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

Abstract

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 (0.01-0.25l). 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.

Super-Resolution Defect Characterization Using Microwave Near-Field Resonance Reflectometry and Cross-correlation Image Processing

“…The technique was applied for various standoff distances and measurement steps Δx, Δy. It has been observed that in the range of standoff distances λ/25-λ/10 the imaging resolution deteriorates proportionally to z -1.5 where z is an imaging distance [14], [16], [17]. It should be stressed that the measurement step defines the defect resolution and should be of order of the spatial defect deviation [17].…”

“…The probe generates a dipole-mode transmitted near field pattern, Fig.2a) with the E-field significantly enhanced in the dipole-mode dual hot spots [14], [16], [17]. The probe supports very high-quality transmission resonance around 4.1GHz, Fig.2b).…”

“…Experiments carried out in [14], [16], [17] demonstrate that in typical imaging scenarios involving dielectric materials and surfaces with conductive elements, the characteristic raw image resolution contrast (defined as variation of the S11 parameter from feature to feature or from the feature to background) achievable with the LA probe exceeds 10-20dB in amplitude and 100 degrees in phase of the reflection parameter …”

“…The dipole insert can be folded which results in subdiffraction spatial resolution in two dimensions, [16], [17]. Due to efficient EM coupling the imaging contrast (the contrast between the imaged features and/or the features and the background) can exceed 10-20dB in amplitude and 100 degrees in phase in typical imaging scenarios [14], [16], [17]. Even though other resonance near field reflectometry probes, very small (~λ/10) in all three dimensions have been proposed previously, e.g.…”

“…At the same time, due to the extended length dimension (~λ/4) of the insert as compared to its width, very high raw image resolution is only possible in the direction orthogonal to the LA length [17]. Additionally, ghost dual-peak image features appear as a result of dipole-mode near field coupling which can be considered as a disadvantage for image interpretation [16]- [18].…”