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The photoexcitation life cycle from incident photon (and creation of photoexcited electron–hole pair) to ultimate extraction of electrical current is a complex multiphysics process spanning across a range of spatiotemporal scales of quantum materials. Photocurrent is sensitive to a myriad of physical processes across these spatiotemporal scales, and over the past decade it has emerged as a versatile probe of electronic states, Bloch band quantum geometry, quantum kinetic processes and device characteristics of quantum materials. This Technical Review outlines the key multiphysics principles of photocurrent diagnostics, for resolving band structure and characterizing topological materials, for disentangling distinct types of carrier scattering that can range from femtosecond to nanosecond timescales and for enabling new types of remote-sensing protocols and photocurrent nanoscopy. These distinctive capabilities underscore photocurrent diagnostics as a novel multiphysics probe for a growing class of quantum materials with properties governed by physics spanning multiple spatiotemporal scales. Ministry of Education (MOE) Q.M. was supported through NSF Career DMR-2143426 and the CIFAR Azrieli Global Scholars programme. R.K.K. acknowledges the EU Horizon 2020 programme under the MarieSkłodowska-Curie grant numbers 754510 and 893030. S.-Y.X. was supported through NSF Career (Harvard fund 129522) DMR-2143177. F.H.L.K. acknowledges support from the ERC TOPONANOP (726001), the Government of Spain (PID2019-106875GB-I00; Severo Ochoa CEX2019-000910-S (MCIN/AEI/10.13039/501100011033)), Fundació Cellex, Fundació Mir-Puig and Generalitat de Catalunya (CERCA, AGAUR, SGR 1656). Furthermore, the research leading to these results has received funding from the European Union’s Horizon 2020 under grant agreement numbers 881603 (Graphene flagship Core3) and 820378 (Quantum flagship). J.C.W.S. acknowledges support from the Singapore MOE Academic Research Fund Tier 3 Grant number MOE2018-T3-1-002.

Photonic integrated circuits (PICs) for next-generation optical communication interconnects and all-optical signal processing require efficient (∼A/W) and fast (≥25 Gbs–1) light detection at low (–1) power consumption, in devices compatible with Si processing, so that the monolithic integration of electro-optical materials and electronics can be achieved consistently at the wafer scale. Graphene-based photodetectors can meet these criteria, thanks to their broadband absorption, ultra-high mobility, ultra-fast electron interactions, and strong photothermoelectric effect. High responsivities (∼ 1 A/W), however, have only been demonstrated in biased configurations, which introduce dark current, noise, and power consumption, while unbiased schemes, with low noise and zero consumption, have remained in the ∼ 0.1 A/W regime. Here, we consider the unbiased asymmetric configuration and show that optimized plasmonic enhanced devices can reach for both transverse-electric and transverse-magnetic modes (at λ = 1550 nm), ∼A/W responsivity, and ∼ 100 GHz operation speed at zero power consumption. We validate the model and material parameters by simulating experimental devices and derive analytical expressions for the responsivity. Our comprehensive modeling paves the way for efficient, fast, and versatile optical detection in PICs with zero power consumption.

Imaging the vibrations of nanomechanical resonators means measuring their flexural mode shapes from the dependence of their frequency response on in-plane position. Applied to two-dimensional resonators, this technique provides a wealth of information on the mechanical properties of atomically-thin membranes. We present a simple and robust system to image the vibrations of few layer graphene (FLG) resonators at room temperature and in vacuum with an in-plane displacement precision of ≈0.20 μm. It consists of a sturdy vacuum enclosure mounted on a three-axis micropositioning stage and designed for free space optical measurements of vibrations. The system is equipped with ultraflexible radio frequency transmission lines to electrically actuate resonators. With it we characterize the lowest frequency mode of FLG resonators by measuring its frequency response as a function of position on the membrane and by extracting its effective mass. We use the background noise of the undriven vibrational spectrum to calibrate in-plane displacement. Finally, we measure the first three vibration modes of a resonator whose membrane is partially folded and find that folds locally suppress vibrations.

