Magnetic probe for material characterization at optical frequencies

Rapid development of novel, functional metamaterials made of purely dielectric, plasmonic, or composite structures which exhibit tunable optical frequency magnetic responses creates a need for new measurement techniques. We propose a method of actively measuring magnetic responses, i.e. magnetic dispersion, of such metamaterials within a wide range of optical frequencies with a single probe by exciting individual elementary cells within a larger matrix. The probe is made of a tapered optical fiber with a radially corrugated metal coating. It concentrates azimuthally polarized light in the near-field below the apex into a subwavelength size focus of the longitudinal magnetic field component. An incident azimuthally polarized beam propagates in the core until it reaches the metal stripes of constant angular width running parallel to the axis. For a broad frequency range light-to-plasmon coupling is assured as the lattice constant changes with the radius due to constant angular width. Bound plasmonic modes in slits between the metal stripes propagate toward the apex where circular currents in stripes and displacement currents in slits generate a strong longitudinal magnetic field. The energy density of the longitudinal magnetic component in the vicinity of the axis is much stronger than that of all the other components combined, what allows for pure magnetic excitation of magnetic resonances rather than by the electric field. The scattered signal is then measured in the far-field and analyzed

Bi-metal coated aperture SNOM probes

Aperture probes of scanning near-field optical microscopes (SNOM) offer resolution which is limited by a sum of the aperture diameter at the tip of a tapered waveguide probe and twice the skin depth in metal used for coating. An increase of resolution requires a decrease of the aperture diameter. However, due to low energy throughput of such probes aperture diameters usually are larger than 50 nm. A groove structure at fiber core-metal coating interface for photon-to-plasmon conversion enhances the energy throughput 5-fold for Al coated probes and 30-fold for Au coated probes due to lower losses in the metal. However, gold coated probes have lower resolution, first due to light coupling from the core to plasmons at the outside of the metal coating, and second due to the skin depth being larger than for Al. Here we report on the impact of a metal bilayer of constant thickness for coating aperture SNOM probes. The purpose of the bilayer of two metals of which the outer one is aluminum and the inner is a noble metal is to assure low losses, hence larger transmission. Using body-of-revolution finite-difference time-domain simulations we analyze properties of probes without corrugations to measure the impact of using a metal bilayer and choose an optimum bi-metal configuration. Additionally we investigate how this type of metalization works in the case of grooved probes

Optimization of transmission and focusing properties of plasmonic nanolenses

We consider two kinds of plasmonic nanolenses which focus radially polarized Laguerre-Gauss beam into subwavelength spot. The first one is free-standing opaque metal layer with concentric grooves on both sides [Phys. Rev. Lett. 102, 183902 (2009)]. The second has slits instead of grooves thus concentric rings have to be integrated with dielectric matrix. Constructive interference of far-field radiation of SPPs scattered on the back side of the lenses gives subwavelength size foci approaching the Rayleigh resolution limit. We investigate transmission and focusing properties of considered metal structures. Choice of appropriate metal such as silver, gold, copper or aluminum strongly affects transmission. Parameters of surface structure determine efficient photon-plasmon coupling and plasmon scattering phenomenon thus influence both transmission and focusing effect. Finally, the choice of dielectric function of surrounding medium gives another degree of freedom to fulfill momentum matching condition for resonant photon-plasmon interaction. In this paper, taking into account the above parameters, we show an optimization procedure, which leads to high transmission, tight focal spot and large focal length of the considered plasmonic nanolenses

Plasmonic concentrator of magnetic field of light

We propose an efficient concentrator of the magnetic component of evanescent field of light for measuring magnetic responses of nanostructures. It is in the form of a tapered fiber probe, which in its final part has corrugations along the angular dimension and is coated with metal except for the aperture at the tip. Internal, azimuthally polarized illumination is concentrated into a subwavelength spot with a strong longitudinal magnetic component H-z. Within the visual range of wavelengths 400-700 nm, the energy density of H-z is up to 50 times larger than that of the azimuthal electric E-phi one. This dominant H-z contribution may be used for magnetic excitation of elementary cells of metamaterials with a single probe guiding a wide spectrum of generated plasmons

Concentrator of magnetic field of light

In the recent decade metamaterials with magnetic permeability different than unity and unusual response to the magnetic field of incident light have been intensively explored. Existence of magnetic artificial materials created an interest in a scanning near-field magnetic microscope for studies of magnetic responses of subwavelength elementary cells of those metamaterials. We present a method of measuring magnetic responses of such elementary cells within a wide range of optical frequencies with single probes of two types. The first type probe is made of a tapered silica fiber with radial metal stripes separated by equidistant slits of constant angular width. The second type probe is similar to metal coated, corrugated, tapered fiber apertured SNOM probe, but in this case corrugations are radially oriented. Both types of probes have internal illumination with azimuthally polarized light. In the near-field they concentrate into a subwavelength spot the longitudinal magnetic field component which is much stronger than the perpendicular electric one

Special Issue of Opto-Electronics Review on Nanophotonics

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Fabrication of corrugated Ge-doped silica fibers

We present a method of fabricating Ge-doped SiO2 fibers with corrugations around their full circumference for a desired length in the longitudinal direction. The procedure comprises three steps: hydrogenation of Ge-doped SiO2 fibers to increase photosensitivity, recording of Bragg gratings with ultraviolet light to achieve modulation of refractive index, and chemical etching. Finite-length, radially corrugated fibers may be used as couplers. Corrugated tapered fibers are used as high energy throughput probes in scanning near-field optical microscopy

Performance of Scanning Near-Field Optical Microscope Probes with Single Groove and Various Metal Coatings

We investigate the performance of a simple corrugated aperture scanning near-field optical microscope (SNOM) probe with various cladding metals. The probes have only one corrugation, however, they offer increased transmission over both uncorrugated probes and those with many grooves. Enhancement of light throughput results from excitation of surface plasmons at the corrugation at the core–cladding interface. We show how the choice of metal influences radiation properties of grooved probes

Plasmonic lenses with long focal lengths

We report on recent progress made in the development of plasmonic nanolenses. These lenses exhibit intensity transmittance close to 80%, focal lengths equal to one or more wavelengths, and foci with full-widths at halfmaximum close to the diffraction limit. We consider lenses in the form of (i) a silver layer with no hole on the optical axis and double-sided concentric corrugations, (ii) a silver layer with no hole on the axis and single-sided corrugations, and (iii) a lens composed of several concentric metallic rings and on-axis stop with external layer of transparent dielectric that integrates all elements. Investigations are carried out using the Finite-Difference Time-Domain method and the Transfer Matrix Method. The nanolenses are diffractive optical elements that concentrate radially polarized Laguerre-Gauss illumination as tightly as high-NA refractive optical systems