Exciton polaritons based on atomically thin semiconductors are essential building blocks of quantum optoelectronic devices. Their properties are governed by an ultrafast and oscillatory energy transfer between their excitonic and photonic constituents, resulting in the formation of polaritonic quasiparticles with pronounced nonlinearities induced by the excitonic component. In metallic, plasmonic nanoresonators, dissipation phenomena limit the polariton lifetime to a few tens of femtoseconds, so short that the role of these polaritons for the nonlinear response of such hybrids is yet unexplored. Here we use ultrafast two-dimensional electronic spectroscopy (2DES) to uncover coherent polariton dynamics in a hybrid monolayer (1L) WS2/plasmonic nanostructure. With respect to an uncoupled WS2 flake, we observe an over 20-fold, polarization-dependent enhancement of the optical nonlinearity and a rapid evolution of the 2DES spectra within 70 fs. We relate these dynamics to a transition from coherent polaritons to incoherent excitations, unravel the microscopic origin of the optical nonlinearities and show the potential of coherent polaritons for ultrafast all-optical switching. Strongly enhanced optical nonlinearities and an ultrafast transition from coherent to incoherent polariton excitations are demonstrated by coupling an atomically thin WS2 layer to a plasmonic nanostructure using ultrafast multidimensional spectroscopy.

Laser processing with ultrashort pulses has previously been demonstrated as a useful mechanism for ultra-high precision fabrication of colour centre defects, such as the nitrogen vacancy in diamond 1,2. Recently spin-photon interfaces based on different colour centres in diamond, such as those involving group IV atoms, have been shown to offer particular benefits for future quantum networks3. However, new techniques for precise single-defect positioning and activation is critical for scalable device fabrication. Here we address this problem by demonstrating a two-step fabrication method for tin vacancy (SnV-) centres that uses site-controlled ion implantation followed by local femtosecond laser annealing with in-situ spectral monitoring4.

The increased demand for high-speed (terabit-per-second) wireless data transmission has driven the shift of the frequency carrier from ubiquitous radio frequency systems toward the 1-5 THz range, triggering a new interest for THz quantum cascade laser (QCL)-based free-space optical (FSO) links. As compared to standard telecom-band FSO links, platforms based on THz frequency sources are inherently robust against Rayleigh scattering. Atmospheric absorption, mainly due to water vapor, limits the achievable link distance range, but at the same time, it shifts channel security on the physical layer. THz QCL-based FSO links are reported with setups requiring cryogenic cooling, seriously limiting their development for mass applications. Here, a cryogen-free, transportable THz FSO communication system is presented relying on a directly modulated 2.83 THz QCL transmitter, hosted in a closed-cycle Stirling cryocooler, and exploiting a room-temperature graphene-based receiver, implementing a binary on-off keying modulation scheme with Manchester encoding. Power-versus-distance measurements and communication tests are performed, and propose a propagation model to extrapolate the performances of the THz link in an optimized configuration. This approach reduces complexity and costs, as compared to the state-of-the-art THz FSO links, and paves the way for the deployment of optical wireless communication systems exploiting the 1-5 THz frequency range.

We report an environmentally robust, dissipative-soliton, mode-locked Tm-doped fiber laser operating in the short-wavelength infrared region at 1876 nm, using all-polarization-maintaining (all-PM) fibers. Self-starting, mode-locked operation is enabled by a single-wall carbon nanotube (SWNT) based saturable absorber (SA). Cavity dispersion is managed by using a commercially available PM dispersion compensating fiber (DCF). A PM fiber Lyot filter enables dissipative-soliton mode locking at a central wavelength of 1876 nm. The laser generates stable pulses at a repetition rate - 19.2 MHz with an average power - 21.5 mW, corresponding to a pulse energy - 1.1 nJ. The output pulse has a duration of 4.2 ps and can be compressed down to 391 fs using a gratingbased compressor. A higher pulse energy - 3.2 nJ can be obtained with a compressed pulse width of 566 fs by varying the net cavity dispersion. To the best of our knowledge, this is the first nJ-energy-level, all-fiberized PM dissipative-soliton mode-locked Tm-doped fiber laser based on SWNT, with potential for applications in highpenetration nonlinear biomedical imaging microscopy.

