P. James Schuck

C–H terminated nanometer scale diamonds ( d = 1 to 15 nm) are synthesized from 1-fluoroadamantane at high pressure (6–8 GPa) and high temperature (500–1500 °C) in a multianvil press. High resolution transmission electron microscopy, X-ray diffraction, Raman, diffuse reflectance Fourier transform infrared, and X-ray absorption spectroscopies demonstrate the excellent crystallinity and atomically flat C–H terminated surfaces of nanodiamonds with (111) and (110) facets. The importance of hydrogen to the synthesis of nanodiamond and its faceting is discussed. Following vacancy generation, annealing and oxidation of the nanodiamonds, optically detected magnetic resonance and electron spin resonance coherence times ( T 2 = 0.9 and 2.1 μs) of nitrogen vacancy (NV) centers are measured. The obtained T 2 values are equivalent to the shallow NV centers (depth <10 nm) in bulk diamond crystals and larger nanocrystals prepared by mechanical milling.

Nanocavity optical trapping has enabled the isolation and precise positioning of nanoscale objects, from single molecules to quantum emitters. Yet, practically deployable single quantum sources require more than trapping, i.e., they require real-time control over position, polarization, brightness, and wavelength. Achieving such dynamic modulation remains a critical challenge, but is key to realizing ultra-secure quantum communication and adaptive quantum sensing. Here, we present tip-enhanced nano-optical trapping spectroscopy, utilizing shear-force atomic force microscopy in liquid, to achieve deterministic and dynamically reconfigurable single quantum emitters. This approach enables precise positioning and dipole alignment of cavity-coupled single quantum dots (QDs), driven by nano-optical gradient forces and field-induced torque, with simultaneous nano-spectroscopic analysis. Moreover, dynamic control of the tip-cavity mode volume and tip-induced pressure allows further tuning of trapping and coupling behaviors, modulating the quantum emission characteristics, e.g., brightness and photon energy, from the weak to the strong coupling regime. This work represents a significant advancement toward realizing the vast potential of QDs in quantum applications, such as tuning emission properties of single photon sources for quantum switches and modulators, or implementing quantum gates via plexciton state manipulation.

Abstract Cavities provide a means to manipulate the optical and electronic responses of quantum materials by selectively enhancing light-matter interaction at specific frequencies and momenta. While cavities typically involve external structures, exfoliated flakes of van der Waals (vdW) materials can form intrinsic self-cavities due to their small finite dimensions, confining electromagnetic fields into plasmonic cavity modes, characterized by standing-wave current distributions. While cavity-enhanced phenomena are well-studied at optical frequencies, the impact of self-cavities on nonlinear electronic responses—such as directional photocurrent—remains largely unexplored, particularly in the terahertz regime, critical for emerging ultrafast optoelectronic technologies. Here, we report a self-cavity-induced Purcell enhancement of directional photocurrents in the vdW semimetal WTe 2 . Using ultrafast optoelectronic circuitry, we measured coherent near-field THz emission resulting from nonlinear photocurrents excited at the sample edges. We observed enhanced emission at finite frequencies, tunable via excitation fluence and sample geometry, which we attribute to plasmonic interference effects controlled by the cavity boundaries. We developed an analytical theory that captures the cavity resonance conditions and spectral response across multiple devices. Our findings establish WTe 2 as a bias-free, geometry-tunable THz emitter and demonstrate the potential of self-cavity engineering for controlling nonlinear, nonequilibrium dynamics in quantum materials.

Electron microscopy (EM) is fundamental to nanocrystal characterization, but some structures degrade quickly under electron beams, limiting advanced structural characterization. Here, we introduce a synthetic strategy that combines layer-by-layer shell growth with in situ annealing to produce NaYb 0.8 Er 0.2 F 4 alloyed upconverting nanoparticles (UCNPs) with enhanced structural stability and optical properties. Using an automated synthesis platform to control precursor delivery and annealing cycles, high rare-earth ion concentrations are maintained during shell growth and annealing at high temperature, reducing luminescence quenching and degradation under electron beams. This in situ annealing layer-by-layer (ISA-LBL) approach gives rise to alloyed UCNPs (aUCNPs) with exceptional electron-beam stability, reducing beam-induced fractures and voids by >90% compared to conventionally synthesized aUCNPs. ISA-LBL aUCNPs also exhibit enhanced photoluminescence intensity and extended lifetimes, consistent with fewer quenching defects. This demonstrates a synthetic route to nanocrystals with enhanced structural integrity, increasing their compatibility with EM studies and their utility in ionizing environments.

