42 research outputs found
Photon super-bunching from a generic tunnel junction
Generating 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 photon stream in the visible spectrum whose super-bunching
index is 17 when measured with a 53 picosecond instrumental resolution limit.
These photon bunches contain true photon pairs of plasmonic origin, distinct
from accidental photon coincidences. The effect is electrically rather than
optically driven - completely absent are pulsed lasers, down-conversions, and
four-wave mixing schemes. 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
Single charge and exciton dynamics probed by molecular-scale-induced electroluminescence
Excitons and their constituent charge carriers play the central role in
electroluminescence mechanisms determining the ultimate performance of organic
optoelectronic devices. The involved processes and their dynamics are often
studied with time-resolved techniques limited by spatial averaging that
obscures the properties of individual electron-hole pairs. Here we overcome
this limit and characterize single charge and exciton dynamics at the nanoscale
by using time-resolved scanning tunnelling microscopy-induced luminescence
(TR-STML) stimulated with nanosecond voltage pulses. We use isolated defects in
C thin films as a model system into which we inject single charges and
investigate the formation dynamics of a single exciton. Tuneable hole and
electron injection rates are obtained from a kinetic model that reproduces the
measured electroluminescent transients. These findings demonstrate that TR-STML
can track dynamics at the quantum limit of single charge injection and can be
extended to other systems and materials important for nanophotonic devices
Character of electronic states in the transport gap of molecules on surfaces
We report on scanning tunneling microscopy (STM) topographs of individual
metal phthalocyanines (MPc) on a thin salt (NaCl) film on a gold substrate, at
tunneling energies within the molecule's electronic transport gap. Theoretical
models of increasing complexity are discussed. The calculations for MPcs
adsorbed on a thin NaCl layer on Au(111) demonstrate that the STM pattern
rotates with the molecule's orientations - in excellent agreement with the
experimental data. Thus, even the STM topography obtained for energies in the
transport gap represent the structure of a one atom thick molecule. It is shown
that the electronic states inside the transport gap can be rather accurately
approximated by linear combinations of bound molecular orbitals (MOs). The gap
states include not only the frontier orbitals but also surprisingly large
contributions from energetically much lower MOs. These results will be
essential for understanding processes, such as exciton creation, which can be
induced by electrons tunneling through the transport gap of a molecule
Growth and surface alloying of Fe on Pt(997)
The growth of ultra-thin layers of Fe on the vicinal Pt(997) surface is
studied by thermal energy He atom scattering (TEAS) and Auger electron
spectroscopy (AES) in the temperature range between 175K and 800K. We find
three distinct regimes of qualitatively different growth type: Below 450K the
formation of a smooth first monolayer, at and above 600K the onset of bulk
alloy formation, and at intermediate temperature 500K - 550K the formation of a
surface alloy. Monatomic Fe rows are observed to decorate the substrate steps
between 175K and 500K. The importance of the high step density is discussed
with respect to the promotion of smooth layer growth and with respect to the
alloying process and its kinetics
Anionic Character of the Conduction Band of Sodium Chloride
The alkali halides are ionic compounds. Each alkali atom donates an electron
to a halogen atom, leading to ions with full shells. The valence band is mainly
located on halogen atoms, while, in a traditional picture, the conduction band
is mainly located on alkali atoms. Scanning tunnelling microscopy of NaCl at 4
K actually shows that the conduction band is located on Cl because the
strong Madelung potential reverses the order of the Na 3s and Cl 4s
levels. We verify this reversal is true for both atomically thin and bulk NaCl,
and discuss implications for II-VI and I-VII compounds
Top-layer superstructures of the reconstructed Pt(100) surface
The structures of the two reconstructed phases of the Pt(100) surface have been studied by high-resolution helium diffraction. In contrast to earlier investigations, we show that for both phases the superstructure in the approximate 〈011〉 direction is not fivefold but much larger. The mean distance between atom rows in the top layer, however, is very close to that of a fivefold superstructure. This supports the description of the surface layer in a model which assumes static oscillations about a flat and equidistant atom arrangement. The results are discussed in comparison with low-energy electron diffraction, scanning-tunneling-microscopy, and x-ray-diffraction results
Atomic-Scale Imaging and Spectroscopy of Electroluminescence at Molecular Interfaces
The conversion of electric power to light is an important scientific and technological challenge. Advanced experimental methods have provided access to explore the relevant microscopic processes at the nanometer scale. Here, we review state-of-the-art studies of electroluminescence induced on the molecular scale by scanning tunneling microscopy. We discuss the generation of excited electronic states and electron hole pairs (excitons) at molecular interfaces and address interactions between electronic states, local electromagnetic fields (tip-induced plasmons), and molecular vibrations. The combination of electronic and optical spectroscopies with atomic-scale spatial resolution is able to provide a comprehensive picture of energy conversion at the molecular level. A recently developed aspect is the characterization of electroluminescence emitters as quantum light sources, which can be studied with high time resolution, thus providing access to picosecond dynamics at the atomic scale
Electroluminescence properties of organic nanostructures studied by scanning tunnelling microscopy
The control of light emission on the scale of individual quantum systems, like molecules or quantum dots, is a field of intense current research. One way to induce light emission from these systems is the local charge injection through the tip of a scanning tunnelling microscope (STM). Studies which employ this method have to address one basic question: Does the detected luminescence provide information precisely from the molecule into which charge is injected by the STM tip apex or are the luminescence properties determined by a larger volume? In this article, we focus on the investigation of organic nanocrystals and discuss the relation between the local excitation, the intermolecular coupling and the influence of the STM as a measuring instrument. Choosing pentacene as an organic emitter, we present results, which suggest that the STM-induced luminescence cannot be attributed simply to the emission by a single molecule. We discuss how information about locality can be obtained and comment on the present experimental limitations and possible future improvements