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We explore the optical dipole orientation of single photon emitters in monolayer MoS2 as produced by a focused helium ion beam. The single photon emitters can be understood as single sulfur vacancies. The corresponding far-field luminescence spectra reveal several photoluminescence lines below the dominating luminescence of the exciton in MoS2. These sub-bandgap emission lines were predicted by ab initio theory, but they have never been resolved in luminescence experiments because of their small amplitude. We reveal the lines by their dependence as a function of the photon energy and momentum as measured in the back focal plane of the optical circuitry. The agreement between theory and experiment suggests that the defect states interact strongly within the Brillouin zone.

Interlayer excitons (IXs), composed of electron and hole states localized in different layers, excel in bilayers composed of atomically thin van der Waals materials such as semiconducting transition metal dichalcogenides (TMDs) due to drastically enlarged exciton binding energies, exciting spin-valley properties, elongated lifetimes, and large permanent dipoles. The latter allows modification by electric fields and the study of thermalized bosonic quasiparticles, from the single particle level to interacting degenerate dense ensembles. Additionally, the freedom to combine bilayers of different van der Waals materials without lattice or relative twist angle constraints leads to layer hybridized and moir\'e excitons which can be widely engineered. This review covers fundamental aspects of IXs including correlation phenomena as well as the consequence of moir\'e superlattices with a strong focus on TMD homo- and hetero-bilayers., Comment: Review article

The question of macroscopic occupation and spontaneous emergence of coherence for exciton ensembles has gained renewed attention due to the rise of van der Waals heterostructures made of atomically thin semiconductors. The hosted interlayer excitons exhibit nanosecond lifetimes, long enough to allow for excitonic thermalization in time. Several experimental studies reported signatures of macroscopic occupation effects at elevated exciton densities. With respect to theory, excitons are composite particles formed by fermionic constituents, and a general theoretical argument for a bosonic thermalization of an exciton gas beyond the linear regime is still missing. Here, we derive an equation for the phonon mediated thermalization at densities above the classical limit, and identify which conditions favor the thermalization of fermionic or bosonic character, respectively. In cases where acoustic, quasielastic phonon scattering dominates the dynamics, our theory suggests that transition metal dichalcogenide (TMDC) excitons might be bosonic enough to show bosonic thermalization behaviour and decreasing dephasing for increasing exciton densities. This can be interpreted as a signature of an emerging coherence in the exciton ground state, and agrees well with the experimentally observed features, such as a decreasing linewidth for increasing densities.

We report on the spatial coherence of interlayer exciton ensembles as formed in MoSe$_2$/WSe$_2$ heterostructures and characterized by point-inversion Michelson-Morley interferometry. Below 10 K, the measured spatial coherence length of the interlayer excitons reaches values equivalent to the lateral expansion of the exciton ensembles. In this regime, the light emission of the excitons turns out to be homogeneously broadened in energy with a high temporal coherence. At higher temperatures, both the spatial coherence length and the temporal coherence time decrease, most likely because of thermal processes. The presented findings point towards a spatially extended, coherent many-body state of interlayer excitons at low temperature.

We report lasing of moir\'e trapped interlayer excitons (IXs) by integrating a pristine hBN-encapsulated MoSe$_2$/WSe$_2$ heterobilayer into a high-$Q$ ($>10^4$) nanophotonic cavity. We control the cavity-IX detuning using a magnetic field and measure their dipolar coupling strength to be $78 \pm 4\ \mathrm{\mu eV}$, fully consistent with the 82 $\mathrm{\mu eV}$ predicted by theory. The emission from the cavity mode shows clear threshold-like behavior as the transition is tuned into resonance with the cavity. We observe a superlinear power dependence accompanied by a narrowing of the linewidth as the distinct features of lasing. The onset and prominence of these threshold-like behaviors are pronounced at resonance while weak off-resonance. Our results show that a lasing transition can be induced in interacting moir\'e IXs with macroscopic coherence extending over the length scale of the cavity mode. Such systems raise interesting perspectives for low-power switching and synaptic nanophotonic devices using two-dimensional materials.

