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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.

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.

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

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

Intrinsic and extrinsic disorder from lattice imperfections, substrate and environment has a strong effect on the local electronic structure and hence the optical properties of atomically thin transition metal dichalcogenides that are determined by strong Coulomb interaction. Here, we examine the role of the substrate material and intrinsic defects in monolayer MoS2 crystals on SiO2 and hBN substrates using a combination of scanning tunneling spectroscopy, scanning tunneling microscopy, optical absorbance, and low-temperature photoluminescence measurements. We find that the different substrates significantly impact the optical properties and the local density of states near the conduction band edge observed in tunneling spectra. While the SiO2 substrates induce a large background doping with electrons and a substantial amount of band tail states near the conduction band edge of MoS2, such states as well as the high doping density are absent using high quality hBN substrates. By accounting for the substrate effects we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about 480 meV. We identify several intrinsic lattice defects that are ubiquitious in MoS2, but we find that on hBN substrates the impact of these defects appears to be passivated. We conclude that the choice of substrate controls both the effects of intrinsic defects and extrinsic disorder, and thus the electronic and optical properties of MoS2. The correlation of substrate induced disorder and defects on the electronic and optical properties of MoS2 contributes to an in-depth understanding of the role of the substrates on the performance of 2D materials and will help to further improve the properties of 2D materials based quantum nanosystems., Comment: 23 pages, 4 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

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.

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.

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

We study laterally trapped dipolar exciton ensembles in coupled GaAs quantum wells at high magnetic fields in the Faraday configuration. In photoluminescence experiments, we identify three magnetic field regimes. At low fields, the exciton density is increased by a reduced charge carrier escape from the trap, and additionally, the excitons' emission energy is corrected by a positive diamagnetic shift. At intermediate fields, magnetic field dependent correction terms apply which follow the characteristics of a neutral magnetoexciton. Due to a combined effect of an increasing binding energy and lifetime, the exciton density is roughly doubled from zero to about seven Tesla. At the latter high field value, the charge carriers occupy only the lowest Landau level. In this situation, the exciton trap can overflow independently from the electrostatic depth of the trapping potential, and the energy shift of the excitons caused by the so-called quantum confined Stark effect is effectively compensated. Instead, the exciton energetics seem to be driven by the magnetic field dependent renormalization of the many-body interaction terms. In this regime, the impact of parasitic in-plane fields at the edge of trapping potential is eliminated., Comment: 18 pages, 4 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.

Micromechanically exfoliated mono- and multilayers of molybdenum disulfide (MoS2) are investigated by spectroscopic imaging ellipsometry. In combination with knife edge illumination, MoS2 flakes can be detected and classified on arbitrary flat and also transparent substrates with a lateral resolution down to 1 to 2 um. The complex dielectric functions from mono- and trilayer MoS2 are presented. They are extracted from a multilayer model to fit the measured ellipsometric angles employing an anisotropic and an isotropic fit approach. We find that the energies of the critical points of the optical constants can be treated to be independent of the utilized model, whereas the magnitude of the optical constants varies with the used model. The anisotropic model suggests a maximum absorbance for a MoS2 sheet supported by sapphire of about 14 % for monolayer and of 10 % for trilayer MoS2. Furthermore, the lateral homogeneity of the complex dielectric function for monolayer MoS2 is investigated with a spatial resolution of 2 um. Only minor fluctuations are observed. No evidence for strain, for a significant amount of disorder or lattice defects can be found in the wrinkle-free regions of the MoS2 monolayer from complementary Raman spectroscopy measurements. We assume that the minor lateral variation in the optical constants are caused by lateral modification in the van der Waals interaction presumably caused by the preparation using micromechanical exfoliation and viscoelastic stamping., Comment: Manuscript and Supplementary Material

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs-core-shell-nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photo-thermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

We investigate the optoelectronic properties of single indium arsenide nanowires, which are grown vertically on p-doped silicon substrates. We apply a scanning photocurrent microscopy to study the optoelectronic properties of the single heterojunctions. The measured photocurrent characteristics are consistent with an excess charge carrier transport through mid-gap trap states, which form at the Si/InAs heterojunctions. Namely, the trap states add an additional transport path across a heterojunction, and the charge of the defects changes the band bending at the junction. The bending gives rise to a photovoltaic effect at a small bias voltage. In addition, we observe a photoconductance effect within the InAs nanowires at large biases.

We describe a photogating effect in mono- and few-layer MoS2, which allows the control of the charge carrier density by almost two orders of magnitude without electrical contacts. Our Raman studies are consistent with physisorbed environmental molecules, that effectively deplete the intrinsically n-doped charge carrier system via charge transfer, and which can be gradually removed by the exposure to light. This photogating process is reversible and precisely tunable by the light intensity. The photogating efficiency is quantified by comparison with measurements on electrostatically gated MoS2.

To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAs-nanowire-based circuits, an on-chip photocurrent pump-probe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photo-thermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAs-nanowires, which is interpreted in terms of a dominating photo-Dember effect. The results are relevant for nanowire-based optoelectronic and photovoltaic applications as well as for the design of nanowire-based THz sources.

InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps.

In recent years, a class of solid state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We re-veal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators., Comment: 19 pages, 4 figures, supplementary information

We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK., Comment: 5 pages, 3 figures

Nonradiative transfer processes are often regarded as loss channels for an optical emitter1, since they are inherently difficult to be experimentally accessed. Recently, it has been shown that emitters, such as fluorophores and nitrogen vacancy centers in diamond, can exhibit a strong nonradiative energy transfer to graphene. So far, the energy of the transferred electronic excitations has been considered to be lost within the electron bath of the graphene. Here, we demonstrate that the trans-ferred excitations can be read-out by detecting corresponding currents with picosecond time resolution. We electrically detect the spin of nitrogen vacancy centers in diamond electronically and con-trol the nonradiative transfer to graphene by electron spin resonance. Our results open the avenue for incorporating nitrogen vacancy centers as spin qubits into ultrafast electronic circuits and for harvesting non-radiative transfer processes electronically.

We report on the photoconductance in two-dimensional arrays of gold nanorods which is strongly enhanced at the frequency of the longitudinal surface plasmon of the nanorods. The arrays are formed by a combination of droplet deposition and stamping of gold nanorod solutions on SiO2 substrates. We find that the plasmon induced photoconductance is sensitive to the linear polarization of the exciting photons. We interpret the occurrence of the photoconductance as a bolometric enhancement of the arrays' conductance upon excitation of the longitudinal surface plasmon resonance of the nanorods., Comment: 15 pages, 3 figures

Undoped diamond, a remarkable bulk electrical insulator, exhibits a high surface conductivity in air when the surface is hydrogen-terminated. Although theoretical models have claimed that a two-dimensional hole gas is established as a result of surface energy band bending, no definitive experimental demonstration has been reported so far. Here, we prove the two-dimensional character of the surface conductivity by low temperature characterization of diamond in-plane gated field-effect transistors that enable the lateral confinement of the transistor's drain-source channel to nanometer dimensions. In these devices, we observe Coulomb blockade effects of multiple quantum islands varying in size with the gate voltage. The charging energy and thus the size of these zero-dimensional islands exhibits a gate voltage dependence which is the direct result of the two-dimensional character of the conductive channel formed at hydrogen-terminated diamond surfaces.