Electron microscopy is fundamental to nanocrystal characterization, but some structures are quickly degraded in electron beams, limiting advanced structural characterization. Here, we introduce a synthetic strategy that combines layer-by-layer shell growth with in situ annealing to produce NaYbErF4 alloyed upconverting nanoparticles (UCNPs) with enhanced structural stability and optical properties. Using an automated synthesis platform to precisely control precursor delivery and annealing cycles, high rare earth ion concentrations are maintained during shell growth and annealing at high temperature, reducing luminescence quenching and degradation under electron beams. This in situ annealing layer-by-layer (ISA-LBL) approach gives rise to alloyed UCNPs with exceptional electron beam stability, reducing beam-induced fractures and voids by over 90% compared to conventionally synthesized aUCNPs. ISA-LBL aUCNPs also exhibit enhanced photoluminescence intensity and extended lifetimes, consistent with fewer quenching defects. This demonstrates a synthetic route to nanocrystals with enhanced structural integrity, increasing their compatibility with EM studies and their utility in highly ionizing environments.

The BiTeX family of polar van der Waals (vdW) semiconductors offers a unique platform for exploring the interplay between polar crystalline structure and nonlinear optical phenomena. Here, we utilize second harmonic generation (SHG) polarimetry to demonstrate giant anisotropic optical nonlinearities in BiTeBr and BiTeI driven by contributions to the crystals' nonlinear polarizability originating from their permanent dipole moment. In addition, using SHG microscopy, we show that BiTeI displays a distinctive SHG spatial texture consisting of thread-like regions of reduced harmonic intensity. These features demark the boundaries between phase and anti-phase polar domains, confirmed via piezoresponse force microscopy and are attributable to disorder-driven restoration of inversion symmetry and concomitant optical interference effects. Our results unveil the power of polarization-resolved SHG microscopy in elucidating the intricate relationship between structure, symmetry, and nonlinear optical responses in polar vdW materials and highlight the promise of BiTeX as a material platform for domain-engineered nanoscale nonlinear photonics.

The BiTeX family of polar van der Waals (vdW) semiconductors offers a unique platform for exploring the interplay between polar crystalline structure and nonlinear optical phenomena. Here, we utilize second harmonic generation (SHG) polarimetry to demonstrate giant anisotropic optical nonlinearities in BiTeBr and BiTeI driven by contributions to the crystals' nonlinear polarizability originating from their permanent dipole moment. In addition, using SHG microscopy, we show that BiTeI displays a distinctive SHG spatial texture consisting of thread-like regions of reduced harmonic intensity. These features demark the boundaries between phase and anti-phase polar domains, confirmed via piezoresponse force microscopy and are attributable to disorder-driven restoration of inversion symmetry and concomitant optical interference effects. Our results unveil the power of polarization-resolved SHG microscopy in elucidating the intricate relationship between structure, symmetry, and nonlinear optical responses in polar vdW materials and highlight the promise of BiTeX as a material platform for domain-engineered nanoscale nonlinear photonics.

Tunability in solid-state materials is essential for testing theory, discovering quantum phases, and enabling functionality. Layered van der Waals (vdW) semiconductors offer a unique platform, providing new degrees of freedom at the two-dimensional (2D) limit through exfoliation and external controls. Here, we demonstrate tunability of symmetry and electronic structure via halogen substitution in a family of layered vdW tungsten oxyhalides. Substituting the halogens in WO 2 X 2 (X = I, Br, Cl) tunes the bandgap across a broad energy range and modifies the structural symmetry from centrosymmetric to noncentrosymmetric. By alloying WO 2 I 2– y Br y, we continuously tune the polar distortion and optical gap across the visible range. These insights into halogen substitution effects on symmetry and electronic structure lay the foundation for new tunable vdW semiconductors for optoelectronics and nonlinear optics.

I will discuss our use of tip-enhanced spectroscopies and spectroscopic imaging to probe localized exciton properties in 2D semiconductors. These include the study of engineered nanowrinkles in monolayer TMDs, strain-localized excitons in 1D TMD nanoribbons, and naturally formed nanobubbles. In the latter, we observe for the first time a tunable, strain-induced bandgap transition, in which nano-optical approaches enable the brightening of momentum-dark excitons. Our work demonstrates the potential to control and tailor exciton states localized in monolayer (1L) TMDs, paving the way for on-chip polariton-based nanophotonic applications spanning quantum information processing to photochemistry.

