![]() The challenge is to outcouple the highly confined MIM SPP mode to a single interface SPP mode that propagates along a dielectric–metal interface of a plasmonic waveguide because the momentum of the MIM SPP is one order of magnitude larger than that of the single interface SPPs,. We note that MIM-TJs are commercially available (e.g., magnetic tunnel junctions, Josephson junctions, tunnel diodes, millimeter wave detectors, or rectennas), ,, , and can be scaled to subhundred nanometers. Such systems have been used to demonstrate directional light emission of photons in free space, , but directional control over SPPs excited by MIM-TJs has not been demonstrated. These efficiencies are high enough to drive and modulate optical antennae, ,. Theoretical models predict internal excitation efficiencies of 10%, so far the highest reported external efficiencies of SPPs and photons generated by MIM-TJs are 1–2%. Direct excitation of plasmons by tunneling electrons is attractive because the process occurs at tunneling time scales on the order of fs, which is much faster than electron–hole recombination processes (>1 ps) for photon generation. It is well-known that SPPs and photons in free space can be electrically excited via inelastic quantum mechanical tunneling in metal–insulator–metal tunnel junctions (MIM-TJs), ,. We demonstrate, for the first time, unidirectional SPP excitation along a plasmonic waveguide from an aperiodic groove array (designed to reflect propagating SPPs by 180°) electrically driven by a quantum mechanical tunnel junction. For many applications, however, especially in nano-optoelectronics or sensing, directional control over the propagation of SPPs excited by electrical means is needed. Directional excitation of SPPs by optical means has been achieved in plasmonic structures such as asymmetric (slot) nanoantennae, , aperiodic gratings, , or Bragg mirrors. Usually, SPPs are excited by optical means using, for instance, lasers and prisms or simple gratings, but such approaches using external light sources are diffraction limited and normally do not provide control over the propagation direction of the SPPs. Surface plasmon polaritons (SPPs) confine electromagnetic fields at dielectric–metal interfaces and are promising candidates for applications in subwavelength imaging, sensing, and other areas in nanotechnology, ,, because of strong field enhancement and their capability to overcome the diffraction limit. In our experiments, we achieved a directionality (i.e., + x/− x ratio) of close to 8, and all of our experimental findings are supported by detailed theoretical simulations. Leakage radiation microscopy (Fourier and real plane imaging) shows that the propagation direction of selectively only one SPP mode (propagating along the metal–substrate interface) is controlled. ![]() ![]() We used constrained nonlinear optimization of the groove array based on the sequential quadratic programming algorithms coupled with finite-difference time-domain (FDTD) simulations to achieve the optimal structures. The aperiodic groove array consists of six grooves and is optimized to specifically reflect the SPPs by 180° in the desired direction (+ x or − x) along the plasmonic strip waveguide. This paper describes directional excitation of surface plasmon polaritons propagating along a plasmonic strip waveguide integrated with an aperiodic groove array electrically driven by an Al–Al 2O 3–Au tunnel junction. Access to surface plasmon polaritons (SPPs) with directional control excited by electrical means is important for applications in (on-chip) nano-optoelectronic devices and to circumvent limitations inherent to approaches where SPPs are excited by optical means (e.g., diffraction limit).
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