Figure 3. Three representative knife-edge measurements of THz beam diameter d.

(a) d = 20 μm at z = 0 mm. (b) d = 30 μm at z = 3 mm. (c) d = 1 mm at z = 6 mm. z axis corresponds to the laser propagation direction and z = 0 is defined as the starting position where significant THz signal was able to be detected.