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS$_2$ generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below $10^{11}$ cm$^{-2}$. The corresponding spectra reveal a broad sub-gap absorption, being consistent with theoretical predictions related to sulfur vacancy-bound excitons in MoS$_2$. Our results highlight cavity-enhanced extinction spectroscopy as efficient means for the detection of optical transitions in nanoscale thin films with weak absorption, applicable to a broad range of materials., Comment: 17 pages main manuscript, 7 pages supporting information

The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent centres with emission energies of 2 eV which exhibit pronounced phonon sidebands. We investigate the microscopic origin of these luminescent centres by combining ab initio calculations with non-perturbative open quantum system theory to study the emission and absorption properties of 26 defect transitions. Comparing the calculated line shapes with experiments we narrow down the microscopic origin to three carbon-based defects: $\mathrm{C_2C_B}$, $\mathrm{C_2C_N}$, and $\mathrm{V_NC_B}$. The theoretical method developed enables us to calculate so-called photoluminescence excitation (PLE) maps, which show excellent agreement with our experiments. The latter resolves higher-order phonon transitions, thereby confirming both the vibronic structure of the optical transition and the phonon-assisted excitation mechanism with a phonon energy 170 meV. We believe that the presented experiments and polaron-based method accurately describe luminescent centres in hBN and will help to identify their microscopic origin.

Semiconducting two-dimensional materials and their heterostructures gained a lot of interest for applications as well as fundamental studies due to their rich optical properties. Assembly in van der Waals heterostacks can significantly alter the intrinsic optical properties as well as the wavelength-dependent absorption and emission efficiencies making a direct comparison of e.g. photoluminescence intensities difficult. Here, we determine the dielectric function for the prototypical MoSe2/WSe2 heterobilayer and their individual layers. Apart from a redshift of 18 meV - 44 meV of the energetically lowest interband transitions, we find that for larger energies the dielectric function can only be described by treating the van der Waals heterobilayer as a new artificial homobilayer crystal rather than a stack of individual layers. The determined dielectric functions are applied to calculate the Michelson contrast of the individual layers and the bilayer in dependence of the oxide thickness of often used Si/SiO2 substrates. Our results highlight the need to consider the altered dielectric functions impacting the Michelson interference in the interpretation of intensities in optical measurements such as Raman scattering or photoluminescence., Comment: 6 pages, 4 figures

Single spin defects in 2D transition-metal dichalcogenides are natural spin-photon interfaces for quantum applications. Here we report high-field magneto-photoluminescence spectroscopy from three emission lines (Q1, Q2 and Q*) of He-ion induced sulfur vacancies in monolayer $\text{MoS}_2$. Analysis of the asymmetric PL lineshapes in combination with the diamagnetic shift of Q1 and Q2 yields a consistent picture of localized emitters with a wavefunction extent of $\sim$ 3.5 nm. The distinct valley-Zeeman splitting in out-of-plane $B$-fields and the brightening of dark states through in-plane $B$-fields necessitates spin-valley selectivity of the defect states and lifted spin-degeneracy at zero field. Comparing our results to ab-initio calculations identifies the nature of Q1 and Q2 and suggests that Q* is the emission from a chemically functionalized defect. Analysis of the optical degree of circular polarization reveals that the Fermi level is a parameter that enables the tunability of the emitter. These results show that defects in 2D semiconductors may be utilized for quantum technologies.

Negatively charged boron vacancies ($V_B^-$) in hexagonal boron nitride (hBN) exhibit a broad emission spectrum due to strong electron-phonon coupling and Jahn-Teller mixing of electronic states. As such, the direct measurement of zero-phonon line (ZPL) of $V_B^-$ has remained elusive. Here, we measure the room-temperature ZPL wavelength to be $773\pm2$ nm by coupling the hBN layer to the high-Q nanobeam cavity. As the wavelength of cavity mode is tuned, we observe a pronounced intensity resonance, indicating the coupling to $V_B^-$. Our observations are consistent with the spatial redistribution of $V_B^-$ emission. Spatially resolved measurements show a clear Purcell effect maximum at the midpoint of the nanobeam, in accord with the optical field distribution of the cavity mode. Our results are in good agreement with theoretical calculations, opening the way to using $V_B^-$ as cavity spin-photon interfaces.

