Roadmap for Photonics with 2D Materials

Abstract

Triggered by advances in atomic-layer exfoliation and growth techniques, along with the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or a few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals now constitute a broad research field expanding in multiple directions through the combination of layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary subset of those directions, where 2D materials contribute remarkable nonlinearities, long-lived and ultraconfined polaritons, strong excitons, topological and chiral effects, susceptibility to external stimuli, accessibility, robustness, and a completely new range of photonic materials based on layer stacking, gating, and the formation of moiré patterns. These properties are being leveraged to develop applications in electro-optical modulation, light emission and detection, imaging and metasurfaces, integrated optics, sensing, and quantum physics across a broad spectral range extending from the far-infrared to the ultraviolet, as well as enabling hybridization with spin and momentum textures of electronic band structures and magnetic degrees of freedom. The rapid expansion of photonics with 2D materials as a dynamic research arena is yielding breakthroughs, which this Roadmap summarizes while identifying challenges and opportunities for future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

Keywords: 2D polaritons; electro-optical modulation; excitons in van der Waals materials; layer stacking and moiré photonics; nonlinear optics; photonics with 2D materials; quantum photonics.

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Chameleonic properties and uses of 2D materials. Driven by their exceptional properties and unique physical behavior (top), 2D materials are unlocking disruptive applications in photonics (bottom), as discussed in this Roadmap.

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Overview of 2D polaritons. (a) Universal scaling of the out-of-plane confinement length (1/2π)­λ_polariton in the local electrostatic limit for polaritons in planar films of small thickness d compared with their wavelength λ_polariton, sandwiched by media of arbitrary permittivities ϵ₁ and ϵ₂. (b) General parametrization of the frequency-dependent surface conductivity σ­(ω) in terms of a Drude weight ω _D , an intrinsic resonance frequency ω _g , and a phenomenological lifetime τ. The polariton-to-photon wavelength ratio, expressed in terms of the fine-structure constant α ≈ 1/137 and the average environmental permittivity ϵ̅ = (ϵ₁ + ϵ₂)/2, is much smaller than 1 for actual material parameters. (c) Range of optical parameters in common types of 2D polaritonic materials. We have ω _g = 0 for plasmons, while ω _D scales like the product of the Fermi velocity v _F and the Fermi wave vector k _F in conducting materials. For phonons/excitons, ω _D is expressed as a fraction f _ph /f _ex of ω _g . (d) Experimental techniques available to synthesize 2D polaritonic materials based on geometry/composition and the application of external actions such as doping, electromagnetic fields, or heating. (e,f) Selection of challenges and opportunities associated with 2D polaritonics, as discussed in more detail in the main text.

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Illustration of isofrequency contours and the hyperbolic on-chip nanophotonic platform. (a) Isofrequency surface of a dielectric isotropic medium. (b) Isofrequency surface of a Type I hyperbolic material that is characterized by ϵ _xx = ϵ _yy > 0 and ϵ _z < 0. Notably, for any pair of coordinates (k _x ,k _y ), it is always possible to find a point on the normal surface. (c) On-chip nanophotonic-platform generation: A high-bias current is driven through a graphene channel encapsulated with hBN, producing hyperbolic phonon polaritons (HPhPs) through electroluminescence. The schematic represents the cross section of a configuration in which HPhPs bypass the source contact and propagate for further processing. (d) On-chip nanophotonic-platform manipulation: A tunable polaritonic or topological crystal for molecule detection. The schematic illustrates the cross section of an hBN/graphene/hBN polaritonic lattice (hyperlattice) in which the polaritonic density of states can be electrically tuned by adjusting the gate voltage. This concept could enable molecular selectivity based on their interaction with the crystal lattice. (e) On-chip nanophotonic-platform detection: A graphene-based photothermoelectric detector. The schematic illustrates the processed HPhPs incoming from one side of the device being absorbed at the graphene PN junction defined by a split-gate structure. This absorption generates a photothermoelectric response, converting the polaritonic signal into an electrical output.

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Manifesto of plasmonics in twisted 2D materials. We list future challenges and potential outcomes of plasmonics applied to twisted 2D materials.

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Harnessing ultraconfined graphene plasmons to probe collective excitations and strongly correlated phenomena in condensed-matter systems. (a) Schematic of a 2D material (here represented by graphene) in the vicinity of a probing material, separated by a thin insulating slab. The field provided by the 2D polaritons can couple to quasiparticles in the nearby medium, thereby probing the response and collective excitations of the material. (b) Illustration of the out-of-plane component of the electric field associated with acoustic graphene plasmons (AGPs) in a graphene−dielectric−metal (GDM) configuration. (c) Collective modes, emerging as features in Im r _p, of a 2DSC−G−2DSC heterostructure. The 2D superconductor (2DSC) is FeSe, with a superconducting gap of 23 meV at zero temperature and supporting a Bardasis−Schrieffer mode at 18.2 meV. See ref for details. (d) Dispersion relation of AGPs in a GDM configuration. The metallic quantum surface response (encoded through the Feibelman d _⊥-parameter; see inset) leads to sizable spectral shifts, underscoring the potential of AGPs to infer quantum nonlocal effects in nanoplasmonics. Adapted with permission from ref (Copyright 2021 Springer Nature). (e) Dispersion relation exhibiting the hybridization between graphene plasmons and the magnon mode of monolayer CrI₃ in a heterostructure composed of three single layers: graphene−hBN−CrI₃. Adapted with permission from ref (Copyright 2023 American Chemical Society).