Since the discovery of graphene in 2004, there has been rapid progress in research on two-dimensional (2D) material’s potential applications with significant impact. Efforts to bring 2D materials closer to industrial production, quality assessment, and standardization still need to be expanded upon. This remains challenging because sophisticated analytics techniques are required to analyze a limited amount of material with high variability due to differences in the synthesis process and the lack of a clear standard for comparing results. Modern methods can evaluate domain size, surface coverage, defects, dopants, mobility, and photoluminescence parameters. Still, it is challenging to understand the structure–property relationship due to external influences such as adsorbates and strain, which can alter results. To address this problem, the Graphene Flagship project conducted an extensive benchmarking study of monolayer molybdenum disulfide grown through chemical vapor deposition by various groups. The study employed a variety of techniques to characterize the material, including optical and transmission electron microscopy, Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, and capacitance-voltage measurements. Herein, we present the results of this comprehensive study and the correlation between various methods

Over the past two decades, 2D materials have rapidly evolved into a diverse and expanding family of material platforms. Many members of this materials class have demonstrated their potential to deliver transformative impact on fundamental research and technological applications across different fields. In this roadmap, we provide an overview of the key aspects of 2D material research and development, spanning synthesis, properties and commercial applications. We specifically present roadmaps for high impact 2D materials, including graphene and its derivatives, transition metal dichalcogenides, MXenes as well as their heterostructures and moiré systems. The discussions are organized into thematic sections covering emerging research areas (e.g., twisted electronics, moiré nano-optoelectronics, polaritronics, quantum photonics, and neuromorphic computing), breakthrough applications in key technologies (e.g., 2D transistors, energy storage, electrocatalysis, filtration and separation, thermal management, flexible electronics, sensing, electromagnetic interference shielding, and composites) and other important topics (computational discovery of novel materials, commercialization and standardization). This roadmap focuses on the current research landscape, future challenges and scientific and technological advances required to address, with the intent to provide useful references for promoting the development of 2D materials.

We report Raman forbidden layer-breathing modes (LBMs) in layered semiconductor materials (LSMs). The intensity distribution of all observed LBMs depends on layer number, incident light wavelength, and refractive index mismatch between LSM and underlying substrate. These results are understood by a Raman scattering theory via the proposed spatial interference model, where the naturally occurring optical and phonon cavities in LSMs enable spatially coherent photon-phonon coupling mediated by the corresponding one-dimensional periodic electronic states. Our work reveals the impact of spatial coherence of photon and phonon fields on phonon excitation via photon or phonon cavity engineering.

We investigate interlayer adhesion and relaxation at interfaces between graphene and hexagonal boron nitride (hBN) monolayers in van der Waals heterostructures. The adhesion potential between graphene and hBN is calculated as a function of local lattice offset using diffusion quantum Monte Carlo methods, which provide an accurate treatment of van der Waals interactions. Combining the adhesion potential with elasticity theory, we determined the relaxed structures of graphene and hBN layers at interfaces, finding no metastable structures. The adhesion potential is well described by simple Lennard-Jones pair potentials that we parametrize using our quantum Monte Carlo data. Encapsulation of graphene between near-aligned crystals of hBN gives rise to a moiré pattern whose period is determined by the misalignment angle between the hBN crystals superimposed over the moiré superlattice previously studied in graphene on an hBN substrate. We model minibands in such supermoiré superlattices and find them to be sensitive to the 180° rotation of one of the encapsulating hBN crystals. We find that monolayer and bilayer graphene placed on a bulk hBN substrate and bulk hBN/graphene/bulk hBN systems do not relax to adopt a common lattice constant. The energetic balance is much closer for free-standing monolayer graphene/hBN bilayers and hBN/graphene/hBN trilayers. The layers in an alternating stack of graphene and hBN are predicted to strain to adopt a common lattice constant, and hence, we obtain a stable three-dimensional crystal with a distinct electronic structure.

This perspective addresses the topic of harnessing the tools of artificial intelligence (AI) for boosting innovation in functional materials design and engineering as well as discovering new materials for targeted applications in energy storage, biomedicine, composites, nanoelectronics or quantum technologies. It gives a current view of experts in the field, insisting on challenges and opportunities provided by the development of large materials databases, novel schemes for implementing AI into materials production and characterization as well as progress in the quest of simulating physical and chemical properties of realistic atomic models reaching the trillion atoms scale and with near ab initio accuracy.

Spin-photon interfaces based on group-IV colour centres in diamond offer a promising platform for quantum networks. A key challenge in the field is realizing precise single-defect positioning and activation, which is crucial for scalable device fabrication. Here we address this problem by demonstrating a two-step fabrication method for tin vacancy (SnV-) centres that uses site-controlled ion implantation followed by local femtosecond laser annealing with in-situ spectral monitoring. The ion implantation is performed with sub-50 nm resolution and a dosage that is controlled from hundreds of ions down to single ions per site, limited by Poissonian statistics. Using this approach, we successfully demonstrate site-selective creation and modification of single SnV- centres. The technique opens a window onto materials tuning at the single defect level, and provides new insight into defect structures and dynamics during the annealing process. While demonstrated for SnV- centres, this versatile approach can be readily generalised to other implanted colour centres in diamond and wide-bandgap materials.