A conventional optical cavity supports modes which are confined because they are unable to leak out of the cavity. Bound state in continuum (BIC) cavities are an unconventional alternative, where light can leak out, but is confined by multimodal destructive interference. BICs are a general wave phenomenon, of particular interest to optics, but BICs and multimodal interference have never been demonstrated at the nanoscale. Here, we demonstrate the first nanophotonic cavities based on BIC-like multimodal interference. This novel confinement mechanism for deep sub-wavelength light shows orders of magnitude improvement in several confinement metrics. Specifically, we obtain cavity volumes below 100x100x3nm^3 with quality factors about 100, with extreme cases having 23x23x3nm^3 volumes or quality factors above 400. Key to our approach, is the use of pristine crystalline hyperbolic dispersion media (HyM) which can support large momentum excitations with relatively low losses. Making a HyM cavity is complicated by the additional modes that appear in a HyM. Ordinarily, these serve as additional channels for leakage, reducing cavity performance. But, in our experiments, we find a BIC-like cavity confinement enhancement effect, which is intimately related to the ray-like nature of HyM excitations. In fact, the quality factors of our cavities exceed the maximum that is possible in the absence of higher order modes. The alliance of HyM with BICs in our work yields a radically novel way to confine light and is expected to have far reaching consequences wherever strong optical confinement is utilized, from ultra-strong light-matter interactions, to mid-IR nonlinear optics and a range of sensing applications.

We report on the far-field photoluminescence intensity distribution of interlayer excitons in MoSe$_{2}$-WSe$_{2}$ heterostacks as measured by back focal plane imaging in the temperature range between 1.7 K and 20 K. By comparing the data with an analytical model describing the dipolar emission pattern in a dielectric environment, we are able to obtain the relative contributions of the in- and out-of-plane transition dipole moments associated to the interlayer exciton photon emission. We determine the transition dipole moments for all observed interlayer exciton transitions to be (99 $\pm$ 1)% in-plane for R- and H-type stacking, independent of the excitation power and therefore the density of the exciton ensemble in the experimentally examined range. Finally, we discuss the limitations of the presented measurement technique to observe correlation effects in exciton ensembles.

We report magneto-photoluminescence spectroscopy of gated MoS$_2$ monolayers in high magnetic fields to 28 T. At B = 0T and electron density $n_s\sim 10^{12}cm^-2$, we observe three trion resonances that cannot be explained within a single-particle picture. Employing ab initio calculations that take into account three-particle correlation effects as well as local and non-local electron-hole exchange interaction, we identify those features as quantum superpositions of inter- and intravalley spin states. We experimentally investigate the mixed character of the trion wave function via the filling factor dependent valley Zeeman shift in positive and negative magnetic fields. Our results highlight the importance of exchange interactions for exciton physics in monolayer MoS$_2$ and provide new insights into the microscopic understanding of trion physics in 2D multi-valley semiconductors for low excess carrier densities., Comment: Main manuscript: 7 pages, 3 figures ; Supplemental material: 16 pages, 4 figures

Atomically thin semiconductors can be readily integrated into a wide range of nanophotonic architectures for applications in quantum photonics and novel optoelectronic devices. We report the observation of nonlocal interactions of \textit{free} trions in pristine hBN/MoS$_2$/hBN heterostructures coupled to single mode (Q $>10^4$) quasi 0D nanocavities. The high excitonic and photonic quality of the interaction system stems from our integrated nanofabrication approach simultaneously with the hBN encapsulation and the maximized local cavity field amplitude within the MoS$_2$ monolayer. We observe a nonmonotonic temperature dependence of the cavity-trion interaction strength, consistent with the nonlocal light-matter interactions in which the extent of the center-of-mass wavefunction is comparable to the cavity mode volume in space. Our approach can be generalized to other optically active 2D materials, opening the way towards harnessing novel light-matter interaction regimes for applications in quantum photonics.