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Topological phases in 2D materials. (a) A Chern insulator is related to charge transport and generates a voltage V _H across the sample. (b) A quantum spin Hall insulator is connected to spin transport, which results in a current I _s across the sample. Both Chern and quantum spin Hall insulators are manifestations of the 2D Dirac Hamiltonian. (c) In contrast, in certain 2D materials, an effective photon mass term can enter the Maxwell Hamiltonian arising from viscous hydrodynamic light−matter interactions. This leads to an optical topological number: N-invariant which captures the winding number of the microscopic optical response tensor. Adapted with permission from ref (CC BY 4.0. International license).

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Multimessenger scanning probe nanoimaging is based on AFM and STM platforms. Top panel: Schematic representation of scanning probe imaging modalities commonly applied to the exploration of 2D materials and their heterostructures. Bottom panel: Examples of images of different classes of 2D materials reported by coauthors. From left to right: (A) MFM imaging of atomically layered magnetism in semiconducting CrSBr; (B) Nanophotocurrent in minimally twisted graphene; (C) Nano-THz response of few layer WTe₂ microcrystals; (D and E) Co-located nano-IR and STM studies of nonlocal relaxation dynamics in twisted trilayer graphene moiré superlattices; (F) Nano-PL image of a strained nanobubbles in a WSe₂ monolayer on Au substrate; (G) Nano-Raman image of WS₂ monolayer bubbles on a Au substrate; (H) Nano-SHG characterization of layer stacking in WSe₂. Panels A-H are reproduced with permission from (A) ref ; (B) ref ; (C) ref ; (D,E) ref ; (F) ref ; (G) ref ; (H) ref (Copyright 2022 Wiley; 2021, 2021, 2022, 2020 Springer Nature; 2021 American Chemical Society; 2022 Wiley; respectively).

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Schematic illustration of tip-enhanced nanoscopies applied to the study of photonics in 2D materials. The tip−sample nanocavity can confine electromagnetic fields and couple near- to far-field photons, and vice versa, enabling local probing of optical and electronic excitations with atomic-scale spatial resolution. This can be done by using photons or tunneling electrons for excitation and detection. The intrinsic surface sensitivity and subnanometer spatial resolution of tip-enhanced techniques make them especially suited for studying and controlling light−matter interaction phenomena in 2D systems. The spatial resolution is particularly useful for the study of systems exhibiting nanometer-scale lateral modulations or inhomogeneities such as moiré superlattices, nanostructures, lateral heterostructures, or defects. The application of external stimuli such as strain, electrical gating, and magnetic fields can be incorporated for the manipulation of the local properties and correlating it with mesoscopic properties via in operando studies. Incorporating fast detection or pump−probe schemes adds time resolution to study quasiparticle dynamics with the same spatial resolution. All these elements converge in a toolbox of techniques that enable deterministic characterization and manipulation of photonic phenomena at the ultimate spatiotemporal limits of picometers and attoseconds.

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Chemistry, electronic structure, and optics of 2D materials with fast electron spectroscopies. (a-b) Electron-microscopy images and model for nitrogen atoms in graphene in different atomic configurations. (c) Electron energy-loss spectroscopy (EELS) core-loss spectra and simulation for the nitrogen K edge of atoms in the graphitic and pyridinic configurations. (d) WS₂ trion (X⁻) cathodoluminescence (CL) intensity map. High-intensity spots are numbered and marked by dashed circles to be compared with (e). (e) Spectral profile along the arrow in (d), showing how the trion-to-neutral exciton CL emission evolves on the local maxima. Panels (a-c) are adapted with permission from ref (Copyright 2018 American Association for the Advancement of Science); Panels (d-e) are adapted with permission from ref (Copyright 2020 American Association for the Advancement of Science).

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Study of strong exciton−photon coupling in 2D vdW crystals combines different classes of materials with optical resonators to investigate intriguing phenomena related to optics and photonics.

