57 research outputs found
Valley-selective energy transfer between quantum dots in atomically thin semiconductors
In monolayers of transition metal dichalcogenides the nonlocal nature of the effective dielectric screening leads to large binding energies of excitons. Additional lateral confinement gives rise to exciton localization in quantum dots. By assuming parabolic confinement for both the electron and the hole, we derive model wave functions for the relative and the center-of-mass motions of electronhole pairs, and investigate theoretically resonant energy transfer among excitons localized in two neighboring quantum dots. We quantify the probability of energy transfer for a direct- gap transition by assuming that the interaction between two quantum dots is described by a Coulomb potential, which allows us to include all relevant multipole terms of the interaction. We demonstrate the structural control of the valley-selective energy transfer between quantum dots
Observation of dressed excitonic states in a single quantum dot
We report the observation of dressed states of a quantum dot. The optically
excited exciton and biexciton states of the quantum dot are coupled by a strong
laser field and the resulting spectral signatures are measured using
differential transmission of a probe field. We demonstrate that the anisotropic
electron-hole exchange interaction induced splitting between the x- and
y-polarized excitonic states can be completely erased by using the AC-Stark
effect induced by the coupling field, without causing any appreciable
broadening of the spectral lines. We also show that by varying the polarization
and strength of a resonant coupling field, we can effectively change the
polarization-axis of the quantum dot
Plasmon-Exciton Coupling Using DNA Templates
Coherent energy exchange between plasmons and excitons is a phenomenon that
arises in the strong coupling regime resulting in distinct hybrid states. The
DNA-origami technique provides an ideal framework to custom-tune
plasmon-exciton nanostructures. By employing this well controlled self-assembly
process, we realized hybrid states by precisely positioning metallic
nanoparticles in a defined spatial arrangement with fixed nanometer-sized
interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60
nm while keeping their separation distance constant allowed us to precisely
adjust the plasmon resonance of the structure to accurately match the energy
frequency of a J-aggregate exciton. With this system we obtained strong
plasmon-exciton coupling and studied far-field scattering at the
single-structure level. The individual structures displayed normal mode
splitting up to 170 meV. The plasmon tunability and the strong field
confinement attained with nanodimers on DNA-origami renders an ideal tool to
bottom-up assembly plasmon-exciton systems operating at room temperature.Comment: This document is the unedited Author's version of a Submitted Work
that was subsequently accepted for publication in Nano Letters, copyright
\copyright American Chemical Society after peer review. To access the final
edited and published work see
http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.6b03015, Nano Letters 201
Antenna-enhanced Optoelectronic Probing of Carbon Nanotubes
We report on the first antenna-enhanced optoelectronic microscopy studies on
nanoscale devices. By coupling the emission and excitation to a scanning
optical antenna, we are able to locally enhance the electroluminescence and
photocurrent along a carbon nanotube device. We show that the emission source
of the electroluminescence can be point-like with a spatial extension below 20
nm. Topographic and antenna-enhanced photocurrent measurements reveal that the
emission takes place at the location of highest local electric field indicating
that the mechanism behind the emission is the radiative decay of excitons
created via impact excitation
Cavity-enhanced Raman Microscopy of Individual Carbon Nanotubes
Raman spectroscopy reveals chemically specific information and provides
label-free insight into the molecular world. However, the signals are
intrinsically weak and call for enhancement techniques. Here, we demonstrate
Purcell enhancement of Raman scattering in a tunable high-finesse microcavity,
and utilize it for molecular diagnostics by combined Raman and absorption
imaging. Studying individual single-wall carbon nanotubes, we identify crucial
structural parameters such as nanotube radius, electronic structure and
extinction cross-section. We observe a 320-times enhanced Raman scattering
spectral density and an effective Purcell factor of 6.2, together with a
collection efficiency of 60%. Potential for significantly higher enhancement,
quantitative signals, inherent spectral filtering and absence of intrinsic
background in cavity-vacuum stimulated Raman scattering render the technique a
promising tool for molecular imaging. Furthermore, cavity-enhanced Raman
transitions involving localized excitons could potentially be used for gaining
quantum control over nanomechanical motion and open a route for molecular
cavity optomechanics
Photon Antibunching in the Photoluminescence Spectra of a Single Carbon Nanotube
We report the first observation of photon antibunching in the
photoluminescence from single carbon nanotubes. The emergence of a fast
luminescence decay component under strong optical excitation indicates that
Auger processes are partially responsible for inhibiting two-photon generation.
Additionally, the presence of exciton localization at low temperatures ensures
that nanotubes emit photons predominantly one by one. The fact that multiphoton
emission probability can be smaller than 5% suggests that carbon nanotubes
could be used as a source of single photons for applications in quantum
cryptography.Comment: content as publishe
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