Optical harmonic generation occurs when high intensity light (>10 W m) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

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.

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties [1][2][3][4][5][6][7]. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting [8][9][10][11][12][13][14][15][16] require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons [17][18][19] in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.Owing to its large in-plane thermal conductivity, graphene has been suggested as material for the thermal management of nanoscale devices [8]. At the same time, graphene is well-known for its ability to convert incident light into electrical heat, i.e. hot electrons that can be used to generate photocurrent, with applications in photodetection, data communication and light harvesting [10,20,21]. Understanding, and ultimately controlling, heat flow in graphene-van der Waals heterostructures is therefore of paramount importance. For example, a short cooling time of graphene hot carriers is advantageous for thermal management and for high switching rates of photodetectors (PDs) for data communication, whereas a long cooling time is favorable for photodetection sensitivity [10,20,21]. Of particular relevance are heterostructure devices that contain high-quality graphene encapsulated by layered materials, such as hexagonal BN (hBN) and MoS 2 , which have the potential to crucially improve the performance of electronic and optoelectronic devices [1,2]. * Electronic address: Correspondence: klaas-jan.tielrooij@icfo.eu, frank.koppens@icfo.eu ‡ Equal contribution.A number of cooling pathways for graphene carriers have been proposed, involving among others strongly coupled optical phonons [22][23][24], acoustic phonons [25][26][27][28], substrate phonons [29] and plasmons [30] (see also Appendix 1). Here, using several experimental approaches, we show that cooling in graphene encapsulated by hBN is governed by out-of-plane coupling of graphene electrons to special polar phonon modes that occur in layered materials (LMs): hyperbolic phonon polaritons, where xx zz < 0, with xx and zz the permittivity parallel and perpendicular to the LM plane. Owing to this property, these materials carry deep sub-wavelength, raylike optical phonon polaritons. For hBN, within the two Rest...

Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron–hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton–exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive as electronic correlations are expected to greatly accelerate the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and frequency range of transient reflectivity measurements up to an unprecedented level, enabling us to directly observe the ∼16 fs build-up of the e ective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations.A fter almost 30 years of intensive experimental and theoretical efforts to understand the origin of high-temperature superconductivity in copper oxides, a consensus about the microscopic process responsible for the superconducting pairing is still lacking. The large Coulomb repulsion U 1 eV between two electrons occupying the same lattice site is believed to have fundamental consequences for the normal state of these systems 1 , and it is not clear whether a BCS-like bosonic glue that mediates the electron interactions and eventually leads to pairing can still be defined [2][3][4] . The fundamental issue can be reduced to the question whether the electronic interactions are essentially unmediated and instantaneous, or whether the low-energy physics, including superconductivity, can be effectively described in terms of interactions among the fermionic charge carriers mediated by the exchange of bosons. The problem can be rationalized by considering the Hubbard model, in which the instantaneous virtual hopping of holes into already occupied sites (with an energy cost of U ) inherently favours an antiferromagnetic (AF) coupling J = 4t 2 h /U between neighbouring sites, where t h is the nearestneighbour hopping energy. As a consequence, antiferromagnetic fluctuations with a high-energy cutoff of 2J U naturally emerge as a candidate 5 for mediating the low-energy electronic interactions, on a characteristic retarded timescale of the order ofh/2J .In principle, time-resolved optical spectroscopy 6 may be used to prove the existence of an effective retarded boson-mediated interaction, provided that the temporal resolution is of the order of the inverse bosonic-fluctuation scale (for example,h/2J for AF fluctuations) and the optical properties are probed over a sufficiently broad frequency range, to extract the dynamics of the electron-boson coupling. Recent advances in ultrafast optical spectroscopy have succeeded in separately fulfilling these requirements. For example, high-temporal-resolution (<15 fs) experiments 7,8 have been carried out to investigate the...

We investigate the role of excess excitation energy on the nature of photoexcitations in donor-acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b']dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor-acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics.

Layered hybrid metal-halide perovskites with non-centrosymmetric crystal structure are predicted to show spin-selective band splitting from Rashba effects. Thus, fabrication of metal-halide perovskites with defined crystal symmetry is desired to control the spin-splitting in their electronic states. Here, we report the influence of halogen parasubstituents on the crystal structure of benzylammonium lead iodide perovskites (4-XC 6 H 4 CH 2 NH 3 ) 2 PbI 4 (X = H, F, Cl, Br). Using X-ray diffraction and second-harmonic generation, we study structure and symmetry of single crystal and thin film samples. We report that introduction of a halogen atom lowers the crystal symmetry such that the chlorine-and bromine-substituted structures are non-centrosymmetric. The differences can be attributed to the nature of the intermolecular interactions between the organic molecules. We calculate electronic band structures and find good control of Rashba splittings. Our results present a facile approach to tailor hybrid layered metal halide perovskites with potential for spintronic and non-linear optical applications.

In monolayer (1L) transition metal dichalcogenides (TMDs) the valence and conduction bands are spin-split because of the strong spin−orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark excitons, consisting of electrons and holes with opposite spin orientation, have lower energy than A excitons. The transition from bright to dark excitons involves the scattering of electrons from the upper to the lower conduction band at the K point of the Brillouin zone, with detrimental effects for the optoelectronic response of 1L-TMDs, since this reduces their light emission efficiency. Here, we exploit the valley selective optical selection rules and use two-color helicity-resolved pump−probe spectroscopy to directly measure the intravalley spin−flip relaxation dynamics in 1L-WS 2 . This occurs on a sub-ps time scale, and it is significantly dependent on temperature, indicative of phonon-assisted relaxation. Time-dependent ab initio calculations show that intravalley spin−flip scattering occurs on significantly longer time scales only at the K point, while the occupation of states away from the minimum of the conduction band significantly reduces the scattering time. Our results shed light on the scattering processes determining the light emission efficiency in optoelectronic and photonic devices based on 1L-TMDs.