| 1. |
- Konig-Otto, J. C., et al.
(författare)
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Slow Noncollinear Coulomb Scattering in the Vicinity of the Dirac Point in Graphene
- 2016
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 117:8
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Tidskriftsartikel (refereegranskat)abstract
- The Coulomb scattering dynamics in graphene in energetic proximity to the Dirac point is investigated by polarization resolved pump-probe spectroscopy and microscopic theory. Collinear Coulomb scattering rapidly thermalizes the carrier distribution in k directions pointing radially away from the Dirac point. Our study reveals, however, that, in almost intrinsic graphene, full thermalization in all directions relying on noncollinear scattering is much slower. For low photon energies, carrier-optical-phonon processes are strongly suppressed and Coulomb mediated noncollinear scattering is remarkably slow, namely on a ps time scale. This effect is very promising for infrared and THz devices based on hot carrier effects.
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| 2. |
- Mihnev, M. T., et al.
(författare)
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Microscopic origins of the terahertz carrier relaxation and cooling dynamics in graphene
- 2016
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 7
-
Tidskriftsartikel (refereegranskat)abstract
- The ultrafast dynamics of hot carriers in graphene are key to both understanding of fundamental carrier-carrier interactions and carrier-phonon relaxation processes in two-dimensional materials, and understanding of the physics underlying novel high-speed electronic and optoelectronic devices. Many recent experiments on hot carriers using terahertz spectroscopy and related techniques have interpreted the variety of observed signals within phenomenological frameworks, and sometimes invoke extrinsic effects such as disorder. Here, we present an integrated experimental and theoretical programme, using ultrafast timeresolved terahertz spectroscopy combined with microscopic modelling, to systematically investigate the hot-carrier dynamics in a wide array of graphene samples having varying amounts of disorder and with either high or low doping levels. The theory reproduces the observed dynamics quantitatively without the need to invoke any fitting parameters, phenomenological models or extrinsic effects such as disorder. We demonstrate that the dynamics are dominated by the combined effect of efficient carrier-carrier scattering, which maintains a thermalized carrier distribution, and carrier-optical-phonon scattering, which removes energy from the carrier liquid.
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| 3. |
- Wendler, Florian, 1982, et al.
(författare)
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Symmetry-Breaking Supercollisions in Landau-Quantized Graphene
- 2017
-
Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 119:6, s. Article no 067405 -
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Tidskriftsartikel (refereegranskat)abstract
- Recent pump-probe experiments performed on graphene in a perpendicular magnetic field have revealed carrier relaxation times ranging from picoseconds to nanoseconds depending on the quality of the sample. To explain this surprising behavior, we propose a novel symmetry-breaking defect-assisted relaxation channel. This enables scattering of electrons with single out-of-plane phonons, which drastically accelerate the carrier scattering time in low-quality samples. The gained insights provide a strategy for tuning the carrier relaxation time in graphene and related materials by orders of magnitude.
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| 4. |
- Aeschlimann, S., et al.
(författare)
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Ultrafast momentum imaging of pseudospin-flip excitations in graphene
- 2017
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:2
-
Tidskriftsartikel (refereegranskat)abstract
- The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time-and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n-doped samples. By simulating the Coulomb-and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications.
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| 5. |
- Bange, Jan Philipp, et al.
(författare)
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Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure
- 2024
-
Ingår i: Science advances. - 2375-2548. ; 10:6
-
Tidskriftsartikel (refereegranskat)abstract
- In two-dimensional semiconductors, cooperative and correlated interactions determine the material’s excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlated exciton state formed by the Coulomb interaction between single-particle holes and electrons. We find that the ultrafast transfer of an exciton’s hole across a type II band-aligned semiconductor heterostructure leads to an unexpected sub-200-femtosecond upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this upshift is a clear fingerprint of the correlated interaction of the electron and hole parts of the exciton. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access electron-hole correlations and cooperative behavior in quantum materials. Our work highlights this capability and motivates the future study of optically inaccessible correlated excitonic and electronic states of matter.
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| 6. |
- Berghäuser, Gunnar, 1983, et al.
(författare)
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Inverted valley polarization in optically excited transition metal dichalcogenides
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 9:1
-
Tidskriftsartikel (refereegranskat)abstract
- Large spin-orbit coupling in combination with circular dichroism allows access to spin-polarized and valley-polarized states in a controlled way in transition metal dichalcogenides. The promising application in spin-valleytronics devices requires a thorough understanding of intervalley coupling mechanisms, which determine the lifetime of spin and valley polarizations. Here we present a joint theory-experiment study shedding light on the Dexter-like intervalley coupling. We reveal that this mechanism couples A and B excitonic states in different valleys, giving rise to an efficient intervalley transfer of coherent exciton populations. We demonstrate that the valley polarization vanishes and is even inverted for A excitons, when the B exciton is resonantly excited and vice versa. Our theoretical findings are supported by energy-resolved and valley-resolved pump-probe experiments and also provide an explanation for the recently measured up-conversion in photoluminescence. The gained insights might help to develop strategies to overcome the intrinsic limit for spin and valley polarizations.
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| 7. |
- Berghäuser, Gunnar, 1983, et al.
(författare)
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Mapping of the dark exciton landscape in transition metal dichalcogenides
- 2018
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 98:2
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Berghäuser, Gunnar, 1983, et al.
