Figure 3. Mapping of the 3.4 THz focused beam by the near-field probe with the BP detector.

(a) Schematic of the experimental system: the QCL beam is focused on the near-field probe using a pair of Picarin lenses (focal lengths: 2 cm and 5 cm). The near-field probe is translated in the yz-plane. (b,c) THz intensity maps normalized to the incident THz power recorded by the 20 μm aperture BP probe with its surface oriented normally to the optical axis (b) and at a small angle (c).