Modelling of surface waves on a THz antenna detected by a near-field probe

Abstract

We have modelled the experimental system based on the sub-wavelength aperture probe employed in our previous work for terahertz (THz) surface plasmon wave imaging on a bowtie antenna. For the first time we demonstrate the accuracy of the proposed interpretation of the images mapped by the probe. The very good agreement between numerical and experimental results proves that the physical quantity detected by the probe is the spatial derivative of the electric field normal component. The achieved understanding of the near-field probe response allows now a correct interpretation of the images and the distribution of the electric field to be extracted. We have also carried out the first assessment of the probe invasiveness and found that the pattern of the surface plasmon wave on the antenna is not modified significantly by the proximity of the probe. This makes the experimental system an effective tool for near-field imaging of THz antennas and other metallic structures.

Terahertz imaging of sub-wavelength particles with Zenneck surface waves

“…3,4 In this THz microscopy system, a flat metallic surface with a small aperture is interacting with the antenna. It was found that placing the metallic plane of the probe close to a surface, which supports a Z-TSW, enhances the field of this wave (by of factor of 10 for a 3 lm separation between antenna and the probe) whereas the wave properties (the angular distribution of wave vectors) remain determined by the supporting surface geometry and stay practically unaffected by the metallic plane of the probe.…”

“…It was found that placing the metallic plane of the probe close to a surface, which supports a Z-TSW, enhances the field of this wave (by of factor of 10 for a 3 lm separation between antenna and the probe) whereas the wave properties (the angular distribution of wave vectors) remain determined by the supporting surface geometry and stay practically unaffected by the metallic plane of the probe. 4 This configuration, consisting of a planar antenna and the metallic plane of the NF probe placed parallel to the antenna plane, offers a possibility for THz imaging and spectroscopy of small (subwavelength size) dielectric objects.…”

“…A subwavelength size particle is attached to the surface of one of the bow-tie arms and the sample is scanned (together with the output end of the waveguide) with respect to an integrated sub-wavelength aperture THz NF probe. 3,4,26,27 The THz NF probe has an aperture of d ¼ 10 lm and an integrated photoconductive antenna detector. 23,27 The overall spatial resolution of this NF probe is determined by the aperture size and the distance to the sample.…”

“…When the NF probe-sample separation is less than d/3 and thus the evanescent fields of high spatial frequency can be collected by the NF probe, the spatial resolution equals approximately the aperture size d. 23 The probe can detect THz surface waves as well as the incident THz wave, producing a sum of two signals: one is proportional to the temporal derivative of the horizontally polarized electric field (E x ) of the incoming THz pulse 28 and the other is proportional to the spatial derivative of the out-of-plane electric field (dE z /dx) of the Z-TSW. 3,4,26 In the area of the bow-tie antenna, where the incident THz wave is blocked by the metal, only the contribution of the Z-TSW, dE z /d x , remains. Typical detected surface wave patterns on plain bow-tie antennas can be found in Refs.…”

Near-Field Analysis of Terahertz Pulse Generation From Photo-Excited Charge Density Gradients

“…For each position, the simulated voltage across the gap of the antenna is recorded in time, providing a spacetime map of the radiated field, as in the experiment. The simulation of the probe geometry ensures that the numerical results can be compared directly to the experiment despite the fact that these maps display a signal, which combines two electric field components, E x and E z [21,22].…”