(författare)
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Optical fingerprint of dark 2p-states in transition metal dichalcogenides
- 2017
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- Atomically thin transition metal dichalcogenides exhibit a remarkably strong Coulomb interaction. This results in a fascinating many-particle physics including a variety of bright and dark excitonic states that determine optical and electronic properties of these materials. So far, the impact of dark states has remained literally in the dark to a large extent, since a measurement of these optically forbidden states is very challenging. Here we demonstrate a strategy to measure a direct fingerprint of dark states even in standard linear absorption spectroscopy. We present a microscopic study on bright and dark higher excitonic states in the presence of disorder for the exemplary material of tungsten disulfide (WS2). We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of these bright and dark states offers a strategy to circumvent optical selection rules. As a proof, we show a clear fingerprint of dark 2p states in the absorption spectrum of WS2. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors.
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| 9. |
- Bernal, Ivan, 1984, et al.
(författare)
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Exciton broadening and band renormalization due to Dexter-like intervalley coupling
- 2018
-
Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 5:2
-
Tidskriftsartikel (refereegranskat)abstract
- A remarkable property of atomically thin transition metal dichalcogenides (TMDs) is the possibility to selectively address single valleys by circularly polarized light. In the context of technological applications, it is very important to understand possible intervalley coupling mechanisms. Here, we show how the Dexter-like intervalley coupling mixes A and B states from opposite valleys leading to a significant broadening γB 1s of the B 1s exciton. The effect is much more pronounced in tungsten-based TMDs, where the coupling excitonic states are quasi-resonant. We calculate a ratio γB B 1s /γA B 1s ≈ 4.0, which is in good agreement with the experimentally measured value of 3.9 ± 0.7. In addition to the broadening effect, the Dexter-like intervalley coupling also leads to a considerable energy renormalization resulting in an increased energetic distance between A 1s and B 1s states.
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| 10. |
- Brem, Samuel, 1991, et al.
(författare)
-
Electrically pumped graphene-based Landau-level laser
- 2018
-
Ingår i: Physical Review Materials. - 2475-9953. ; 2:3
-
Tidskriftsartikel (refereegranskat)abstract
- Graphene exhibits a nonequidistant Landau quantization with tunable Landau-level (LL) transitions in the technologically desired terahertz spectral range. Here, we present a strategy for an electrically driven terahertz laser based on Landau-quantized graphene as the gain medium. Performing microscopic modeling of the coupled electron, phonon, and photon dynamics in such a laser, we reveal that an inter-LL population inversion can be achieved resulting in the emission of coherent terahertz radiation. The presented paper provides a concrete recipe for the experimental realization of tunable graphene-based terahertz laser systems.
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| 11. |
- Brem, Samuel, 1991, et al.
(författare)
-
Exciton Relaxation Cascade in two-dimensional Transition Metal Dichalcogenides
- 2018
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 8:1
-
Tidskriftsartikel (refereegranskat)abstract
- Monolayers of transition metal dichalcogenides (TMDs) are characterized by an extraordinarily strong Coulomb interaction giving rise to tightly bound excitons with binding energies of hundreds of meV. Excitons dominate the optical response as well as the ultrafast dynamics in TMDs. As a result, a microscopic understanding of exciton dynamics is the key for a technological application of these materials. In spite of this immense importance, elementary processes guiding the formation and relaxation of excitons after optical excitation of an electron-hole plasma has remained unexplored to a large extent. Here, we provide a fully quantum mechanical description of momentum- and energy-resolved exciton dynamics in monolayer molybdenum diselenide (MoSe2) including optical excitation, formation of excitons, radiative recombination as well as phonon-induced cascade-like relaxation down to the excitonic ground state. Based on the gained insights, we reveal experimentally measurable features in pump-probe spectra providing evidence for the exciton relaxation cascade.
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| 12. |
- Brem, Samuel, 1991, et al.
(författare)
-
Hybridized intervalley moiré excitons and flat bands in twisted WSe(2)bilayers
- 2020
-
Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 12:20, s. 11088-11094
-
Tidskriftsartikel (refereegranskat)abstract
- The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe(2)bilayers. In particular, we show that the strong hybridization of electrons at the ? point leads to a drastic redshift of the momentum-dark K-? exciton, accompanied by the emergence of flat moire exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moire potential of the twisted bilayer lattice.
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| 13. |
- Brem, Samuel, 1991, et al.
(författare)
-
Intrinsic lifetime of higher excitonic states in tungsten diselenide monolayers
- 2019
-
Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 11:25, s. 12381-12387
-
Tidskriftsartikel (refereegranskat)abstract
- © 2019 The Royal Society of Chemistry. The reduced dielectric screening in atomically thin transition metal dichalcogenides allows to study the hydrogen-like series of higher exciton states in optical spectra even at room temperature. The width of excitonic peaks provides information about the radiative decay and phonon-assisted scattering channels limiting the lifetime of these quasi-particles. While linewidth studies so far have been limited to the exciton ground state, encapsulation with hBN has recently enabled quantitative measurements of the broadening of excited exciton resonances. Here, we present a joint experiment-theory study combining microscopic calculations with spectroscopic measurements on the intrinsic linewidth and lifetime of higher exciton states in hBN-encapsulated WSe2 monolayers. Surprisingly, despite the increased number of scattering channels, we find both in theory and experiment that the linewidth of higher excitonic states is similar or even smaller compared to the ground state. Our microscopic calculations ascribe this behavior to a reduced exciton-phonon scattering efficiency for higher excitons due to spatially extended orbital functions.
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| 14. |
- Brem, Samuel, 1991, et al.
(författare)
-
Microscopic modeling of tunable graphene-based terahertz Landau-level lasers
- 2017
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:4
-
Tidskriftsartikel (refereegranskat)abstract
- In the presence of strong magnetic fields the electronic band structure of graphene drastically changes. The Dirac cone collapses into discrete nonequidistant Landau levels, which can be externally tuned by changing the magnetic field. In contrast to conventional materials, specific Landau levels are selectively addressable using circularly polarized light. Exploiting these unique properties, we propose the design of a tunable laser operating in the technologically promising terahertz spectral range. To uncover the many-particle physics behind the emission of light, we perform a fully quantum mechanical investigation of the nonequilibrium dynamics of electrons, phonons, and photons in optically pumped Landau-quantized graphene embedded in a high-quality optical cavity. The microscopic insights gained allow us to predict optimal experimental conditions to realize a technologically promising terahertz laser.
