A home-built scattering-type scanning near-field millimeter-wave microscope based on a 110-GHz continuous-wave solid-state source is demonstrated with a spatial resolution of 1 µm, approximately 1/3000 of the incident wavelength, and a signal-to-noise ratio of 23.8 dB. The relationship between the length of the tip (antenna) and the wavelength for resonant enhancement, and the near-field distribution around the tip apex at different tip-sample separations were explored using finite-difference time-domain electromagnetic simulations to facilitate the design of the microscope. The dependence of the spatial resolution on the tip-sample separation and the harmonic order of the modulation frequency at which the near-field signal was extracted has been investigated experimentally and discussed in terms of the signal-tonoise ratio and the standard deviation of the demodulated signal. INDEX TERMS Millimeter wave, near-field, FDTD, high resolution, background noise.

Photoconductive antenna microprobe (PCAM)‐based terahertz (THz) near‐field imaging technique is promising for biomedical detection due to its excellent biocompatibility and high resolution; yet it is limited by its imaging speed and the difficulty in the control of the PCAM tip‐sample separation. In this work, we successfully realized imaging of mouse brain tissue slices using an improved home‐built PCAM‐based THz near‐field microscope. In this system, the imaging speed was enhanced by designing and applying a voice coil motor‐based delay‐line. The tip‐sample separation control was implemented by developing an image analysis‐based technique. Compared with conventional PCAM‐based THz near‐field systems, our improved system is 100 times faster in imaging speed and the tip‐sample separation can be controlled to a few micrometers (e.g., 3 μm), satisfying the requirements of THz near‐field imaging of biological samples. It took about ~30 min (not the tens of hours it took to acquire the same kind of image previously) to collect a THz near‐field image of brain tissue slices of BALb/c mice (500 μm × 500 μm) with pixel size of 20 μm × 20 μm. The results show that the mouse brain slices can be properly imaged and different regions in the slices (i.e., the corpus callosum region and the cerebrum region) can be identified unambiguously. Evidently, the work demonstrated here provides not only a convincing example but a useful technique for imaging biological samples with THz near‐field microscopy. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2741, 2019.

Objectives: Terahertz (THz)-based imaging techniques hold great potential for biological and biomedical applications, which nevertheless are hampered by the low spatial resolution of conventional THz imaging systems. In this work, we report a high-performance photoconductive antenna microprobe-based near-field THz timedomain spectroscopy scanning microscope. Materials and methods:A single watermelon pulp cell was prepared on a clean quartz slide and covered by a thin polyethylene film. The high performance near-field THz microscope was developed based on a coherent THz time-domain spectroscopy system coupled with a photoconductive antenna microprobe. The sample was imaged in transmission mode. Results:We demonstrate the direct imaging of the morphology of single watermelon pulp cells in the natural dehydration process with our near-field THz microscope. Conclusions:Given the label-free and non-destructive nature of THz detection techniques, our near-field microscopy-based single-cell imaging approach sheds new light on studying biological samples with THz.

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