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Tunable properties of excitons and trions. (a) Exciton wave function of excitons (right top) and trions (left bottom) in monolayer MoS₂. The hole is fixed in the center, and the (averaged) electron distribution is shown in a top view. Adapted with permission from ref (Copyright 2017 Springer Nature). (b) Exciton energies with respect to an applied vertical electric field in a bilayer MoS₂. While intralayer states are almost constant in energy, states with strong interlayer contributions show an X-like shape. The colored data are ab initio results overlaying experimental reflectance measurements. Adapted with permission from ref (Copyright 2021 Springer Nature). (c) Representative relaxation channels and momentum dependent excitations in TMDs. While transitions between valence and conduction bands at K have a momentum close to Γ, momentum-indirect transitions involving phonons (arrows) are also possible with a pronounced temperature dependence. Adapted with permission from ref (Copyright 2017 American Physical Society). (d) Strain dependent photoluminescence of WSe₂ monolayer featuring bright (X⁰, X) and dark (D1, D2) excitons. Adapted with permission from ref (Copyright 2022 Springer Nature). (e) Magnetic tuning of the ground and excited states in monolayer WS₂. Adapted with permission from ref (Copyright 2019 Springer Nature). (f) Tuning of interlayer excitons in a MoSe₂/WSe₂ heterobilayer with electric field. (g) Corresponding injection of free charges demonstrating p- and n-type interlayer trions. Panels (f,g) are adapted with permission from ref (Copyright 2019 American Association for the Advancement of Science).

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Overview of growth, characterization, and applications in doping, alloying, and intercalation of 2D vdW materials. (a) Schematic of structural changes in vdW materials and heterostructures for doping, alloying, and intercalation. (b.1) The current status of growth efforts includes doped and alloyed high-quality wafer-scale 2D materials by bottom-up approaches such as chemical vapor deposition. However, producing wafer-scale 2D materials with minimal defects and achieving controlled dopant density and alloy stoichiometry with minimal strain is still a challenge. Adapted from ref (Copyright 2021 Springer Nature). (b.2) The remaining challenges include low-temperature growth, reversible doping strategies, and precise postgrowth patterning that are critical for applications relying on heterojunctions and BEOL processing. Adapted from ref (Copyright 2016 Springer Nature). (c.1) The current status of characterization methods for quasiparticles is exemplified by STEM (left) and far-field optical spectroscopy techniques (right) that reveal dopant and defect distributions, chemical composition, and excitonic properties in 2D materials. Left and right schematics are adapted from ref (Copyright 2020 American Chemical Society) and ref (Copyright 2024 Wiley), respectively. (c.2) A key challenge in characterizing intercalated, doped, and alloyed samples is achieving the spatial resolution necessary to probe both the atomically resolved crystal structure and the dynamics of excitons at the Bohr radius length scale (1−2 nm) at low temperatures. (d.1) An example of a current application enabled by solution-processed 2D materials is electroluminescence from megasonicated MoS₂ inks that retain monolayer direct bandgap character in percolating films. Adapted from ref (Copyright 2023 American Chemical Society). (d.2) Challenges for photonic applications include the ordering of dopants, substitutional species, and intercalated molecules. The specific example depicted here is ordered intercalated chiral molecules in MoS₂ that have the potential to realize macroscopic chiral optical response. Adapted from ref (Copyright 2022 Springer Nature). (e) Schematic illustrating opportunities for integrating in situ high-resolution imaging and spectroscopy techniques for real-time monitoring of crystallinity, defect states, and electronic structures during growth, which can accelerate the realization of patterned doped regions without disturbing the underlying material lattice. The incorporation of advanced artificial intelligence and machine learning approaches with large-scale data sets will accelerate data analysis, while an integrated feedback mechanism will further refine the understanding of the physical mechanisms influencing device performance.

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Strain and electrostatic engineering for monolayer exciton confinement. (a,b) The principle of strain-induced quantum confinement (a) and representative site-controlled quantum emitters (b). Reproduced with permission from ref (Copyright 2017 Springer Nature). (c,d) Structures for 0D electrostatic quantum confinement of intralayer excitons with position and energy tunability. Reproduced with permission from ref (Copyright 2024 The American Association for the Advancement of Science). (e) Some future possibilities for scaled-up arrays of controlled QEs, interfacing with photonic integrated circuits, and new interfaces with 2D material platforms. Adapted with permission from ref (Copyright 2021 American Chemical Society).