The scalable synthesis and transfer of large-area graphene underpins the development of nanoscale photonic devices ideal for new applications in a variety of fields, ranging from biotechnology, to wearable sensors for healthcare and motion detection, to quantum transport, communications, and metrology. We report room-temperature zero-bias thermoelectric photodetectors, based on single- and polycrystal graphene grown by chemical vapor deposition (CVD), tunable over the whole terahertz range (0.1-10 THz) by selecting the resonance of an on-chip patterned nanoantenna. Efficient light detection with noise equivalent powers

The single-particle and many-body properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, particularly when the two layers are rotated relative to each other by a small angle (θ ≈ 1°), owing to the moire potential induced by the twist. Here we probe the collective excitations of TBG with a spatial resolution of 20 nm, by applying mid-infrared near-field optical microscopy. We find a propagating plasmon mode in charge-neutral TBG for θ = 1.1−1.7°, which is different from the intraband plasmon in single-layer graphene. We interpret it as an interband plasmon associated with the optical transitions between minibands originating from the moire superlattice. The details of the plasmon dispersion are directly related to the motion of electrons in the moire superlattice and offer an insight into the physical properties of TBG, such as band nesting between the flat band and remote band, local interlayer coupling, and losses. We find a strongly reduced interlayer coupling in the regions with AA stacking, pointing at screening due to electron–electron interactions. Optical nano-imaging of TBG allows the spatial probing of interaction effects at the nanoscale and potentially elucidates the contribution of collective excitations to many-body ground states. Moire potentials substantially alter the electronic properties of twisted bilayer graphene at a magic twist angle. A propagating plasmon mode, which can be observed with optical nano-imaging, is associated with transitions between the moire minibands.

Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges1–13. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge14–22. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm2 s−1, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm2 s−1, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management., Spatiotemporal thermoelectric microscopy enables the observation of electronic heat flow in graphene in diffusive and hydrodynamic regimes at room temperature, as well as a controlled transition from a Fermi liquid to Dirac fluid.

Independent control of carrier density and out-of-plane displacement field is essential for accessing novel phenomena in two-dimensional (2D) material heterostructures. While this is achieved with independent top and bottom metallic gate electrodes in transport experiments, it remains a challenge for near-field optical studies as the top electrode interferes with the optical path. Here, we characterize the requirements for a material to be used as the top-gate electrode and demonstrate experimentally that few-layer WSe

Imaging polariton dynamics Two-dimensional (2D) materials can confine light to volumes much shorter than the wavelength, and, together, the long propagation lengths make them attractive materials for developing nanophotonic platforms. Characterizing the spatiotemporal control of 2D polariton wave packets has been hindered for the same reasons that make their potential applications exciting: They have extremely small wavelengths and are strongly confined inside the material. Kurman et al. developed a new pump-probe technique based on electron emission that provides access to the spatiotemporal dynamics of 2D polaritons. The nanometric spatial resolution and femtosecond temporal resolution will be useful for probing the excitation dynamics of these materials. Science , abg9015, this issue p. 1181

A quantitative understanding of the electromagnetic response of materials is essential for the precise engineering of maximal, versatile, and controllable light-matter interactions. Material surfaces, in particular, are prominent platforms for enhancing electromagnetic interactions and for tailoring chemical processes. However, at the deep nanoscale, the electromagnetic response of electron systems is significantly impacted by quantum surface-response at material interfaces, which is challenging to probe using standard optical techniques. Here, we show how ultraconfined acoustic graphene plasmons in graphene-dielectric-metal structures can be used to probe the quantum surface-response functions of nearby metals, here encoded through the so-called Feibelman d-parameters. Based on our theoretical formalism, we introduce a concrete proposal for experimentally inferring the low-frequency quantum response of metals from quantum shifts of the acoustic graphene plasmons dispersion, and demonstrate that the high field confinement of acoustic graphene plasmons can resolve intrinsically quantum mechanical electronic length-scales with subnanometer resolution. Our findings reveal a promising scheme to probe the quantum response of metals, and further suggest the utilization of acoustic graphene plasmons as plasmon rulers with angstrom-scale accuracy. Knowledge of the quantum response of materials is essential for designing light-matter interactions at the nanoscale. Here, the authors report a theory for understanding the impact of metallic quantum response on acoustic graphene plasmons and how such response could be inferred from measurements., N.A.M. is a VILLUM Investigator supported by VILLUM FONDEN (Grant No. 16498) and Independent Research Fund Denmark (Grant No. 7026-00117B). The Center for Nano Optics is financially supported by the University of Southern Denmark (SDU 2020 funding). The Center for Nanostructured Graphene (CNG) is sponsored by the Danish National Research Foundation (Project No. DNRF103). This work was partly supported by the Army Research Office through the Institute for Soldier Nanotechnologies under Contract No. W911NF-18-2-0048. N.M.R.P. acknowledges support from the European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (No. 881603, Core 3), COMPETE 2020, PORTUGAL 2020, FEDER and the Portuguese Foundation for Science and Technology (FCT) through project POCI-01-0145-FEDER028114 and through the framework of the Strategic Financing UID/FIS/04650/2019. F.H. L.K. acknowledges financial support from the Government of Catalonia through the SGR grant and from the Spanish Ministry of Economy and Competitiveness (MINECO) through the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-20150522), support by Fundacio Cellex Barcelona, Generalitat de Catalunya through the CERCA program, and the MINECO grants Plan Nacional (FIS2016-81044-P) and the Agency for Management of University and Research Grants (AGAUR) 2017 SGR 1656. Furthermore, the research leading to these results has received funding from the European Union's Horizon 2020 program under the Graphene Flagship Grant Agreements No. 785219 (Core 2) and no. 881603 (Core 3), and the Quantum Flagship Grant No. 820378. This work was also supported by the ERC TOPONANOP (Grant No. 726001).