This talk focuses on the development of photon avalanching (PA) in various upconverting nanoparticle systems. Extraordinary properties and applications have already been demonstrated in these materials, with progress accelerated by combining recent advances in lanthanide-based nanomaterials synthesis, the modeling of optical properties and energy transfer, and new characterization methods. Here, I highlight the recent demonstration of piconewton to micronewton force sensing with these next-generation nanocrystals and the development of ANPs capable of operating in three different response modalities. In addition, I will touch on their use in applications spanning superresolution imaging, photoswitching, optical patterning and memory, and ultrasensitive nanothermometry. I will then finish with a brief discussion of future prospects, key challenges, and the potential promise of nanoscale PA.

I will discuss our recent results demonstrating exceptional nonlinear optical frequency conversion in transition metal dichalcogenides (TMDs). By fabricating nano-photonic elements such as waveguides, metasurfaces, and periodically poled stacks, we achieve macroscopic frequency conversion efficiencies over microscopic distances, 10 to 100× smaller than current systems with similar performances. Further, we report the broadband generation of entangled photon pairs at telecom wavelength via quasi-phase-matched spontaneous parametric down-conversion, showing the highest coincidence-to-accidental-ratio (CAR) ever demonstrated in a vdW material. This work opens the new and unexplored field of phase-matched nonlinear optics with microscopic van der Waals crystals, unlocking applications that require simple, ultra-compact technologies such as on-chip entangled photon-pair sources for integrated quantum circuitry and sensing.

We unveil intrinsic terahertz (THz) emission from Floquet-Bloch states in T<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</inf>-WTe<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> using an on-chip optoelectronic platform. Under intense optical driving, we observe robust THz radiation whose origin is localized to the material’s edges, scaling linearly with the driving field intensity—a hallmark of photocurrents excited in the Floquet-Bloch regime.

In addition to a plethora of emergent phenomena, the spatial topology of optical vortices enables an array of applications in optical communications and quantum information science. Multibeam nonlinear optical processes, augmented by optical vortices, are essential in this context, providing robust access to an infinitely large set of quantum states associated with the orbital angular momentum of light. Here, we push the boundaries of vortex nonlinear optics to the ultimate limits of material dimensionality. By exploiting multipulse difference frequency, sum frequency, and four-wave mixing in monolayer quantum materials, we demonstrate their ability to independently control the orbital angular momentum and radial distribution of vortex light-fields in addition to their wavelength. Due to the atomically thin nature of the host crystal, this control spans a broad spectral bandwidth in a highly integrable platform that is unconstrained by the traditional limits of bulk nonlinear optical materials. Our work heralds an innovative path for ultracompact and scalable hybrid nanophotonic technologies empowered by twisted nonlinear light-matter interactions in van der Waals nanomaterials.

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.

Collective modes in multilayer graphene, such as plasmons and phonons, exhibit sensitivity to displacement fields and interlayer coupling, distinguishing them from their counterparts in single-layer graphene. Here, we engineer collective modes in charge-transfer heterostructures composed of multilayer graphene and <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>α</a:mi> <a:mtext>−</a:mtext> <a:msub> <a:mi>RuCl</a:mi> <a:mn>3</a:mn> </a:msub> </a:math> . In heterostructures with a single <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi>α</c:mi> <c:mtext>−</c:mtext> <c:msub> <c:mrow> <c:mi>RuCl</c:mi> </c:mrow> <c:mrow> <c:mn>3</c:mn> </c:mrow> </c:msub> </c:mrow> </c:math> interface, the charge transfer generates displacement fields up to 7 V/nm at the interface between <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>α</e:mi> <e:mtext>−</e:mtext> <e:msub> <e:mi>RuCl</e:mi> <e:mn>3</e:mn> </e:msub> </e:math> and the adjacent graphene layer—the highest value achieved through charge-transfer methods. As a result of the broken inversion symmetry, we discover enhanced nonlinear optical response and modified phonon selection rules. Conversely, we find that multilayer graphene sandwiched between two <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>α</g:mi> <g:mtext>−</g:mtext> <g:msub> <g:mi>RuCl</g:mi> <g:mn>3</g:mn> </g:msub> </g:math> flakes causes displacement fields to cancel. There, we achieve carrier densities as high as <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mn>8</i:mn> <i:mo>×</i:mo> <i:msup> <i:mn>10</i:mn> <i:mn>13</i:mn> </i:msup> <i:mtext> </i:mtext> <i:mtext> </i:mtext> <i:mrow> <i:msup> <i:mrow> <i:mi>cm</i:mi> </i:mrow> <i:mrow> <i:mo>−</i:mo> <i:mn>2</i:mn> </i:mrow> </i:msup> </i:mrow> </i:math> in multilayer graphene and restore the phonon selection rules to their unperturbed state. Meanwhile, we demonstrate that plasmonic properties derive from the depletion of multiple valence bands. As a result of the quasilinear band dispersion, these “Dirac multiband plasmons” are relatively unaffected by displacement fields. On the other hand, the inverted heterostructure sequence—two multilayer graphene sheets encapsulating <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>α</k:mi> <k:mtext>−</k:mtext> <k:msub> <k:mi>RuCl</k:mi> <k:mn>3</k:mn> </k:msub> </k:math> —activates significant alteration of the plasmons via interlayer plasmon-plasmon coupling. Hence, multilayer graphene and <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>α</m:mi> <m:mtext>−</m:mtext> <m:msub> <m:mi>RuCl</m:mi> <m:mn>3</m:mn> </m:msub> </m:math> heterostructures offer a gate-free platform for engineering collective modes derived from inversion symmetry and interlayer coupling.