Full experimental control of local spin-charge interconversion is of primary interest for spintronics. Heterostructures combining graphene with a strongly spin-orbit coupled two-dimensional (2D) material enable such functionality by design. Electric spin valve experiments have provided so far global information on such devices, while leaving the local interplay between symmetry breaking, charge flow across the heterointerface and aspects of topology unexplored. Here, we utilize magneto-optical Kerr microscopy to resolve the gate-tunable, local current-induced spin polarisation in graphene/WTe$_2$ van der Waals (vdW) heterostructures. It turns out that even for a nominal in-plane transport, substantial out-of-plane spin accumulation is induced by a corresponding out-of-plane current flow. We develop a theoretical model which explains the gate- and bias-dependent onset and spatial distribution of the massive Kerr signal on the basis of interlayer tunnelling, along with the reduced point group symmetry and inherent Berry curvature of the heterostructure. Our findings unravel the potential of 2D heterostructure engineering for harnessing topological phenomena for spintronics, and constitute an important further step toward electrical spin control on the nanoscale., Comment: 6 pages, 4 figures

The relaxation dynamics of hot carriers in silicon (100) is studied via a novel holistic approach based on phase-resolved transient absorption spectroscopy with few-cycle optical pulses. After excitation by a sub-5 fs light pulse, strong electron-phonon coupling leads to an ultrafast momentum relaxation with time constant of 10 fs. The thermalization of the hot carriers occurs on a time constant of 150 fs, visible in the temporal evolution of the collision time as extracted from the Drude model. We find an increase of the collision time from 3 fs for the shortest timescales with a saturation at approximately 18 fs. Moreover, the optical effective mass of the hot carrier ensemble evolves on ultrafast timescales as well, with a bi-exponential decrease from 0.7 $m_e$ to about 0.125 $m_e$ and time constants of 4 fs and 58 fs. The presented information on the electron mass dynamics as well as the momentum-, energy-, and collision-scattering times with unprecedented time resolution is important for all hot carrier optoelectronic devices., Comment: 6 pages, 5 figures

Interest in building dedicated Quantum Information Science and Engineering (QISE) education programs has greatly expanded in recent years. These programs are inherently convergent, complex, often resource intensive and likely require collaboration with a broad variety of stakeholders. In order to address this combination of challenges, we have captured ideas from many members in the community. This manuscript not only addresses policy makers and funding agencies (both public and private and from the regional to the international level) but also contains needs identified by industry leaders and discusses the difficulties inherent in creating an inclusive QISE curriculum. We report on the status of eighteen post-secondary education programs in QISE and provide guidance for building new programs. Lastly, we encourage the development of a comprehensive strategic plan for quantum education and workforce development as a means to make the most of the ongoing substantial investments being made in QISE., Comment: 18 pages, 2 figures, 1 table

We report magneto-optical spectroscopy of gated monolayer MoS$_2$ in high magnetic fields up to 28T and obtain new insights on the many-body interaction of neutral and charged excitons with the resident charges of distinct spin and valley texture. For neutral excitons at low electron doping, we observe a nonlinear valley Zeeman shift due to dipolar spin-interactions that depends sensitively on the local carrier concentration. As the Fermi energy increases to dominate over the other relevant energy scales in the system, the magneto-optical response depends on the occupation of the fully spin-polarized Landau levels in both $K/K^{\prime}$ valleys. This manifests itself in a many-body state. Our experiments demonstrate that the exciton in monolayer semiconductors is only a single particle boson close to charge neutrality. We find that away from charge neutrality it smoothly transitions into polaronic states with a distinct spin-valley flavour that is defined by the Landau level quantized spin and valley texture., Comment: Main manuscript: 7 pages, 4 figures ; Supplemental material: 20 pages, 8 figures

Integration of semiconducting transition metal dichalcogenides (TMDs) into functional optoelectronic circuitries requires an understanding of the charge transfer across the interface between the TMD and the contacting material. Here, we use spatially resolved photocurrent microscopy to demonstrate electronic uniformity at the epitaxial graphene/molybdenum disulfide (EG/MoS2) interface. A 10x larger photocurrent is extracted at the EG/MoS2 interface when compared to metal (Ti/Au) /MoS2 interface. This is supported by semi-local density-functional theory (DFT), which predicts the Schottky barrier at the EG/MoS2 interface to be ~2x lower than Ti/MoS2. We provide a direct visualization of a 2D material Schottky barrier through combination of angle resolved photoemission spectroscopy with spatial resolution selected to be ~300 nm (nano-ARPES) and DFT calculations. A bending of ~500 meV over a length scale of ~2-3 micrometer in the valence band maximum of MoS2 is observed via nano-ARPES. We explicate a correlation between experimental demonstration and theoretical predictions of barriers at graphene/TMD interfaces. Spatially resolved photocurrent mapping allows for directly visualizing the uniformity of built-in electric fields at heterostructure interfaces, providing a guide for microscopic engineering of charge transport across heterointerfaces. This simple probe-based technique also speaks directly to the 2D synthesis community to elucidate electronic uniformity at domain boundaries alongside morphological uniformity over large areas.