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| 15. |
- Brem, Samuel, 1991, et al.
(författare)
-
Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides
- 2019
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The photoluminescence (PL) spectrum of transition metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of momentum-dark excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as result of different configurations of bright and dark states. In good agreement with experiments, we show that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark excitons.
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| 16. |
- Brem, Samuel, 1991, et al.
(författare)
-
Phonon-Assisted Photoluminescence from Indirect Excitons in Monolayers of Transition-Metal Dichalcogenides
- 2020
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 20:4, s. 2849-2856
-
Tidskriftsartikel (refereegranskat)abstract
- The photoluminescence (PL) spectrum of transition-metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line, and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of indirect excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon, and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as the result of different configurations of bright and momentum-dark states. In good agreement with experiments, our calculations predict that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark K-K′ excitons.
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| 17. |
- Brem, Samuel, 1991, et al.
(författare)
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Terahertz Fingerprint of Monolayer Wigner Crystals
- 2022
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 22:3, s. 1311-1315
-
Tidskriftsartikel (refereegranskat)abstract
- The strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields. In this work, we predict that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. We apply the density matrix formalism to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multipeak shape of the obtained terahertz spectrum. Moreover, we predict a characteristic shift of the peak position as a function of charge density for different atomically thin materials and show how our results can be generalized to an arbitrary two-dimensional system.
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| 18. |
- Brem, Samuel, 1991, et al.
(författare)
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Tunable Phases of Moiré Excitons in van der Waals Heterostructures
- 2020
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 20:12, s. 8534-8540
-
Tidskriftsartikel (refereegranskat)abstract
- Stacking monolayers of transition metal dichalcogenides into a heterostructure with a finite twist-angle gives rise to artificial moiré superlattices with a tunable periodicity. As a consequence, excitons experience a periodic potential, which can be exploited to tailor optoelectronic properties of these materials. Whereas recent experimental studies have confirmed twist-angle-dependent optical spectra, the microscopic origin of moiré exciton resonances has not been fully clarified yet. Here, we combine first-principles calculations with the excitonic density matrix formalism to study transitions between different moiré exciton phases and their impact on optical properties of the twisted MoSe2/WSe2 heterostructure. At angles smaller than 2°, we find flat, moiré-trapped states for inter- and intralayer excitons. This moiré exciton phase changes into completely delocalized states at 3°. We predict a linear and quadratic twist-angle dependence of excitonic resonances for the moiré-trapped and delocalized exciton phases, respectively.
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| 19. |
- Choi, Junho, et al.
(författare)
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Fermi Pressure and Coulomb Repulsion Driven Rapid Hot Plasma Expansion in a van der Waals Heterostructure
- 2023
-
Ingår i: Nano Letters. - 1530-6992 .- 1530-6984. ; 23:10, s. 4399-4405
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal dichalcogenide heterostructures provide a versatile platform to explore electronic and excitonic phases. As the excitation density exceeds the critical Mott density, interlayer excitons are ionized into an electron-hole plasma phase. The transport of the highly non-equilibrium plasma is relevant for high-power optoelectronic devices but has not been carefully investigated previously. Here, we employ spatially resolved pump-probe microscopy to investigate the spatial-temporal dynamics of interlayer excitons and hot-plasma phase in a MoSe2/WSe2 twisted bilayer. At the excitation density of ∼1014 cm-2, well exceeding the Mott density, we find a surprisingly rapid initial expansion of hot plasma to a few microns away from the excitation source within ∼0.2 ps. Microscopic theory reveals that this rapid expansion is mainly driven by Fermi pressure and Coulomb repulsion, while the hot carrier effect has only a minor effect in the plasma phase.
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| 20. |
- Christiansen, D., et al.
(författare)
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Phonon Sidebands in Monolayer Transition Metal Dichalcogenides
- 2017
-
Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 119:18
-
Tidskriftsartikel (refereegranskat)abstract
- Excitons dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). Besides optically accessible bright exciton states, TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we show that efficient exciton-phonon scattering couples bright and dark states and gives rise to an asymmetric excitonic line shape. The observed asymmetry can be traced back to phonon-induced sidebands that are accompanied by a polaron redshift. We present a joint theory-experiment study investigating the microscopic origin of these sidebands in different TMD materials taking into account intra and intervalley scattering channels opened by optical and acoustic phonons. The gained insights contribute to a better understanding of the optical fingerprint of these technologically promising nanomaterials.
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| 21. |
- Christiansen, Dominik, et al.
(författare)
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Theory of exciton dynamics in time-resolved ARPES: Intra- and intervalley scattering in two-dimensional semiconductors
- 2019
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 100:20
-
Tidskriftsartikel (refereegranskat)abstract
- 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 exceptionally strong Coulomb interaction such as monolayer transition-metal dichalcogenides exhibit tightly bound excitons, which dominate their optical properties. This raises the question of whether excitons, in particular their formation and relaxation dynamics, can be detected in photoemissions. Here, we develop a fully microscopic theory of the temporal dynamics of excitonic time- and angle-resolved photoemission with a 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.
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| 22. |
- de Amorim Ferreira, Beatriz, 1996, et al.
