| 1. |
- Ferreira, Beatriz, 1996, et al.
(författare)
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Signatures of dark excitons in exciton–polariton optics of transition metal dichalcogenides
- 2023
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 10
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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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| 2. |
- Lengers, F., et al.
(författare)
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Spatiotemporal dynamics of Coulomb-correlated carriers in semiconductors
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:15
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Tidskriftsartikel (refereegranskat)abstract
- When the excitation of carriers in real space is focused down to the nanometer scale, the carrier system can no longer be viewed as homogeneous, and ultrafast transport of the excited carrier wave packets occurs. In state-of-the-art semiconductor structures such as low-dimensional heterostructures or monolayers of transition-metal dichalcogenides, the Coulomb interaction between excited carriers becomes stronger due to confinement or reduced screening. This demands a fundamental understanding of strongly interacting electrons and holes and the influence of Coulomb correlations. To study the corresponding particle dynamics in a controlled way, we consider a system of up to two electron-hole pairs exactly within a wave-function approach. We show that the excited wave packets contain a nontrivial mixture of free-particle and excitonic states. We further scrutinize the influence of the Coulomb interaction on the wave-packet dynamics, revealing its different role for below- and above-band-gap excitation.
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| 3. |
- Perea Causin, Raul, 1995, et al.
(författare)
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Exciton Propagation and Halo Formation in Two-Dimensional Materials
- 2019
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 19:10, s. 7317-7323
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Tidskriftsartikel (refereegranskat)abstract
- The interplay of optics, dynamics, and transport is crucial for the design of novel optoelectronic devices, such as photodetectors and solar cells. In this context, transition-metal dichalcogenides (TMDs) have received much attention. Here, strongly bound excitons dominate optical excitation, carrier dynamics, and diffusion processes. While the first two have been intensively studied, there is a lack of fundamental understanding of nonequilibrium phenomena associated with exciton transport that is of central importance (e.g., for high-efficiency light harvesting). In this work, we provide microscopic insights into the interplay of exciton propagation and many-particle interactions in TMDs. On the basis of a fully quantum mechanical approach and in excellent agreement with photoluminescence measurements, we show that Auger recombination and emission of hot phonons act as a heating mechanism giving rise to strong spatial gradients in excitonic temperature. The resulting thermal drift leads to an unconventional exciton diffusion characterized by spatial exciton halos.
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| 4. |
- Rosati, Roberto, 1984, et al.
(författare)
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Effective detection of spatio-temporal carrier dynamics by carrier capture
- 2019
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Ingår i: Journal of Physics Condensed Matter. - 0953-8984 .- 1361-648X. ; 31:28
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Tidskriftsartikel (refereegranskat)abstract
- The spatio-temporal dynamics of electrons moving in a 2D plane is challenging to detect when the required resolution shrinks simultaneously to nanometer length and subpicosecond time scale. We propose a detection scheme relying on phonon-induced carrier capture from 2D unbound states into the bound states of an embedded quantum dot. This capture process happens locally and here we explore if this locality is sufficient to use the carrier capture process as detection of the ultrafast diffraction of electrons from an obstacle in the 2D plane. As an example we consider an electronic wave packet traveling in a semiconducting monolayer of the transition metal dichalcogenide MoSe2, and we study the scattering-induced dynamics using a single particle Lindblad approach. Our results offer a new way to high resolution detection of the spatio-temporal carrier dynamics.
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| 5. |
- Rosati, Roberto, 1984, et al.
(författare)
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Negative effective excitonic diffusion in monolayer transition metal dichalcogenides
- 2020
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Ingår i: Nanoscale. - : Royal Society of Chemistry (RSC). - 2040-3372 .- 2040-3364. ; 12:1, s. 356-363
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Tidskriftsartikel (refereegranskat)abstract
- While exciton relaxation in monolayers of transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton diffusion has received only a little attention-in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours (e.g. spatial halos). Here, we study the spatiotemporal dynamics in TMD monolayers and track optically excited excitons in time, momentum, and space. In particular, we investigate the temperature-dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative effective diffusion characterized by a shrinking of the spatial exciton distributions. This phenomenon can be traced back to the existence of dark exciton states in TMD monolayers and is a result of an interplay between spatial exciton diffusion and intervalley exciton-phonon scattering.
