Directional launching of surface plasmon polaritons by electrically driven aperiodic groove array reflectors

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). 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-Al2O3-Au tunnel junction. 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. 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. 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. 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