Optical Anisotropy in van der Waals materials: Impact on Direct Excitation of Plasmons and Photons by Quantum Tunneling

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.

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

“…At the same time, the plasmonic mode (M1), which is generally nonradiative and confined to the metal−dielectric interface, scatters from the ±y edges of the Au strip and results in an enhanced intensity at these locations. 22,29,30 Figure 4b shows the light scattering from the pl-WG and the ph-WG ends at x = 10 and 20 μm, respectively. Interestingly, we also note that the etched boundaries of the SiO 2 −SiN substrate along the y-axis show a weak scattering contribution, probably from M2, which isotropically propagates in all directions from the TJ before it couples to the ph-WG.…”

“…The flakes were assembled via the PMGI/PMMA sacrificial layer transfer method. 30 The FLG flake was etched as a 5-μm-wide strip using electron beam lithography (EBL) (JEOL, Model JBX-6300FS) and O 2 plasma etching (Femto Science, Model VITA). The hBN-FLG heterostructure was annealed in a vacuum at 220 °C for 6 h. The flake thicknesses were determined by using atomic force microscopy (AFM) (Bruker FastScan).…”

The Luminescence Hypothesis of Olfaction

“…Noble-metal films can also exhibit electroluminescence through inelastic electron tunneling [ 43 ]. The optical properties of IET tunnel junctions have been controlled by changing the shape or the material of the electrodes with the aim to improve photon emission efficiencies [ 44 ]. In addition, the resonant peak of emitted photons from IET can be spectrally tuned [ 45 ].…”

Waveguide-Integrated Light-Emitting Metal–Insulator–Graphene Tunnel Junctions

“…The efficiency of this process was estimated to be as low as ∼10 –3 . By contrast, for the MIG-TJ (Figure E,G), plasmonic mode of the nanowire–graphene structure having a much higher propagation length (∼6.5 μm) can be directly excited by the dipolar tunneling source. , Furthermore, the modes reaching the junction edge can couple to the mode of the plasmonic nanowire with an efficiency of ∼0.7 benefiting from a small mode mismatch between the modes (cf. Figure J,K).…”

“…Figure E presents an emission spectrum of the tunneling device at V bias = 2.2 V measured from the nanowire tip. The emission spectrum is essentially broadband, which is fundamentally related to the nature of the IET process, producing photonic and/or plasmonic excitations of all energies below eV bias . ,,, From the measured emission power at the nanowire tip regions together with the tunneling current of ∼7 μA at 2.2 V, the EQE for the waveguided plasmonic output channel in both directions was estimated to be 2 × 10 –6 (Supporting Information Section 6). This value is about 10 times lower than that of tunneling devices with waveguided output based on nanoantenna designs , or scanning tunneling microscope tips (tunneling current ∼10 nA), which is mainly due to the lower LDOS of the MIG-TJs.…”