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| 6. |
- Rosati, Roberto, 1984, et al.
(författare)
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Spatial control of carrier capture in two-dimensional materials: Beyond energy selection rules
- 2018
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 98:19
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Tidskriftsartikel (refereegranskat)abstract
- © 2018 American Physical Society. The carrier capture from a two-dimensional transition metal dichalcogenide monolayer into a quasi-zero-dimensional potential is a decisive process to exploit these remarkable materials as, e.g., single-photon sources. Here, we study theoretically the phonon-induced carrier capture in a MoSe2 monolayer using a Lindblad single-particle approach. Although one decisive control parameter of the capture efficiency is the energy selection rule, which links the energy of the incoming carriers to that of the final state via the emitted phonon, we show that additionally the spatiotemporal dynamics plays a crucial role. By varying the direction of the incoming carriers with respect to the orientation of the localized potential, we introduce a new control mechanism for the carrier capture: the spatial control.
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| 7. |
- Rosati, Roberto, 1984, et al.
(författare)
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Strain-dependent exciton diffusion in transition metal dichalcogenides
- 2021
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Monolayers of transition metal dichalcogenides have a remarkable excitonic landscape with deeply bound bright and dark exciton states. Their properties are strongly affected by lattice distortions that can be created in a controlled way via strain. Here, we perform a joint theory-experiment study investigating exciton diffusion in strained tungsten disulfide (WS2) monolayers. We reveal a non-trivial and non-monotonic influence of strain. Lattice deformations give rise to different energy shifts for bright and dark excitons changing the excitonic landscape, the efficiency of intervalley scattering channels and the weight of single exciton species to the overall exciton diffusion. We predict a minimal diffusion coefficient in unstrained WS2 followed by a steep speed-up by a factor of 3 for tensile biaxial strain at about 0.6% strain - in excellent agreement with our experiments. The obtained microscopic insights on the impact of strain on exciton diffusion are applicable to a broad class of multi-valley 2D materials.
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| 8. |
- Rosati, Roberto, 1984, et al.
(författare)
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Temporal Evolution of Low-Temperature Phonon Sidebands in Transition Metal Dichalcogenides
- 2020
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Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 7:10, s. 2756-2764
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Tidskriftsartikel (refereegranskat)abstract
- Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe2) shows a multitude of sharp emission peaks below the bright exciton. Some of them have been recently identified as phonon sidebands of momentum-dark states. However, the exciton dynamics behind the emergence of these sidebands has not been revealed yet. In this joint theory-experiment study, we theoretically predict and experimentally observe time-resolved PL, providing microscopic insights into the thermalization of hot excitons formed after optical excitation. In very good agreement between theory and experiment, we demonstrate a spectral red-shift of phonon sidebands on a time scale of tens of picoseconds, reflecting the phonon-driven thermalization of hot excitons in momentum-dark states. Furthermore, we predict the emergence of a transient phonon sideband that vanishes in the stationary PL. The obtained microscopic insights are applicable to a broad class of 2D materials with multiple exciton valleys.
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| 9. |
- Zipfel, Jonas, et al.
(författare)
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Exciton diffusion in monolayer semiconductors with suppressed disorder
- 2020
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 101:11
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Tidskriftsartikel (refereegranskat)abstract
- Tightly bound excitons in monolayer semiconductors represent a versatile platform to study two-dimensional propagation of neutral quasiparticles. Their intrinsic properties, however, can be severely obscured by spatial energy fluctuations due to a high sensitivity to the immediate environment. Here, we take advantage of the encapsulation of individual layers in hexagonal boron nitride to strongly suppress environmental disorder. Diffusion of excitons is then directly monitored using time and spatially resolved emission microscopy at ambient conditions. We consistently find very efficient propagation with linear diffusion coefficients up to 10 cm(2)/s, corresponding to room-temperature effective mobilities as high as 400 cm(2)/Vs as well as a correlation between rapid diffusion and short population lifetime. At elevated densities we detect distinct signatures of many-particle interactions and consequences of strongly suppressed Auger-type exciton-exciton annihilation. A combination of analytical and numerical theoretical approaches is employed to provide pathways toward comprehensive understanding of the observed linear and nonlinear propagation phenomena. We emphasize the role of dark exciton states and present a mechanism for diffusion facilitated by free-electron hole plasma from entropy-ionized excitons.
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