Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs.

Reliable methods to individually track large numbers of cells in real time are urgently needed to advance our understanding of important biological processes like cancer metastasis, neuronal network development and wound healing. It has recently been suggested to introduce microscopic whispering gallery mode lasers into the cytoplasm of cells and to use their characteristic, size-dependent emission spectrum as optical barcode but so far there is no evidence that this approach is generally applicable. Here, we describe a method that drastically improves intracellular delivery of resonators for several cell types, including mitotic and non-phagocytic cells. In addition, we characterize the influence of resonator size on the spectral characteristics of the emitted laser light and identify an optimum size range that facilitates tagging and tracking of thousands of cells simultaneously. Finally, we observe that the microresonators remain internalized by cells during cell division, which enables tagging several generations of cells.

Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio many-body calculations, we demonstrate how a strong coupling of conduction electrons with collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping.

The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue toward engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. Direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines the effect of electrical doping on the electronic band structure of twisted bilayer graphene using a back‐gated device architecture for angle‐resolved photoemission measurements with a nano‐focused light spot. A twist angle of 12.2° is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene.

Screening in reduced dimensions has strong consequences on the electronic properties in vander Waals semiconductors, impacting the quasiparticle band gap and exciton binding energy.Screening in these materials is typically treated isotropically, yet black phosphorus exhibits inplane electronic anisotropy seen in its effective mass, carrier mobility, excitonic wavefunctions, and plasmonic dispersion. Here, we use the adsorption of individual potassium atoms on the surface of black phosphorus to vary the near-surface doping over a wide range, while simultaneously probing the dielectric screening via the ordering of the adsorbed atoms. Using scanning tunneling microscopy, we visualize the role of strongly anisotropic screening which leads to the formation of potassium chains with a well-defined orientation and spacing. We quantify the mean interaction potential utilizing statistical methods and find that the dimensionality and anisotropy of the screening is consistent with the presence of a band-bending induced confinement potential near the surface. We corroborate the observed behavior with coverage-dependent studies of the electronic structure with angle-resolved photoemission.

Inversion-symmetric crystals are optically isotropic and thus naively not expected to show dichroism effects in optical absorption and photoemission processes. Here, we find a strong linear dichroism effect (up to 42.4%) in the conduction band of inversion-symmetric bilayer MoS2, when measuring energy-and momentum-resolved snapshots of excited electrons by time-and angle-resolved photoemission spectroscopy. We model the polarization-dependent photoemission intensity in the transiently-populated conduction band using the semiconductor Bloch equations and show that the observed dichroism emerges from intralayer single-particle effects within the isotropic part of the dispersion. This leads to optical excitations with an anisotropic momentum-dependence in an otherwise inversion symmetric material.Optical selection rules in absoprtion experiments are powerful tools that can be used to determine the symmetry of electronic states in solids [1]. Given the similarity of the processes underlying optical absorption and photoemission, selection rules have also been exploited in angle-resolved photoemission spectroscopy (ARPES) for decades [2]. More recently, optical dichroism has been used in ARPES to study orbital degrees of freedom [3][4][5], as well as the Berry curvature of the initial Bloch states [6, 7]. A particularly interesting opportunity for polarization-dependent excitations arises in single-layer (SL) transition metal dichalcogenides (TMDCs) such as MoS 2 , where the helicity of circularly polarized light strongly couples to the valley and spin degrees of freedom [8, 9], permitting the generation of a finite valley polarization [10, 11].Adding time-resolution (TR) to ARPES in a pumpprobe experiment leads to a process involving two optical excitations. This opens the possibility of exploiting not only the selection rules governing the photoemission process, but also those giving rise to the initial optical excitation into a transiently populated conduction band (CB) state [12][13][14][15][16][17][18]. Indeed, the creation of a finite valleypolarization in SL WS 2 has recently been followed in momentum space by TR-ARPES using circularly polarized pump pulses [19,20]. Here, we extend such an experiment to the case of bilayer (BL) MoS 2 in the 2H structure, which is inversion-symmetric and hence an optically isotropic material. Surprisingly, we observe a substantial linear dichroism effect. Our results can be reconciled with the momentum-dependent excited state population determined by solving the semiconductor Bloch equations within the framework of a low energy k · p model that ac-counts for intra-and interlayer interactions between the d orbitals forming the valence band maximum (VBM) and conduction band minimum (CBM) aroundK (K ). These findings underline the necessity of accounting for selection rules governing the optical excitation in addition to the photoemission matrix elements when interpreting the intensity in dichroic TR-ARPES.Our BL MoS 2 sample is grown on Ag(111) and has predominantly one domain orie...

The experimental realization of large-scale, homogeneous semiconducting films with a single-layer thickness is of major importance for next-generation devices. Especially in view of the compatibility with state-of-the-art semiconductor technology, Si-based monolayer crystals are of particular interest. Here, the successful epitaxial growth of monolayer Si 2 Te 2 (MLSi 2 Te 2 ) films on semiconducting Sb 2 Te 3 thin film substrates is reported. High-quality (1 × 1) ML-Si 2 Te 2 films with a coverage as high as 95% are obtained as revealed by scanning tunneling microscopy. X-ray photoelectron spectroscopy confirms the existence of the SiTe bonds in the obtained films. By combining scanning tunneling spectroscopy with density functional theory calculations, the existence of a semiconducting bandgap is demonstrated on the order of 370 meV for the MLSi 2 Te 2 films which reside in an important mid-infrared spectral range. The results pave the way for practical applications of this novel artificial two-dimensional material.

We show that the presence of a transiently excited hot electron gas in graphene leads to a substantial broadening of the C 1s line probed by time-resolved x-ray photoemission spectroscopy. The broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron gas, rather than by vibrational excitations. This interpretation is supported by a quantitative line-shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in good agreement with electronic temperature values obtained from valence band data. Furthermore, we show how the momentum change of the outgoing core electrons leads to a detectable but very small change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the presence of a narrow σ band in the C 1s state.