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We demonstrate a nonlinear plasmonic metasurface that exhibits strongly asymmetric second-harmonic generation: nonlinear scattering is efficient upon excitation in one direction and it is substantially suppressed when the excitation direction is reversed, thus enabling a diode-like functionality. A significant (approximately 10 dB) extinction ratio of SHG upon opposite excitations is measured experimentally and those findings are substantiated with full-wave simulations. The combination of two commonly used metals - aluminium and silver - produces a material composition asymmetry that results into a bianisotropic response of the system, as confirmed by performing homogenization analysis and extracting an effective susceptibility tensor. Finally, we discuss the implications of our results from the more fundamental perspectives of reciprocity and time-reversal asymmetry., Comment: 7 pages, 4 figures in the main text; 4 pages and 6 figures in the supporting information

We report thorough measurements of surface plasmon polaritons (SPPs) running along nearly perfect air-gold interfaces formed by atomically flat surfaces of chemically synthesized gold monocrystals. By means of amplitude- and phase-resolved near-field microscopy, we obtain their propagation length and effective mode index at visible wavelengths (532, 594, 632.8, 729, and 800 nm). The measured values are compared with the values obtained from the dielectric functions of gold that are reported in literature. Importantly, a reported dielectric function of monocrystalline gold implies $\sim 1.5$ times shorter propagation lengths than those observed in our experiments, whereas a dielectric function reported for properly fabricated polycrystalline gold leads to SPP propagation lengths matching our results. We argue that the SPP propagation lengths measured in our experiments signify the ultimate limit of optical losses in gold, encouraging further comprehensive characterization of optical material properties of pure gold as well as other plasmonic materials.

Cathodoluminescence spectroscopy in conjunction with second-order auto-correlation measurements of $g_2(\tau)$ allows to extensively study the synchronization of quantum light sources in low-dimensional structures. Co-existing excitons in two-dimensional transition metal dichalcogenide monolayers provide a great source of identical quantum emitters which can be simultaneously excited by an electron. In this article, we demonstrate large photon bunching with $g_2(0)$ up to $156\pm16$ of a tungsten disulfide monolayer, exhibiting a strong dependence on the electron-beam current density. To further improve the excitation synchronization and the electron-emitter interaction, we show exemplary that the careful selection of a simple and compact geometry -- a thin, monocrystalline gold nanodisk -- can be used to realize a record-high bunching $g_2(0)$ of up to $2152\pm236$. This approach to control the electron excitation of excitons in a \ce{WS2} monolayer allows for the synchronization of quantum emitters in an ensemble, which is important to further advance quantum information processing and computing technologies., Comment: Supporting information is available upon request to authors

Historically, the field of plasmonics has been relying on the framework of classical electrodynamics, with the local-response approximation of material response being applied even when dealing with nanoscale metallic structures. However, when approaching the atomic-scale confinement of the electromagnetic radiation, mesoscopic effects are anticipated to become observable, e.g., those associated with the nonlocal electrodynamic surface response of the electron gas. We investigate nonlocal effects in propagating gap surface plasmon modes in ultrathin metal--dielectric--metal planar waveguides, exploiting monocrystalline gold flakes separated by atomic-layer-deposited aluminum oxide. We use scanning near-field optical microscopy to directly access the near-field of such confined gap plasmon modes and measure their dispersion relation (via their complex-valued propagation constants). We compare our experimental findings with the predictions of the generalized nonlocal optical response theory to unveil signatures of nonlocal damping, which becomes appreciable for smaller dielectric gaps., Comment: Supporting information is available upon request to authors

Near-field mapping has been widely used to study hyperbolic phonon-polaritons in van der Waals crystals. However, an accurate measurement of the polaritonic loss remains challenging because of the inherent complexity of the near-field signal and the substrate-mediated loss. Here we demonstrate that large-area monocrystalline gold flakes, an atomically-flat low-loss substrate for image polaritons, provide a platform for precise near-field measurement of the complex propagation constant of polaritons in van der Waals crystals. As a topical example, we measure propagation loss of the image phonon-polaritons in hexagonal boron nitride, revealing that their normalized propagation length exhibits a parabolic spectral dependency. Further, we show that image phonon-polaritons exhibit up to a twice lower normalized propagation loss, while being 2.4 times more compressed compared to the case of dielectric substrate. We conclude that the monocrystalline gold flakes provide a unique nanophotonic platform for probing and exploitation of the image modes in low-dimensional materials.

