Cylindrical Surface Plasmon Research Articles

Subwavelength proximity nanolithography using a plasmonic lens

This article presents a novel device, the plasmonic lens (PL), consisting of equally spaced ring apertures in a metal film deposited on a fused silica substrate. It was fabricated by electron-beam lithography (EBL) and reactive-ion etching (RIE). When illuminated by a collimated laser, a cylindrical surface plasmon (SP) is excited in the PL, scattered by the structure, and propagates. As a result, the PL focuses a subwavelength spot in the midfield, i.e., the focal length is several microns. The authors experiment demonstrated that 90–300 nm spots (up to λ∕4) with pitches of 400–500 nm, focal length of 1.7 μm, were printed by a PL using 405 nm laser. The authors three-dimensional electromagnetic simulation predicted a full width at half maximum (FWHM) of 210 nm, equivalent to an aberration-free lens having an unity numerical aperture (NA=1). The experimental result agreed well with the simulation. A theoretical model is also presented. Given its small footprint and subwavelength resolution, the PL holds great promise in direct-writing and scanning microscopy.

Extraordinary optical transmission with coaxial apertures

Recently it has been predicted that “cylindrical” surface plasmons (CSP’s) on cylindrical interfaces of coaxial ring apertures produce a different form of extraordinary optical transmission that extends to ever increasing wavelengths as the dielectric ring narrows. This letter presents experimental confirmation of this CSP assisted extraordinary transmission. Nanoarrays of submicron coaxial apertures are fabricated in a thin silver film on a glass substrate and far-field transmission spectra are measured. The experimental spectrum is in close agreement with predictions from finite-difference time-domain simulations and CSP dispersion theory. The role of cylindrical surface plasmons in producing extraordinary transmission is thus confirmed.

NEAR-FIELD OPTICAL VORTEXES AT NANOSTRUCTURED METALLIC FILMS

The scattering of linearly polarized monochromatic light by a small protrusion on a metal surface is analyzed within the framework of perturbation theory. It is shown that even at normal incidence of light slight deviations of the protrusion shape from a circularly symmetric one can lead to formation of optical vortexes in the near-field region. The effect is due to resonant excitation of cylindrical surface plasmon waves. This agrees with the fact that the in-plane near-field intensity distribution experimentally observed by scanning near-field optical microscopy has distinct spiral patterns near metallized nanostructured polymer substrates.

Enhanced transmission with coaxial nanoapertures: Role of cylindrical surface plasmons

We simulate the optical fields and optical transmission through nanoarrays of silica rings embedded in thin gold films using the finite-difference-time-domain method. By examining the optical transmission spectra for varying ring geometries we uncover large enhancements in the transmission at wavelengths much longer than the usual cutoffs for cylindrical apertures or where the usual planar surface plasmons or other periodic effects from the array could play a role. We attribute these enhancements to closely coupled cylindrical surface plasmons on the inner and outer surfaces of the rings, and this coupling is more efficient as the inner and outer ring radii approach each other. We confirm this hypothesis by comparing the transmission peaks of the simulation with cylindrical surface plasmon (CSP) dispersion curves calculated for the geometries of interest. One important result is that a transmission peak appears in the simulations close to the frequency where the longitudinal wave number ${k}_{\mathrm{z}}$ in the ring satisfies ${k}_{\mathrm{z}}=m\ensuremath{\pi}∕\mathrm{L}$, where $m$ is an integer and $L$ the length of the aperture, for a normal CSP ${\mathrm{TE}}_{1}$ or ${\mathrm{TM}}_{1}$ mode. The behavior of the CSP dispersion is such that propagating modes can be sent through the rings for ever longer wavelengths as the ring radii approach, whereas the transmission decreases only in proportion to the ring area.

Role of cylindrical surface plasmons in enhanced transmission

Utilizing normal mode analysis of Maxwell’s equations and finite-difference-time-domain simulations we find extraordinary optical transmission in nanoarrays of insulating coaxial cylindrical rings embedded in metal films. As the rings become narrower we find transmission peaks at longer wavelengths, with the peak wavelength increasing indefinitely as the rings narrow. This behavior results from the excitation of cylindrical surface plasmon resonant modes on the cylindrical insulator-metal interfaces of the rings. These findings indicate that the excitation of cylindrical surface plasmons in these structures can produce propagating modes and enhanced transmission at wavelengths longer than those predicted previously.

Body-of-revolution FDTD simulations of improved tip performance for scanning near-field optical microscopes

Recently, it has been proposed to attach a nanometric metallic sphere at the apex of a scanning near-field optical microscope (SNOM) tip in order to enhance its efficiency [Appl. Phys. Lett. 76 (15) (2000) 2134]. In this paper, we use a home-made body-of-revolution FDTD code to compare the emission of conventional metal coated tip with and without this nanoparticle. In both cases, our calculation establishes the occurrence of a cylindrical surface plasmon that propagates along the taper. When it is excited, the surface plasmon of the sphere strongly modified both the temporal and the spectral responses of the tip. In the near field, a 100× exaltation of the field intensity, at the resonant wavelength, is shown. In the far field, an interference phenomenon between the core guided pulse and the cylindrical surface plasmon leads to a channeled transmitted spectrum.

Surface plasmon polaritons on metal cylinders with dielectric core

Metal-cladded dielectric cylinders with submicron diameters may serve to model coated tips used in near-field scanning optical microscopy. The signal measured may be greatly influenced by resonance effects due to eigenmodes of the probe. Especially, using a photon scanning tunneling microscope setup, gold-coated tips have been found to detect a signal proportional to the magnetic field distributions [E. Devaux et al., Phys. Rev. B 62, 10 504 (2000)]. This effect is attributed to cylindrical surface plasmons. We present here fully retarded calculations of the dispersion and field patterns of the nonradiative plasmon modes in cylindrical geometry. We study the effect of varying the cylinder radius on the surface plasmon dispersion, thus justifying that the cylinder is a useful model for near-field probes in spite of their slightly conical shape.

Near field by subwavelength particles on metallic substrates with cylindrical surface plasmon excitation

The near electromagnetic field scattered by a spherical subwavelength particle on a flat metallic substrate is calculated when the incident light is a plane monochromatic linearly polarized wave. The complete solution to the scattering problem that we consider includes the possible surface wave components of the field. Cylindrical surface plasmon-polaritons are shown to be excited by the particle on the substrate when this is metallic and the particle is close enough. The shape of the field associated to the surface plasmon-polariton depends on the incident polarization and angle of incidence. In a real situation, the plasmon field interferes with the incident field. The spatial distribution of the interference pattern is analyzed in terms of the incident polarization and the angle of incidence and differences from the case of bulk propagating waves are shown. Incident polarization parallel to the plane of incidence is proved to be more efficient than the perpendicular one for surface plasmon generation because of the vertical component of the particle dipole moment. The propagation characteristics of the cylindrical surface waves (wave number, phase) are obtained from the interference pattern.