Real-world quantum applications, eg. on-chip quantum networks and quantum cryptography, necessitate large scale integrated single-photon sources with nanoscale footprint for modern information technology. While on-demand and high fidelity implantation of atomic scale single-photon sources in conventional 3D materials suffer from uncertainties due to the crystals dimensionality, layered 2D materials can host point-like centers with inherent confinement to a sub-nm plane. However, previous attempts to truly deterministically control spatial position and spectral homogeneity while maintaining the 2D character have not been realized. Here, we demonstrate the on-demand creation and precise positioning of single-photon sources in atomically thin MoS2 with very narrow ensemble broadening and near-unity fabrication yield. Focused ion beam irradiation creates 100s to 1000s of mono-typical atomistic defects with anti-bunched emission lines with sub-10 nm lateral and 0.7 nm axial positioning accuracy. Our results firmly establish 2D materials as a scalable platform for single-photon emitters with unprecedented control of position as well as photophysical properties owing to the all-interfacial nature., Comment: 13 pages, 4 figures

Atomistic van der Waals heterostacks are ideal systems for high-temperature exciton condensation because of large exciton binding energies and long lifetimes. Charge transport and electron energy-loss spectroscopy showed first evidence of excitonic many-body states in such two-dimensional materials. Pure optical studies, the most obvious way to access the phase diagram of photogenerated excitons have been elusive. We observe several criticalities in photogenerated exciton ensembles hosted in MoSe2-WSe2 heterostacks with respect to photoluminescence intensity, linewidth, and temporal coherence pointing towards the transition to a coherent quantum state. For this state, the occupation is 100 percent and the exciton diffusion length is increased. The phenomena survive above 10 kelvin, consistent with the predicted critical condensation temperature. Our study provides a first phase-diagram of many-body interlayer exciton states including Bose Einstein condensation., Comment: 26 pages, 16 figures

Valley selective hybridization and residual coupling of electronic states in commensurate van der Waals heterobilayers enable the control of the orbital character of interlayer excitons. We demonstrate electric field control of layer index, orbital character, lifetime and emission energy of indirect excitons in MoS2/WS2 heterobilayers embedded in an vdW field effect structure. Different excitonic dipoles normal to the layers are found to stem from bound electrons and holes located in different valleys of MoS2/WS2 with a valley selective degree of hybridization. For the energetically lowest emission lines, coupling of electronic states causes a field-dependent level anticrossing that goes along with a change of the IX lifetime from 400 ns to 100 ns. In the hybridized regime the exiton is delocalized between the two constituent layers, whereas for large positive or negative electric fields, the layer index of the bound hole is field-dependent. Our results demonstrate the design of novel van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields., Comment: 5 pages, 3 figures

Quantum light sources in solid-state systems are of major interest as a basic ingredient for integrated quantum device technologies. The ability to tailor quantum emission through deterministic defect engineering is of growing importance for realizing scalable quantum architectures. However, a major difficulty is that defects need to be positioned site-selectively within the solid. Here, we overcome this challenge by controllably irradiating single-layer MoS$_{2}$ using a sub-nm focused helium ion beam to deterministically create defects. Subsequent encapsulation of the ion bombarded MoS$_{2}$ flake with high-quality hBN reveals spectrally narrow emission lines that produce photons at optical wavelengths in an energy window of one to two hundred meV below the neutral 2D exciton of MoS$_{2}$. Based on ab-initio calculations we interpret these emission lines as stemming from the recombination of highly localized electron-hole complexes at defect states generated by the helium ion bombardment. Our approach to deterministically write optically active defect states in a single transition metal dichalcogenide layer provides a platform for realizing exotic many-body systems, including coupled single-photon sources and exotic Hubbard systems., Comment: Main: 9 pages, 3 figures + SI: 19 pages, 10 figures

