D. Van Labeke

Light transmission by subwavelength square coaxial aperture arrays in metallic films

Using the Fourier modal method, we study the enhanced transmission exhibited by arrays of square coaxial apertures in a metallic film. The calculated transmission spectrum is in good agreement with FDTD calculations. We show that the enhanced transmission can be explained when we consider a few guided modes of a coaxial waveguide.

Bessel beams as virtual tips for near-field optics.

In the previous NFO meeting, we proposed the use of confined evanescent light beams as ