Ultrafast Coherent Exciton Couplings and Many-Body Interactions in Monolayer WS2.

Transition metal dichalcogenides (TMDs) are quantum confined systems with interesting optoelectronic properties, governed by Coulomb interactions in the monolayer (1L) limit, where strongly bound excitons provide a sensitive probe for many-body interactions. Here, we use two-dimensional electronic spectroscopy (2DES) to investigate many-body interactions and their dynamics in 1L-WS2 at room temperature and with sub-10 fs time resolution. Our data reveal coherent interactions between the strongly detuned A and B exciton states in 1L-WS2. Pronounced ultrafast oscillations of the transient optical response of the B exciton are the signature of a coherent 50 meV coupling and coherent population oscillations between the two exciton states. Supported by microscopic semiconductor Bloch equation simulations, these coherent dynamics are rationalized in terms of Dexter-like interactions. Our work sheds light on the role of coherent exciton couplings and many-body interactions in the ultrafast temporal evolution of spin and valley states in TMDs.

Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor.

Graphene is a unique platform for tunable opto-electronic applications thanks to its linear band dispersion, which allows electrical control of resonant light-matter interactions. Tuning the nonlinear optical response of graphene is possible both electrically and in an all-optical fashion, but each approach involves a trade-off between speed and modulation depth. Here, lattice temperature, electron doping, and all-optical tuning of third-harmonic generation are combined in a hexagonal boron nitride-encapsulated graphene opto-electronic device and demonstrate up to 85% modulation depth along with gate-tunable ultrafast dynamics. These results arise from the dynamic changes in the transient electronic temperature combined with Pauli blocking induced by the out-of-equilibrium chemical potential. The work provides a detailed description of the transient nonlinear optical and electronic response of graphene, which is crucial for the design of nanoscale and ultrafast optical modulators, detectors, and frequency converters.

High efficiency graphene-silicon hybrid-integrated thermal and electro-optical modulators.

Graphene modulators are considered a potential solution for achieving high-efficiency light modulation, and graphene-silicon hybrid-integrated modulators are particularly favorable due to their CMOS compatibility and low cost. The exploitation of graphene modulator latent capabilities remains an ongoing endeavour to improve the modulation and energy efficiency. Here, high-efficiency graphene-silicon hybrid-integrated thermal and electro-optical modulators are realized using gold-assisted transfer. We fabricate and demonstrate a microscale thermo-optical modulator with a tuning efficiency of 0.037 nm mW-1 and a high heating performance of 67.4 K µm3 mW-1 on a small active area of 7.54 µm2 and a graphene electro-absorption modulator featuring a high speed data rate reaching 56 Gb s-1 and a low power consumption of 200 fJ per bit. These devices show superior performance compared to the state of the art devices in terms of high efficiency, low process complexity, and compact device footage, which can support the realization of high-performance graphene-silicon hybrid-integrated photonic circuits with CMOS compatibility.

Ultrafast all-optical second harmonic wavefront shaping.

Optical communication can be revolutionized by encoding data into the orbital angular momentum of light beams. However, state-of-the-art approaches for dynamic control of complex optical wavefronts are mainly based on liquid crystal spatial light modulators or miniaturized mirrors, which suffer from intrinsically slow (µs-ms) response times. Here, we experimentally realize a hybrid meta-optical system that enables complex control of the wavefront of light with pulse-duration limited dynamics. Specifically, by combining ultrafast polarization switching in a WSe2 monolayer with a dielectric metasurface, we demonstrate second harmonic beam deflection and structuring of orbital angular momentum on the femtosecond timescale. Our results pave the way to robust encoding of information for free space optical links, while reaching response times compatible with real-world telecom applications.

Ultrafast Charge Transfer and Recombination Dynamics in Monolayer-Multilayer WSe2 Junctions Revealed by Time-Resolved Photoemission Electron Microscopy.

