The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing, and nanoscale lasers. Although plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the length scale of one atom. This is achieved through far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric spacer between graphene and metal rods. A theoretical model that takes into account the nonlocal optical response of both graphene and metal is used to describe the results. These ultraconfined plasmonic modes, addressed with far-field light excitation, enable a route to new regimes of ultrastrong light-matter interactions.

Acoustic graphene plasmons are highly confined electromagnetic modes carrying large momentum and low loss in the mid-infrared and terahertz spectra. However, until now they have been restricted to micrometer-scale areas, reducing their confinement potential by several orders of magnitude. Using a graphene-based magnetic resonator, we realized single, nanometer-scale acoustic graphene plasmon cavities, reaching mode volume confinement factors of ~5 × 10–10. Such a cavity acts as a mid-infrared nanoantenna, which is efficiently excited from the far field and is electrically tunable over an extremely large broadband spectrum. Our approach provides a platform for studying ultrastrong-coupling phenomena, such as chemical manipulation via vibrational strong coupling, as well as a path to efficient detectors and sensors operating in this long-wavelength spectral range.

We demonstrate the generation of self-accelerating surface plasmon beams along arbitrary caustic curvatures. These plasmonic beams are excited by free-space beams through a two-dimensional binary plasmonic phase mask, which provides the missing momentum between the two beams in the direction of propagation, and sets the required phase for the plasmonic beam in the transverse direction. We examine the cases of paraxial and nonparaxial curvatures and show that this highly versatile scheme can be designed to produce arbitrary plasmonic self-accelerating beams. Several different plasmonic beams, which accelerate along polynomial and exponential trajectories, are demonstrated both numerically and experimentally, with a direct measurement of the plasmonic light intensity using a near-field-scanning-optical-microscope. PACS numbers:Surface-plasmon-polaritons (SPPs) are surface electromagnetic waves that are coupled to electron waves, which propagate at the interface between a dielectric and a metallic medium [1]. The ability to control and guide plasmonic light waves opens exciting new possibilities in photonics and electronics [2,3]. Specifically, nanoscale on-chip technologies such as surface plasmon circuitry [4], sub-wavelength optical devices [5,6] and nanoscale electro-optics [7], as well as new applications in biology and chemistry such as bio-sensing, optical trapping and micro-manipulation at the nanoscale [8] have attracted great interest in recent years.In the last several years, new types of plasmonic beams have been realized having unique properties. These beams can be "non-spreading" -i.e. preserve their spatial shape with propagation, as well as "self-accelerating" -i.e. propagate along curved trajectories. For example, the plasmonic CosineGauss beam [9] is a non-spreading beam which propagates along a straight trajectory, whereas the plasmonic Airy beam [10-13] is non-spreading and propagates along a parabolic trajectory. The latter, is the only self-accelerating plasmonic beam demonstrated until now and is restricted to a parabolic trajectory. In this paper, we address the question of whether it is possible to create self-accelerating surface plasmon beams that propagate along arbitrary curved trajectories.The Airy function is an exact solution of the paraxial Helmholtz equation, or equivalently, of the Schrodinger equation for a free particle [14] which carries infinite energy. An actual Airy beam, however, carries finite energy and is obtained by truncating the infinitely long tail of the Airy function by using an exponential or Gaussian window [15]. The truncated Airy beam preserves its shape and self-accelerates, but only over a finite distance. Recently it was shown [16,17] that free-space non-spreading beams, propagating along arbitrary convex trajectories over finite distances, can be realized. However, the question still remains whether this concept, demonstrated so far only with free-space beams, can be used for the case of surface plasmons waves. If so, several fundamental challenges, owing t...

Surface plasmon polaritons and free-space beams are often coupled through periodic gratings. Here we show that by employing holographic-based techniques for modulating the grating, one can systematically control the amplitude and phase of the free-space beam. Alternatively, arbitrarily shaped surface plasmon can be generated. By using gratings with different periods for the input and output coupling, we obtain a planar beam transformer, whose resonance angles are related through a generalized form of the Bragg law. Specifically, we demonstrate the coupling of surface plasmon polaritons into focused free-space beams, as well as into accelerating Airy beams and vortex beams.

The field of 2D materials-based nanophotonics has been growing at a rapid pace, triggered by the ability to design nanophotonic systems with in-situ control 1 , unprecedented degrees of freedom, and to build material heterostructures from bottom up with atomic precision 2 . A wide palette of polaritonic classes [3][4][5][6] have been identified, comprising ultra-confined optical fields, even approaching characteristic length-scales of a single atom 7 . These advances have been a real boost for the emerging field of quantum nanophotonics, where the quantum mechanical nature of the electrons and/or polaritons and their interactions become relevant. Examples include, quantum nonlocal effects [8][9][10][11] , ultrastrong light-matter interactions [11][12][13][14][15][16] , Cherenkov radiation 13,17,18 , access to forbidden transitions 11 , hydrodynamic effects [19][20][21] , single-plasmon nonlinearities 22,23 , polaritonic quantization 24 , topological effects etc. 3,4 . In addition to these intrinsic quantum nanophotonic phenomena, the 2D material system can also be used as a sensitive probe 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, e.g. in superconductors 25 , or as a new tool to monitor the existence of Berry curvature in topological materials and superlattice effects in twisted 2D materials.In this article, 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 (cf. Figure 1): i) quantum sensing, featuring polaritons to probe superconductivity and explore new electronic transport hydrodynamic behaviours, 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 nano-scale (e.g., quantum chemistry 26 , nonlocal effects, high Purcell enhancement).

Nanomechanical resonators have emerged as sensors with exceptional sensitivities. These sensing capabilities open new possibilities in the studies of the thermodynamic properties in condensed matter. Here, we use mechanical sensing as a novel approach to measure the thermal properties of low-dimensional materials. We measure the temperature dependence of both the thermal conductivity and the specific heat capacity of a transition metal dichalcogenide (TMD) monolayer down to cryogenic temperature, something that has not been achieved thus far with a single nanoscale object. These measurements show how heat is transported by phonons in two-dimensional systems. Both the thermal conductivity and the specific heat capacity measurements are consistent with predictions based on first-principles.

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions inTMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides nearunity excitonic absorption, and emission of excitonic complexes that are observed at ultra-low excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, non-radiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices. * These authors contributed equally †

We introduce a new class of plasmonic holograms for the near field. These holograms provide complete control of the amplitude and phase of surface plasmon polaritons (SPPs), thereby enabling the generation of any desired plasmonic light beam. The scheme is based on a two-dimensional near-field plasmonic hologram, which couples the SPP from free space into the metal-dielectric interface, and also sets the attributes of the plasmonic beam. We demonstrate the concept for a wide variety of plasmonic beams with different qualities-in particular, "self-similar" Hermite-Gauss beams, "nondiffracting" cosine-Gauss beams, and "self-accelerating" Airy beams.