A new frontier in van der Waals twistronics is the development of three-dimensional (3D) supertwisted materials, where each successive atomic layer rotates by the same angle. While two-dimensional (2D) moire systems have been extensively studied, the unique phenomena arising from 3D twistronics remain largely unexplored. In this work, we report the discovery of flat-band excitons in 3D supertwisted WS2, revealed by systematic photoluminescence (PL) experiments and electronic structure calculations. These excitons retain key features of 2D moire transition metal dichalcogenides (TMDs)-such as layer confinement, moire-driven localization, and strong Coulomb interactions-while also offering advantages in scalability and enhanced optical responses in three dimensions. Beyond the PL signatures reminiscent of 2D A excitons, we observe novel direct and indirect exciton emission uniquely tied to the supertwist geometry. Using generalized Bloch band theory and local density of states calculations that incorporate screw rotational symmetry, we uncovered the coexistence of 2D and 3D flatband gaps. These flat-band excitons serve as sensitive probes of the electronic properties of 3D supertwisted semiconductors and open new pathways for applications in quantum optoelectronics.

Single-layer WSe 2 hosts localized states that are technologically promising solid-state quantum light sources. Their formation requires localized tensile strain and is robust to substrate, material quality, and nanoscale stressors. How these factors affect the quantum emitters is obscured by intrasample heterogeneity that hinders the comparison of individual emitters across different samples. Here, by statistically comparing emitter populations from samples with unique permutations of material quality, substrate composition, and nanoscale stressors, we show that sample architecture affects emitter dynamics at the ensemble level. Systematic differences are revealed by formulating simple numerical descriptors of emitter photoluminescence transients from the different populations. Whereas the substrate does not significantly alter emitter dynamics, both nonlinear relaxation and exciton diffusion to emitter sites are reduced in high-quality WSe 2 on gold nanocones. This study demonstrates how statistical comparisons can reveal important structure–property relationships of the emitters that are important for further developing them into quantum light sources.

Atomically layered van der Waals (vdW) materials exhibit remarkable properties, including highly confined infrared waveguide modes and the capacity for infrared emission in the monolayer limit. Here we engineered structures that leverage both of these nano-optical functionalities. Specifically, we encased a photoluminescing atomic sheet of MoTe2 within two bulk crystals of WSe2, forming a vdW waveguide for the embedded light-emitting monolayer. The modified electromagnetic environment offered by the WSe2 waveguide alters MoTe2 spontaneous emission—a phenomenon we directly image with our interferometric nano-photoluminescence technique. We captured spatially oscillating nanoscale patterns prompted by spontaneous emission from MoTe2 into waveguide modes of WSe2 slabs. We quantify the resulting Purcell-enhanced emission rate within the framework of a waveguide quantum electrodynamics model, relating the MoTe2 spontaneous emission rate to the measured waveguide dispersion. Our work marks a substantial advance in the implementation of all-vdW quantum electrodynamics waveguides. A nano-optical probe of the Purcell effect in a van der Waals waveguide is demonstrated, exploiting its highly confined infrared waveguide modes and the capacity for infrared emission in the monolayer limit of atomically layered van der Waals materials.