The ultrafast carrier dynamics of junctions between two chemically identical, but electronically distinct, transition metal dichalcogenides (TMDs) remains largely unknown. Here, we employ time-resolved photoemission electron microscopy (TR-PEEM) to probe the ultrafast carrier dynamics of a monolayer-to-multilayer (1L-ML) WSe2 junction. The TR-PEEM signals recorded for the individual components of the junction reveal the sub-ps carrier cooling dynamics of 1L- and 7L-WSe2, as well as few-ps exciton-exciton annihilation occurring on 1L-WSe2. We observe ultrafast interfacial hole (h) transfer from 1L- to 7L-WSe2 on an ∼0.2 ps time scale. The resultant excess h density in 7L-WSe2 decays by carrier recombination across the junction interface on an ∼100 ps time scale. Reminiscent of the behavior at a depletion region, the TR-PEEM image reveals the h density accumulation on the 7L-WSe2 interface, with a decay length ∼0.60 ± 0.17 µm. These charge transfer and recombination dynamics are in agreement with ab initio quantum dynamics. The computed orbital densities reveal that charge transfer occurs from the basal plane, which extends over both 1L and ML regions, to the upper plane localized on the ML region. This mode of charge transfer is distinctive to chemically homogeneous junctions of layered materials and constitutes an additional carrier deactivation pathway that should be considered in studies of 1L-TMDs found alongside their ML, a common occurrence in exfoliated samples.

Brightening and Directionality Control of Dark Excitons through Quasi-Bound States in the Continuum.

Thanks to their long lifetime, spin-forbidden dark excitons in transition metal dichalcogenides are promising candidates for storage applications in opto-electronics and valleytronics. To date, their study has been hindered by inefficient generation mechanisms and the necessity for elaborate detection schemes. In this work, we propose a new hybrid platform that simultaneously addresses both challenges. We study an all-dielectric metasurface with two symmetrically protected quasi-bound states in the continuum to enhance both the excitation and emission of dark excitons in a tungsten diselenide monolayer under normal light incidence. Our simulations show a giant photoluminescence signal enhancement (∼520) along with directional emission, thus offering distinct advantages for opto-electronic and valleytronic devices.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling.

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Nonlinear All-Optical Coherent Generation and Read-Out of Valleys in Atomically Thin Semiconductors.

With conventional electronics reaching performance and size boundaries, all-optical processes have emerged as ideal building blocks for high speed and low power consumption devices. A promising approach in this direction is provided by valleytronics in atomically thin semiconductors, where light-matter interaction allows to write, store, and read binary information into the two energetically degenerate but non-equivalent valleys. Here, nonlinear valleytronics in monolayer WSe2 is investigated and show that an individual ultrashort pulse with a photon energy tuned to half of the optical band-gap can be used to simultaneously excite (by coherent optical Stark shift) and detect (by a rotation in the polarization of the emitted second harmonic) the valley population.

A Monolithic Graphene-Functionalized Microlaser for Multispecies Gas Detection.

Optical-microcavity-enhanced light-matter interaction offers a powerful tool to develop fast and precise sensing techniques, spurring applications in the detection of biochemical targets ranging from cells, nanoparticles, and large molecules. However, the intrinsic inertness of such pristine microresonators limits their spread in new fields such as gas detection. Here, a functionalized microlaser sensor is realized by depositing graphene in an erbium-doped over-modal microsphere. By using a 980 nm pump, multiple laser lines excited in different mode families of the microresonator are co-generated in a single device. The interference between these splitting mode lasers produce beat notes in the electrical domain (0.2-1.1 MHz) with sub-kHz accuracy, thanks to the graphene-induced intracavity backward scattering. This allows for lab-free multispecies gas identification from a mixture, and ultrasensitive gas detection down to individual molecule.

Nonlinear co-generation of graphene plasmons for optoelectronic logic operations.

Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene's plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectronic devices. Here, we use two counter propagating frequency combs with temporally synchronized pulses to demonstrate deterministic all-optical generation and electrical control of multiple plasmon polaritons, excited via difference frequency generation (DFG). Electrical tuning of a hybrid graphene-fibre device offers a precise control over the DFG phase-matching, leading to tunable responses of the graphene's plasmons at different frequencies across a broadband (0 ~ 50 THz) and provides a powerful tool for high-speed logic operations. Our results offer insights for plasmonics on hybrid photonic devices based on layered materials and pave the way to high-speed integrated optoelectronic computing circuits.

Tuning nanowire lasers via hybridization with two-dimensional materials.