(författare)
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Revealing Dark Exciton Signatures in Polariton Spectra of 2D Materials
- 2024
-
Ingår i: ACS Photonics. - 2330-4022. ; 11:6, s. 2215-2220
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Tidskriftsartikel (refereegranskat)abstract
- Dark excitons in transition metal dichalcogenides (TMDs) have been so far neglected in the context of polariton physics due to their lack of oscillator strength. However, in tungsten-based TMDs, dark excitons are known to be the energetically lowest states and could thus provide important scattering partners for polaritons. In this joint theoretical-experimental work, we investigate the impact of the full exciton energy landscape on polariton absorption and reflectance. By changing the cavity detuning, we vary the polariton energy relative to the unaffected dark excitons in such a way that we open or close specific phonon-driven scattering channels. We demonstrate both in theory and experiment that this controlled switching of scattering channels manifests in characteristic sharp changes in the optical spectra of polaritons. These spectral features can be exploited to extract the position of dark excitons. Our work suggests new possibilities for exploiting polaritons for fingerprinting nanomaterials via their unique exciton landscape.
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| 23. |
- Dwedari, Magdulin, et al.
(författare)
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Disorder-induced broadening of excitonic resonances in transition metal dichalcogenides
- 2019
-
Ingår i: Physical Review Materials. - 2475-9953. ; 3:7
-
Tidskriftsartikel (refereegranskat)abstract
- The presence of impurities and disorder has an important impact on the optical response of monolayer transition metal dichalcogenides (TMDs). Here, we investigate elastic exciton-impurity scattering and its influence on the linewidth of excitonic resonances in different TMD materials. We derive an analytic expression for the linewidth broadening within the density matrix formalism. We find that the exciton linewidth increases for states up to the 3s exciton due to the scattering with impurities. For higher states, the impurity contribution decreases, reflecting the reduced scattering cross section. Furthermore, we reveal that the scattering efficiency is the largest for transitions between s and p exciton states. Finally, different TMDs show generally a similar behavior. The quantitatively smaller broadening in tungsten-based TMDs can be ascribed to their smaller effective masses resulting in a less efficient scattering.
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| 24. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Dark exciton-exciton annihilation in monolayer WSe2
- 2021
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 104:24
-
Tidskriftsartikel (refereegranskat)abstract
- The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer WSe2. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors.
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| 25. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Electrically tunable dipolar interactions between layer-hybridized excitons
- 2023
-
Ingår i: Nanoscale. - 2040-3372 .- 2040-3364. ; 15:26, s. 11064-11071
-
Tidskriftsartikel (refereegranskat)abstract
- Transition-metal dichalcogenide bilayers exhibit a rich exciton landscape including layer-hybridized excitons, i.e. excitons which are of partly intra- and interlayer nature. In this work, we study hybrid exciton-exciton interactions in naturally stacked WSe2 homobilayers. In these materials, the exciton landscape is electrically tunable such that the low-energy states can be rendered more or less interlayer-like depending on the strength of the external electric field. Based on a microscopic and material-specific many-particle theory, we reveal two intriguing interaction regimes: a low-dipole regime at small electric fields and a high-dipole regime at larger fields, involving interactions between hybrid excitons with a substantially different intra- and interlayer composition in the two regimes. While the low-dipole regime is characterized by weak inter-excitonic interactions between intralayer-like excitons, the high-dipole regime involves mostly interlayer-like excitons which display a strong dipole-dipole repulsion and give rise to large spectral blue-shifts and a highly anomalous diffusion. Overall, our microscopic study sheds light on the remarkable electrical tunability of hybrid exciton-exciton interactions in atomically thin semiconductors and can guide future experimental studies in this growing field of research.
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| 26. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Exciton-exciton interaction in transition metal dichalcogenide monolayers and van der Waals heterostructures
- 2021
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 103:4
-
Tidskriftsartikel (refereegranskat)abstract
- Due to a strong Coulomb interaction, excitons dominate the excitation kinetics in two-dimensional (2D) materials. While Coulomb scattering between electrons has been well studied, the interaction of excitons is more challenging and remains to be explored. As neutral composite bosons consisting of electrons and holes, excitons show nontrivial scattering dynamics. Here, we study exciton-exciton interaction in transition-metal dichalcogenides and related van der Waals heterostructures on microscopic footing. We demonstrate that the crucial criterion for efficient scattering is a large electron/hole mass asymmetry, giving rise to internal charge inhomogeneities of excitons and emphasizing their cobosonic substructure. Furthermore, both exchange and direct exciton-exciton interactions are boosted by enhanced exciton Bohr radii. We also predict an unexpected temperature dependence that is usually associated with phonon-driven scattering, and we reveal an orders of magnitude stronger interaction of interlayer excitons due to their permanent dipole moment. The developed approach can be generalized to arbitrary material systems and will help to study strongly correlated exciton systems, such as moire super lattices.
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| 27. |
- Erkensten, Daniel, 1995, et al.
(författare)
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Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures
- 2022
-
Ingår i: Physical Review Materials. - 2475-9953. ; 6:9
-
Tidskriftsartikel (refereegranskat)abstract
- Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) heterobilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly nonlinear exciton propagation at elevated densities in the MoSe2 - WSe2 heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counterintuitive, we reveal that interlayer excitons in freestanding samples propagate slower than excitons in hBN-encapsulated TMDs-due to an enhancement of the net Coulomb drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors.
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| 28. |
- Fang, Hanlin, 1992, et al.
