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Heterostructures of molecules and two-dimensional materials feature emergent properties not seen in their individual components. Here, we study excitons in bilayer transition metal dichalcogenides exposed to an intense electric field produced by charge transfer from proximal molecules. Our approach allows for reaching an electric field strength of 0.35 V nm −1 , up to a factor of two higher than previously achieved in purely solid-state gated devices. Under this field, inter- and intralayer excitons are brought into an energetic resonance, allowing us to explore a new physical regime. We detect a previously unseen interlayer exciton that only becomes visible at high electric field through hybridization with the intralayer A exciton. Moreover, the system experiences an ultra-strong Stark splitting of  > 350 meV with exciton energies tunable over a large range of the optical spectrum, holding potential for optoelectronics. Our work paves the way for using strong electric fields to study new physical phenomena and control exciton hybridization in 2D semiconductors. The authors develop an on-chip molecular deposition method to generate large electric fields in bilayer transition metal dichalcogenides, enabling the hybridisation of intralayer and hitherto unobserved interlayer excitons.

Excitons, Coulomb‐bound electron–hole pairs, are the fundamental excitations governing the optoelectronic properties of semiconductors. Although optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum‐, energy‐, and time‐resolved perspective on excitons in the layered semiconductor WSe2. By tuning the excitation wavelength, we determine the energy–momentum signature of bright exciton formation and its difference from conventional single‐particle excited states. The multidimensional data allow to retrieve fundamental exciton properties like the binding energy and the exciton–lattice coupling and to reconstruct the real‐space excitonic distribution function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton properties and is applicable to bright and dark excitons in semiconducting materials, heterostructures, and devices. Key points The full life cycle of excitons is recorded with time‐ and angle‐resolved photoemission spectroscopy. The real‐space distribution of the excitonic wave function is visualized. Direct measurement of the exciton‐phonon interaction. Real‐space density distribution of an excitonic wave function retrieved from time‐ and angle‐resolved photoemission spectroscopy.

Atomically thin layered van der Waals heterostructures feature exotic and emergent optoelectronic properties. With growing interest in these novel quantum materials, the microscopic understanding of fundamental interfacial coupling mechanisms is of capital importance. Here, using multidimensional photoemission spectroscopy, we provide a layer- and momentum-resolved view on ultrafast interlayer electron and energy transfer in a monolayer-WSe 2 /graphene heterostructure. Depending on the nature of the optically prepared state, we find the different dominating transfer mechanisms: while electron injection from graphene to WSe 2 is observed after photoexcitation of quasi-free hot carriers in the graphene layer, we establish an interfacial Meitner-Auger energy transfer process following the excitation of excitons in WSe 2 . By analysing the time-energy-momentum distributions of excited-state carriers with a rate-equation model, we distinguish these two types of interfacial dynamics and identify the ultrafast conversion of excitons in WSe 2 to valence band transitions in graphene. Microscopic calculations find interfacial dipole-monopole coupling underlying the Meitner-Auger energy transfer to dominate over conventional Förster- and Dexter-type interactions, in agreement with the experimental observations. The energy transfer mechanism revealed here might enable new hot-carrier-based device concepts with van der Waals heterostructures. Here, the authors investigate the interfacial charge/energy transfer dynamics in a WSe 2 /graphene heterostructure. They unveil an energy transfer mechanism from WSe 2 to graphene mediated by an interfacial Meitner-Auger process, resulting in a transient hole distribution in the Dirac cone at energies larger than the photon energy of the optical excitation.