Ensembles of indirect or interlayer excitons (IXs) are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles. IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electron-hole wave functions. We demonstrate electric Field control of indirect excitons in MoS2/WS2 hetero-bilayers embedded in a field effect structure with few-layer hexagonal boron nitrite as insulator and few-layer graphene as gate-electrodes. The different strength of the excitonic dipoles and a distinct temperature dependence identify the indirect excitons to stem from optical interband transitions with electrons and holes located in different valleys of the hetero-bilayer featuring highly hybridized electronic states. For the energetically lowest emission lines, we observe a field-dependent level anticrossing at low temperatures. We discuss this behavior in terms of coupling of electronic states from the two semiconducting monolayers resulting in spatially delocalized excitons of the hetero-bilayer behaving like an artificial van der Waals solid. Our results demonstrate the design of novel nano-quantum materials prepared from artificial van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields., Comment: 4 figures

A direct band gap, remarkable light-matter coupling as well as strong spin-orbit and Coulomb interaction establish two-dimensional (2D) crystals of transition metal dichalcogenides (TMDs) as an emerging material class for fundamental studies as well as novel technological concepts. Valley selective optical excitation allows for optoelectronic applications based on the momentum of excitons. In addition to lattice imperfections and disorder, scattering by phonons is a significant mechanism for valley depolarization and decoherence in TMDs at elevated temperatures preventing high-temperature valley polarization required for realistic applications. Thus, a detailed knowledge about strength and nature of the interaction of excitons with phonons is vital. We directly access exciton-phonon coupling in charge tunable single layer MoS2 devices by polarization resolved Raman spectroscopy. We observe a strong defect mediated coupling between the long-range oscillating electric field induced by the longitudinal optical (LO) phonon in the dipolar medium and the exciton. We find that this so-called Fr\"ohlich exciton LO-phonon interaction is suppressed by doping. This suppression correlates with a distinct increase of the degree of valley polarization of up to 20 % even at elevated temperatures of 220 K. Our result demonstrates a promising strategy to increase the degree of valley polarization towards room temperature valleytronic applications., Comment: 18 pages, 3 figures, SI

Using Hall photovoltage measurements, we demonstrate that an anomalous Hall-voltage can be induced in few layer WTe2 under circularly polarized light illumination. By applying a bias voltage along different crystal axes, we find that the photo-induced anomalous Hall conductivity coincides with a particular crystal axis. Our results are consistent with the underlying Berry-curvature exhibiting a dipolar distribution due to the breaking of crystal inversion symmetry. Using a time-resolved optoelectronic auto-correlation spectroscopy, we find that the decay time of the anomalous Hall voltage exceeds the electron-phonon scattering time by orders of magnitude but is consistent with the comparatively long spin-lifetime of carriers in the momentum-indirect electron and hole pockets in WTe2. Our observation suggests, that a helical modulation of an otherwise isotropic spin-current is the underlying mechanism of the anomalous Hall effect.

Crystals with symmetry-protected topological order, such as topological insulators, promise coherent spin and charge transport phenomena even in the presence of disorder at room temperature. Still, a major obstacle towards an application of topological surface states in integrated circuits is a clear, reliable, and straightforward read-out independent of a prevailing charge carrier density in the bulk. Here, we demonstrate how to image and read-out the local conductance of helical surface modes in the prototypical topological insulators Bi2Se3 and BiSbTe3. We apply the so-called Shockley-Ramo theorem to design an optoelectronic probe circuit for the gapless surface states, and surprisingly find a precise conductance quantization at 1e2/h. The unprecedented response is a clear signature of local spin-polarized transport, and it can be switched on and off via an electrostatic field effect. The macroscopic, global read-out scheme is based on the displacement current resistivity, and it does not require coherent transport between electrodes.6 It provides a generalizable platform for studying further non-trivial gapless systems such as Weyl-semimetals and quantum spin-Hall insulators.