Atomistic simulations of matter, especially those that leverage first-principles (ab initio) electronic structure theory, provide a microscopic view of the world, underpinning much of our understanding of chemistry and materials science. Over the last decade or so, machine-learned force fields have transformed atomistic modeling by enabling simulations of ab initio quality over unprecedented time and length scales. However, early machine-learning (ML) force fields have largely been limited by (i) the substantial computational and human effort required to develop and validate potentials for each particular system of interest and (ii) a general lack of transferability from one chemical system to the next. Here, we show that it is possible to create a general-purpose atomistic ML model, trained on a public dataset of moderate size, that is capable of running stable molecular dynamics for a wide range of molecules and materials. We demonstrate the power of the MACE-MP-0 model—and its qualitative and at times quantitative accuracy—on a diverse set of problems in the physical sciences, including properties of solids, liquids, gases, chemical reactions, interfaces, and even the dynamics of a small protein. The model can be applied out of the box as a starting or “foundation” model for any atomistic system of interest and, when desired, can be fine-tuned on just a handful of application-specific data points to reach ab initio accuracy. Establishing that a stable force-field model can cover almost all materials changes atomistic modeling in a fundamental way: experienced users obtain reliable results much faster, and beginners face a lower barrier to entry. Foundation models thus represent a step toward democratizing the revolution in atomic-scale modeling that has been brought about by ML force fields.

The development of scalable techniques for large-area growth of layered materials unlocks technological opportunities for the implementation of devices and components suitable for on-chip integration on photonic integrated platforms. Here, terahertz (THz) photoreceivers based on large-area hexagonal boron nitride/single-layer graphene (SLG)/hexagonal boron nitride heterostructures, prepared by chemical vapor deposition and realized by means of an industrially scalable method, are reported. The photo-thermoelectric sensors are integrated on-chip with planar antennas, on-chip radio frequency circuitry, a low-pass hammer-head filter and coplanar strip lines, combining nanosecond response time and large sensitivity. Room temperature responsivities of approximate to 4 V W-1, with noise equivalent power approximate to 4 nWHz-1/2 at high (2.86 THz) frequencies are reached, in a fully frequency-scalable architecture. This paves the way for multiplexed hyperspectral THz cameras and optical communication systems.

Quantum confinement and reduced dielectric screening lead to strong excitonic effects in atomically thin transition metal dichalcogenides (TMDs). Encapsulation of TMD monolayers in hexagonal boron nitride (hBN) unveils the excitonic Rydberg series below the free particle bandgap. The nonequilibrium response and the dynamics of these higher order exciton states and their multiparticle complexes remain almost unexplored. Here we use ultrafast pump-probe optical microscopy to measure the dynamics of excited-state (2s) excitons in hBN-encapsulated monolayer WSe2. 2s excitons form through an ultrafast relaxation process from high-energy states and exhibit longer decay dynamics than ground state excitons due to their higher spatial extension. We detect light-induced formation of 2s trions with significant oscillator strength and faster decay dynamics than 2s excitons, attributed to an intra-excitonic Auger effect causing an additional decay channel. Our results shed light on the dynamics of excited state excitons in TMDs and their interactions with free carriers.

In twisted layered materials (t-LMs), an interlayer rotation can break inversion symmetry and create an interfacial array of staggered out-of-plane polarization due to AB/BA stacking registries. This symmetry breaking can also trigger the formation of edge in-plane polarizations localized along the perimeter of AB/BA regions (i.e., saddle point domains). However, a comprehensive experimental investigation of these features is still lacking. Here, we use piezo force microscopy to probe the electromechanical behavior of twisted hexagonal boron nitride (t-hBN). For parallel stacking alignment of t-hBN, we reveal very narrow (width ∼ 10 nm) saddle point in-plane polarizations, which we also measure in the antiparallel configuration. These localized polarizations can still be found on a multiply stacked t-hBN structure, determining the formation of a double moiré. Our findings imply that polarizations in t-hBN do not only point in the out-of-plane direction but also show an in-plane component, giving rise to a much more complex 3D polarization field.

Micromechanical resonators with very low mass are highly desirable for sensing and transduction applications. Layered materials (LMs) can be used to fabricate single- to few-atom thick suspended membranes, representing the ultimate limit to low mass. Transition-metal dichalcogenides (TMDs), such as WSe2, are particularly compelling because they can host spatially confined excitons in single layer (1L), potentially enabling the creation of nonclassical mechanical states and interconnects between quantum networks and processors. However, these exciting prospects have been tempered by low device yields, invasive methods for detecting resonator motion, and high mechanical damping. Here, we report the creation of mechanical resonators by suspending 1L-WSe2 across a 90 x 90 array of 2.5-mu m diameter holes with a > 75% success rate. We detect the resonator room-temperature (RT) Brownian motion and measure resonator mass to quantify contamination, using below-bandgap laser interferometry. We investigate the relation between frequency, diameter, and mechanical quality factor, which can exceed 1000 in our devices. We find the dependence agrees with the effect of dissipation dilution, highlighting the importance of reducing mechanical mode-bending. Key to this is the large-scale, high-quality arrays that make it possible to access a frequency range that surpasses previous works. Further, the ability to fabricate large numbers of 1L resonators, and the simplicity of probing their motion without electrodes or an underlying reflective substrate, facilitates previously hard-to-reach configurations, such as resonators in phononic crystals or within optical cavities.