Floquet engineering uses time-periodic electromagnetic fields to modify the electronic properties of quantum materials via the creation of Floquet-Bloch states. These photon-dressed states inherit features from both the material and the driving field, enabling the exploration and control of quantum phenomena in light-matter hybrid systems. In non-centrosymmetric materials, shift currents can arise from the quantum geometric properties of electronic wavefunctions. However, shift currents from Floquet-Bloch states remain experimentally unexplored. Here, we employ an on-chip optoelectronic circuit to detect intrinsic terahertz emission from Floquet-Bloch states in T d -WTe 2 under intense optical driving. We observe strong edge-localized terahertz emission that scales linearly with the driving field, consistent with the theoretical prediction for shift currents generated by Floquet-Bloch states. The results advance our understanding of strongly driven quantum materials and provide insights for developing efficient, bias-free terahertz sources for future optoelectronic technologies.

Abstract Polaritons are quantum mechanical superpositions of photon states with elementary excitations in molecules and solids. The light–matter admixture causes a characteristic frequency‐momentum dispersion shared by all polaritons irrespective of the microscopic nature of material excitations that could entail charge, spin, lattice or orbital effects. Polaritons retain the strong nonlinearities of their matter component and simultaneously inherit ray‐like propagation of light. Polaritons prompt new properties, enable new opportunities for spectroscopy/imaging, empower quantum simulations and give rise to new forms of synthetic quantum matter. Here, we review the emergent effects rooted in polaritonic quasiparticles in a wide variety of their physical implementations. We present a broad portfolio of the physical platforms and phenomena of what we term polaritonic quantum matter. We discuss the unifying aspects of polaritons across different platforms and physical implementations and focus on recent developments in: polaritonic imaging, cavity electrodynamics and cavity materials engineering, topology and nonlinearities, as well as quantum polaritonics.

Abstract Graphene is a privileged 2D platform for hosting confined light-matter excitations known as surface plasmon polaritons (SPPs), as it possesses low intrinsic losses and a high degree of optical confinement. However, the isotropic nature of graphene limits its ability to guide and focus SPPs, making it less suitable than anisotropic elliptical and hyperbolic materials for polaritonic lensing and canalization. Here, we present graphene/CrSBr as an engineered 2D interface that hosts highly anisotropic SPP propagation across mid-infrared and terahertz energies. Using scanning tunneling microscopy, scattering-type scanning near-field optical microscopy, and first-principles calculations, we demonstrate mutual doping in excess of 10 13 cm –2 holes/electrons between the interfacial layers of graphene/CrSBr. SPPs in graphene activated by charge transfer interact with charge-induced electronic anisotropy in the interfacial doped CrSBr, leading to preferential SPP propagation along the quasi-1D chains that compose each CrSBr layer. This multifaceted proximity effect both creates SPPs and endows them with anisotropic propagation lengths that differ by an order-of-magnitude between the in-plane crystallographic axes of CrSBr.

We demonstrate a technique for the fabrication of p-type transistors based on monolayer molybdenum telluride (MoTe 2 ). In the device structure, monolayer hexagonal boron nitride ( h BN) protects the channel from oxidation and acts as a tunnel barrier for Pd contacts. P-doping is achieved through charge transfer from oxidized WSe 2 . The contacts show low resistance and Ohmic behavior down to cryogenic temperatures. The resulting FET mobility is the highest reported to date for monolayer MoTe 2 . This architecture can be utilized for electronic and optoelectronic applications of MoTe 2 and will be useful for electrical transport studies of exotic quantum phenomena in MoTe 2 twisted bilayers.

Van der Waals (vdW) semiconductors have emerged as promising platforms for efficient nonlinear optical conversion, including harmonic and entangled photon generation. Although major efforts are devoted to integrating vdW materials in nanoscale waveguides for miniaturization, the realization of efficient, phase-matched conversion in these platforms remains challenging. Here, to address this challenge, we report a far-field ultrafast imaging method to track the propagation of both fundamental and harmonic waves within vdW waveguides with femtosecond and sub-50 nanometre spatiotemporal precision. We focus on light propagation in slab waveguides of rhombohedral-stacked MoS2, a vdW semiconductor with large nonlinear susceptibility. Our method allows systematic optimization of nonlinear conversion by determining the phase-matching angles, mode profiles and losses in waveguides without prior knowledge of material optical constants. Using this approach, we show that both multimode and single-mode rhombohedral-stacked MoS2 waveguides support birefringent phase matching, demonstrating the material’s potential for efficient on-chip nonlinear optics. An optical method images nonlinear optical processes in waveguides with femtosecond and nanometre precision. The approach is used to achieve phase-matched second-harmonic generation in 3R-MoS2 waveguides.

