High-index dielectric nanostructures have recently become prominent forefront alternatives for manipulating light at the nanoscale. Their electric and magnetic resonances with intriguing characteristics endow them with a unique ability to strongly enhance near-field effects with minimal absorption. Similar to their metallic counterparts, dielectric oligomers consisting of two or more coupled particles are generally employed to create localized optical fields. Here we show that individual all-dielectric nanostructures, with rational designs, can produce strong electric fields with intensity enhancements exceeding 3 orders of magnitude. Such a striking effect is demonstrated within a Si nanodisk by fully exploiting anapole generation and simultaneously introducing a slot area with high-contrast interfaces. By performing finite-difference time-domain simulations and multipole decomposition analysis, we systematically investigate both far-field and near-field properties of the slotted disk and reveal a subtle interplay among different resonant modes of the system. Furthermore, while electric fields at anapole modes are typically internal, i.e., found inside nanostructures, our slotted configuration generates external hotspots with electric fields additionally enhanced by virtue of boundary conditions. These electric hotspots are thereby directly accessible to nearby molecules or quantum emitters, opening up new possibilities for single-particle enhanced spectroscopies or singlephoton emission enhancement due to large Purcell effects. Our presented design methodology is also readily extendable to other materials and other geometries, which may unlock enormous potential for sensing and quantum nanophotonic applications.

Anapole states associated with the resonant suppression of electric-dipole scattering exhibit minimized extinction and maximized storage of electromagnetic energy inside a particle. Using numerical simulations, optical extinction spectroscopy and amplitude-phase near-field mapping of silicon dielectric disks, we demonstrate high-order anapole states in the near-infrared wavelength range (900-1700 nm). We develop the procedure for unambiguously identifying anapole states by monitoring the normal component of the electric near-field and experimentally detect the first two anapole states as verified by far-field extinction spectroscopy and confirmed with the numerical simulations. We demonstrate that higher-order anapole states possess stronger energy concentration and narrower resonances, a remarkable feature that is advantageous for their applications in metasurfaces and nanophotonics components, such as non-linear higher-harmonic generators and nanoscale lasers.

Metasurfaces enable exceptional control over the light with surface-confined planar components, offering the fascinating possibility of very dense integration and miniaturization in photonics. Here, we design, fabricate and experimentally demonstrate chip-size plasmonic spectropolarimeters for simultaneous polarization state and wavelength determination. Spectropolarimeters, consisting of three gap-plasmon phasegradient metasurfaces that occupy 120° circular sectors each, diffract normally incident light to six predesigned directions, whose azimuthal angles are proportional to the light wavelength, while contrasts in the corresponding diffraction intensities provide a direct measure of the incident polarization state through retrieval of the associated Stokes parameters. The proof-of-concept 96-μm-diameter spectropolarimeter operating in the wavelength range of 750 -950 nm exhibits the expected polarization selectivity and high angular dispersion (0.0133 °/nm). Moreover, we show that, due to the circular-sector design, polarization analysis can be conducted for optical beams of different diameters without prior calibration, demonstrating thereby the beam-size invariant functionality.The proposed spectropolarimeters are compact, cost-effective, robust, and promise highperformance real-time polarization and spectral measurements.