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Ultrafast optical processes in vdW heterostructures. (a) Sketch of a TMD heterostructure illustrating intralayer, interlayer, and hybrid excitons, along with the simplified representation of the heterostructure band structure with the different band alignments hosting different excitonic species. Hybrid exciton results from resonant interlayer electron tunneling. Adapted with permission from ref (Copyright 2021, American Chemical Society). (b) Graphical representation of the interlayer-exciton (ILX) formation process in a MoSe₂/WSe₂ heterostructure upon resonant photoexcitation of the bright exciton in MoSe₂. Hole transfer results in the formation of a hot ILX population with finite momentum Q. Hot ILXs quickly relax to the optically bright ILX ground state by exchanging energy and momentum with phonons. Adapted from ref (Creative Commons CC BY). (c) Sketch of the single-particle valleys of MoS₂/WSe₂ heterostructures at some high-symmetry points (i.e., K and Σ). After resonant photoexcitation of the intralayer exciton in MoS₂, an ILX at the K valleys forms via intermediate phonon-mediated scattering with hybrid exciton (hΣ). Adapted with permission from ref (Copyright 2023 IOP Publishing). (d) Exciton diffusion in TMD heterostructures with and without moiré superlattice (left and center panels). θ is the twist angle between the two monolayers. The moiré potential (sketched in the right panel) tends to localize ILXs and to hinder their diffusion.

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Ultrafast 2D electronic spectroscopy of 2D materials. The sample is resonantly excited with a phase-locked pulse pair with delay τ. A probe pulse at waiting time T induces a nonlinear polarization in the material. (a) Coherent couplings induce temporally oscillating diagonal and cross peaks in the 2DES map. (b) Valley-selective 2DES of A and B excitons using circularly polarized light pulses. (c) Intrinsic many-body interactions result in a rich variety of quasiparticles and dominate their optical properties. Such interactions can be tuned by moiré potentials. (d) New quantum states are created by hybridization with fields in external microcavities and plasmonic nanoresonators.

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Representative path toward next-generation photonics based on high-order nonlinearities in 2D layered materials (2DLMs). Stacking engineering, χ⁽ⁿ⁾ engineering, field enhancement, and hybrid integration methods can be used for the overall enhancement of the nonlinear conversion efficiency of 2D materials. With new emergent 2D materials exhibiting better nonlinear performance, disruptive advances and revolutionary findings can be foreseen in integrated photonics, attosecond and strong-field physics, petahertz electronics, and ultrafast lasers.

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Nanoscale platforms for efficient SPDC generation. (a) Periodically poled vdW materials. (b) vdW waveguide. (c) vdW metasurface.

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Nonlinear polaritonics in 2D materials. (a) Intense high-harmonic generation from graphene nanostructures (upper part) is predicted to arise from the combination of plasmonic field enhancement and anharmonic electron motion associated with the conical band structure of graphene (lower-left part), with the latter attribute leading to an intrinsically nonlinear optical response that contrasts the harmonic response exhibited by a parabolic electron dispersion (lower-right part). Adapted with permission from refs and (Copyright 2014 and 2017 Springer Nature). (b,c) The large transient photothermal response of graphene and ultrathin metals can be harnessed to trigger incoherent nonlinear light−matter interactions that enable the creation of transient optical responses (b) and optical switching at the single- or few-photon level (c). Adapted with permission from refs (Copyright 2020 Springer Nature) and (Copyright 2019 American Chemical Society). (d,e) In crystalline ultrathin metal films (d), anharmonicity can be engineered by vertical quantum confinement to enhance the nonlinear response of plasmon polaritons (e). Adapted with permission from ref (Copyright 2021 de Gruyter). (f) The nonlinear response of phonon polaritons in hBN originates from the anharmonicity of the potential landscape under atomic displacements (upper part), which can produce nonperturbative effects (lower left) and enables electrical modulation of the phonon frequency through wave mixing with a static field in the V/nm range (lower right). Adapted with permission from ref (Copyright 2021 American Chemical Society).

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Schematic of all-optical valleytronic operations and time-reversal symmetry (TRS) modulation/detection. A valley imbalance can be generated by resonant one-photon absorption of circularly polarized light (a) or the coherent valley-selective optical Stark effect (b). The valley imbalance is typically measured via helicity-resolved PL (c) or ultrafast resonant Kerr rotation (both linear and nonlinear) (d). Future valleytronic devices could use local field enhancement via metallic tips (e) and potentially probe broken TRS even in centrosymmetric crystals by nonlinear third-harmonic Kerr rotation.

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Symmetry-sensitive nonlinear optics as a powerful tool for probing 2D magnets under external stimuli. PM and AFM represent paramagnetic and antiferromagnetic, respectively. T _c denotes the Néel temperature of the magnetic phase transition. The symbols i and t indicate spatial inversion and time reversal operations, respectively.

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Bulk quantum geometric photocurrents. (a,b) Injection (a) and shift photocurrents (b) arise from the change in electron group velocity (v _c and v _v in the conduction and valence bands) and real space displacement (R _c and R _v in conduction and valence bands) when electrons are photoexcited from valence to conduction bands in noncentrosymmetric materials. (c−e) In the presence of a Fermi surface, a variety of other mechanisms become activated including (c) an intrinsic Fermi surface effect (arising from virtual interband processes), (d) a Berry curvature dipole (inset; asymmetric distribution of Berry curvature across a Fermi surface), and (e) skew scattering contributions (produced when the transition rates W between Bloch states at momenta k _1,2 become imbalanced). These mechanisms provide a means for achieving a type of optoelectronics based on bulk noncentrosymmetric metals.

