Two-dimensional subwavelength-focused imaging using a near-field probe at a λ/4 working distance

Two-dimensional (2D) subwavelength imaging using a near-field antenna array probe is demonstrated experimentally at a probe-to-object separation distance of λ/4. Field perturbations caused by the presence of small objects are detected by monitoring the input reflection coefficient as the probe is scanned. The probe is designed to produce a subwavelength focal spot with a 0.217λ full-width half-maximum beam width. Propagating components are suppressed through destructive interference, enhancing the evanescent field enough to resolve objects spaced by 0.259λ. The poor resolving capability of a conventional single-element probe at this working distance is also addressed; it is shown that even objects spaced over a wavelength apart are inaccurately detected. A full 2D imaging experiment is carried out using eight scattered objects. The single element probe is unable to resolve any of the objects, while all but the two most closely spaced pairs are resolved by the array probe.