arXiv subjects

Samuel Brem

Publications and source records attributed to Samuel Brem.

At least 55 records · Page 3Linked to original sources

arXiv2020

Phonon-assisted Exciton Dissociation in Transition Metal Dichalcogenides

Monolayers of transition metal dichalcogenides (TMDs) have been established in the last years as promising materials for novel optoelectronic devices. However, the performance of such devices is often limited by the dissociation of tightly bound excitons into free electrons and holes. While previous studies have investigated tunneling at large electric fields, we focus in this work on phonon-assisted exciton dissociation that is expected to be the dominant mechanism at small fields. We present a microscopic model based on the density matrix formalism providing access to time- and momentum-resolved exciton dynamics including phonon-assisted dissociation. We track the pathway of excitons from optical excitation via thermalization to dissociation, identifying the main transitions and dissociation channels. Furthermore, we find intrinsic limits for the quantum efficiency and response time of a TMD-based photodetector and investigate their tunability with externally accessible knobs, such as excitation energy, substrate screening, temperature and strain. Our work provides microscopic insights in fundamental mechanisms behind exciton dissociation and can serve as a guide for the optimization of TMD-based optoelectronic devices.

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arXiv2020

Criteria for deterministic single-photon emission in two-dimensional atomic crystals

The deterministic production of single-photons from two dimensional materials promises to usher in a new generation of photonic quantum devices. In this work, we outline criteria by which single-photon emission can be realised in two dimensional materials: spatial isolation, spectral filtering and low excitation of quantum emitters. We explore how these criteria can be fulfilled in atomically thin transition metal dichalcogenides, where excitonic physics dictates the observed photoemission. In particular, we model the effect of defects and localised strain, in accordance with the most common experimental realisations, on the photon statistics of emitted light. Moreover, we demonstrate that an optical cavity has a negative impact on the photon statistics, suppressing the single-photon character of the emission by diminishing the effect of spectral filtering on the emitted light. Our work provides a theoretical framework revealing criteria necessary to facilitate single-photon emission in two-dimensional materials and thus can guide future experimental studies in this field.

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arXiv2020

Strain-dependent exciton diffusion in transition metal dichalcogenides

Monolayers of transition metal dichalcogenides (TMDs) 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 (WS$_2$) 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 WS$_2$ 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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arXiv2020

Tunable Phases of Moiré Excitons in van der Waals Heterostructures

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. While 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 principle calculations with the excitonic density matrix formalism to study transitions between different moiré exciton phases and their impact on optical properties of the twisted MoSe$_2$/WSe$_2$ heterostructure. At angles smaller than 2$^{\circ}$ we find flat, moiré trapped states for inter- and intralayer excitons. This moiré exciton phase drastically changes into completely delocalized states already at 3$^{\circ}$. We predict a linear and quadratic twist-angle dependence of excitonic resonances for the moiré-trapped and delocalized exciton phase, respectively. Our work provides microscopic insights opening the possibility to tailor moiré exciton phases in van der Waals superlattices.

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arXiv2020

Temporal evolution of low-temperature phonon sidebands in WSe$_2$ monolayers

Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe$_2$) 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 thermalization of hot excitons formed after optical excitation. In good agreement between theory and experiment, we demonstrate a spectral red-shift of phonon sidebands on a timescale 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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arXiv2020

Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides

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 WS$_2$, WSe$_2$ and MoS$_2$, while the photoluminescence of MoSe$_2$ is only determined by the bright exciton. In particular, we reveal the mechanism behind the brightening of states that are both spin- \textit{and} momentum-dark in MoS$_2$. 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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arXiv2020

Hybridized intervalley moiré excitons and flat bands in twisted WSe$_2$ bilayers

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 $Λ$ point leads to a drastic redshift of the momentum-dark K-$Λ$ exciton, accompanied by the emergence of flat moiré 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 moiré potential of the twisted bilayer lattice.

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arXiv2020

Exciton diffusion in monolayer semiconductors with suppressed disorder

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-like exciton-exciton annihilation. A combination of analytical and numerical theoretical approaches is employed to provide pathways towards comprehensive understanding of the observed linear and non-linear 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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arXiv2019

Ultrafast transition between exciton phases in van der Waals heterostructures

Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic, Mott insulating, or superconducting phases. In transition metal dichalcogenide heterostructures, electrons and holes residing in different monolayers can bind into spatially indirect excitons with a strong potential for optoelectronics, valleytronics, Bose condensation, superfluidity, and moiré-induced nanodot lattices. Yet these ideas require a microscopic understanding of the formation, dissociation, and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers; phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s-2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s-2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.