Mixed-dimensional hybrid structures have recently gained increasing attention as promising building blocks for novel electronic and optoelectronic devices. In this context, hybridization of semiconductor nanowires with two-dimensional materials could offer new ways to control and modulate lasing at the nanoscale. In this work, we deterministically fabricate hybrid mixed-dimensional heterostructures composed of ZnO nanowires and MoS2 monolayers with micrometer control over their relative position. First, we show that our deterministic fabrication method does not degrade the optical properties of the ZnO nanowires. Second, we demonstrate that the lasing wavelength of ZnO nanowires can be tuned by several nanometers by hybridization with CVD-grown MoS2 monolayers. We assign this spectral shift of the lasing modes to an efficient carrier transfer at the heterointerface and the subsequent increase of the optical band gap in ZnO (Moss-Burstein effect).

Electrically Tunable Nonequilibrium Optical Response of Graphene.

The ability to tune the optical response of a material via electrostatic gating is crucial for optoelectronic applications, such as electro-optic modulators, saturable absorbers, optical limiters, photodetectors, and transparent electrodes. The band structure of single layer graphene (SLG), with zero-gap, linearly dispersive conduction and valence bands, enables an easy control of the Fermi energy, EF, and of the threshold for interband optical absorption. Here, we report the tunability of the SLG nonequilibrium optical response in the near-infrared (1000-1700 nm/0.729-1.240 eV), exploring a range of EF from -650 to 250 meV by ionic liquid gating. As EF increases from the Dirac point to the threshold for Pauli blocking of interband absorption, we observe a slow-down of the photobleaching relaxation dynamics, which we attribute to the quenching of optical phonon emission from photoexcited charge carriers. For EF exceeding the Pauli blocking threshold, photobleaching eventually turns into photoinduced absorption, because the hot electrons' excitation increases the SLG absorption. The ability to control both recovery time and sign of the nonequilibrium optical response by electrostatic gating makes SLG ideal for tunable saturable absorbers with controlled dynamics.

Multispecies and individual gas molecule detection using Stokes solitons in a graphene over-modal microresonator.

Soliton frequency combs generate equally-distant frequencies, offering a powerful tool for fast and accurate measurements over broad spectral ranges. The generation of solitons in microresonators can further improve the compactness of comb sources. However the geometry and the material's inertness of pristine microresonators limit their potential in applications such as gas molecule detection. Here, we realize a two-dimensional-material functionalized microcomb sensor by asymmetrically depositing graphene in an over-modal microsphere. By using one single pump, spectrally trapped Stokes solitons belonging to distinct transverse mode families are co-generated in one single device. Such Stokes solitons with locked repetition rate but different offsets produce ultrasensitive beat notes in the electrical domain, offering unique advantages for selective and individual gas molecule detection. Moreover, the stable nature of the solitons enables us to trace the frequency shift of the dual-soliton beat-note with uncertainty <0.2 Hz and to achieve real-time individual gas molecule detection in vacuum, via an optoelectronic heterodyne detection scheme. This combination of atomically thin materials and microcombs shows the potential for compact photonic sensing with high performances and offers insights toward the design of versatile functionalized microcavity photonic devices.

Hot carriers in graphene - fundamentals and applications.

Hot charge carriers in graphene exhibit fascinating physical phenomena, whose understanding has improved greatly over the past decade. They have distinctly different physical properties compared to, for example, hot carriers in conventional metals. This is predominantly the result of graphene's linear energy-momentum dispersion, its phonon properties, its all-interface character, and the tunability of its carrier density down to very small values, and from electron- to hole-doping. Since a few years, we have witnessed an increasing interest in technological applications enabled by hot carriers in graphene. Of particular interest are optical and optoelectronic applications, where hot carriers are used to detect (photodetection), convert (nonlinear photonics), or emit (luminescence) light. Graphene-enabled systems in these application areas could find widespread use and have a disruptive impact, for example in the field of data communication, high-frequency electronics, and industrial quality control. The aim of this review is to provide an overview of the most relevant physics and working principles that are relevant for applications exploiting hot carriers in graphene.

Low-Loss Integrated Nanophotonic Circuits with Layered Semiconductor Materials.

Monolayer transition-metal dichalcogenides with direct bandgaps are emerging candidates for optoelectronic devices, such as photodetectors, light-emitting diodes, and electro-optic modulators. Here we report a low-loss integrated platform incorporating molybdenum ditelluride monolayers with silicon nitride photonic microresonators. We achieve microresonator quality factors >3 × 106 in the telecommunication O- to E-bands. This paves the way for low-loss, hybrid photonic integrated circuits with layered semiconductors, not requiring heterogeneous wafer bonding.

