Coherence Between Different Propagating Surface Plasmon Polariton Modes Excited by Quantum Mechanical Tunnel Junctions

Coherence between different surface plasmon polariton (SPP) modes excited by inelastically tunneling electrons in biased metal–insulator–metal tunnel junctions (MIM-TJs) is demonstrated. By employing a dedicated SPP stripe waveguide with MIM-TJ, an effective double-slit configuration similar to the Young's experiment is realized for an electrically biased SPP source. The spatial correlation between different SPP modes originates from a single inelastic tunneling event and leads to strong interference in the far-field, observed as alternate bright and dark fringes in the Fourier plane. The measured fringe-spacing inversely follows the stripe waveguide length, with upper limit dictated by the SPP propagation length, confirming the SPP mediated spatial correlation. Finite difference time domain simulations support the experimental findings. Also, the experimental and simulation results unambiguously demonstrate the two-step plasmonic decay process in plasmonic MIM-TJs. The results presented here provide a simple and robust demonstration of the inherent coherence existing between different decay channels (plasmons and photons) of the inelastically tunneling electrons and can be exploited for plasmonic applications with tailored spatial coherence with implications in plasmon amplification and quantum information processing.