Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding

Abstract

Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2–SiN–glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 ± 1.74)% and (29.07 ± 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of μm) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms.

Electrically Driven Deterministic Plasmon Light Sources Based on Arrays of Molecular Tunnel Junctions

“…Under a small external electrical bias (<2 V), the inelastic electron tunnelling (IET) inside tunnel junctions could drive the electrical excitation of surface plasmons on noble metal electrodes, leading to tunnel junction light sources with small footprints and ultrafast response speeds . This tunnelling-based light emission has the potential to be used as a nanoscale light source for applications such as subnanometer probes for light-matter interactions, , electrically driven optical antennas − and plasmonic circuits. , To realize the above applications, tunnel junctions are usually formed in different fashions ranging from scanning tunnelling microscopy (STM) junctions, − to metal–insulator–metal (MIM) tunnel junctions, − or molecular tunnel junctions (MTJ). − Although many exciting works have been demonstrated in improving the electron-to-photon coupling efficiency − and coupling these tunnelling-based light sources to optical waveguides, − so far most tunnel junction devices still suffer from low yield and nonuniform emission, as the quantum tunnelling is highly sensitive to the thickness or local defects of the tunnel barrier. However, toward the chip-scale applications, it will require up-scaling and integration of arrays of tunnel junctions, with uniform light emission from each functional unit.…”