Nonlinear optics lies at the heart of classical and quantum light generation. The invention of periodic poling revolutionized nonlinear optics and its commercial applications by enabling robust quasi-phase-matching in crystals such as lithium niobate. However, reaching useful frequency conversion efficiencies requires macroscopic dimensions, limiting further technology development and integration. Here we realize a periodically poled van der Waals semiconductor (3R-MoS2). Owing to its large nonlinearity, we achieve a macroscopic frequency conversion efficiency of 0.03% at the relevant telecom wavelength over a microscopic thickness of 3.4 μm (that is, 3 poling periods), 10–100× thinner than current systems with similar performances. Due to intrinsic cavity effects, the thickness-dependent quasi-phase-matched second harmonic signal surpasses the usual quadratic enhancement by 50%. Further, we report the broadband generation of photon pairs at telecom wavelength via quasi-phase-matched spontaneous parametric down-conversion, showing a maximum coincidence-to-accidental ratio of 638 ± 75. This work opens the new and unexplored field of phase-matched nonlinear optics with microscopic van der Waals crystals, unlocking applications that require simple, ultra-compact technologies such as on-chip entangled photon-pair sources for integrated quantum circuitry and sensing. Researchers created a periodically poled van der Waals semiconductor (3R-MoS2) and achieved a macroscopic frequency conversion efficiency of 0.03% over a thickness of 3.4 μm. The quasi-phase-matched second harmonic signal surpasses the usual quadratic enhancement by 50% and broadband generation of photon pairs at telecom wavelength is demonstrated with a coincidence-to-accidental ratio of 638 ± 75.

Optically bistable materials respond to a single input with two possible optical outputs, contingent on excitation history. Such materials would be ideal for optical switching and memory, but the limited understanding of intrinsic optical bistability (IOB) prevents the development of nanoscale IOB materials suitable for devices. Here we demonstrate IOB in Nd3+-doped KPb2Cl5 avalanching nanoparticles, which switch with high contrast between luminescent and non-luminescent states, with hysteresis characteristic of bistability. We elucidate a non-thermal mechanism in which IOB originates from suppressed non-radiative relaxation in Nd3+ ions and from the positive feedback of photon avalanching, resulting in extreme, >200th-order optical nonlinearities. The modulation of laser pulsing tunes the hysteresis widths, and dual-laser excitation enables transistor-like optical switching. This control over nanoscale IOB establishes avalanching nanoparticles for photonic devices in which light is used to manipulate light. Intrinsic optical bistability in Nd3+-doped KPb2Cl5 avalanching nanoparticles enables high-contrast switching between luminescent and non-luminescent states and transistor-like optical responses. A non-thermal mechanism is discussed and >200th-order optical nonlinearities are shown to be possible.

Mechanical force is an essential feature for many physical and biological processes1, 2, 3, 4, 5, 6–7, and remote measurement of mechanical signals with high sensitivity and spatial resolution is needed for diverse applications, including robotics8, biophysics9,10, energy storage11 and medicine12,13. Nanoscale luminescent force sensors excel at measuring piconewton forces, whereas larger sensors have proven powerful in probing micronewton forces14, 15–16. However, large gaps remain in the force magnitudes that can be probed remotely from subsurface or interfacial sites, and no individual, non-invasive sensor is capable of measuring over the large dynamic range needed to understand many systems14,17. Here we demonstrate Tm3+-doped avalanching-nanoparticle18 force sensors that can be addressed remotely by deeply penetrating near-infrared light and can detect piconewton to micronewton forces with a dynamic range spanning more than four orders of magnitude. Using atomic force microscopy coupled with single-nanoparticle optical spectroscopy, we characterize the mechanical sensitivity of the photon-avalanching process and reveal its exceptional force responsiveness. By manipulating the Tm3+ concentrations and energy transfer within the nanosensors, we demonstrate different optical force-sensing modalities, including mechanobrightening and mechanochromism. The adaptability of these nanoscale optical force sensors, along with their multiscale-sensing capability, enable operation in the dynamic and versatile environments present in real-world, complex structures spanning biological organisms to nanoelectromechanical systems. An avalanching-nanoparticle force sensor that can operate in the piconewton-to-micronewton range with exceptional force responsiveness is achieved by using the mechanical sensitivity of the photon-avalanching process.

