DIRECTIONAL COUPLING IN DUAL-BRANCH ELECTRON-WAVE-GUIDE JUNCTIONS

We present a detailed analysis of a quantum directional coupler. The innovative aspect of the proposal comes from a dual coupling scheme. With respect to structures with a single interaction window, the crucial advantage is that the phase in the output leads can be strongly modulated through the distinct coupling paths. As a consequence, the proposed structure is highly directional in a four-terminal configuration. In order to address the above idea, a theoretical analysis is conducted by solving the two-dimensional Schrodinger equation using a mode-matching technique. Transmission spectra and conductance variations are calculated and interpreted paying attention to the influence of structural parameters such as wire widths and branch-line coupling lengths. On this basis, a parametric analysis is carried out including notably studies of the multimode operation and of the influence of electrostatic potential variations along the direction of propagation. Various modes of operation are pointed out. First, we illustrate a 3-dB coupling situation with a directivity as high as 35 dB in the monomode limit where the dearest interference effects are expected. Second, a real-space transfer mechanism with over 90% of transferred electrons is proposed as the operating mechanism of a quantum interference electronic switch. At last, the time response of mechanisms is discussed by viewing the transfer of electrons as a resonance process.