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Apart from suggesting an approach for directional enhancement of nonlinear response in plasmonic systems, we anticipate that our findings can be used as a rapid and non-destructive method for characterization of crystal quality and orientation that may be of significant importance in future applications., Comment: 5 pages, 4 figures, color

Striving for nanometer-sized solid-state single-photon sources, we investigate atom-like quantum emitters based on single germanium vacancy (GeV) centers isolated in crystalline nanodiamonds (NDs). Cryogenic characterization indicated symmetry-protected and bright (> 10^6 counts/s with off-resonance excitation) zero-phonon optical transitions with up to 6-fold enhancement in energy splitting of their ground states as compared to that found for GeV centers in bulk diamonds (i.e., up to 870 GHz in highly strained NDs vs 150 GHz in bulk). Utilizing lithographic alignment techniques, we demonstrate an integrated nanophotonic platform for deterministic interfacing plasmonic waveguides with isolated GeV centers in NDs that enables 10-fold enhancement of single-photon decay rates along with the emission direction control by judiciously designing and positioning a Bragg reflector. This approach allows one to realize the unidirectional emission from single-photon dipolar sources introducing a novel method that is alternative to the propagation-direction-dependent techniques based on chiral interactions or topological protection. The developed plasmon-based nanophotonic platform opens thereby new perspectives for quantum nanophotonics in general and for realizing entanglement between single photons and spin qubits, in particular.

We observe strongly dissimilar scattering from two types of edges in hexagonal quasi-monocrystalline gold flakes with thicknesses around 1 micron. We identify as the origin the interference between a direct, quasi-specular scattering and an indirect scattering process involving an intermediate surface-plasmon state. The dissimilarity between the two types of edges is a direct consequence of the three-fold symmetry around the [111]-axis and the intrinsic chirality of a face-centered cubic lattice. We propose that this effect can be used to estimate flake thickness, crystal morphology, and surface contamination., Comment: 18 pages, including 5 figures

Colouration by surface nanostructuring has attracted a great deal of attention by the virtue of making use of environment-friendly recyclable materials and generating non-bleaching colours [1-8]. Recently, it was found possible to delegate the task of colour printing to laser post-processing that modifies carefully designed and fabricated nanostructures [9,10]. Here we take the next crucial step in the development of structural colour printing by dispensing with preformed nanostructures and using instead near-percolation metal films atop dielectric-metal sandwiches, i.e., near-percolation plasmonic reflector arrays. Scanning rapidly (~ 20 {\mu}m/s) across 4-nm-thin island-like gold films supported by 30-nm-thin silica layers atop 100-nm-thick gold layers with a strongly focused Ti-sapphire laser beam, while adjusting the average laser power from 1 to 10 mW, we produce bright colours varying from green to red by laser-heating-induced merging and reshaping of gold islands. Selection of strongly heated islands and their reshaping, both originating from the excitation of plasmonic resonances, are strongly influenced by the polarization direction of laser illumination, so that the colours produced are well pronounced only when viewed with the same polarization. Conversely, the laser colour writing with circular polarizations results in bright polarization-independent colour images. The fabrication procedure for near-percolation reflector arrays is exceedingly simple and scalable to mass production, while the laser-induced modification occurs inherently with the subwavelength resolution. This unique combination of remarkable features makes the approach developed for laser colour writing readily amenable for practical implementation and use in diverse applications ranging from nanoscale patterning for security marking to large-scale colour printing for decoration., Comment: 29 pages, 17 figures

Metasurfaces are paving the way to improve traditional optical components by integrating multiple functionalities into one optically flat metasurface design. We demonstrate the implementation of a multifunctional gap surface plasmon-based metasurface which, in reflection mode, splits orthogonal linear light polarizations and focuses into different focal spots. The fabricated configuration consists of 50 nm thick gold nanobricks with different lateral dimensions, organized in an array of 240 nm x 240 nm unit cells on the top of a 50 nm thick silicon dioxide layer, which is deposited on an optically thick reflecting gold substrate. Our device features high efficiency (up to ~65%) and polarization extinction ratio (up to ~30 dB), exhibiting broadband response in the near-infrared band (750-950 nm wavelength) with the focal length dependent on the wavelength of incident light. The proposed optical component can be forthrightly integrated into photonic circuits or fiber optic devices., Comment: 18 pages, including 5 figures