We present an ultra-thin source of down-converted photon pairs. These semiconductor-based devices are periodically poled to enhance generation efficiencies, yielding high-fidelity, maximally-entangled, tunable polarization states, without any walk-off compensation or interferometric techniques.

This study characterizes the influence of self-assembly conditions on the aggregation pathway and resulting photophysical properties of one-dimensional aggregates of the simple imide-substituted perylene diimide, N, N'-didodecyl-3,4,9,10-perylenedicarboximide (ddPDI). We show that ddPDI, which has symmetric alkyl chains at the imide positions, assembles into fibers with distinct morphology, emission spectra, and temperature-dependent behavior as a function of preparation conditions. In all conditions explored, aggregates are one-dimensional; however, assembly conditions can bias formation to either J-like or H-like aggregates. Specifically, a solvent phase interfacial (SPI) method yields two types of aggregates with distinct morphology and photophysical properties while a surface and solvent vapor assisted method (SSVA) generates more uniform aggregates with H-dominant behavior. A combined SPI and SSVA approach facilitates the simultaneous generation and in situ characterization of distinct ddPDI assemblies, some of which assemble via seeded growth. Microscopic and spectroscopic imaging unveil the heterogeneity among ddPDI aggregates, each with unique photophysical properties including H-dominant aggregates with a very high degree of molecular alignment and uniformity in intermolecular organization. Overall, this study highlights the pathway complexity in self-assembly of even the simplest PDI molecules, paving the way for utilization of simple PDI aggregates in applications that demand diverse photophysical behavior.

Photon avalanche (PA)-where the absorption of a single photon initiates a 'chain reaction' of additional absorption and energy transfer events within a material-is a highly nonlinear optical process that results in upconverted light emission with an exceptionally steep dependence on the illumination intensity. Over 40 years following the first demonstration of photon avalanche emission in lanthanide-doped bulk crystals, PA emission has been achieved in nanometer-scale colloidal particles. The scaling of PA to nanomaterials has resulted in significant and rapid advances, such as luminescence imaging beyond the diffraction limit of light, optical thermometry and force sensing with (sub)micron spatial resolution, and all-optical data storage and processing. In this review, we discuss the fundamental principles underpinning PA and survey the studies leading to the development of nanoscale PA. Finally, we offer a perspective on how this knowledge can be used for the development of next-generation PA nanomaterials optimized for a broad range of applications, including mid-IR imaging, luminescence thermometry, (bio)sensing, optical data processing and nanophotonics.

Van der Waals (vdW) semiconductors have emerged as promising platforms for efficient nonlinear optical conversion, including harmonic and entangled photon generation. Although major efforts are devoted to integrating vdW materials in nanoscale waveguides for miniaturization, the realization of efficient, phase-matched conversion in these platforms remains challenging. To address this challenge, we develop a far-field ultrafast imaging method to track the propagation of both fundamental and harmonic waves within vdW waveguides with extreme spatiotemporal resolution. Our approach allows systematic optimization of nonlinear conversion by determining the phase-matching angles, mode profiles, and losses in waveguides without a priori knowledge of material properties. We focus on light propagation in slab waveguides of rhombohedral-stacked MoS2, an emerging vdW semiconductor with giant nonlinear susceptibility. Our results reveal that these waveguides support birefringent phase-matching, demonstrating the material's potential for efficient on-chip nonlinear optics. This work establishes spatiotemporal imaging of light propagation in waveguides as an incisive and general method to identify new materials and architectures for efficient nonlinear nanophotonics.