(författare)
-
Localization and interaction of interlayer excitons in MoSe 2 /WSe 2 heterobilayers
- 2023
-
Ingår i: Nature Communications. - 2041-1723 .- 2041-1723. ; 14:1
-
Tidskriftsartikel (refereegranskat)abstract
- Transition metal dichalcogenide (TMD) heterobilayers provide a versatile platform to explore unique excitonic physics via the properties of the constituent TMDs and external stimuli. Interlayer excitons (IXs) can form in TMD heterobilayers as delocalized or localized states. However, the localization of IX in different types of potential traps, the emergence of biexcitons in the high-excitation regime, and the impact of potential traps on biexciton formation have remained elusive. In our work, we observe two types of potential traps in a MoSe2/WSe2 heterobilayer, which result in significantly different emission behavior of IXs at different temperatures. We identify the origin of these traps as localized defect states and the moiré potential of the TMD heterobilayer. Furthermore, with strong excitation intensity, a superlinear emission behavior indicates the emergence of interlayer biexcitons, whose formation peaks at a specific temperature. Our work elucidates the different excitation and temperature regimes required for the formation of both localized and delocalized IX and biexcitons and, thus, contributes to a better understanding and application of the rich exciton physics in TMD heterostructures.
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| 29. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides
- 2021
-
Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 8:1
-
Tidskriftsartikel (refereegranskat)abstract
- Monolayer transition metal dichalcogenides (TMDs) have been in focus of current research, among others due to their remarkable exciton landscape consisting of bright and dark excitonic states. Although dark excitons are not directly visible in optical spectra, they have a large impact on exciton dynamics and hence their understanding is crucial for potential TMD-based applications. Here, we study brightening mechanisms of dark excitons via interaction with phonons and in-plane magnetic fields. We show clear signatures of momentum- and spin-dark excitons in WS2, WSe2 and MoS2, while the photoluminescence of MoSe2 is only determined by the bright exciton. In particular, we reveal the mechanism behind the brightening of states that are both spin- and momentum-dark in MoS2. Our results are in good agreement with recent experiments and contribute to a better microscopic understanding of the exciton landscape in TMDs.
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| 30. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Dark exciton based strain sensing in tungsten-based transition metal dichalcogenides
- 2019
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:19
-
Tidskriftsartikel (refereegranskat)abstract
- The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultrathin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes which can not be achieved by conventional semiconductors. This makes these materials optimal candidates for sensing applications. Here, we propose a dark exciton based concept for ultrasensitive strain sensors. By investigating both dark and bright excitons in tungsten-based monolayer transition metal dichalcogenides, we demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. The predicted opposite strain-induced shifts for dark and bright excitons result in a pronounced change in photoluminescence stemming from dark excitons even at very small strain values. The predicted high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.
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| 31. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Impact of strain on the optical fingerprint of monolayer transition-metal dichalcogenides
- 2017
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:4
-
Tidskriftsartikel (refereegranskat)abstract
- Strain presents a straightforward tool to tune electronic properties of atomically thin nanomaterials that are highly sensitive to lattice deformations. While the influence of strain on the electronic band structure has been intensively studied, there are only a few works on its impact on optical properties of monolayer transition-metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearest-neighbor tight-binding approximation, we present an analytical view on how uni- and biaxial strain influences the optical fingerprint of TMDs, including their excitonic binding energy, oscillator strength, optical selection rules, and the radiative broadening of excitonic resonances. We show that the impact of strain can be reduced to changes in the lattice structure (geometric effect) and in the orbital functions (overlap effect). In particular, we demonstrate that the valley-selective optical selection rule is softened in the case of uniaxial strain due to the introduced asymmetry in the lattice structure. Furthermore, we reveal a considerable increase of the radiative dephasing due to strain-induced changes in the optical matrix element and the excitonic wave functions.
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| 32. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Molecule signatures in photoluminescence spectra of transition metal dichalcogenides
- 2018
-
Ingår i: Physical Review Materials. - 2475-9953. ; 2:1
-
Tidskriftsartikel (refereegranskat)abstract
- Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this paper, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature, as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage, and distribution. We show that under certain optimal conditions even room-temperature detection of molecules can be achieved.
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| 33. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Optical fingerprint of bright and dark localized excitonic states in atomically thin 2D materials
- 2019
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 21:47, s. 26077-26083
-
Tidskriftsartikel (refereegranskat)abstract
- Point defects, local strain or impurities can crucially impact the optical response of atomically thin two-dimensional materials as they offer trapping potentials for excitons. These trapped excitons appear in photoluminescence spectra as new resonances below the bright exciton that can even be exploited for single photon emission. While large progress has been made in deterministically introducing defects, only little is known about their impact on the optical fingerprint of 2D materials. Here, based on a microscopic approach we reveal direct signatures of localized bright excitonic states as well as indirect phonon-assisted side bands of localized momentum-dark excitons. The visibility of localized excitons strongly depends on temperature and disorder potential width. This results in different regimes, where either the bright or dark localized states are dominant in optical spectra. We trace back this behavior to an interplay between disorder-induced exciton capture and intervalley exciton-phonon scattering processes.
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| 34. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Optical fingerprint of non-covalently functionalized transition metal dichalcogenides
- 2017
-
Ingår i: Journal of Physics Condensed Matter. - 0953-8984 .- 1361-648X. ; 29:38, s. Article no 384003 -
-
Tidskriftsartikel (refereegranskat)abstract
- Atomically thin transition metal dichalcogenides (TMDs) hold promising potential forapplications in optoelectronics. Due to their direct band gap and the extraordinarily strong Coulomb interaction, TMDs exhibit efficient light-matter coupling and tightly bound excitons. Moreover, large spin orbit coupling in combination with circular dichroism allows for spin and valley selective optical excitation. As atomically thin materials, they are very sensitive to changes in the surrounding environment. This motivates a functionalization approach, where external molecules are adsorbed to the materials surface to tailor its optical properties. Here, we apply the density matrix theory to investigate the potential of non-covalently functionalized monolayer TMDs. Considering exemplary molecules with a strong dipole moment, we predict spectral redshifts and the appearance of an additional side peak in the absorption spectrum of functionalized TMDs. We show that the molecular characteristics, e.g. coverage, orientation and dipole moment, crucially influence the optical properties of TMDs, leaving a unique optical fingerprint in the absorption spectrum. Furthermore, we find that the molecular dipole moments open a channel for coherent intervalley coupling between the high-symmetry K and K' points which may create new possibilities for spin-valleytronics application.