Quantized vortices are topological defects found in different two-dimensional geometries, from liquid crystals to ferromagnets, famously involved in spontaneous symmetry breaking and phase transitions. Their optical counterparts appear in planar geometries as a universal wave phenomenon, possessing topologically protected orbital angular momentum (OAM). So far, the spatiotemporal dynamics of optical vortices, including vortex-pair creation and annihilation, has been observed only in Bose–Einstein condensates. Here we observe optical vortices in van der Waals materials and measure their dynamics, including events of pair creation and annihilation. Vortices of opposite OAM are involved in pair creation/annihilation events, and their relative signs determine the surrounding field profile throughout their motion. The vortices are made of phonon polaritons in hexagonal boron nitride, which we directly probe using free electrons in an ultrafast transmission electron microscope. Our findings promote future investigations of vortex phenomena in van der Waals platforms, toward their use for chiral plasmonics, quantum simulators, and control over selection rules in light–matter interactions.

Integrating and manipulating the nano-optoelectronic properties of Van der Waals heterostructures can enable unprecedented platforms for photodetection and sensing. The main challenge of infrared photodetectors is to funnel the light into a small nanoscale active area and efficiently convert it into an electrical signal. Here, we overcome all of those challenges in one device, by efficient coupling of a plasmonic antenna to hyperbolic phonon-polaritons in hexagonal-BN to highly concentrate mid-infrared light into a graphene pn-junction. We balance the interplay of the absorption, electrical and thermal conductivity of graphene via the device geometry. This approach yields remarkable device performance featuring room temperature high sensitivity (NEP of 82 pW/Hz−−−√) and fast rise time of 17 nanoseconds (setup-limited), among others, hence achieving a combination currently not present in the state-of-the-art graphene and commercial mid-infrared detectors. We also develop a multiphysics model that shows very good quantitative agreement with our experimental results and reveals the different contributions to our photoresponse, thus paving the way for further improvement of these types of photodetectors even beyond mid-infrared range., F.H.L.K. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R& D (SEV-2015-0522), support by Fundacio Cellex Barcelona, Generalitat de Catalunya through the CERCA program, and the Agency for Management of University and Research Grants (AGAUR) 2017 SGR 1656. Furthermore, the research leading to these results has received funding from the European Union Seventh Framework Programme under grant agreement no. 785219 and no. 881603 Graphene Flagship for Core2 and Core3. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). K.J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 804349. R.H. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (national project RTI2018-094830-B-100 and the project MDM-2016-0618 of the Marie de Maeztu Units of Excellence Program) and the Basque Government (grant No. IT1164-19). S.C. acknowledges financial support from the Barcelona Institute of Science and Technology (BIST), the Secretaria d’Universitats i Recerca del Departament d’Empresa i Coneixement de la Generalitat de Catalunya and the European Social Fund (L’FSE inverteix en el teu futur)—FEDER. D.E. acknowledges partial support from the Army Research Office MURI “Ab-Initio Solid-State Quantum Materials” Grant No. W911NF18-1-0431. J.G. was supported by the ARL-MIT Institute for Soldier Nanotechnologies (ISN). T.S. and L.M.M. acknowledge support by Spain’s MINECO under Grant No. MAT2017-88358-C3-1-R and the Aragon Government through project Q-MAD.

