arXiv subjects

Alexander Högele

Publications and source records attributed to Alexander Högele.

49 records · Page 3Linked to original sources

arXiv2017

The role of momentum-dark excitons in the elementary optical response of bilayer WSe$_{2}$

Monolayer (ML) transition metal dichalcogenides (TMDs) undergo substantial changes in the single-particle band structure and excitonic optical response upon the addition of just one layer. As opposed to the single-layer limit, the bandgap of bilayer (BL) TMD semiconductors is indirect which results in reduced photoluminescence with richly structured spectra that have eluded a detailed understanding to date. Here, we provide a closed interpretation of the elementary optical responses of BL WSe$_2$ as a representative material for the wider class of TMD semiconductors. By combining theoretical calculations with comprehensive spectroscopy experiments, we identify the crucial role of momentum-indirect excitons for the understanding of basic absorption and emission spectra ubiquitously exhibited by various TMD BLs. Our results shed light on the origin of quantum dot (QD) formation in monolayer and bilayer crystals and will facilitate further advances directed at opto-electronic applications of layered TMD semiconductors in van der Waals heterostructures and devices.

cond-mat.mes-hall↗

arXiv2017

Cavity-control of bright and dark interlayer excitons in van der Waals heterostructures

Monolayer (ML) transition metal dichalcogenides (TMDs) integrated in optical microcavities host exciton-polaritons as a hallmark of the strong light-matter coupling regime. Analogous concepts for hybrid light-matter systems employing spatially indirect excitons with a permanent electric dipole moment in heterobilayer (HBL) crystals promise realizations of exciton-polariton gases and condensates with immanent dipolar interactions. Here, we identify optical signatures of spatially indirect momentum-bright and momentum-dark interlayer excitons in vertical MoSe$_2$-WSe$_2$ heterostructures and implement cavity-control of both exciton manifolds. Our experiments quantify the strength of light-matter coupling for both zero and finite momentum excitons residing in Moiré superlattices of TMD HBLs and demonstrate that both exciton species are susceptible to Purcell enhancement in cavity-modified photonic environments. Our results form the basis for further developments of dipolar exciton-polariton gases and condensates in hybrid cavity -- van der Waals heterostructure systems.

cond-mat.mes-hall↗

arXiv2017

Contamination of polymethylmethacrylate by organic quantum emitters

We report the observation of ubiquitous contamination of polymethylmethacrylate by organic molecules with optical activity in the visible spectral range. Contamination sites of individual solvent-specific fluorophores in thin films of polymethylmethacrylate constitute fluorescence hot-spots with quantum emission statistics and quantum yields approaching 30% at cryogenic temperatures. Our findings not only resolve prevalent puzzles in the assignment of spectral features to various nanoemitters in polymer matrices, they also identify means for simple and cost-efficient realization of single-photon sources in the visible spectral range.

cond-mat.mes-hall↗

arXiv2017

Dipolar and charged localized excitons in carbon nanotubes

We study both experimentally and theoretically the fundamental interplay of exciton localization and polarization in semiconducting single-walled carbon nanotubes. From Stark spectroscopy of individual carbon nanotubes at cryogenic temperatures we identify localized excitons as permanent electric dipoles with dipole moments of up to 1eÅ. Moreover, we demonstrate field-effect doping of localized excitons with an additional charge which results in defect-localized trions. Our findings provide not only fundamental insight into the microscopic nature of localized excitons in carbon nanotubes, they also signify their potential for sensing applications and may serve as guidelines for molecular engineering of exciton-localizing quantum dots in other atomically thin semiconductors including transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2016

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.

cond-mat.mes-hall↗

arXiv2016

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.

physics.optics↗

arXiv2014

Highly coherent electron beam from a laser-triggered tungsten needle tip

We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain $(0.80\pm 0.05)\,$nm in laser-triggered and $(0.55\pm 0.02)\,$nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.

physics.optics↗

arXiv2012

Optical signatures of quantum dot excitons in carbon nanotubes

We report optical studies of quantum dot excitons in individual suspended carbon nanotubes at cryogenic temperatures. Narrow optical linewidths, strongly suppressed spectral wandering, and photoluminescence lifetimes in the range of nanoseconds emerge as key signatures of exciton localization. We infer exciton quantum dot formation with a characteristic length of a few exciton Bohr radii. Localization inhibits exciton diffusion and protects the exciton from dephasing by structural or environmental inhomogeneities as well as from exploring nonradiative quenching sites. In consequence, quantum dot excitons in carbon nanotubes exhibit intrinsic radiative lifetimes, long coherence times and a quantum yield of 100%. Our results underpin the potential of carbon nanotube excitons for both fundamental studies and applications that scale advantageously with enhanced spectral resolution and coherence.

cond-mat.mes-hall↗

arXiv2011

DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response

Surface plasmon resonances generated in metallic nanostructures can be utilized to tailor electromagnetic fields. The precise spatial arrangement of such structures can result in surprising optical properties that are not found in any naturally occurring material. Here, the designed activity emerges from collective effects of singular components equipped with limited individual functionality. Top-down fabrication of plasmonic materials with a predesigned optical response in the visible range by conventional lithographic methods has remained challenging due to their limited resolution, the complexity of scaling, and the difficulty to extend these techniques to three-dimensional architectures. Molecular self-assembly provides an alternative route to create such materials which is not bound by the above limitations. We demonstrate how the DNA origami method can be used to produce plasmonic materials with a tailored optical response at visible wavelengths. Harnessing the assembly power of 3D DNA origami, we arranged metal nanoparticles with a spatial accuracy of 2 nm into nanoscale helices. The helical structures assemble in solution in a massively parallel fashion and with near quantitative yields. As a designed optical response, we generated giant circular dichroism and optical rotary dispersion in the visible range that originates from the collective plasmon-plasmon interactions within the nanohelices. We also show that the optical response can be tuned through the visible spectrum by changing the composition of the metal nanoparticles. The observed effects are independent of the direction of the incident light and can be switched by design between left- and right-handed orientation. Our work demonstrates the production of complex bulk materials from precisely designed nanoscopic assemblies and highlights the potential of DNA self-assembly for the fabrication of plasmonic nanostructures.

cond-mat.mes-hall↗

arXiv2008

Non-Markovian decoherence of localized nanotube excitons by acoustic phonons

We demonstrate that electron-phonon interaction in quantum dots embedded in one-dimensional systems leads to pronounced, non-Markovian decoherence of optical transitions. The experiments we present focus on the lineshape of photoluminescence from low-temperature axially localized carbon nanotube excitons. The independent boson model that we use to model the phonon interactions reproduces with very high accuracy the broad and asymmetric emission lines and the weak red-detuned radial breathing mode replicas observed in the experiments. The intrinsic phonon-induced pure-dephasing of the zero-phonon line is two orders of magnitude larger than the lifetime broadening and is a hallmark of the reduced dimensionality of the phonon bath. The non-Markovian nature of this decoherence mechanism may have adverse consequences for applications of one-dimensional systems in quantum information processing.

cond-mat.mes-hall↗

arXiv2008

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.

cond-mat.mes-hall↗

arXiv2007

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.

cond-mat.mes-hall↗