Novel configuration of aperture-type terahertz near-field imaging probe

A near-field imaging probe based on a size-optimized tapered frustum with concave aperture is designed and demonstrated. The performance of the new probe is evaluated by finite difference time-domain electromagnetic simulation and near-field imaging experiments. The new probe structure eliminates the large amount of reflection of the terahertz (THz) wave at the probe aperture due to impedance mismatch, and increases the electric field intensity that can be transmitted through the aperture by a factor of three within the near-field domain outside the aperture. These beneficial changes lead to an increase in the spatial resolution of the new probe, while concurrently prolonging the decay length of the electric field intensity transmitted from the apex, so that the THz imaging beam can transmit a longer distance and the stringent near-field conditions are moderated. An imaging resolution of at least 10 mu m (lambda/270) at 0.11 THz is obtained experimentally, with excellent repeatability, while the workable near-field distance is also increased substantially, making the new probe feasible for imaging subsurface microstructures.