When graphene is placed in the near vicinity of a metal substrate, graphene plasmons are screened by the metal thereby giving rise to acoustic graphene plasmons. These exhibit record-high field confinement, squeezing light down to nanometer dimensions. Hence, plasmons in such heterostructures make ideal candidates to probe the quantum nonlocal electrodynamic response of the nearby metal. We treat graphene at the level of the RPA and describe the nonclassical optical response of the metal using a framework of mesoscopic electrodynamics based on microscopic surface-response functions, known as Feibelman d-parameters, which embody quantum corrections in the metal’s response. We show that the graphene plasmons’ resonances exhibit quantum shifts due to the quantum surface-response of the metal, and show how these spectral shifts can be used to interrogate the quantum surface-response of metals, thus provide a theoretical basis for experimentally inferring the said quantum surface-response.

The development of silicon semiconductor technology has produced breakthroughs in electronics—from the microprocessor in the late 1960s to early 1970s, to automation, computers and smartphones—by downscaling the physical size of devices and wires to the nanometre regime. Now, graphene and related two-dimensional (2D) materials offer prospects of unprecedented advances in device performance at the atomic limit, and a synergistic combination of 2D materials with silicon chips promises a heterogeneous platform to deliver massively enhanced potential based on silicon technology. Integration is achieved via three-dimensional monolithic construction of multifunctional high-rise 2D silicon chips, enabling enhanced performance by exploiting the vertical direction and the functional diversification of the silicon platform for applications in opto-electronics and sensing. Here we review the opportunities, progress and challenges of integrating atomically thin materials with silicon-based nanosystems, and also consider the prospects for computational and non-computational applications. Progress in integrating atomically thin two-dimensional materials with silicon-based technology is reviewed, together with the associated opportunities and challenges, and a roadmap for future applications is presented.

We report high room-temperature mobility in single layer graphene grown by Chemical Vapor Deposition (CVD) after wet transfer on SiO$_2$ and hexagonal boron nitride (hBN) encapsulation. By removing contaminations trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to$\sim70000cm^2 V^{-1} s^{-1}$ at room temperature and$\sim120000cm^2 V^{-1} s^{-1}$ at 9K. These are over twice those of previous wet transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room temperature mobilities$\sim30000 cm^2 V^{-1} s^{-1}$. These results show that, with appropriate encapsulation and cleaning, room temperature mobilities well above $10000cm^2 V^{-1} s^{-1}$ can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

The ability to concentrate light into nanometric volumes enables access to an ultrastrong light-matter coupling regime. Graphene-based platforms are extremely appealing for achieving this goal. Indeed, this unique 2D material possesses a strong third-order nonlinear response, which, thanks to the intense near-fields associated with the excitation of surface plasmons, can be made substantial even at the single-photon level [1] , [2].

Single organic molecules have recently seen increased interest for use as single photon sources [1] . They emit photons with high efficiency and at favourable wavelengths for coupling to other quantum systems. While the excitation of molecules and their subsequent radiative emission is efficient [2] , the generated photons can be difficult to efficiently collect. There is therefore a large amount of ongoing work on coupling organic molecules to nanophotonic structures to modify their emission. Evanescent coupling to nanophotonic [3] , [4] and hybrid plasmonic [5] waveguides has shown promise but has limitations; the molecules must be very close to the waveguide to be in the evanescent field of the guided mode which can cause the molecules to become unstable. Here I will present our recent work on coupling organic molecules to interrupted waveguides using on chip micro-capillaries [6].

Acoustic-graphene-plasmons (AGPs) are highly confined electromagnetic modes, which carry extreme momentum and low loss in the Mid-infrared (MIR) to Terahertz (THz) spectra. In this work [1], we demonstrate a new way to excite AGPs from the far-field, which are confined to nanometric-scale cavities, via localized graphene-plasmon-magnetic-resonators (GPMRs). This approach enables the efficient excitation of single AGP cavities, which are able to confine MIR light to record-breaking ultra-small mode-volumes that are ∼5·1010 times smaller than their free-space volume.