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| 35. |
- Feierabend, Maja, 1990, et al.
(författare)
-
Proposal for dark exciton based chemical sensors
- 2017
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 8
-
Tidskriftsartikel (refereegranskat)abstract
- The rapidly increasing use of sensors throughout different research disciplines and the demand for more efficient devices with less power consumption depends critically on the emergence of new sensor materials and novel sensor concepts. Atomically thin transition metal dichalcogenides have a huge potential for sensor development within a wide range of applications. Their optimal surface-to-volume ratio combined with strong light-matter interaction results in a high sensitivity to changes in their surroundings. Here, we present a highly efficient sensing mechanism to detect molecules based on dark excitons in these materials. We show that the presence of molecules with a dipole moment transforms dark states into bright excitons, resulting in an additional pronounced peak in easy accessible optical spectra. This effect exhibits a huge potential for sensor applications, since it offers an unambiguous optical fingerprint for the detection of molecules-in contrast to common sensing schemes relying on small peak shifts and intensity changes.
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| 36. |
- Feldstein, David, et al.
(författare)
-
Microscopic Picture of Electron-Phonon Interaction in Two-Dimensional Halide Perovskites
- 2020
-
Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:23, s. 9975-9982
-
Tidskriftsartikel (refereegranskat)abstract
- Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study of excitonic properties of monolayered hybrid organic perovskites - supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wave functions and binding energies. Exploiting the generalized Elliot formula, we calculate the photoluminescence spectra demonstrating a pronounced contribution of a phonon sideband for temperatures up to 50 K, in agreement with experimental measurements. Finally, we predict temperature-dependent line widths of the three energetically lowest excitonic transitions and identify the underlying phonon-driven scattering processes.
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| 37. |
- Ferreira, Beatriz, 1996, et al.
(författare)
-
Microscopic modeling of exciton-polariton diffusion coefficients in atomically thin semiconductors
- 2022
-
Ingår i: Physical Review Materials. - 2475-9953. ; 6:3
-
Tidskriftsartikel (refereegranskat)abstract
- In the strong light-matter coupling regime realized, e.g., by integrating semiconductors into optical microcavities, polaritons as new hybrid light-matter quasiparticles are formed. The corresponding change in the dispersion relation has a large impact on optics, dynamics, and transport behavior of semiconductors. In this paper, we investigate the strong-coupling regime in hBN-encapsulated MoSe2 monolayers focusing on exciton-polariton diffusion. Applying a microscopic approach based on the exciton density matrix formalism combined with the Hopfield approach, we predict a drastic increase of the diffusion coefficients by two to three orders of magnitude in the strong-coupling regime. We explain this behavior by the much larger polariton group velocity and suppressed polariton-phonon scattering channels with respect to the case of bare excitons. Our study contributes to a better microscopic understanding of polariton diffusion in atomically thin semiconductors.
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| 38. |
- Ferreira, Beatriz, 1996, et al.
(författare)
-
Signatures of dark excitons in exciton–polariton optics of transition metal dichalcogenides
- 2023
-
Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10
-
Tidskriftsartikel (refereegranskat)abstract
- Integrating 2D materials into high-quality optical microcavities opens the door to fascinating many-particle phenomena including the formation of exciton-polaritons. These are hybrid quasi-particles inheriting properties of both the constituent photons and excitons. In this work, we investigate the so-far overlooked impact of dark excitons on the momentum-resolved absorption spectra of hBN-encapsulated WSe2 and MoSe2 monolayers in the strong-coupling regime. In particular, thanks to the efficient phonon-mediated scattering of polaritons into energetically lower dark exciton states, the absorption of the lower polariton branch in WSe2 is much higher than in MoSe2. It shows unique step-like increases in the momentum-resolved profile indicating opening of specific scattering channels. We study how different externally accessible quantities, such as temperature or mirror reflectance, change the optical response of polaritons. Our study contributes to an improved microscopic understanding of exciton-polaritons and their interaction with phonons, potentially suggesting experiments that could determine the energy of dark exciton states via momentum-resolved polariton absorption.
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| 39. |
- Fitzgerald, Jamie, 1992, et al.
(författare)
-
Twist Angle Tuning of Moiré Exciton Polaritons in van der Waals Heterostructures
- 2022
-
Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 22:11, s. 4468-4474
-
Tidskriftsartikel (refereegranskat)abstract
- Twisted atomically thin semiconductors are characterized by moiré excitons. Their optical signatures and selection rules are well understood. However, their hybridization with photons in the strong coupling regime for heterostructures integrated in an optical cavity has not been the focus of research yet. Here, we combine an excitonic density matrix formalism with a Hopfield approach to provide microscopic insights into moiré exciton polaritons. In particular, we show that exciton-light coupling, polariton energy, and even the number of polariton branches can be controlled via the twist angle. We find that these new hybrid light-exciton states become delocalized relative to the constituent excitons due to the mixing with light and higher-energy excitons. The system can be interpreted as a natural quantum metamaterial with a periodicity that can be engineered via the twist angle. Our study presents a significant advance in microscopic understanding and control of moiré exciton polaritons in twisted atomically thin semiconductors.
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| 40. |
- Funk, H., et al.
