Fig. 2.

Polarization-dependent tunneling in ring-shaped quantum barriers. a SEM image of quantum barrier loop in the shape of a triangle. THz-field-driven surface current K charges the triangular loop. b Schematic description of the polarization-dependent current response in a triangular barrier. The first column shows the incident polarization of the surface current. The second and third columns show the polarization-dependent potential V(l) and tunneling current density J_t(l) (Eq. 3 of Methods) as a function of arc length l (x-axis) defined in (a) and polarization (row). The final column shows the total current direction (inward or outward from the loop). The direction of the surface current determines the potential distribution, which in turn drives the tunneling currents across each side of the triangle. The final output current is the contour integral of tunneling currents along the whole quantum barrier loop. c Experimentally measured tunneling currents across triangular (side length of 70 μm, perimeter of 210 μm) and square (side length of 25 μm, perimeter of 100 μm) nanogap loops (gap size of 2 nm) are shown as a function of the THz polarizer angle. To directly compare the currents from the two loops of different sizes, a factor of 2.1 (=210 μm/100 μm) is multiplied to the current measured from the square loop. d Polar plot of the reconstructed current (red and blue filled curves) obtained from the results in (c). The black triangle and square are guides to the eye