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×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. 1 arXiv:1709.04257v2 [cond-mat.mes-hall]

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 postprocessing 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 dielectricmetal sandwiches, i.e., near-percolation plasmonic reflector arrays. Scanning rapidly (~ 20 μm/s) across 4-nm-thin island-like gold films supported by 30-nm-thin silica layers atop 100nm-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.*e-mail: seib@mci.sdu.dk Ultrafast laser processing of materials holds important implications for both applied and fundamental research 11 , including novel possibilities for post-processing and reconfiguring nanophotonic structures 12,13 . Laser processing at the nanoscale often involves resonant (local) field enhancement and photo-thermal effects, for example, to modify the morphology of individual plasmonic resonators 14 or dielectric nanoparticles (NPs) 10 . Fascinating applications range from the ultrafast delivery of heat at the nanoscale 15 to laser writing of plasmonic colours with subdiffraction-limit resolution 9 . Laser ablation, achieved typically at high radiation fluences, can also be used for colouration of plasmonic surfaces 1 , although with considerably lower spatial resolutions. Previous works on plasmonic colours 2,3 have extensively utilised progress in nanofabrication technologies to accurately pattern surfaces with carefully designed plasmonic nanostructures 4-6 , using also replication techniques oriented towards mass production 7,8 .Considering a very large body of research conducted in the field of structural colours 1-10 , it seems unavoidable...

Striving for nanometer-sized solid-state single-photon sources, we investigate atomlike 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 singlephoton 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 quasimonocrystalline 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.

Controlled and reliable field enhancement (FE) effects associated with the excitation of plasmons in resonant metal nanostructures constitute an essential prerequisite for the development of various sensing configurations, especially those utilizing surface-enhanced Raman scattering (SERS) spectroscopy techniques. Leveraging advantages of random nanostructures in providing strong collective resonances in a broad wavelength range with the design flexibility of individual gap plasmon resonators, we experimentally investigate fractal-shaped arrays of gap plasmon resonators and characterize the occurring FE effects by mapping SERS signals from uniformly spread Rhodamine 6G with high-resolution Raman microscopy. In such a geometry, the total FE is expected to benefit from both FE associated with gap plasmon excitation and FE due to constructive interference of the surface plasmon modes reflected and diffracted by fractal-shaped boundaries. Linear reflection imaging spectroscopy is used to verify that the fabricated nanostructures exhibit spatially distributed resonances (bright spots) close to the excitation wavelengths used for the Raman microscopy. The positions of bright spots are argued to be influenced by fractal-shaped boundaries, particle dimensions, polarization, and wavelength of the incident and scattered light. Experimentally obtained SERS images from similar fractal (gold) structures fabricated with different dielectric SiO 2 spacer thicknesses (0, 20, and 40 nm) featured diffraction-limited bright spots corresponding to local SERS enhancements of up to ∼10 7 (relative to Raman signals obtained with a glass substrate) for 40 nm thick SiO 2 layers. Our results indicate that the strategy of combining fractal array geometry with gap plasmon resonances is promising for the design of highly efficient SERS substrates for potential applications in surface-enhanced multichannel sensing, including single-molecule spectroscopy.