Ultraschnelle Dynamik elektronischer Korrelationen in Dichalkogenid-Monolagen und van-der-Waals-Heterostrukturen

Die Untersuchung von Exzitonen in atomar dünnen Wolframdiselenidschichten und deren Dynamik bildet den wissenschaftlichen Kern dieser Arbeit. Mittels zeitaufgelöster Anrege-Multi-THz-Abtastspektroskopie kann ein umfassendes Bild der fundamentalen elektronischen Korrelationen erstellt werden, um so grundlegende Fragestellungen zu beantworten. Zum einen: Wie schnell bilden sich Exzitonen in einer Wolframdiselenid-Monolage aus ursprünglich ungebundenen Elektron-Loch-Paaren? Zum anderen: Wie lassen sich durch Heterostrukturierung die elektronischen Korrelationen, sprich die Exzitonen, in einer Wolframdiselenid-Monolage gezielt manipulieren? Dass sich die Methode der Anrege-Multi-THz-Abtast Spektroskopie auch auf weitere hochinteressante, atomar dünne Probensysteme anwenden lässt, zeigen die am Ende der Arbeit vorgestellten weiterführenden Experimente

Mapping of the dark exciton landscape in transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including bright and optically forbidden dark excitonic states. Here, we show how dark excitons can be experimentally revealed by probing the intraexcitonic 1s-2p transition. Distinguishing the optical response shortly after the excitation and after the exciton thermalization allows us to reveal the relative position of bright and dark excitons. We find both in theory and experiment a clear blueshift in the optical response demonstrating the transition of bright exciton populations into lower-lying dark excitonic states

Direct Observation of ultrafast exciton Formation in a monolayer of WSe2

Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like states called excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron hole pairs in single monolayers of WSe2 on a diamond substrate following femtosecond nonresonant optical excitation. To this end, phase locked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the nonequilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While the spectral shape of the infrared response immediately after above-bandgap injection is dominated by free charge carriers, up to 60% of the electron-hole pairs are bound into excitons already on a subpicosecond time scale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a nonequilibrium state of the photoexcited electron-hole system

Ultrabroadband etalon-free detection of infrared transients by van-der-Waals contacted sub-10-µm GaSe detectors

We demonstrate ultrabroadband electro-optic detection of multi-THz transients using mechanically exfoliated flakes of gallium selenide of a thickness of less than 10 mu m, contacted to a diamond substrate by van-der-Waals bonding. While the low crystal thickness allows for extremely broadband phase matching, the excellent optical contact with the index-matched substrate suppresses multiple optical reflections. The high quality of our structure makes our scheme suitable for the undistorted and artifact-free observation of electromagnetic waveforms covering the entire THz spectral range up to the near-infrared regime without the need for correction for the electro-optic response function. With the current revolution of chemically inert quasi-two-dimensional layered materials, we anticipate that exfoliated vander-Waals materials on index-matched substrates will open new flexible ways of ultrabroadband electro-optic detection at unprecedented frequencies. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Resonant internal Quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2

Atomically thin two-dimensional crystals have revolutionized materials science1, 2, 3. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range4, 5, 6, 7, 8, 9. Their electronic and optical properties are dominated by Coulomb-bound electron–hole pairs called excitons10, 11, 12, 13, 14, 15, 16, 17, 18, whose unusual internal structure13, symmetry15, 16, 17, many-body effects18 and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum—inside and outside the radiative cone—in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s–2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers19, 20, 21, 22, 23, 24

Internal structure and ultrafast dynamics of tailored excitons in van der Waals heterostructures

Phase-locked few-cycle mid-infrared pulses trace how a capping layer of hexagonal boron nitride renormalizes the internal structure of photoexcited excitons in a WSe2 monolayer and how dark excitons form from initially bright species