The recent rise of two-dimensional materials attracted a tremendous amount of interest in the felds of photonics and optoelectronics, as they are promising candidates for nextgeneration devices. Materials, which are under extensive investigation, are graphene and monolayer transition-metal dichalcogenides (TMDC). While graphene is a semi-metal, certain monolayer TMDCs are semiconductors exhibiting tightly bound electron-hole pairs – excitons, which dominate their optical response to a large extent. The light-matter interaction of twodimensional materials composes the footing of their excitation dynamics. This incorporates the excitation of excitons in the visible range of the electromagnetic spectrum. Because of exciton-phonon interaction a subsequent transfer from optically excited coherent to incoherent excitons occurs. For molybdenum-based TMDCs calculations and optical experiments suggest that the excitons at the optical active K± points form the global minimum. In contrast, for tungsten-based TMDCs it seems that momentum-indirect dark excitons with electron and hole at diferent valleys in the Brillouin zone are the energetically lowest lying states. Time and angle resolved photoemission spectroscopy (tr-ARPES) is a method to measure the ultrafast electron dynamics directly in momentum space. In case that tr-ARPES is also able to access Coulomb-correlated two-particle states this experimental method might be the smoking gun to proof the presence of the momentum-indirect states. In this thesis, we develop a microscopic description of the temporal dynamics of excitonic time and angle resolved photoemission spectroscopy with the focus on phonon-mediated relaxation of optically excited excitons. We show that tr-ARPES is able to access excitons and quantify the spectroscopic signatures of coherent and incoherent excitons in tr-ARPES. Additionally, we suggest coherent pump Fourier transform ARPES to measure the exciton coherence lifetime with great accuracy. Here, we combine a coherent tr-ARPES experiment with a second pump pulse and investigate the tr-ARPES signal as function of the time delay to the second pump pulse.Moreover, van der Waals materials enable the construction of heterostructures of diferent atomically-thin materials. Here, after excitation of the distinct materials diferent interfacial energy and charge transfer mechanisms occur. In particular, we investigate a WSe2-graphene stack and study the coupling mechanisms of Förster, Dexter, and phonon-assisted tunneling. In addition, we propose a new energy transfer mechanism: interlayer Meitner-Auger energy transfer. Here, a non-radiative exciton recombination leads to intraband transitions in graphene and therefore to a hot hole distribution deep in the valence band.Besides intralayer excitons, van der Waals heterostructures exhibit also interlayer excitons, where electron and hole are in diferent materials. We propose that interlayer excitons in certain hybrid inorganic/organic systems might form a new many-body excitonic ground state. Therefore, semiconductors functionalized by organic molecules might be the ideal candidate for the experimental realization of the elusive excitonic insulator. Using a proper description for the new many-body ground state based on a Bogoliubov description, we calculate the excitonic phase diagram of a WS2-F6TCNNQ stack as function of the relevant experimental parameters band gap (tunable by applied voltage), temperature, and dielectric environment. We show that all excitonic phases, namely semi-metal, semiconductor, and excitonic insulator have unique optical signatures in the far-infrared to terahertz (THz) regime.Besides monolayer TMDC excitons excited by visible wavelengths and intraexcitonic transitions, present in excitonic insulators, and excited by long wavelength radiation, also core electrons can be excited by X-ray radiation. X-ray absorption spectroscopy is divided into X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fne structure spectroscopy (EXAFS). The former constitute transitions of core electrons into unoccupied conduction band states and is typically described by Fermi’s golden rule. The latter accounts for ionization of core electrons from the material and is typically described in a high-order multiple-scattering theory. Here, we aim for a consistent description of both processes within a Maxwell coupled spatio-temporal Bloch formalism. Within this formalism we describe the polarization dependence of core transitions, the radiative and Meitner-Auger recombination channels of core electrons and give microscopic insights into the spectral signatures observed in EXAFS beyond point scattering theory. Moreover, the correct inclusion of the Bloch character of solid state core electrons allows us to assign so far uninterpreted features in the Fourier transformed EXAFS spectrum of graphene.

We present a data-driven approach to efficiently approximate nonlinear transient dynamics in solid-state systems. Our proposed machine-learning model combines a dimensionality reduction stage with a nonlinear vector autoregression scheme. We report an outstanding time-series forecasting performance combined with an easy to deploy model and an inexpensive training routine. Our results are of great relevance as they have the potential to massively accelerate multi-physics simulation software and thereby guide to future development of solid-state based technologies.

Time- and angle-resolved photoemission spectroscopy (trARPES) is a powerful spectroscopic method to measure the ultrafast electron dynamics directly in momentum-space. However, band gap materials with exceptional strong Coulomb interaction such as monolayer transition metal dichlacogenides (TMDC) exhibit tightly bound excitons, which dominate their optical properties. This rises the question whether excitons, in particular their formation and relaxation dynamics, can be detected in photoemission. Here, we develope a fully microscopic theory of the temporal dynamics of excitonic time- and angle resolved photoemission with particular focus on the phonon-mediated thermalization of optically excited excitons to momentum-forbidden dark exciton states. We find that trARPES is able to probe the ultrafast exciton formation and relaxation throughout the Brillouin zone.

The combination of Maxwell and X-ray Bloch equations forms an appropriate framework to describe ultrafast time-resolved X-ray experiments on attosecond time scale in crystalline solids. However, broadband experiments such as X-ray absorption near edge spectroscopy or resonant inelastic X-ray scattering require a detailed knowledge of the electronic structure and transition matrix elements. Here, we show how to fill this gap by combining the Maxwell-X-ray Bloch formalism with first-principles calculations treating explicitly the core states. The resulting X-ray absorption spectrum recovers key spectral signatures which were missing in our previous work relying on a semi-empirical tight-binding approach.

The interplay of the non-equivalent corners in the Brillouin zone of transition metal dichalcogenides have been investigated extensively. While experimental and theoretical works contributed to a detailed understanding of the relaxation of selective optical excitations and the related relaxation rates, only limited microscopic descriptions of stationary experiments are available so far. In this manuscript we present microscopic calculations for the non-equilibrium steady state properties of excitons during continuous wave pumping. We find sharp features in photoluminescence excitation spectra and degree of polarization which result from phonon assisted excitonic transitions dominating over exciton recombination and intervalley exchange coupling.

We develop a self-consistent Maxwell-Bloch formalism for the interaction of X-rays with two-dimensional crystalline materials by incorporating the Bloch theorem and Coulomb many-body interaction. This formalism is illustrated for graphene, by calculating the polarization-dependent XANES, formulating expressions for the radiative and Meinter-Auger recombination of core-holes, and the discussion of microscopic insights into the spectral oscillations of EXAFS beyond point scattering theory. In particular, the correct inclusion of lattice periodicity in our evaluation allows us to assign so far uninterpreted spectral features in the Fourier transformed EXAFS spectrum.