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Aperture-type terahertz near-field imaging with a cylindrical frustum-shaped plastic probe.

We designed an aluminum-coated plastic probe made from the combination of cylinder and a frustum of a cone for terahertz near-field imaging. The bunching width of the probe is 12 µm and the electric field intensity enhancement factor just outside the probe tip reaches up to 25. An imaging resolution obtained through near-field imaging experiments on a variety of targets, including narrow metals and narrow media, is 6 µm (λ/450) at 0.11 THz. By adopting an adaptive threshold segmentation image processing algorithm that combines Hilbert scanning and wavelet transform, the influence of a large area of metal near the imaging area on the imaging result is successfully suppressed. Imaging of a scratch on the plastic substrate makes the new probe feasible for quality identification of nonmetallic cultural relics.