Figure 5. Schematics of indicating dipole positions for coupling simulations of the MIM-SPP to the SPP Au-glass via a) pathway 2 and b) pathways 2 and 3. The simulated corresponding coupling efficiencies for the four different MIM-TJs with a solid line indicating rough MIM-TJs with σ m = 5 nm and a dashed line indicating smooth MIM-TJs with σ m = 0 nm are shown for c) pathway 2 and d) pathways 2 and 3.

No abstract

Inelastic quantum mechanical tunneling of electrons across plasmonic tunnel junctions can lead to surface plasmon polariton (SPP) and photon emission. So far, the optical properties of such junctions have been controlled by changing the shape, or the type of the material, of the electrodes, primarily with the aim to improve SPP or photon emission efficiencies. Here we show that by tuning the tunneling barrier itself, the efficiency of the inelastic tunneling rates can be improved by a factor of 3. We exploit the anisotropic nature of hexagonal boron nitride (hBN) as the tunneling barrier material in Au//hBN//graphene tunnel junctions where the Au electrode also serves as a plasmonic strip waveguide. As this junction constitutes an optically transparent hBN–graphene heterostructure on a glass substrate, it forms an open plasmonic system where the SPPs are directly coupled to the dedicated strip waveguide and photons outcouple to the far field. We experimentally and analytically show that the photon emission rate per tunneling electron is significantly improved (~ ×3) in Au//hBN//graphene tunnel junction due to the enhancement in the local density of optical states (LDOS) arising from the hBN anisotropy. With the dedicated strip waveguide, SPP outcoupling efficiency is quantified and is found to be ∼ 80% stronger than the radiative outcoupling in Au//hBN//graphene due to the high LDOS of the SPP decay channel associated with the inelastic tunneling. The new insights elucidated here deepen our understanding of plasmonic tunnel junctions beyond the isotropic models with enhanced LDOS.

Light emission from metal−insulator−semiconductor junctions (MISJs) has been explored for decades as a possible on-chip light source; however it is not clear whether the mechanism of light emission is plasmonic in nature or is dominated by electroluminescence. Previous studies only investigated silicon with low doping levels, but here we show that only highly doped silicon allows us to excite surface plasmon polaritons (SPPs) in MISJs via inelastic tunneling. This paper describes the mechanism of charge transport and light emission from siliconbased Au-SiO 2 -nSi MISJs as a function of the doping level N d varying from 1.6 × 10 15 cm −3 to 1.0 × 10 20 cm −3 . At low doping levels (N d ∼ 10 15 cm −3 ), the MISJs behave as Schottky diodes, and the mechanism of light emission involves a radiative recombination of electrons and holes from minority carrier injection under high applied bias (>5.5 V). With increasing doping levels, the current−voltage characteristics of the MISJs change, resulting in symmetrical current−voltage curves with parabolic conductance behavior characteristic of quantum mechanical tunneling. MISJs with the highest doping level (N d ∼ 10 20 cm −3 ) are dominated by quantum mechanical tunneling, and light emission originates from radiative decay of surface plasmon polaritons (SPPs) via scattering at threshold voltages as low as 1.5 V. Our simulations indicate that tunneling over the thin SiO 2 barrier between the Au and highly doped nSi excites a hybrid-SPP mode localized to the Au whose dispersion depends on the effective index induced by the SiO 2 −nSi interface. Our studies show that Si needs to be sufficiently doped to be conductive enough to enable SPP excitation via inelastic tunneling.

The spectral distribution of light emitted from a scanning tunnelling microscope junction not only bears its intrinsic plasmonic signature but is also imprinted with the characteristics of optical frequency fluc- tuations of the tunnel current. Experimental spectra from gold-gold tunnel junctions are presented that show a strong bias (V b) dependence, curiously with emission at energies higher than the quantum cut-off (eV b); a component that decays monotonically with increasing bias. The spectral evolution is explained by developing a theoretical model for the power spectral density of tunnel current fluctuations, incorporating finite temperature contribution through consideration of the quantum transport in the system. Notably, the observed decay of the over cut-off emission is found to be critically associated with, and well explained in terms of the variation in junction conductance with V b. The investigation highlights the scope of plasmon-mediated light emission as a unique probe of high frequency fluctuations in electronic systems that are fundamental to the electrical generation and control of plasmons.