We show that the Higgs mode of a superconductor, which is usually challenging to observe by far-field optics, can be made clearly visible using near-field optics by harnessing ultraconfined graphene plasmons. As near-field sources we investigate two examples: graphene plasmons and quantum emitters. In both cases the coupling to the Higgs mode is clearly visible. In the case of the graphene plasmons, the coupling is signaled by a clear anticrossing stemming from the interaction of graphene plasmons with the Higgs mode of the superconductor. In the case of the quantum emitters, the Higgs mode is observable through the Purcell effect. When combining the superconductor, graphene, and the quantum emitters, a number of experimental knobs become available for unveiling and studying the electrodynamics of superconductors., N.M.R.P. acknowledges support from the European Commission through the project "Graphene-Driven Revolutions in Information and Communication Technology (ICT) and Beyond" (881603-Core 3) and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2019. N.M.R.P. also acknowledges COMPETE2020, PORTUGAL2020, Fundo Europeu de Desenvolvimento Regional (FEDER), and the Portuguese FCT through Project POCI-01-0145-FEDER-028114. N.A.M. is a VILLUM Investigator supported by VILLUM FONDEN (Grant 16498) and Independent Research Fund Denmark (Grant 7026-00117B). The Center for Nano Optics is financially supported by the University of Southern Denmark (SDU) (SDU 2020 funding). The Center for Nanostructured Graphene is sponsored by the Danish National Research Foundation (Project DNRF103). Work on hybrid heterostructures at Columbia was supported entirely by the Center on Precision-Assembled Quantum Materials, funded through the US National Science Foundation Materials Research Science and Engineering Centers (Award DMR-2011738). D.N.B. is Moore Investigator in Quantum Materials, Emergent Phenomena in Quantum Systems (EPiQS) 9455. D.N.B. is the Vannevar Bush Faculty Fellow ONR-VB: N00014-19-1-2630. F.H.L.K. acknowledges financial support from the Government of Catalonia trough the SGR grant and from the Span-ish Ministry of Economy and Competitiveness (MINECO) through the Severo Ochoa Program for Centers of Excellence in Research & Development (SEV2015-0522); support by Fundacio ' Cellex Barcelona, Generalitat de Catalunya through the Centres de Recerca de Catalunya (CERCA) program; and the MINECO grants Plan Nacional (FIS2016-81044-P) and the Agency for Management of University and Research Grants 2017 SGR 1656. Furthermore, the research leading to these results has received funding from the European Union's Horizon 2020 program under the Graphene Flagship Grants 785219 (Core 2) and 881603 (Core 3) and the Quantum Flagship Grant 820378. This work was also supported by

Electro-absorption (EA) waveguide-coupled modulators are essential building blocks for on-chip optical communications. Compared to state-of-the-art silicon (Si) devices, graphene-based EA modulators promise smaller footprints, larger temperature stability, cost-effective integration and high speeds. However, combining high speed and large modulation efficiencies in a single graphene-based device has remained elusive so far. In this work, we overcome this fundamental trade-off by demonstrating the 2D-3D dielectric integration in a high-quality encapsulated graphene device. We integrated hafnium oxide (HfO2) and two-dimensional hexagonal boron nitride (hBN) within the insulating section of a double-layer (DL) graphene EA modulator. This combination of materials allows for a high-quality modulator device with high performances: a ~39 GHz bandwidth (BW) with a three-fold increase in modulation efficiency compared to previously reported high-speed modulators. This 2D-3D dielectric integration paves the way to a plethora of electronic and opto-electronic devices with enhanced performance and stability, while expanding the freedom for new device designs., Here, three-dimensional hafnium oxide and two-dimensional hexagonal boron nitride are integrated in the insulating section of double-layer graphene optical modulators, leading to a maximum bandwidth of 39 GHz and enhanced modulation efficiency.

We predict the existence of a new type of in-plane propagating exciton-polaritons that are supported by monolayer transition-metal-dichalcogenides (TMDs), which can carry large momentum in the visible spectrum. This 2D exciton-polariton (2DEP) is a result of the coupling between the electromagnetic light field and collective oscillations of excitons in TMDs. We experimentally show that the conditions required for the excitation of the 2DEP are attainable if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and is cryogenically cooled. In addition, a comparison of the properties of the 2DEP twith those of surface-plasmon-polaritons at the same spectral range reveals that the 2DEP exhibits two orders-of-magnitude larger wavelength confinement.

Graphene-based moiré superlattices have recently emerged as a unique class of tuneable solid-state systems that exhibit significant optoelectronic activity. Local probing at length scales of the superlattice should provide deeper insight into the microscopic mechanisms of photoresponse and the exact role of the moiré lattice. Here, we employ a nanoscale probe to study photoresponse within a single moiré unit cell of minimally twisted bilayer graphene. Our measurements reveal a spatially rich photoresponse, whose sign and magnitude are governed by the fine structure of the moiré lattice and its orientation with respect to measurement contacts. This results in a strong directional effect and a striking spatial dependence of the gate-voltage response within the moiré domains. The spatial profile and carrier-density dependence of the measured photocurrent point towards a photo-thermoelectric induced response that is further corroborated by good agreement with numerical simulations. Our work shows sub-diffraction photocurrent spectroscopy is an exceptional tool for uncovering the optoelectronic properties of moiré superlattices., Here, the authors use a nanoscale probe to study the photoresponse within a single moiré unit cell of minimally twisted bilayer graphene, and observe an intricate photo-thermoelectric response attributed to the Seebeck coefficient variation at AB-BA domain boundaries.