Nanoribbons (NRs) of atomic layer transition metal dichalcogenides (TMDs) can boost the rapidly emerging field of quantum materials owing to their width-dependent phases and electronic properties. However, the controllable downscaling of width by direct growth and the underlying mechanism remain elusive. Here, we demonstrate the vapor-liquid-solid growth of single crystal of single layer NRs of a series of TMDs (MeX2: Me = Mo, W; X = S, Se) under chalcogen vapor atmosphere, seeded by pre-deposited and respective transition metal-alloyed nanoparticles that also control the NR width. We find linear dependence of growth rate on supersaturation, known as a criterion for continues growth mechanism, which decreases with decreasing of NR width driven by the Gibbs-Thomson effect. The NRs show width-dependent photoluminescence and strain-induced quantum emission signatures with up to ≈ 90% purity of single photons. We propose the path and underlying mechanism for width-controllable growth of TMD NRs for applications in quantum optoelectronics. Size control in quantum materials by direct growth is still difficult to achieve. Here, the authors present the width-dependent growth of single-layer nanoribbons of transition metal dichalcogenides from nanoalloy seeds, achieving strain-induced quantum emission with a purity of up to 90 % for single photons.

Two-dimensional semiconductors exhibit pronounced many-body effects and intense optical responses due to strong Coulombic interactions. Consequently, subtle differences in photoexcitation conditions can strongly influence how the material dissipates energy during thermalization. Here, using multiple excitation spectroscopies, we show that a distinct thermalization pathway emerges at elevated excitation energies, enhancing the formation of trions and charged biexcitons in single-layer WSe 2 by up to 2× and 5×, respectively. Power- and temperature-dependent measurements lend insights into the origin of the enhancement. These observations underscore the complexity of excited state relaxation in monolayer semiconductors, provide insights for the continued development of carrier thermalization models, and highlight the potential to precisely control excitonic yields and probe nonequilibrium dynamics in 2D semiconductors.

Intense electromagnetic fields can result in dramatic changes in the electronic properties of solids. These changes are commonly studied using optical probes of the modified electronic structure. Here we use optical-scanning tunneling microscopy (optical-STM) equipped with near-field continuous wave (CW) laser excitation to directly measure the electronic structure of light-dressed states in a monolayer transition metal dichalcogenide (TMD) semiconductor, WSe2. We find that the effective tunneling gap and tunneling density of states are strongly influenced by the intensity of the electromagnetic field when the applied field is resonant with the bandgap of the semiconductor. Our findings indicate that CW laser excitation can be used to generate light-dressed electronic states of quantum materials when confined strongly to the nanoscale.

Optical technologies enable real-time, noninvasive analysis of complex systems but are limited to discrete regions of the optical spectrum. While wavelengths in the short-wave infrared (SWIR) window (typically, 1700–3000 nm) should enable deep subsurface penetration and reduced photodamage, there are few luminescent probes that can be excited in this region. Here, we report the discovery of lanthanide-based upconverting nanoparticles (UCNPs) that efficiently convert 1740 or 1950 nm excitation to wavelengths compatible with conventional silicon detectors. Screening of Ln 3+ ion combinations by differential rate equation modeling identifies Ho 3+ /Tm 3+ or Tm 3+ dopants with strong visible or NIR-I emission following SWIR excitation. Experimental upconverted photoluminescence excitation (U-PLE) spectra find that 10% Tm 3+ -doped NaYF 4 core/shell UCNPs have the strongest 800 nm emission from SWIR wavelengths, while UCNPs with an added 2% or 10% Ho 3+ show the strongest red emission when excited at 1740 or 1950 nm. Mechanistic modeling shows that addition of a low percentage of Ho 3+ to Tm 3+ -doped UCNPs shifts their emission from 800 to 652 nm by acting as a hub of efficient SWIR energy acceptance and redistribution up to visible emission manifolds. Parallel experimental and computational analysis shows rate equation models are able to predict compositions for specific wavelengths of both excitation and emission. These SWIR-responsive probes open a new IR bioimaging window, and are responsive at wavelengths important for vision technologies.

The advancement of quantum photonic technologies relies on the ability to precisely control the degrees of freedom of optically active states. First, motivated by recent evidence showing that nanowrinkles generate strain-localized room-temperature emitters, we demonstrate a method to intentionally induce wrinkles with collections of stressors. We show that long-range wrinkle direction and position are controllable with patterned array design, forming quantum emitters as evidenced by cryogenic anti-bunched emission. Next, we realize real-time, room-temperature tunable strong plasmon-exciton coupling in 2D semiconductor monolayers enabled by a general approach that combines strain engineering plus force- and voltage-adjustable plasmonic nanocavities. We show that the exciton energy and nanocavity plasmon resonance can be controllably toggled in concert by applying pressure with a plasmonic nanoprobe, allowing in operando control of detuning and coupling strength, with observed Rabi splittings >100 meV. We identify distinct polariton bands and dark polariton states, and map their evolution as a function of nanogap and strain tuning.