Miniaturised on-chip light sources are critical to realise optoelectronic circuits comparable in size to solely electronic ones. Optoelectronic circuitry that operates at distances smaller than the diffraction limit of light will result in ultra-high-density nano-circuitry capable of high data bandwidths with low energy consumption. While such sub-diffraction limit optoelectronic circuitry has been demonstrated using surface plasmon polaritons (SPPs), nanoscale SPP sources such as metal-insulator-metal tunnelling junctions (MIM-TJs) have low outcoupling efficiencies (i.e. the ratio of light energy emitted relative to that dissipating within the source) which constrain practical applications of MIM-TJs. Improvements to the outcoupling efficiency is impeded, among others, by uncertainty over the specific physical mechanism(s) leading to outcoupling.
This work combines experiments and theoretical models to show that MIM-TJs have only three possible outcoupling mechanisms, and that, out of the three mechanisms, the surface roughness-induced momentum matching mechanism is the dominant contributor to outcoupled light. This enhanced understanding is significantly different from that resulting from the computational model of smooth MIM-TJs (i.e. zero surface roughness) described in Nature Nanotechnology, vol 10, pp. 1058-1063 (2015).
The computational challenges are two-fold: first, straddling 3-4 orders of magnitude in length-scale between the micrometre-sized structure and nanometre-sized surface roughness; and second, modelling surface roughness with no symmetry or repeated unit. IHPC proposed customised models to overcome these well-known computational challenges to build and run a numerical model that explicitly modelled nanoscale MIM-TJ interface surface roughness, in addition to the easily modelled parameter of electrode thickness. To the best of our knowledge, this is the first computational model that accounted for surface roughness in MIM-TJs.
This work arose out of IHPC's involvement in the five-year NRF Competitive Research Programme (CRP) project entitled "Integration of Electrically Driven Plasmonic Components in High Speed Electronics" with start date 1 Jun 17. Since the completion of this work, the IHPC team has gone on to propose a nanoantenna-based design that leverages on the nanocavity-induced momentum matching mechanism to achieve an outcoupling efficiency sufficient for practical devices, and is currently awaiting experimental verification of the performance of a prototype. The enhanced understanding of outcoupling mechanisms can also guide the rational design of future energy-efficient, fast, compact CMOS integrated plasmonic devices for on-chip data communications and sensing. Similar modelling capabilities can also be applied to understand and design single photon sources, a critical component in quantum systems, based on cavity quantum electrodynamics.