Bright hot plasmon emission is observed in graphene due to the ultrafast relaxation of hot carriers that were excited by femtosecond laser pulses of visible light.

A quantitative and predictive theory of quantum light-matter interactions in ultra thin materials involves several fundamental challenges. Any realistic model must simultaneously account for the ultra-confined plasmonic modes and their quantization in the presence of losses, while describing the electronic states from first principles. Herein we develop such a framework by combining density functional theory (DFT) with macroscopic quantum electrodynamics, which we use to show Purcell enhancements reaching 107 for intersubband transitions in few-layer transition metal dichalcogenides sandwiched between graphene and a perfect conductor. The general validity of our methodology allows us to put several common approximation paradigms to quantitative test, namely the dipole-approximation, the use of 1D quantum well model wave functions, and the Fermi’s Golden rule. The analysis shows that the choice of wave functions is of particular importance. Our work lays the foundation for practical ab initio-based quantum treatments of light-matter interactions in realistic nanostructured materials.

The field of two-dimensional (2D) materials-based nanophotonics has been growing at a rapid pace, triggered by the ability to design nanophotonic systems with in situ control, unprecedented number of degrees of freedom, and to build material heterostructures from the bottom up with atomic precision. A wide palette of polaritonic classes have been identified, comprising ultraconfined optical fields, even approaching characteristic length-scales of a single atom. These advances have been a real boost for the emerging field of quantum nanophotonics, where the quantum mechanical nature of the electrons and polaritons and their interactions become relevant. Examples include quantum nonlocal effects, ultrastrong light–matter interactions, Cherenkov radiation, access to forbidden transitions, hydrodynamic effects, single-plasmon nonlinearities, polaritonic quantization, topological effects, and so on. In addition to these intrinsic quantum nanophotonic phenomena, 2D material systems can also be used as sensitive probes for the quantum properties of the material that carries the nanophotonics modes or quantum materials in its vicinity. Here, polaritons act as a probe for otherwise invisible excitations, for example, in superconductors, or as a new tool to monitor the existence of Berry curvature in topological materials and superlattice effects in twisted 2D materials. In this Perspective, we present an overview of the emergent field of 2D-material quantum nanophotonics and provide a future perspective on the prospects of both fundamental emergent phenomena and emergent quantum technologies, such as quantum sensing, single-photon sources, and quantum emitters manipulation. We address four main implications: (i) quantum sensing, featuring polaritons to probe superconductivity and explore new electronic transport hydrodynamic behaviors, (ii) quantum technologies harnessing single-photon generation, manipulation, and detection using 2D materials, (iii) polariton engineering with quantum materials enabled by twist angle and stacking order control in van der Waals heterostructures, and (iv) extreme light−matter interactions enabled by the strong confinement of light at atomic level by 2D materials, which provide new tools to manipulate light fields at the nanoscale (e.g., quantum chemistry, nonlocal effects, high Purcell enhancement)., H.L.K. acknowledges support from the Government of Spain (FIS2017-91599-EXP; Severo Ochoa CEX2019-000910-S), Fundacio ' Cellex, Fundacio ' Mir-Puig, and Generalitat de Catalunya (CERCA, AGAUR, SGR 1656). Furthermore, the research leading to these results has received funding from the European Union's Horizon 2020 under Grant Agreements 785219 (Graphene flagship Core2), 881603 (Graphene flagship Core3), and 820378 (Quantum flagship). This work was also supported by the ERC TOPONANOP under Grant Agreement No. 726001. I.T. acknowledges funding from the Spanish Ministry of Science, Innovation and Universities (MCIU) and State Research Agency (AEI) via the Juan de la Cierva Fellowship No. FJC2018-037098-I. F.H.L. K. and A.R.-P. acknowledge BIST Ignite Programme Grant from the Barcelona Institute of Science and Technology (QEE2DUP). N.M.R.P. acknowledges support from the European Commission through the project "Graphene-Driven Revolutions in ICT and Beyond" (ref. No. 881603, CORE 3), COMPETE 2020, PORTUGAL 2020, FEDER, and the Portuguese Foundation for Science and Technology (FCT) through Project POCI-01-0145-FEDER028114, and the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Financing UID/FIS/04650/2019. N.A.M. is a VILLUM Investigator supported by VILLUM FONDEN (Grant No. 16498) and Independent Research Fund Denmark (Grant No. 702600117B). The Center for Nano Optics is financially supported by the University of Southern Denmark (SDU 2020 funding). The Center for Nanostructured Graphene (CNG) is sponsored by the Danish National Research Foundation (Project No. DNRF103). J.C.W.S. acknowledges support from the National Research Foundation (NRF) Singapore under its NRF fellowship programme Award No. NRF-NRFF2016-05 and the Ministry of Education (MOE) Singapore under its MOE AcRF Tier 3 Award MOE2018-T3-1-002.