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arXiv2019

Quenching of Intervalley Exchange Coupling in the Presence of Momentum-Dark States in TMDCs

Monolayers of transition metal dichalcogenides are promising materials for valleytronic applications, since they possess two individually addressable excitonic transitions at the non-equivalent $K$ and $K'$ points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucial importance to understand the elementary processes determining the lifetime of these optically injected valley excitons. In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism to investigate the efficiency of the intervalley coupling in the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low lying momentum-dark exciton states. While we predict a valley exciton lifetime on the order of some hundreds of fs in the absence of low lying momentum-dark states we demonstrate a strong quenching of the valley lifetime in the presence of such states.

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arXiv2019

Negative excitonic diffusion in transition metal dichalcogenides

While exciton relaxation in transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton propagation has received only 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 TMDs and track the way of 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 transient diffusion. This phenomenon can be traced back to the existence of dark exciton states in TMDs and is a result of an interplay between spatial exciton diffusion and intervalley exciton-phonon scattering.

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arXiv2019

Optical fingerprint of bright and dark localized excitonic states in atomically thin 2D materials

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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arXiv2019

Exciton propagation and halo formation in two-dimensional materials

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 non-equilibrium 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. Based on 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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arXiv2019

Phonon-assisted Photoluminescence from Dark Excitons in Monolayers of Transition Metal Dichalcogenides

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 WSe$_2$ exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark excitons.

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arXiv2019

Intrinsic Lifetime of Higher Excitonic States in Tungsten Diselenide Monolayers

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 WSe$_2$ 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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arXiv2019

Spatio-temporal dynamics in graphene

Temporally and spectrally resolved dynamics of optically excited carriers in graphene has been intensively studied theoretically and experimentally, whereas carrier diffusion in space has attracted much less attention. Understanding the spatio-temporal carrier dynamics is of key importance for optoelectronic applications, where carrier transport phenomena play an important role. In this work, we provide a microscopic access to the time-, momentum-, and space-resolved dynamics of carriers in graphene. We determine the diffusion coefficient to be $D \approx 360$cm$^{2}$/s and reveal the impact of carrier-phonon and carrier-carrier scattering on the diffusion process. In particular, we show that phonon-induced scattering across the Dirac cone gives rise to back-diffusion counteracting the spatial broadening of the carrier distribution.

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arXiv2018

Carrier dynamics in graphene: ultrafast many-particle phenomena

Graphene is an ideal material to study fundamental Coulomb- and phonon-induced carrier scattering processes. Its remarkable gapless and linear band structure opens up new carrier relaxation channels. In particular, Auger scattering bridging the valence and the conduction band changes the number of charge carriers and gives rise to a significant carrier multiplication - an ultrafast many-particle phenomenon that is promising for the design of highly efficient photodetectors. Furthermore, the vanishing density of states at the Dirac point combined with ultrafast phonon-induced intraband scattering results in an accumulation of carriers and a population inversion suggesting the design of graphene-based terahertz lasers. Here, we review our work on the ultrafast carrier dynamics in graphene and Landau-quantized graphene is presented providing a microscopic view on the appearance of carrier multiplication and population inversion.

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arXiv2018

Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure

Heterostructures of van der Waals bonded layered materials offer unique means to tailor dielectric screening with atomic-layer precision, opening a fertile field of fundamental research. The optical analyses used so far have relied on interband spectroscopy. Here we demonstrate how a capping layer of hexagonal boron nitride (hBN) renormalizes the internal structure of excitons in a WSe$_2$ monolayer using intraband transitions. Ultrabroadband terahertz probes sensitively map out the full complex-valued mid-infrared conductivity of the heterostructure after optical injection of $1s$ A excitons. This approach allows us to trace the energies and linewidths of the atom-like $1s$-$2p$ transition of optically bright and dark excitons as well as the densities of these quasiparticles. The fundamental excitonic resonance red shifts and narrows in the WSe$_2$/hBN heterostructure compared to the bare monolayer. Furthermore, the ultrafast temporal evolution of the mid-infrared response function evidences the formation of optically dark excitons from an initial bright population. Our results provide key insight into the effect of non local screening on electron-hole correlations and open new possibilities of dielectric engineering of van der Waals heterostructures.

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