FIGURE 1.

Schematic illustration of the experimental setup. THz radiation emitted from the THz source was firstly collimated by a polymethyl pentene (TPX) lens with an effective focal length of about 50 mm and then focused by another TPX lens onto the back of a quartz slide on which a single watermelon pulp cell (red) was deposited. The diameter of the focused THz radiation spot is about 1 mm@1.0 THz. The near‐field THz signal of the cell was detected by a PCAM detector along the sample surface (indicated by a dashed black line) at a distance of ~10 μm. The sample was controlled to move in the X, Y and Z directions via a precisely controlled 3D translation stage, enabling raster scanning imaging the sample possible