We use ultra-fast electron microscopy to measure the spatio-temporal dynamics of phonon-polariton wavepackets in a thin hBN sample. The electron probe enables recording the wavepacket formation, propagation, and decay, unveiling intriguing acceleration & deceleration phenomena.

Nonlinear optics is an increasingly important field for scientific and technological applications, owing to its relevance and potential for optical and optoelectronic technologies. Currently, there is an active search for suitable nonlinear material systems with efficient conversion and a small material footprint. Ideally, the material system should allow for chip integration and room-temperature operation. Two-dimensional materials are highly interesting in this regard. Particularly promising is graphene, which has demonstrated an exceptionally large nonlinearity in the terahertz regime. Yet, the light–matter interaction length in two-dimensional materials is inherently minimal, thus limiting the overall nonlinear optical conversion efficiency. Here, we overcome this challenge using a metamaterial platform that combines graphene with a photonic grating structure providing field enhancement. We measure terahertz third-harmonic generation in this metamaterial and obtain an effective third-order nonlinear susceptibility with a magnitude as large as 3 × 10–8 m2/V2, or 21 esu, for a fundamental frequency of 0.7 THz. This nonlinearity is 50 times larger than what we obtain for graphene without grating. Such an enhancement corresponds to a third-harmonic signal with an intensity that is 3 orders of magnitude larger due to the grating. Moreover, we demonstrate a field conversion efficiency for the third harmonic of up to ∼1% using a moderate field strength of ∼30 kV/cm. Finally, we show that harmonics beyond the third are enhanced even more strongly, allowing us to observe signatures of up to the ninth harmonic. Grating-graphene metamaterials thus constitute an outstanding platform for commercially viable, CMOS-compatible, room-temperature, chip-integrated, THz nonlinear conversion applications., K.-J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL) and financial support through the MAINZ Visiting Professorship. ICN2 was supported by the Severo Ochoa program from Spanish MINECO Grant No. SEV-2017-0706. Parts of this research were carried out at ELBE at the Helmholtz-Zentrum Dresden-Rossendorf e.V., a member of the Helmholtz Association. N.A., S.K., and I.I. acknowledge support from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 737038 (TRANSPIRE). X.J. acknowledges the support from the Max Planck Graduate Center with the Johannes Gutenberg-Universität Mainz (MPGC).

Graphene supports electrically tunable plasmon excitations capable of confining electromagnetic radiation into deeply subwavelength volumes. Moreover, when a graphene sheet is placed near a metal substrate, graphene plasmons are screened by the metal thereby giving rise to acoustic graphene plasmons (AGPs) with nearly linear dispersion and superlative field confinement [1] , [2] . For this reason, plasmons in such heterostructures make ideal candidates to probe the quantum nonlocal electrodynamic response of the nearby metal in the THz and infrared spectral ranges. With that in mind, here we describe the quantum response of the metal using a formalism of mesoscopic electrodynamics based on microscopic surface-response functions, known as Feibelman d -parameters, which encode the metal’s quantum-surface response [2] , [3] , [4] . Specifically, we present a theory for calculating the spectrum of acoustic plasmons in graphene–dielectric–metal (GDM) heterostructures ( Fig. 1a ), and show that the AGP’s resonances exhibit nonclassical spectral shifts due to the quantum surface-response of the metal: toward the red for Re d ⊥ > 0 (associated with electronic spill-out) and toward the blue for Re d ⊥ < 0 (associated with an inward spill of the induced charges); this is accompanied by a slight increase of plasmon damping (due to electron-hole pair excitations). Importantly, these findings [4] explain and contextualize recent experimental observations of nonclassical optical response in GDM structures [1] . Crucially, using out theory and the concept set forth here, the metal’s quantum surface-response—embodied through the d -parameters—can be directly inferred from the experimental measurements of AGP’s dispersion ( Fig. 1b ). Based on this work [4] , we envision that by harnessing the record-high light confinement brought about by AGPs one could shed light about the physics governing the complex electron dynamics of metals, and, possibly, of strongly-correlated systems.

