In order to decrease the side band leakage, we designed a
1mm thick metal cover around the probe and left a small hole;
the holes with different radius (Rgap) are simulated. We set
the observation plane at x=0.5 mm. Fig. 8 presents the results
for different Rgap. When the Rgap rises, the beam becomes
stronger and the side band leakage becomes stronger at the
same time. When Rgap is less than 500 μm, no beam is
observed. It is not hard to explain this, we set the Rgap=100
μm, and vary the observation plane, the results from the
simulation are shown in Fig. 9. Compared to Fig. 8, the
relationship between Rgap and the observation plane is
indicated. The observation plane has to be placed at a distance
to the ending of the tip less than Rgap, otherwise, we can’t see
the beam coming out. For Rgap much smaller than x (the
distance of tip and observation plane) the beam resembles a
point source and becomes indistinguishable from the
background.