Exciton-phonon coupling strength in single-layer MoSe2 at room temperature.

Single-layer transition metal dichalcogenides are at the center of an ever increasing research effort both in terms of fundamental physics and applications. Exciton-phonon coupling plays a key role in determining the (opto)electronic properties of these materials. However, the exciton-phonon coupling strength has not been measured at room temperature. Here, we use two-dimensional micro-spectroscopy to determine exciton-phonon coupling of single-layer MoSe2. We detect beating signals as a function of waiting time induced by the coupling between A excitons and A'1 optical phonons. Analysis of beating maps combined with simulations provides the exciton-phonon coupling. We get a Huang-Rhys factor ~1, larger than in most other inorganic semiconductor nanostructures. Our technique offers a unique tool to measure exciton-phonon coupling also in other heterogeneous semiconducting systems, with a spatial resolution ~260 nm, and provides design-relevant parameters for the development of optoelectronic devices.

Guided Assembly and Patterning of Intrinsically Fluorescent Amyloid Fibers with Long-Range Order.

Fibrillar amyloids exhibit a fascinating range of mechanical, optical, and electronic properties originating from their characteristic ß-sheet-rich structure. Harnessing these functionalities in practical applications has so far been hampered by a limited ability to control the amyloid self-assembly process at the macroscopic scale. Here, we use core-shell electrospinning with microconfinement to assemble amyloid-hybrid fibers, consisting of densely aggregated fibrillar amyloids stabilized by a polymer shell. Up to centimeter-long hybrid fibers with micrometer diameter can be arranged into aligned and ordered arrays and deposited onto substrates or produced as free-standing networks. Properties that are characteristic of amyloids, including their high elastic moduli and intrinsic fluorescence signature, are retained in the hybrid fiber cores, and we show that they fully persist through the macroscopic fiber patterns. Our findings suggest that microlevel confinement is key for the guided assembly of amyloids from monomeric proteins.

Weak Distance Dependence of Hot-Electron-Transfer Rates at the Interface between Monolayer MoS2 and Gold.

Electron transport across the transition-metal dichalcogenide (TMD)/metal interface plays an important role in determining the performance of TMD-based optoelectronic devices. However, the robustness of this process against structural heterogeneities remains unexplored, to the best of our knowledge. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and atomic force microscopy to investigate the spatially resolved hot-electron-transfer dynamics at the monolayer (1L) MoS2/Au interface. A spatially heterogeneous distribution of 1L-MoS2/Au gap distances, along with the sub-80 nm spatial- and sub-60 fs temporal resolution of TR-PEEM, permits the simultaneous measurement of electron-transfer rates across a range of 1L-MoS2/Au distances. These decay exponentially as a function of distance, with an attenuation coefficient ß âˆ¼ 0.06 ± 0.01 Å-1, comparable to molecular wires. Ab initio simulations suggest that surface plasmon-like states mediate hot-electron-transfer, hence accounting for its weak distance dependence. The weak distance dependence of the interfacial hot-electron-transfer rate indicates that this process is insensitive to distance fluctuations at the TMD/metal interface, thus motivating further exploration of optoelectronic devices based on hot carriers.

Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures.

Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2 . However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room-temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.

Electrically Tunable Four-Wave-Mixing in Graphene Heterogeneous Fiber for Individual Gas Molecule Detection.

Detection of individual molecules is the ultimate goal of any chemical sensor. In the case of gas detection, such resolution has been achieved in advanced nanoscale electronic solid-state sensors, but it has not been possible so far in integrated photonic devices, where the weak light-molecule interaction is typically hidden by noise. Here, we demonstrate a scheme to generate ultrasensitive down-conversion four-wave-mixing (FWM) in a graphene bipolar-junction-transistor heterogeneous D-shaped fiber. In the communication band, the FWM conversion efficiency can change steeply when the graphene Fermi level approaches 0.4 eV. In this condition, we exploit our unique two-step optoelectronic heterodyne detection scheme, and we achieve real-time individual gas molecule detection in vacuum. Such combination of graphene strong nonlinearities, electrical tunability, and all-fiber integration paves the way toward the design of versatile high-performance graphene photonic devices.