Strongly confined surface plasmon-polariton modes can be used for efficiently delivering the electromagnetic energy to nano-sized volumes by reducing the cross sections of propagating modes far beyond the diffraction limit, i.e., by nanofocusing. This process results in significant local-field enhancement that can advantageously be exploited in modern optical nanotechnologies, including signal processing, biochemical sensing, imaging and spectroscopy. Here, we propose, analyze, and experimentally demonstrate on-chip nanofocusing followed by impedance-matched nanowire antenna excitation in the end-fire geometry at telecom wavelengths. Numerical and experimental evidences of the efficient excitation of dipole and quadrupole (dark) antenna modes are provided, revealing underlying physical mechanisms and analogies with the operation of plane-wave Fabry-Pérot interferometers. The unique combination of efficient nanofocusing and nanoantenna resonant excitation realized in our experiments offers a major boost to the field intensity enhancement up to ∼ 12000, with the enhanced field being evenly distributed over the gap volume of 30×30×10 nm 3 , and promises thereby a variety of useful on-chip functionalities within sensing, nonlinear spectroscopy and signal processing. [This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, c American Chemical Society after peer review. To access the final edited and published work see http://dx.doi.org/10.1021/acs.nanolett.5b03593.]Keywords: Surface plasmons polaritons, nanofocusing, field enhancement, tapered waveguide, phase-resolved near-field microscopy, optical antennasThe major aspect of focusing of electromagnetic radiation is the possibility of concentrating the energy in a small volume. Because of diffraction, the focusing of freepropagating optical waves is limited in size to the half of the light wavelength in the medium the diffraction limit of light.1 One approach to overcome this limit is to use surface plasmon polaritons (SPPs) surface electromagnetic modes bound to and propagating along metaldielectric interfaces, with electromagnetic fields in a dielectric being coupled to collective free electron oscillations in a metal.2 Spatial confinement of SPP modes in the cross section perpendicular to the propagation direction depends on the material composition and geometric configuration of a waveguiding structure. Notably, some SPP modes (supported, for example, by metal nanowires 3 ) exhibit a unique scaling property in their spatial confinement: the mode is progressively better confined for smaller lateral waveguide dimensions, opening thereby the possibility for guiding extremely confined (i.e., on a deep subwavelength scale) SPP modes 4 as well as for designing SPP-based nanoantennas.5 This feature can further be used for nanofocusing, which is the process of reducing the cross sections of propagating optical modes far beyond the diffraction limit, simply by gradually decreasing lateral waveguid...

We demonstrate the use of amplitude-and phase-resolved near-field mapping for direct characterization of plasmonic slot waveguide mode propagation and excitation with nanocouplers in the telecom wavelength range. We measure mode's propagation length, effective index and field distribution and directly evaluate the relative coupling efficiencies for various couplers configurations. We report 26-and 15-fold improvements in the coupling efficiency with two serially connected dipole and modified bow-tie antennas, respectively, as compared to that of the short-circuited waveguide termination. [This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, c American Chemical Society after peer review. To access the final edited and published work see http://dx.doi.org/10.1021/nl501207u.] Keywords: nanocoupler, surface plasmon, slot waveguide, nanoantenna, s-SNOM, near-field microscopy Great advantages offered by plasmonics to optical waveguiding are extreme subwavelength localization of guided modes close to the metal interface 1 together with electrical tunability of electromagnetic waves via intrinsic metallic contacts 2 . Plasmonic waveguides are therefore considered as a future generation of optical interconnects in integrated circuits for datacom technologies 3 . Inevitably, with the appearance of nanoscale waveguides, a new challenge has emerged: how to effectively couple the diffraction-limited optical waves into deep-subwavelength plasmonic waveguides. Various approaches have been utilized ranging from lenses to grating couplers 4 . However, the most compact solution is, an antenna based nanocoupler.Antenna is a common tool to capture free-space propagating radio-waves with more than a centurylong history 5 . Employment of metal-based antennas in photonics started only in the last two decades owing to the progress in high-resolution nanofabrication techniques 6,7 . Usage of plasmonic antennas 8 to couple light to plasmonic waveguides has been suggested theoretically 9-15 and then confirmed experimentally with cross-polarization microscopy measurements in the nearinfrared 16 and with near-field microscopy in optical 17 , telecom 18 and mid-infrared 19 ranges. Nevertheless, the amplitude-and phase-resolved measurements of the antenna-excited slot plasmons in the telecom range (with the free-space wavelength around 1.55 µm) have not been reported so far. It should be emphasized that the usage of phase-resolved near-field mapping is indispensable for direct characterization of the mode effective index as well as for revealing the symmetry of excited plasmonic modes 19 .In this Letter we report, for the first time to our knowledge, the amplitude-and phase-resolved near-field characterization of plasmonic slot waveguides and antenna based nanocouplers in the telecom wavelength range. Illumination with a wide laser beam excites both slot plasmons confined within a dielectric gap in a metal film and surface plasmon polaritons (SPP) propagating along ...