Three-dimensional topological insulators are a class of Dirac materials, wherein strong spin-orbit coupling leads to two-dimensional surface states. The latter feature spin-momentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi2Te2Se platelets for perpendicular incidence of light. The same edges exhibit also a finite bias-dependent Kerr angle, indicative of spin accumulation induced by a transversal spin Hall effect in the bulk states of the Bi2Te2Se platelets. A symmetry analysis shows that the helical photoconductance is distinct to common longitudinal photoconductance and photocurrent phenomena, but consistent with the accumulated spins being transported in the side facets of the platelets. Our findings demonstrate that spin effects in the facets of 3D topological insulators can be addressed and read-out in optoelectronic devices even at room temperatures.

To combine the advantages of ultrafast femtosecond optics with an on-chip communcation scheme, optical signals with a frequency of several hundreds of THz need to be down-converted to coherent electronic signals of GHz or less. So far, this has not been achieved because of the impedance mismatch within electronic circuits and their overall slow response-time. Here, we demonstrate that 14 fs optical pulses in the near-infrared (NIR) can drive electronic on-chip circuits with a bandwidth up to 10 THz. The corresponding electronic pulses propagate in microscopic striplines on a millimeter scale. We exploit femtosecond photoswitches based on tunneling barriers in nanoscale metal junctions to drive the pulses. The non-linear ultrafast response is based on a combination of plasmonically enhanced, multi-photon absorption and quantum tunneling, and gives rise to a field emission of ballistic electrons propagating across the nanoscale junctions. Our results pave the way towards femtosecond electronics integrated in waferscale quantum circuits.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have emerged as promising materials for electronic, optoelectronic, and valleytronic applications. Recent work suggests drastic changes of the band gap and exciton binding energies of photo-excited TMDs with ultrafast non-radiative relaxation processes effectively heating the crystal lattice. Such phenomena have not been considered in the context of optoelectronic devices yet. We resolve corresponding ultrafast photo-conductance dynamics within monolayer MoS2 and demonstrate that a bolometric contribution dominates the overall photoconductance. We further reveal that a focused laser illumination, as is used in many standard optoelectronic measurements of MoS2, modifies and anneals the morphology of metal contacts. We show that a junction evolves with lateral built-in electric fields, although Raman- and photoluminescence spectra indicate no significant changes such as a crystal phase transition. We highlight how such optimized devices can drive ultrafast electromagnetic signals in on-chip high-frequency and THz circuits.

Atomically thin semiconductors have dimensions that are commensurate with critical feature sizes of future optoelectronic devices defined using electron/ion beam lithography. Robustness of their emergent optical and valleytronic properties is essential for typical exposure doses used during fabrication. Here, we explore how focused helium ion bombardment affects the intrinsic vibrational, luminescence and valleytronic properties of atomically thin MoS$_{2}$. By probing the disorder dependent vibrational response we deduce the interdefect distance by applying a phonon confinement model. We show that the increasing interdefect distance correlates with disorder-related luminescence arising 180 meV below the neutral exciton emission. We perform ab-initio density functional theory of a variety of defect related morphologies, which yield first indications on the origin of the observed additional luminescence. Remarkably, no significant reduction of free exciton valley polarization is observed until the interdefect distance approaches a few nanometers, namely the size of the free exciton Bohr radius. Our findings pave the way for direct writing of sub-10 nm nanoscale valleytronic devices and circuits using focused helium ions., Comment: 9 pages, 5 figures

We demonstrate the reduction of the inhomogeneous linewidth of the free excitons in atomically thin transition metal dichalcogenides (TMDCs) MoSe$_{2}$, WSe$_{2}$ and MoS$_{2}$ by encapsulation within few nanometer thick hBN. Encapsulation is shown to result in a significant reduction of the 10K excitonic linewidths down to $\sim3.5 \text{ meV}$ for n-MoSe$_{2}$, $\sim5.0 \text{ meV}$ for p-WSe$_{2}$ and $\sim4.8 \text{ meV}$ for n-MoS$_{2}$. Evidence is obtained that the hBN environment effectively lowers the Fermi level since the relative spectral weight shifts towards the neutral exciton emission in n-doped TMDCs and towards charged exciton emission in p-doped TMDCs. Moreover, we find that fully encapsulated MoS$_{2}$ shows resolvable exciton and trion emission even after high power density excitation in contrast to non-encapsulated materials. Our findings suggest that encapsulation of mechanically exfoliated few-monolayer TMDCs within nanometer thick hBN dramatically enhances optical quality, producing ultra-narrow linewidths that approach the homogeneous limit.

