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

We present a detailed analysis of a quantum directional coupler. The i nnovative aspect of the proposal comes from a dual coupling scheme. Wi th respect to structures with a single interaction window, the crucial advantage is that the phase in the output leads can be strongly modul ated through the distinct coupling paths. As a consequence, the propos ed structure is highly directional in a four-terminal configuration. I n order to address the above idea, a theoretical analysis is conducted by solving the two-dimensional Schrodinger equation using a mode-matc hing technique. Transmission spectra and conductance variations are ca lculated and interpreted paying attention to the influence of structur al parameters such as wire widths and branch-line coupling lengths. On this basis, a parametric analysis is carried out including notably st udies of the multimode operation and of the influence of electrostatic potential variations along the direction of propagation. Various mode s of operation are pointed out. First, we illustrate a 3-dB coupling s ituation with a directivity as high as 35 dB in the monomode limit whe re 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 switc h. At last, the time response of mechanisms is discussed by viewing th e transfer of electrons as a resonance process.