(a) SEM picture of the tip, (b) scheme of the near-field antenna, and experimental setups for (c) continuous-wave monochromatic and (d) pulsed broadband measurements. The particle on the flat output facet in Fig. 1(a) is dust.
(a) Measured amplitude of the normalized reflected signal (two-port -parameter ) for monochromatic radiation at around measured by using the setup shown in Fig. 1(b) in comparison to a numerical field simulation. Both curves exhibit standing-wave resonances in the NFA. The resonance values reach up to 2000 in the experiment depending on the coupling. Insert: modification of such a resonance by various materials placed in front of the tip. [(b) and (c)] Simulated electric field for a frequency corresponding to a resonance peak position. The distance between the metallized pads at the tip termination is . (b) shows the electric-field magnitude at the axis of the NFA as function of position along the length of the NFA and demonstrates the standing-wave pattern and the field enhancement in front of the needle. From the field simulation the electric field at the end facet of the tip was found to be a factor of 8.6 higher than in the straight part of the NFA. (c) Simulated distribution of the electric field in the NFA at a position near the tip demonstrating the focusing effect.
Waveforms corresponding to first oscillations of the terahertz pulse reflected from the tip obtained in a contact mode with several media: air (free space), metal, and lanthanum aluminate substrate. In comparison, we show as a reference waveform the signal reflected directly from the input facet of the needle (vertically scaled by a factor of 0.3 and horizontally shifted by for this plot). Inset: entire waveform reflected at the tip obtained for the free space mode; the gray part corresponds to the main plot.
Examples of and scans of the sample (metal stripes deposited on a lanthanum aluminate substrate). [(a)–(c)] Pulsed broadband setup [following Fig. 1(d)]; scans were performed at a fixed delay time of (dash-dotted line in Fig. 3): (a) scan and (b) scan across a metal stripe edge; (c) scan across a pattern of metal stripes separated by . (d) Continuous-wave monochromatic setup at [following Fig. 1(c)]; scan over a metal edge. The vertical offsets of the data were introduced for clarity.
Illustration of the ability to perform two-dimensional scans: image at of a -wide metal stripe and an array of -wide stripes (lower values of intensity) being separated by gaps of . The stripes have been prepared by photolithography of a metal film on a lanthanum aluminate substrate. The scanning speed was ; the separation between two adjacent scanned lines was .
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