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Overview of optical singularities and their correlations. (a) Different types of optical singularities and their experimental demonstrations, including phase singularities, polarization singularities, and more complex types of singularities such as knots and skyrmions. (b) Many systems show natural occurrences of multiple singularities, which can interact and annihilate in pairs, conserving their combined topological charge. Adapted with permission from refs and (Copyright 2018 American Association for the Advancement of Science and 2016 American Physical Society). (c) Ensembles of phase singularities emerging from the interference of random waves have spatial correlations as theoretically predicted in ref and experimentally demonstrated in ref . Adapted with permission from ref (Copyright 2016 American Physical Society). (d) The interference of random wave packets results in spatiotemporal dynamics of phase singularities, characterized by the creation and annihilation of vortex pairs. The spatiotemporal correlations of these phase singularities remain an underexplored area, warranting further investigation.

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Surface phonon polaritonic (SPhP) skyrmions and other topological features with varying spatial compression. (a) Theoretical SPhP dispersion for different SiC membrane thicknesses from 10 nm to 1 μm. Displayed are the odd and even modes, calculated using the method outlined in ref . On the left, the excitation wavenumber is plotted as a function of SPhP wave vector, and on the right the excitation wavenumber is given as a function of the normalized propagation length L/λ_SPhP. There are two even modes, one of them very close to the light line and the other one with propagation length L < λ_SPhP. The odd mode features high propagation lengths and strong wavelength compression. The wavelength compression increases rapidly for thinner membranes. (b) Schematic representation of collective atom displacements in polar dielectrics as the foundations of even (odd) SPhP modes that are illustrated in the center (bottom) image. (c) Exemplary implantation of a skyrmion in the SPhP electric field. SPhPs are launched from a spiral structure with an opening of λ_SPhP, which is illuminated by circularly polarized light in the mid-IR. The SPhPs interfere in the center of the structure, creating a skyrmion vector texture in the electric field, as depicted in the vector plot and the 2D cut on the right. The size of the skyrmion is determined by the SPhP wavelength and can be compressed strongly in comparison to the excitation wavelength by using thin SiC membranes.

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Chiral response in moiré multilayers. (a) Ellipticity or circular dichroism spectra due to the resonant interband transition at the M point as a function of the twist angle observed in twisted bilayer graphene. Adapted with permission from ref (Copyright 2016 Springer Nature). (b) Schematic of a chiral cavity consisting of moiré systems with an enhanced chiral near field suitable for sensing. Adapted with permission from ref (Copyright 2020 American Chemical Society). The electric moment p due to longitudinal current densities is locked to a magnetic moment m due to transverse current densities. (c) The electric field associated with plasmon excitations propagating in the x direction together with its helicity/spin. For ordinary plasmons, the spin points in the y direction, whereas for chiral plasmons there is also a component in the x direction. Adapted with permission from ref (Copyright 2020 American Physical Society). (d) Schematic view of supertwisted multilayers and AFM image of a representative supertwisted WS₂ sample with continuously twisting layers (scale bar 4 μm). Adapted with permission from ref (Copyright 2024 Springer Nature).

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Roadmap for twistoptics. Current and potential functionalities in twistoptics (gate-tunable polariton canalization, moiré physics, twisted polaritonic crystals, topological transitions) that could trigger a wide array of applications in nanophotonics (sensing, information technologies, radiative heat transfer, optoelectronics). The “Chirality” sketch is reprinted with permission from ref (Copyright 2020 American Physical Society); the “Gate-tunable canalization” panel is adapted with permission from ref (Copyright 2023 American Chemical Society).

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Role of crystalline anisotropy on polaritonic response and function. (a) Role of structural anisotropy in the optical properties of 2D vdW materials, including, from top to bottom, different crystal classes, optical axes leading to birefringence, hyperbolic isofrequency contours, and polariton propagation launched by a point dipole. (b,c) Challenges and future directions.

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Polaritonic metasurfaces. (a) Polaritonic metasurfaces based on vdW interfaces to demonstrate a range of extreme polaritonic phenomena at the nanoscale. Adapted from ref (Copyright 2021 Springer Nature). (b) Topological polaritonic metasurface obtained by loading a topological photonic crystal with a thin film of boron nitride. The right panel shows the effect of such loading: the topological edge state splits its dispersion within the bandgap of the photonic crystal due to the coupling with the phonon resonance of boron nitride. The topological phase is transferred to the polaritonic response. Adapted from ref (Adapted from ref (Copyright 2021 American Association for the Advancement of Science).