AbstractOptical resonators are fundamental building blocks of photonic systems, enabling meta-surfaces, sensors, and transmission filters to be developed for a range of applications. Sub-wavelength size (< λ/10) resonators, including planar split-ring resonators, are at the forefront of research owing to their potential for light manipulation, sensing applications and for exploring fundamental light-matter coupling phenomena. Near-field microscopy has emerged as a valuable tool for mode imaging in sub-wavelength size terahertz (THz) frequency resonators, essential for emerging THz devices (e.g. negative index materials, magnetic mirrors, filters) and enhanced light-matter interaction phenomena. Here, we probe coherently the localized field supported by circular split ring resonators with single layer graphene (SLG) embedded in the resonator gap, by means of scattering-type scanning near-field optical microscopy (s-SNOM), using either a single-mode or a frequency comb THz quantum cascade laser (QCL), in a detectorless configuration, via self-mixing interferometry. We demonstrate deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes resolving both amplitude and phase of the supported modes, and unveiling resonant electric field enhancement in SLG, key for high harmonic generation.

Harmonic generation is a result of a strong non-linear interaction between light and matter. It is a key technology for optics, as it allows the conversion of optical signals to higher frequencies. Owing to its intrinsically large and electrically tunable non-linear optical response, graphene has been used for high harmonic generation but, until now, only at frequencies < 2 THz, and with high-power ultrafast table-top lasers or accelerator-based structures. Here, we demonstrate third harmonic generation at 9.63 THz by optically pumping single-layer graphene, coupled to a circular split ring resonator (CSRR) array, with a 3.21 THz frequency quantum cascade laser (QCL). Combined with the high graphene nonlinearity, the mode confinement provided by the optically-pumped CSRR enhances the pump power density as well as that at the third harmonic, permitting harmonic generation. This approach enables potential access to a frequency range (6-12 THz) where compact sources remain difficult to obtain, owing to the Reststrahlenband of typical III-V semiconductors.

We report an environmentally insensitive, all-fiberized, polarization-maintained (PM), self-starting, mode-locked Tm fiber laser cavity enabled by a single-wall carbon nanotube (SWNT) saturable absorber. This produces dissipative solitons at 1876 nm, with a repetition rate of 19.2 MHz, and a maximum average power of 21.5 mW, corresponding to a pulse energy of 1.1 nJ. The output pulse has a duration of 4.2 ps and can be compressed to 391 fs using a grating-based compressor. To the best of our knowledge, this is the first demonstration of an all-fiberized, all-PM, dissipative-soliton, mode-locked thulium fiber laser using a SWNT saturable absorber.

Exposure to altered gravity influences cellular behaviour in cell cultures. Hydrogels are amongst the most common materials used to produce tissue-engineering scaffolds, and their mechanical properties play a crucial role in cell-matrix interaction. However, little is known about the influence of altered gravity on hydrogel properties. Here we study the mechanical properties of Poly (ethylene glycol) diacrylate (PEGDA) and PEGDA incorporated with graphene oxide (GO) by performing tensile tests in micro and hypergravity during a Parabolic flight campaign, and by comparing them to the same tests performed in Earth gravity. We show that gravity levels do not result in a statistically significant difference in Young's modulus.

The integration of optoelectronic devices, such as transistors and photodetectors (PDs), into wearables and textiles is of great interest for applications such as healthcare and physiological monitoring. These require flexible/wearable systems adaptable to body motions, thus materials conformable to non-planar surfaces, and able to maintain performance under mechanical distortions. Here, fibre PDs are prepared by combining rolled graphene layers and photoactive perovskites. Conductive fibres (~500 Ωcm-1) are made by rolling single-layer graphene (SLG) around silica fibres, followed by deposition of a dielectric layer (Al2O3 and parylene C), another rolled SLG as a channel, and perovskite as photoactive component. The resulting gate-tunable PD has a response time~9ms, with an external responsivity~22kAW-1 at 488nm for a 1V bias. The external responsivity is two orders of magnitude higher, and the response time one order of magnitude faster, than state-of-the-art wearable fibre-based PDs. Under bending at 4mm radius, up to~80% photocurrent is maintained. Washability tests show~72% of initial photocurrent after 30 cycles, promising for wearable applications.