Manipulating photon emission efficiency with local electronic states in a tunneling gap

Unveiling the radiative local density of optical states of a plasmonic nanocavity by STM

Atomically-sharp tips in close proximity of metal surfaces create plasmonic nanocavities supporting both radiative (bright) and non-radiative (dark) localized surface plasmon modes. Disentangling their respective contributions to the total density of optical states remains a challenge. Electroluminescence due to tunnelling through the tip-substrate gap could allow the identification of the radiative component, but this information is inherently convoluted with that of the electronic structure of the system. In this work, we present a fully experimental procedure to eliminate the electronic-structure factors from the scanning tunnelling microscope luminescence spectra by confronting them with spectroscopic information extracted from elastic current measurements. Comparison against electromagnetic calculations demonstrates that this procedure allows the characterization of the meV shifts experienced by the nanocavity plasmonic modes under atomic-scale gap size changes. Therefore, the method gives access to the frequency-dependent radiative Purcell enhancement that a microscopic light emitter would undergo when placed at such nanocavity.

Disentangling the radiative and non-radiative plasmon mode contributions to the total photonic density of states is a challenge. Here, the authors report a procedure to eliminate the electronic-structure factors from scanning tunnelling microscope luminescence spectra to isolate the radiative component.

Optical antennas driven by quantum tunneling: a key issues review

Analogous to radio- and microwave antennas, optical nanoantennas are devices that receive and emit radiation at optical frequencies. Until recently, the realization of electrically driven optical antennas was an outstanding challenge in nanophotonics. In this review we discuss and analyze recent reports in which quantum tunneling-specifically inelastic electron tunneling-is harnessed as a means to convert electrical energy into photons, mediated by optical antennas. To aid this analysis we introduce the fundamentals of optical antennas and inelastic electron tunneling. Our discussion is focused on recent progress in the field and on future directions and opportunities.

A Single Hydrogen Molecule as an Intensity Chopper in an Electrically Driven Plasmonic Nanocavity

Photon statistics is a powerful tool for characterizing the emission dynamics of nanoscopic systems and their photophysics. Recent advances that combine correlation spectroscopy with scanning tunneling microscopy induced luminescence (STML) have allowed the measurement of the emission dynamics from individual molecules and defects, demonstrating their nature as single-photon emitters. The application of correlation spectroscopy to the analysis of the dynamics of a well-characterized adsorbate system in an ultrahigh vacuum remained to be demonstrated. Here, we combine single-photon time correlations with STML to measure the dynamics of individual H₂ molecules between a gold tip and an Au(111) surface. An adsorbed H₂ molecule performs recurrent excursions below the tip apex. We use the fact that the presence of the H₂ molecule in the junction modifies plasmon emission to study the adsorbate dynamics. Using the H₂ molecule as a chopper for STM-induced optical emission intensity, we demonstrate bunching in the plasmonic photon train in a single measurement over 6 orders of magnitude in the time domain (from microseconds to seconds) that takes only a few seconds. Our findings illustrate the power of using photon statistics to measure the diffusion dynamics of adsorbates with STML.

Decay channels of gap plasmons in STM tunnel junctions

We study the decay of gap plasmons localized between a scanning tunneling microscope tip and metal substrate, excited by inelastic tunneling electrons. The overall excited energy from the tunneling electrons is divided into two categories in the form of resistive dissipation and electromagnetic radiation, which together can further be separated into four diffierent channels, including SPP channel on the tip, SPP channel on the substrate, air mode channel and direct quenching channel. In this work, we study the enhancement factor, i.e. Purcell factor, of the STM tunnel junctions, which are mediated by the nearby metallic structures. We find that the gap plasmon mode is most likely to couple to the SPP channel on the tip, rather than the SPP channel on the substrate or the air mode. The direct quenching in the apex of tip also takes a considerable portion especially in high frequency region, the enhancement factor of direct quenching in the tip is much higher than the direct quenching in the substrate. We adopt four tips with diffierent apex radii, i.e., 1 nm, 5 nm, 10 nm, 20 nm. When the apex size is small, the frequency dependent enhancement factor from the SPPs contribution has a pronounced peak at 1.55 eV, however, as the radius increases, the peak of enhancement factor in the high frequency region appears, the 1.55 eV peak becomes less dominated. This phenomenon can be attributed to the change of tip shape, in the form of mode coupling. Our results also show a relationship between the direct quenching in the substrate and in the tip. With the larger radius of apex, the ratio of these two part of energy approaches 1, which indicate that the energy distribution of direct quenching is sensitive to the shape of the tip-substrate gap.

Molecular Chessboard Assemblies Sorted by Site-Specific Interactions of Out-of-Plane d-Orbitals with a Semimetal Template

We show that highly ordered two-dimensional (2D) chessboard arrays consisting of a periodic arrangement of two different molecules can be obtained by self-assembly of unsubstituted metal-phthalocyanines (metal-Pcs) on a suitable substrate serving as the template. Specifically, CuPc + MnPc and CuPc + CoPc mixtures sort into highly ordered Cu/Mn and Cu/Co chessboard arrays on the square p(10 × 10) reconstruction of bismuth on Cu(100). Such created bimolecular chessboard assemblies emerge from the site-specific interactions between the central transition-metal ions and the periodically reconstructed substrate. This work provides a conceptually new approach to induce 2D chessboard patterns in that no functionalization of the molecules is needed.