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Comparison of TMD optical constants and schematic illustrations of TMD-based metasurfaces. (a) Comparison of the spectrally varying real parts of the refractive indices for the principal semiconducting TMDs in multilayer form together with Si and GaAs. − (b) Comparison of exciton oscillator strengths versus resonance energies of various monolayer and multilayer TMDs as measured at room temperature. ,,,, A comparison of GaAs quantum wells at a temperature of 6 K is also provided. (c) Schematic approaches to passive metasurfaces using bulk (multilayer) TMD active layers. Both linear and nonlinear approaches are depicted and discussed. (d) Schematic of approaches to active TMD-based metasurfaces through external parameters.

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Strongly correlated electrons in transdimensional materials. (a) Electron confinement-induced plasmonic breakdown in a transdimensional material. We show a schematic of a plasmonic metal film sandwiched between a substrate and a superstrate. As the film thickness d decreases, the interaction potential among charges acquires an in-plane character and increases drastically, leading to electron confinement. (b) Schematic of Wigner crystallization when an interacting electron liquid has a sufficiently low electron density minimizing its total energy via crystallization into a quantum solid phase. (c) Platzman−Fukuyama (PF) ratio given by eqs and plotted as a function of the surface charge density and film thickness with experimentally measured parameters of HfN TD films at room temperature.

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Pathways for the use of MXenes in optical applications. The tunability of MXene stoichiometry and versatile fabrication methods will allow for the manufacture of designer optical films to enhance current applications and pave the way for future uses. Periodic table adapted with permission from ref (Copyright 2022 Springer Nature). Optical application and flake structure figures adapted from ref . Blade coating figure reprinted in part from ref (Copyright 2024 American Chemical Society). Spray coating and vacuum filtration figures adapted from ref (Copyright 2017 American Chemical Society). Dip coating image adapted with permission from ref (Copyright 2018 Wiley). Spin coating figure reprinted in part from ref (Copyright 2016 Wiley).

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Distinct first-order interband and higher-order inelastic optical responses of BP from the visible to UV excitation regions. (a) Schematic excitation of mPP from BP, a representative 2D material. The frequency tunable pump pulse of ∼30 fs duration generates interband and intraband (hot electron) excitations, and the identical delayed probe pulse records their population decay. (b) Representative mPP energy−momentum E(k _||) maps excited along the zigzag direction of BP with photon energies ℏω = 2.14, 2.58, 2.88, and 3.44 eV, respectively. The dashed lines guide the band dispersions. The lower panels are the corresponding line profiles extracted at k _|| = 0 Å⁻¹. The inset for ℏω = 3.44 eV expands the mPP signal above 5.8 eV to show the remaining band features. The bottom arrows indicate the spectral regions that are excited by 2PP-4PP processes. (c) Calculated band structure of bulk BP with band dispersions along the Z-S (k _AC ) and Z-T (k _ZZ ) directions. (d) Illustrative excitation diagram respectively showing band-to-band resonant excitation (left) and inelastic scattering of virtual states promoted from VB1 leading to the production and detection of hot electrons (right). (e) Interferometric two-pulse correlation measurement of the 2PP signal as a function of the intermediate state energy probing the BP polarization dephasing and hot electron population dynamics. (f) Extracted lifetime, τ_e, of hot electrons as a function of the intermediate state energy. At an energy higher than 1.6 eV, the measurements are limited by the time resolution of ∼35 fs at ℏω = 3.49 eV. Below 1.6 eV, the data are fit to τ _e ∼ (E − E _F )^−1.76, which slightly deviates from the τ _e ∼ (E − E _F )⁻² behavior of a 3D Fermi liquid.

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Plasmonics in ultrathin metal films. (a) A single-atomic-layer gold disk with (111) crystallographic orientation exhibits a sharp plasmon red-shifted to a technologically appealing spectral region relative to a gold nanosphere with the same diameter. Adapted with permission from ref (Copyright 2014 Springer Nature). (b) The plasmon energy of few-atomic-layer gold or silver disks scales roughly as $\sqrt{t/D}$ with the disk thickness t and diameter D. Adapted with permission from ref (Copyright 2015 Royal Society of Chemistry). (c,d) The plasmon band of an ultrathin silver layer (1 nm) deposited on graphene is quenched at plasmon energies exceeding the graphene optical gap 2E _F , where E _F is the graphene Fermi energy (c). This effect can be used to modulate the plasmons of a thin silver ribbon array, thus producing a radical change in light reflection depending on E _F (d). Adapted with permission from ref (Copyright 2016 Springer Nature). (e-h) Ultrathin crystalline silver ribbons can be prepared by epitaxial growth on silicon, followed by nanolithography (e). The resulting metal film exhibits high quality and wafer-scale crystallinity (f), reflected in the emergence of sharp electronic quantum-well states observed via angle-resolved photoemission spectroscopy (g) (i.e., the electron intensity as a function of parallel wave vector k _∥ and energy relative to the Fermi level, E − E _F ). The quality factor (energy-to-width ratio, ω _p /γ) of the obtained ribbon plasmons reaches ∼4 (with a measured width ℏγ _ribbons ≈ 230 meV), which is far from the large values predicted from the bulk Drude-model damping for silver, as measured optically (ℏγ _AC ≈ 21 meV) or electrically (ℏγ _DC ≈ 24.6 meV for a bulk conductivity of 6.30 × 10⁷ S/m). This reduction in quality factor can be presumably ascribed to the effect of imperfections introduced during passivation of the film (with a silica coating) and nanolithography (h). Panels (e-h) are adapted from ref (Copyright 2019 American Chemical Society). (i) Prepatterning of silicon followed by epitaxial deposition of silver crystalline films leads to higher quality structures with great flexibility to pattern different morphologies, as shown in the upper sketches and the lower secondary-electron-microscope images. The measured quality factor of the bowties reaches ∼ 10. Adapted with permission from ref (Copyright 2024 Wiley).

