Imaging diffraction-limited electronic collimation from a non-equilibrium one-dimensional ballistic constriction

We report on the use of a low-temperature scanning probe microscope to investigate non-equilibrium electronic transport through a one-dimensional ballistic constriction. Trans-conductance images of electrons backscattered in the adjoining two-dimensional reservoirs show a weak acceptance cone consistent with semiclassical collimation. The images also show a strong highly collimated beam of quasi-particles injected into each reservoir with a divergence consistent with quantum mechanical diffraction. In the lower-chemical-potential reservoir these quasi-particles are interpreted as hot electrons while in the higher-chemical-potential reservoir they are interpreted as conduction-band holes.