(författare)
-
Microscopic view on Landau level broadening mechanisms in graphene
- 2015
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:20
-
Tidskriftsartikel (refereegranskat)abstract
- Placing a two-dimensional sheet of graphene in an external magnetic field the continuous electronic band structure is discretized due to Landau quantization. The resulting optical transitions are subject to a broadening, which can lead to a significant overlap of Landau levels. We investigate the possible microscopic processes that could cause a broadening of the corresponding peaks in the absorption spectrum of Landau-quantized graphene: (i) radiative decay, (ii) Coulomb interaction, (iii) optical phonons, (iv) acoustic phonons, and (v) impurities. Since recent experiments have shown that independent of the magnetic field the resolvable number of Landau levels is constant, we put a special focus on the dependence of the broadening on the external magnetic field B and the Landau level index n. Our calculations reveal the impurities to be the crucial broadening mechanism, where different regimes of well separated and densely spaced Landau levels need to be taken into account. Furthermore, carrier-carrier and carrier-phonon scattering give rise to a very specific dependence on the Landau level index n that has not been observed yet.
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| 41. |
- Hagel, Joakim, 1994, et al.
(författare)
-
Electrical tuning of moiré excitons in MoSe 2 bilayers
- 2023
-
Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10:1
-
Tidskriftsartikel (refereegranskat)abstract
- Recent advances in the field of vertically stacked 2D materials have revealed a rich exciton landscape. In particular, it has been demonstrated that out-of-plane electrical fields can be used to tune the spectral position of spatially separated interlayer excitons. Other studies have shown that there is a strong hybridization of exciton states, resulting from the mixing of electronic states in both layers. However, the connection between the twist-angle dependent hybridization and field-induced energy shifts has remained in the dark. Here, we investigate on a microscopic footing the interplay of electrical and twist-angle tuning of moiré excitons in MoSe2 homobilayers. We reveal distinct energy regions in PL spectra that are clearly dominated by either intralayer or interlayer excitons, or even dark excitons. Consequently, we predict twist-angle-dependent critical electrical fields at which the material is being transformed from a direct into an indirect semiconductor. Our work provides new microscopic insights into experimentally accessible knobs to significantly tune the moiré exciton physics in atomically thin nanomaterials.
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| 42. |
- Hagel, Joakim, 1994, et al.
(författare)
-
Exciton landscape in van der Waals heterostructures
- 2021
-
Ingår i: Physical Review Research. - 2643-1564. ; 3:4
-
Tidskriftsartikel (refereegranskat)abstract
- van der Waals heterostructures consisting of vertically stacked transition-metal dichalcogenides (TMDs) exhibit a rich landscape of bright and dark intra- and interlayer excitons. In spite of a growing literature in this field of research, the type of excitons dominating optical spectra in different van der Waals heterostructures has not yet been well established. The spectral position of exciton states depends strongly on the strength of hybridization and energy renormalization due to the periodic moire potential. Combining exciton density-matrix formalism and density-functional theory, we shed light on the exciton landscape in TMD homo- and heterobilayers at different stackings. This allows us to identify on a microscopic footing the energetically lowest-lying exciton state for each material and stacking. Furthermore, we disentangle the contribution of hybridization and layer polarization-induced alignment shifts of dark and bright excitons in photoluminescence spectra. By revealing the exciton landscape in van der Waals heterostructures, our work provides the basis for further studies of the optical, dynamical, and transport properties of this technologically promising class of nanomaterials.
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| 43. |
- Hagel, Joakim, 1994, et al.
(författare)
-
Impact of atomic reconstruction on optical spectra of twisted TMD homobilayers
- 2024
-
Ingår i: Physical Review Materials. - 2475-9953. ; 8:3
-
Tidskriftsartikel (refereegranskat)abstract
- Twisted bilayers of transition metal dichalcogenides (TMDs) have revealed a rich exciton landscape including hybrid excitons and spatially trapped moiré excitons that dominate the optical response of the material. Recent studies have shown that in the low-twist-angle regime, the lattice undergoes a significant relaxation in order to minimize local stacking energies. Here, large domains of low energy stacking configurations emerge, deforming the crystal lattices via strain and consequently impacting the electronic band structure. However, so far the direct impact of atomic reconstruction on the exciton energy landscape and the optical properties has not been well understood. Here, we apply a microscopic and material-specific approach and predict a significant change in the potential depth for moiré excitons in a reconstructed lattice, with the most drastic change occurring in naturally stacked TMD homobilayers. We show the appearance of multiple flat bands and a significant change in the position of trapping sites compared to the rigid lattice. Most importantly, we predict a multipeak structure emerging in optical absorption of WSe2 homobilayers - in contrast to the single peak that dominates the rigid lattice. This finding can be exploited as an unambiguous signature of atomic reconstruction in optical spectra of moiré excitons in naturally stacked twisted homobilayers.
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| 44. |
- Han, Bo, et al.
(författare)
-
Angle- and polarization-resolved luminescence from suspended and hexagonal boron nitride encapsulated MoSe2 monolayers
- 2022
-
Ingår i: Optica. - 2334-2536. ; 9:10, s. 1190-1196
-
Tidskriftsartikel (refereegranskat)abstract
- The polarized photoluminescence from atomically thin transition metal dichalcogenides is a frequently applied tool to scrutinize optical selection rules and valley physics, yet it is known to sensibly depend on a variety of internal and external material and sample properties. In this work, we apply combined angle- and polarization-resolved spectroscopy to explore the interplay of excitonic physics and phenomena arising from the commonly utilized encapsulation procedure on the optical properties of atomically thinMoSe2.We probe monolayers prepared in both suspended and encapsulated manners.We show that the hBN encapsulation significantly enhances the linear polarization of exciton photoluminescence emission at large emission angles. This degree of linear polarization of excitons can increase up to ∼17% in the hBN encapsulated samples. As we confirm by finite-difference time-domain simulations, it can be directly connected to the optical anisotropy of the hBN layers. In comparison, the linear polarization at finite exciton momenta is significantly reduced in a suspendedMoSe2 monolayer, and becomes notable only in cryogenic conditions. This phenomenon strongly suggests that the effect is rooted in the k-dependent anisotropic exchange coupling inherent in2Dexcitons.Our results have strong implications on further studies on valley contrasting selection rules and valley coherence phenomena using standard suspended and encapsulated samples.