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Two-dimensional materials for integrated photonics. (a-c) Selected demonstrations of 2D materials integrated optical fiber devices (a), on-chip waveguide devices (b), and novel 2D materials-based integrated platforms (c) over the past years. (d) Schematic illustration of future hybrid integration, providing a vision of the combination of various waveguide platforms, active materials, and their engineered structures, along with multifunctional optoelectronic interconnection systems, aimed at developing the next generation of highly integrated photonics.

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Optoelectronic interconnection of polaritons in 2D materials. (a) Interaction between a dipole emitter and plasmons in doped homogeneous graphene (upper panel) and near-field generated by a perpendicular dipole positioned 10 nm from doped graphene (lower panel). Reprinted from ref (Copyright 2011 American Chemical Society). (b) Edge and waveguide THz surface plasmon modes in graphene microribbons. Reprinted from ref (Copyright 2011 American Physical Society). (c) Gate-tunable topological polaritons in vdW heterostructures with high spatial confinement. Reprinted with permission from ref . (Copyright 2023 AAAS). (d) Thermoelectric detection and imaging of propagating graphene plasmons. Continuous-wave laser light is scattered by a movable metallized AFM tip, which excites plasmons in the hBN-graphene-hBN heterostructure. Reprinted with permission from ref (Copyright 2016-Springer Nature). (e) Schematic illustration of the optoelectronic interconnection of polaritons in 2D materials featuring several key components arranged from left to right: the module for converting electrical signals into polariton signals, the polariton waveguide for signal transmission, the polariton signal modulation module, the polariton photonic computing module, and the polariton detection module. Each modulator is individually controlled by an electronic chip to encode the optical signals. Additionally, a silicon waveguide connects the modulators to the photodetector and can be extended and adapted for fiber optics, enabling long-distance signal transmission.

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Controlling quantum materials with optical near fields. (a) Producing structured near-field radiation can be accomplished with polaritons, including subdiffraciton-limited superlattices of IR radiation in moiré-patterned twisted bilayer graphene. Adapted from ref (Copyright 2018 American Association for the Advancement of Science). (b) A 2D material driven by a periodic potential of plasmons with subwavelength periodicity. Adapted from ref (arXiv). (c) Near fields interacting with a Hall bar of GaAs. Adapted from ref (arXiv).

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Single-photon emitters in 2D materials. (a) Schematic illustrating strain engineering via PDMS-assisted transfer of a monolayer TMD onto a prepatterned Si/SiO₂ substrate, creating localized deformations that form quantum emitters. (b) Dark-field optical microscopy image showing pierced and nonpierced defects in a 2D material lattice. The image covers an area of 170 μm by 210 μm. (c) Illustration of a monolayer WSe₂ integrated with a Si₃N₄ waveguide for photon routing, featuring two output channels. This configuration allows excitation either through the waveguide or a standard confocal setup. (d) Cross-sectional schematic of a WSe₂ monolayer on a gold mirror and sapphire substrate, enhancing photon emission. (e) Plot of PL intensity versus excitation power for uncoupled and coupled WSe₂ emitters, demonstrating enhanced emission when coupled to optical structures. (f) Simplified energy-level diagram for the negatively charged boron vacancy in hBN, illustrating ground, excited, and metastable states. (g) Evolution of the ODMR signal with an external magnetic field applied parallel to the hexagonal c axis (B||c) of hBN. (h) AFM topography of hBN/WSe₂ nanobubbles, showing height variations. (i,h) Integrated PL intensity map of WSe₂ nanobubbles (i) over the 1.5−1.6 eV energy range corresponding to localized excitons (LX), correlated with the topography in panel (h). The scale bar indicates 500 nm length. (j) Schematic of scanning tunneling microscopy (STM)-induced photon emission from a monolayer WS₂ on graphene and a SiC substrate, where inelastic tunneling induces light emission. (k) Spectrally integrated photon map of a sulfur vacancy in WS₂, acquired with a constant current of I = 20 nA and a bias voltage of V = 2.9 V. Corresponding photon counts are displayed. Panels (a,b) are adapted from ref (Copyright 2017 Springer Nature); panel (c) is adapted from ref (Copyright 2021 American Chemical Society); panels (d,e) are adapted from ref (Copyright 2018 Springer Nature); panels (f,g) are adapted from ref (Copyright 2020 Springer Nature); panels (h,i) are adapted from ref (Copyright 2020 Springer Nature); panels (j,k) are adapted from ref (Copyright 2020 American Association for the Advancement of Science).

