non-resonant dielectric targets located in free space and at the interface of moderately lossy layered media.