Light at the End of the Tunnel

Abstract

In recent years, tunnel junctions have reemerged as promising candidates for the transduction of electrical to optical signals at the nanoscale. The process of interest is known as inelastic electron tunneling (IET), where a tunneling electron excites an optical mode while traversing the tunnel barrier. The main appeal of tunnel junctions lies in their size and bandwidth, both of which are unmatched by other electronic devices. However, their main disadvantage so far has been the overall low transduction efficiency. Recently, the realization of photon and surface plasmon polariton (SPP) sources based on IET, with efficiencies exceeding 1%, were reported. In this perspective, we critically analyze the factors that limit efficiencies and extract guidelines for the development of efficient photon and SPP sources.

Efficient Surface Plasmon Polariton Excitation and Control over Outcoupling Mechanisms in Metal–Insulator–Metal Tunneling Junctions

“…Despite this recent experimental evidence of efficient SPP excitation in MIM‐TJs, theoretical treatises of these systems continue to argue that these efficiencies cannot be due to outcoupling of the MIM‐SPP mode, and an alternative interpretation is needed 27,28. In this work we demonstrate that, by modeling MIM–TJ interface surface roughness and electrode thickness with a simple ad hoc roughness model, electron‐to‐SPP efficiencies can be within an order of magnitude of experimental measurement.…”

“…Here we show through experiments supported by theory that the MIM‐SPP mode readily outcouples via three possible outcoupling pathways of the MIM‐SPP mode: i) scattering of the MIM‐SPP to photons at the different dielectric–metal interfaces16,38–41,44,49 (Figure 1c,d, pathway 1), ii) coupling of the MIM‐SPP to bound‐SPP modes by spatial mode‐overlap and roughness‐induced momentum matching22,30,35,37,57 (Figure 1c,d, pathway 2), and iii) direct coupling of the MIM‐SPP mode to photons as well as the bound‐SPP modes at the edge of the MIM‐TJ and the adjacent waveguides14,27 (Figure 1c,d, pathway 3). By studying light emission from Al‐AlO x ‐Cr‐Au MIM‐TJs integrated into Al and Au waveguides (Figure 1b) and varying the Al and Au thickness (Figure 1c,d) we demonstrate that the MIM‐SPP mode couples to the bound‐SPP modes (Figure 1c,d), where the coupling efficiency is a function of the effective electrode thickness.…”

“…The MIM‐SPP mode has a wavevector of an order of magnitude larger than the single‐interface SPP modes that exist at the metal/dielectric interfaces (bound‐SPP modes) at visible/near‐IR frequencies. Parzefall et al27 recently theoretically investigated the outcoupling efficiency of the MIM‐SPP mode of an ideally smooth MIM‐TJ without surface roughness or topography, considering only outcoupling via pathway 2 (the only possible pathway in smooth systems; see the following sections), and reported very low outcoupling efficiencies on the order of 10 −8 . Therefore, in order to increase outcoupling efficiencies, momentum‐matching conditions at the metal–dielectric interfaces are required to outcouple the mode to both free space photons and to bound‐SPPs 26,35.…”

Optical antennas driven by quantum tunneling: a key issues review

“…Originally derived by Hone et al, equation (16) The third model describes IET as a spontaneous emission (SE) process, where the transitions occur within the tunnel junction and between electronic states of different energies [114][115][116][117][118][119]. Here, the rate of IET γ SE inel is determined by Fermi's golden rule…”

“…The further implementation varies in that the interaction Hamiltonian is either given byĤ int = −eφ, whereφ is the potential operator of the optical mode [94,[114][115][116], or byĤ int = −e/m ·p, where is the vector potential operator andp is the momentum operator [117][118][119].…”