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Inelastic electron tunneling between graphene monolayers. (a) Schematic of an all 2D quantum-well type light-emitting device. Charge carriers are injected in the device through the two graphene electrodes. Light emission is facilitated by exciton generation and their subsequent radiative decay. (b) First Brillouin zones (1BZ) of two graphene layers overlapping with a twist angle θ. This leads to a momentum mismatch between the two layers of ΔΚ ₁ and ΔΚ ₂ for neighboring K points. (c) A 3D expansion of a band-diagram to accommodate for both energy and momentum conservation, representing an inelastic tunneling process between two graphene electrodes. The three axes represent energy, momentum, and distance along which tunneling occurs. The initial and final states of the tunneling electron are separated by an energy eV_b and momentum ΔΚ. The inelastic tunneling process occurs by exciting modes (e.g., phonons, photons, excitons, plasmons) that can fulfill energy and momentum conservation. (d) Momentum-indirect excitation of excitons can lead to direct exciton formation by phononic relaxation at the light cone.

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Graphene-integrated THz frequency devices (a) Schematic representation of a bandgap opening in bilayer graphene (BLG) induced by an out-of-plane electric field. Adapted from ref (Copyright 2009 Springer Nature). (b) Optical microscope image of an assembled vdW heterostructure comprising a top-hBN flake, BLG, and a bottom hBN flake on a large-area single-layer graphene flake. (c) Schematic of a dual back-gate (BGL, BGR) tunneling field-effect transistor (TFET) with an on-chip planar bowtie antenna connecting the source (S) and top-gate (TG) electrodes of the TFET. The drain and S contacts connect the heterostructure of panel (b). (d) Schematic of 5th- and 6th-order harmonic frequency-comb QCLs in a wire cavity with top graphene-etched trenches acting as frequency filters.

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Materials for UV-C photonics. (a) Graphic of current technologies for UV-C sensing: materials and applications. (b) A diagram of the bulk and monolayer (1L) bandgap energies of 2D semiconductor materials relevant for UV-C photodetection. (c) Examples of different hybrid systems used for UV-C sensing. Images adapted with permission from (leftmost) ref (Copyright 2024 Wiley-VCH); (middle-left) ref (Copyright 2024 American Chemical Society); (middle-right) ref (Copyright 2021 American Chemical Society); and (rightmost) ref (Copyright 2022 American Chemical Society).

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BP properties and its applications in IR sensing. (a) Schematic of black phosphorus crystalline structure. Reproduced from ref (Copyright 2014 IOP Publishing). (b) Illustration of bandgap tunability by vertical external electric field in BP thin film (∼10 nm). (c) Schematic of a hBN encapsulated BP photodetector in dual-gate configuration. Reproduced from ref (Copyright 2017 Springer Nature). (d) Atomic force microscopy (AFM) images of a BP flake after exposure to ambient environment for different times. Reproduced from ref (Copyright 2015 IOP Publishing). (e) AFM images of BP flake exposed to air without (left) and with (right) aluminum oxide (AlO _x ) encapsulation. Reproduced from ref (Copyright 2014 American Chemical Society). (f) The schematic growth setup to achieve a low P₄ pressure for BP and b-AsP synthesis. The diffusion section is filled with silica sand. Adapted from ref (Copyright 2023 Springer Nature). (g) 3D schematic view of the exfoliator of Q-Press, consisting of a sample stage, a press roller, a rewinder, and a vacuum chuck with a heater.

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2D materials have the potential to revolutionize photonic quantum technologies. They are already making significant inroads in the development of (a) single-photon emitters, (b) spontaneous parametric down-conversion, and (c) nonlinear plasmonics, with potential applications in quantum logic gates.

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Photonics meets magnetism in 2D. (a) Exploiting the electrical and strain control of 2D magnets for on-chip sources of helical light needed for protected quantum communications. (b) Self-hybridized polaritons optically read out the multilevel magnetization. (c) Various means by which optics is employed to probe the magnetic properties of 2D atomic crystals. (d) Stokes/anti-Stokes scattering, allowing for the detection of single magnetic excitations and their quantum correlations. Incorporation into photonic structures provides nonlocal corrections. (e) Combining optical, acoustic, and magnonic cavities to enable fine-tuned control of 2D magnets toward novel ground states.