imaging [5] and sensing [6] for biological species, and for spatial differentiation and edge detection without optical lenses that perform the Fourier transform, [7,8] to name a few. SPPs are coherent due to the collective nature of free electron oscillations [9] and are bound to the metal-dielectric interfaces which provides unrivalled field confinement at the nanoscale, [10] opening up a plethora of applications ranging from sensing [11] to quantum information processing. [12,13] Plasmonic metal-insulator-metal tunnel junctions (MIM-TJs) offer a unique platform for the electrical excitation of SPPs by low energy (<2.5 eV) tunneling electrons [14][15][16][17][18] because of their nanoscale footprint and potential for device-integration for subwavelength imaging, or nano-optoelectronics. [19,20] In MIM-TJs, the SPP or photon emission originates from a three-step outcoupling process in which, first, the energy quanta (ℏω) given off by inelastically tunneling electrons can couple predominantly to a highly-confined cavity mode (MIM-SPP, [18] Figure 1a), which then outcouples to SPPs and finally to photons. [21] Here we show that this entire three-step process remains coherent as the excitation originates from a single inelastic tunneling event and is important for areas of research where spatially correlated and actively modulated sources are essential, such as, plasmonic interferometry, [22] plasmonic analogue of quantum optics
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.