arXiv subjects

Gunnar Berghäuser

Publications and source records attributed to Gunnar Berghäuser.

14 recordsLinked to original sources

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.

cond-mat.mtrl-sci↗

arXiv2018

Dark-exciton based strain sensing in transition metal dichalcogenides

The trend towards ever smaller high-performance devices in modern technology requires novel materials with new functionalities. The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultra-thin 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. This makes these materials optimal candidates for sensing applications. Here, we exploit the remarkably diverse exciton landscape in monolayer transition metal dichalcogenides to propose a novel dark-exciton-based concept for ultra sensitive strain sensors. 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. This results in a pronounced intensity change of dark excitons in photoluminescence spectra, when only 0.05 $\%$ strain is applied. The predicted extremely high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.

cond-mat.mes-hall↗

arXiv2018

Optical fingerprint of non-covalently functionalized transition metal dichalcogenides

Atomically thin transition metal dichalcogenides (TMDs) hold promising potential for applications 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 TMDs. Considering exemplary spiropyran 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 open new possibilities for spin-valleytronics application.

cond-mat.mtrl-sci↗

arXiv2017

Phonon Sidebands in Transition Metal Dichalcogenides

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.

cond-mat.mes-hall↗

arXiv2017

The role of momentum-dark excitons in the elementary optical response of bilayer WSe$_{2}$

Monolayer (ML) transition metal dichalcogenides (TMDs) undergo substantial changes in the single-particle band structure and excitonic optical response upon the addition of just one layer. As opposed to the single-layer limit, the bandgap of bilayer (BL) TMD semiconductors is indirect which results in reduced photoluminescence with richly structured spectra that have eluded a detailed understanding to date. Here, we provide a closed interpretation of the elementary optical responses of BL WSe$_2$ as a representative material for the wider class of TMD semiconductors. By combining theoretical calculations with comprehensive spectroscopy experiments, we identify the crucial role of momentum-indirect excitons for the understanding of basic absorption and emission spectra ubiquitously exhibited by various TMD BLs. Our results shed light on the origin of quantum dot (QD) formation in monolayer and bilayer crystals and will facilitate further advances directed at opto-electronic applications of layered TMD semiconductors in van der Waals heterostructures and devices.

cond-mat.mes-hall↗

arXiv2017

Dark excitons in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.

cond-mat.mtrl-sci↗

arXiv2017

Mapping of the dark exciton landscape in transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including optically accessible (bright) as well as spin- and momentum-forbidden (dark) excitonic states. So far the dark exciton landscape has not been revealed leaving in particular the spectral position of momentum-forbidden dark states completely unclear. This has a significant impact on the technological application potential of TMDs, since the nature of the energetically lowest state determines, if the material is a direct-gap semiconductor. Here, we show how dark states can be experimentally revealed by probing the intra-excitonic 1s-2p transition. Distinguishing the optical response shortly after the excitation (< 100$\,$fs) and after the exciton thermalization (> 1$\,$ps) allows us to demonstrate the relative position of bright and dark excitons. We find both in theory and experiment a clear blue-shift in the optical response demonstrating for the first time the transition of bright exciton populations into lower lying momentum- and spin-forbidden dark excitonic states in monolayer WSe$_2$.

cond-mat.mtrl-sci↗

arXiv2017

Impact of strain on the optical fingerprint of monolayer transition metal dichalcogenides

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 few works on its impact on optical properties of monolayer transition metal dichalcogenides (TMDs). Combining microscopic theory based on Wannier and Bloch equations with nearestneighbor 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.

cond-mat.mes-hall↗

arXiv2017

Dark and bright exciton formation, thermalization, and photoluminescence in monolayer transition metal dichalcogenides

The remarkably strong Coulomb interaction in atomically thin transition metal dichalcogenides (TMDs) results in an extraordinarily rich many-particle physics including the formation of tightly bound excitons. Besides optically accessible bright excitonic states, these materials also exhibit a variety of dark excitons. Since they can even lie below the bright states, they have a strong influence on the exciton dynamics, lifetimes, and photoluminescence. While very recently, the presence of dark excitonic states has been experimentally demonstrated, the origin of these states, their formation, and dynamics have not been revealed yet. Here, we present a microscopic study shedding light on time- and energy-resolved formation and thermalization of bright and dark intra- and intervalley excitons as well as their impact on the photoluminescence in different TMD materials. We demonstrate that intervalley dark excitons, so far widely overlooked in current literature, play a crucial role in tungsten-based TMDs giving rise to an enhanced photoluminescence and reduced exciton lifetimes at elevated temperatures.

cond-mat.mes-hall↗

arXiv2016

Out of the dark and into the light: towards dark-state based chemical sensors

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 surrounding. Here, we present a novel, 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 finger print for the detection of molecules - in contrast to common sensing schemes relying on small peak shifts and intensity changes.

cond-mat.mtrl-sci↗

arXiv2016

Optical fingerprint of dark 2p-states in transition metal dichalcogenides

We present a microscopic study on higher excitonic states in transition metal dichalcogenides in the presence of disorder. We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of bright and dark states offers a strategy to circumvent optical selection rules. As a prove, we show a direct fingerprint of dark 2p states in absorption spectra of WS$_2$. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

Atomically thin transition metal dichalcogenides (TMDs) are direct-gap semiconductors with strong light-matter and Coulomb interaction. The latter accounts for tightly bound excitons, which dominate the optical properties of these technologically promising materials. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. In a recent study, an experimental evidence for the existence of such dark states has been demonstrated, as well as their strong impact on the quantum efficiency of light emission in TMDs. Here, we reveal the microscopic origin of the excitonic coherence lifetime in two representative TMD materials (WS$_2$ and MoSe$_2$) within a joint study combining microscopic theory with optical experiments. We show that the excitonic coherence lifetime is determined by phonon-induced intra- and intervalley scattering into dark excitonic states. Remarkably, and in accordance with the theoretical prediction, we find an efficient exciton relaxation in WS$_2$ through phonon emission at all temperatures.

cond-mat.mtrl-sci↗

arXiv2015

Trion Formation Dynamics in Monolayer Transition Metal Dichalcogenides

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in TMDs. The quasiparticle formation and conversion processes are important for interpreting photoluminescence and photoconductivity in TMDs.

cond-mat.mes-hall↗

arXiv2013

Analytical approach to excitonic properties of MoS2

We present an analytical investigation of the optical absorption spectrum of monolayer molybdenumdisulfide. Based on the density matrix formalism, our approach gives insights into the microscopic origin of excitonic transitions, their relative oscillator strength, and binding energy. We show analytical expressions for the carrier-light coupling element, which contains the optical selection rules and well describes the valleyselective polarization in MoS2. In agreement with experimental results, we find the formation of strongly bound electron-hole pairs due to the efficient Coulomb interaction. The absorption spectrum of MoS2 on a silicon substrate features two pronounced peaks at 1.91 eV and 2.05 eV corresponding to the A and B exciton, which are characterized by binding energies of 420 meV and 440 meV, respectively. Our calculations reveal their relative oscillator strength and predict the appearance of further low-intensity excitonic transitions at higher energies. The presented approach is applicable to other transition metal dichalcogenides and can be extended to investigations of trion and biexcitonic effects.

cond-mat.mtrl-sci↗