We investigate the photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe2 and WSe2 at low temperatures. Surprisingly, we find a doublet structure for such interlayer excitons. Both peaks exhibit long photoluminescence lifetimes of several ten nanoseconds up to 100 ns at low temperatures, which verifies the interlayer nature of both. The peak energy and linewidth of both show unusual temperature and power dependences. In particular, we observe a blue-shift of their emission energy for increasing excitation powers. At a low excitation power and low temperatures, the energetically higher peak shows several spikes. We explain the findings by two sorts of interlayer excitons; one that is indirect in real space but direct in reciprocal space, and the other one being indirect in both spaces. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions, Comment: 18 pages, 5 figures

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.

Photogenerated excitonic ensembles confined in coupled GaAs quantum wells are probed by a complementary approach of emission spectroscopy and resonant inelastic light scattering. Lateral electrostatic trap geometries are used to create dense systems of spatially indirect excitons and excess holes with similar densities in the order of 10$^{11}$ cm$^{-2}$. Inelastic light scattering spectra reveal a very sharp low-lying collective mode that is identified at an energy of 0.44 meV and a FWHM of only ~50 $\mu$eV. This mode is interpreted as a plasmon excitation of the excess hole system coupled to the photogenerated indirect excitons. The emission energy of the indirect excitons shifts under the application of a perpendicular applied electric field with the quantum-confined Stark effect unperturbed from the presence of free charge carriers. Our results illustrate the potential of studying low-lying collective excitations in photogenerated exciton systems to explore the many-body phase diagram, related phase transitions, and interaction physics., Comment: 19 pages, 5 figures

The investigation of 2D van der Waals (vdW) materials is a vibrant, fast moving and still growing interdisciplinary area of research. 2D vdW materials are truly 2D crystals with strong covalent in-plane bonds and weak van der Waals interaction between the layers with a variety of different electronic, optical and mechanical properties. A very prominent class of 2D materials are transition metal dichalcogenides (TMDs) and amongst them particularly the semiconducting subclass. Their properties include bandgaps in the near-infrared to the visible range, decent charge carrier mobility together with high (photo-)catalytic and mechanical stability and exotic many body phenomena. These characteristics make the materials highly attractive for both fundamental research as well as innovative device applications. Furthermore, the materials exhibit a strong light matter interaction providing a high sun light absorbance of up to 15% in the monolayer limit, strong scattering cross section in Raman experiments and access to excitonic phenomena in vdW heterostructures. This review focuses on the light matter interaction in MoS2, WS2, MoSe2, and WSe2 that is dictated by the materials complex dielectric functions and on the multiplicity of studying the first order phonon modes by Raman spectroscopy to gain access to several material properties such as doping, strain, defects and temperature. 2D materials provide an interesting platform to stack them into vdW heterostructures without the limitation of lattice mismatch resulting in novel devices for application but also to study exotic many body interaction phenomena such as interlayer excitons. Future perspectives of semiconducting TMDs and their heterostructures for applications in optoelectronic devices will be examined and routes to study emergent fundamental problems and many-body quantum phenomena under excitations with photons will be discussed., Comment: 30 pages, 11 figures

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches.

A strong coupling between the electron spin and its motion is one of the prerequisites of spin-based data storage and electronics. A major obstacle is to find spin-orbit coupled materials where the electron spin can be probed and manipulated on macroscopic length scales, for instance across the gate channel of a spin-transistor. Here, we report on millimeter-scale edge channels with a conductance quantized at a single quantum 1 $\times$ $e^2/h$ at zero magnetic field. The quantum transport is found at the lateral edges of three-dimensional topological insulators made of bismuth chalcogenides. The data are explained by a lateral, one-dimensional quantum confinement of non-topological surface states with a strong Rashba spin-orbit coupling. This edge transport can be switched on and off by an electrostatic field-effect. Our results are fundamentally different from an edge transport in quantum spin Hall insulators and quantum anomalous Hall insula-tors.