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| 45. |
- Hofmann, Niklas, et al.
(författare)
-
Link between interlayer hybridization and ultrafast charge transfer in WS 2 -graphene heterostructures
- 2023
-
Ingår i: 2D Materials. - 2053-1583. ; 10:3
-
Tidskriftsartikel (refereegranskat)abstract
- Ultrafast charge separation after photoexcitation is a common phenomenon in various van-der-Waals (vdW) heterostructures with great relevance for future applications in light harvesting and detection. Theoretical understanding of this phenomenon converges towards a coherent mechanism through charge transfer states accompanied by energy dissipation into strongly coupled phonons. The detailed microscopic pathways are material specific as they sensitively depend on the band structures of the individual layers, the relative band alignment in the heterostructure, the twist angle between the layers, and interlayer interactions resulting in hybridization. We used time- and angle-resolved photoemission spectroscopy combined with tight binding and density functional theory electronic structure calculations to investigate ultrafast charge separation and recombination in WS2-graphene vdW heterostructures. We identify several avoided crossings in the band structure and discuss their relevance for ultrafast charge transfer. We relate our own observations to existing theoretical models and propose a unified picture for ultrafast charge transfer in vdW heterostructures where band alignment and twist angle emerge as the most important control parameters.
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| 46. |
- Jago, Roland, 1990, et al.
(författare)
-
Current enhancement due to field-induced dark carrier multiplication in graphene
- 2017
-
Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 4:2
-
Tidskriftsartikel (refereegranskat)abstract
- We present a microscopic study on current generation in graphene in response to an electric field. While scattering is generally considered to reduce the current, we reveal that in graphene Auger processes give rise to a current enhancement via a phenomenon we denote dark carrier multiplication. Based on a microscopic approach, we show that, if other scattering channels are absent, this prevents the carrier distribution to reach a stationary value. Taking into account scattering with phonons a finite current is restored, however its value exceeds the stationary current without scattering.
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| 47. |
- Jago, Roland, 1990, et al.
(författare)
-
Graphene as gain medium for broadband lasers
- 2015
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 92:8
-
Tidskriftsartikel (refereegranskat)abstract
- Efficient nonradiative carrier recombination strongly counteracts the appearance of optical gain in graphene. Based on a microscopic and fully quantum-mechanical study of the coupled carrier, phonon, and photon dynamics in graphene, we present a strategy to obtain a long-lived gain: Integrating graphene into a high quality photonic crystal nanocavity and applying a high-dielectric substrate suppresses the nonradiative recombination channels and gives rise to pronounced coherent light emission. This suggests the design of graphene-based laser devices covering a broad spectral range.
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| 48. |
- Jago, Roland, 1990, et al.
(författare)
-
Microscopic origin of the bolometric effect in graphene
- 2019
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:3
-
Tidskriftsartikel (refereegranskat)abstract
- While the thermoelectric and photoconduction effects are crucial in pristine and low-doped graphene, the bolometric effect is known to dominate the photoresponse in biased graphene. Here, we present a detailed microscopic investigation of the photoresponse due to the bolometric effect in graphene. Based on the semiconductor Bloch equations, we investigate the time- and momentum-resolved carrier dynamics in graphene in the presence of a constant electric field under optical excitation. The magnitude of the bolometric effect is determined by the optically induced increase of temperature times the conductivity change. Investigating both factors independently, we reveal that the importance of the bolometric effect in the high-doping regime can be mostly ascribed to the latter showing a parabolic dependence on the doping.
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| 49. |
- Jago, Roland, 1990, et al.
(författare)
-
Microscopic understanding of the photoconduction effect in graphene
- 2017
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 96:8, s. Article no 085431 -
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the photoresponse of intrinsic graphene in an in-plane electric field. Toward that end, we employ a microscopic approach that allows us to determine the time-and momentum-resolved charge-carrier distributions as a result of the interplay between the field-induced acceleration of optically excited carriers and Coulomb- and phonon-driven carrier scattering. Calculating the generated photocurrent that is determined by the asymmetry of the carrier distribution, we reveal the microscopic foundation of the photoconduction effect in graphene. In particular, we discuss the possibility of tuning the photocurrent via externally accessible knobs, such as electric field, temperature, and substrate. Furthermore, we study the impact of Auger-induced carrier multiplication on the photocurrent in graphene.
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| 50. |
- Jago, Roland, 1990, et al.
(författare)
-
Recombination channels in optically excited graphene
- 2015
-
Ingår i: Physica Status Solidi (B): Basic Research. - : Wiley. - 1521-3951 .- 0370-1972. ; 252:11, s. 2456-2460
-
Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical study on the efficiency of non-radiative recombination channels in optically excited graphene on a substrate. We find that depending on the strength of the excitation pulse and the dielectric constant of the applied substrate, either Auger or phonon-induced recombination prevails. The favorable conditions for Auger recombination are (i) strong excitation regime providing a large number of scattering partners and (ii) low-dielectric substrates, which only weakly screen the Coulomb interaction. The gained insights are important for achieving a population inversion in graphene that is temporally limited by the presented recombination channels.
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