In this work, we predict the existence of a new type of in-plane propagating, highly-confined exciton-polariton, supported by monolayer transition-metal-dicalcogonides (TMDs) in the visible spectrum. This 2D exciton-polariton (2DEP) is a result of the coupling between the electromagnetic light field and collective oscillations of excitons in TMDs. We experimentally show that the conditions required for the excitation of the 2DEP are attainable if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and is cryogenically cooled. In addition, a comparison of the properties of the 2DEP twith those of surface-plasmon-polaritons at the same spectral range reveals that the 2DEP exhibits two orders-of-magnitude larger wavelength confinement.

Many promising optoelectronic devices, such as broadband photodetectors, nonlinear frequency converters, and building blocks for data communication systems, exploit photoexcited charge carriers in graphene. For these systems, it is essential to understand the relaxation dynamics after photoexcitation. These dynamics contain a sub-100 fs thermalization phase, which occurs through carrier-carrier scattering and leads to a carrier distribution with an elevated temperature. This is followed by a picosecond cooling phase, where different phonon systems play a role: graphene acoustic and optical phonons, and substrate phonons. Here, we address the cooling pathway of two technologically relevant systems, both consisting of high-quality graphene with a mobility >10000 cm2 V-1 s-1 and environments that do not efficiently take up electronic heat from graphene: WSe2-encapsulated graphene and suspended graphene. We study the cooling dynamics using ultrafast pump-probe spectroscopy at room temperature. Cooling via disorder-assisted acoustic phonon scattering and out-of-plane heat transfer to substrate phonons is relatively inefficient in these systems, suggesting a cooling time of tens of picoseconds. However, we observe much faster cooling, on a time scale of a few picoseconds. We attribute this to an intrinsic cooling mechanism, where carriers in the high-energy tail of the hot-carrier distribution emit optical phonons. This creates a permanent heat sink, as carriers efficiently rethermalize. We develop a macroscopic model that explains the observed dynamics, where cooling is eventually limited by optical-to-acoustic phonon coupling. These fundamental insights will guide the development of graphene-based optoelectronic devices., We would like to thank Andrea Tomadin for discussions. The authors acknowledge funding from the European Union Horizon 2020 Programme under Grant Agreement No. 881603 Graphene Core 3. ICN2 was supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). A.P. acknowledges support from the European Commission under the EU Horizon 2020 MSCA-RISE-2019 programme (project 873028 HYDROTRONICS) and from the Leverhulme Trust under grant RPG-2019-363. K.J.T. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under Grant Agreement No. 804349 (ERC StG CUHL), RyC fellowship No. RYC-2017-22330, and IAE project PID2019-111673GB-I00 and financial support through the MAINZ Visiting Professorship. X.J. acknowledges the support from the Max Planck Graduate Center with the Johannes Gutenberg-Universität Mainz (MPGC). J.Z. acknowledges the support from National Natural Science Foundation of China (No. 52072042). Z.L. acknowledges the support from National Natural Science Foundation of China (No. 51520105003). T.S. acknowledges support from the University of Liege under Special Funds for Research, IPD-STEMA Programme. M.J.V. gratefully acknowledges funding from the Belgian Fonds National de la Recherche Scientifique (FNRS) under PDR grant T.0103.19-ALPS. Computational resources were provided by CECI (FRS-FNRS G.A. 2.5020.11) and the Zenobe Tier-1 supercomputer (Gouvernement Wallon G.A. 1117545) and by a PRACE-3IP DECI grant 2DSpin and Pylight on Beskow (G.A. 653838 of H2020). ICFO was supported by the Severo Ochoa program for Centers of Excellence in R&D (CEX2019-000910-S), Fundació Privada Cellex, Fundació Privada Mir-Puig, and the Generalitat de Catalunya through the CERCA program. N.v.H. acknowledges funding by the European Commission (ERC AdG 670949-LightNet), the Spanish Plan Nacional (PGC2018-096875-BI00), and the Catalan AGAUR (2017SGR1369).