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

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.

Photon superbunching from a generic tunnel junction

Generating time-correlated photon pairs at the nanoscale is a prerequisite to creating highly integrated optoelectronic circuits that perform quantum computing tasks based on heralded single photons. Here, we demonstrate fulfilling this requirement with a generic tip-surface metal junction. When the junction is luminescing under DC bias, inelastic tunneling events of single electrons produce a stream of visible photons of plasmonic origin whose superbunching index is 17 (improved to a record of 70 by the authors during publication) when measured with a 53-ps instrumental resolution limit. The effect is driven electrically, rather than optically. This discovery has immediate and profound implications for quantum optics and cryptography, notwithstanding its fundamental importance to basic science and its ushering in of heralded photon experiments on the nanometer scale.

Vibrations of a molecule in an external force field

The oscillation frequencies of a molecule on a surface are determined by the mass distribution in the molecule and the restoring forces that occur when the molecule bends. The restoring force originates from the atomic-scale interaction within the molecule and with the surface, which plays an essential role in the dynamics and reactivity of the molecule. In 1998, a combination of scanning tunneling microscopy with inelastic tunneling spectroscopy revealed the vibrational frequencies of single molecules adsorbed on a surface. However, the probe tip itself exerts forces on the molecule, changing its oscillation frequencies. Here, we combine atomic force microscopy with inelastic tunneling spectroscopy and measure the influence of the forces exerted by the tip on the lateral vibrational modes of a carbon monoxide molecule on a copper surface. Comparing the experimental data to a mechanical model of the vibrating molecule shows that the bonds within the molecule and with the surface are weakened by the proximity of the tip. This combination of techniques can be applied to analyze complex molecular vibrations and the mechanics of forming and loosening chemical bonds, as well as to study the mechanics of bond breaking in chemical reactions and atomic manipulation.

Adsorbate-driven cooling of carbene-based molecular junctions

We study the role of an NH2 adsorbate on the current-induced heating and cooling of a neighboring carbene-based molecular circuit. We use first-principles methods of inelastic tunneling transport based on density functional theory and non-equilibrium Green’s functions to calculate the rates of emission and absorbtion of vibrations by tunneling electrons, the population of vibrational modes and the energy stored in them. We find that the charge rearrangement resulting from the adsorbate gates the carbene electronic structure and reduces the density of carbene states near the Fermi level as a function of bias. These effects result in the cooling of carbene modes at all voltages compared to the “clean” carbene-based junction. We also find that the direct influence of adsorbate states is significantly smaller and tends to heat adsorbate vibrations. Our results highlight the important role of molecular adsorbates not only on the electronic and elastic transport properties but also on the current-induced energy exchange and stability under bias of single-molecule circuits.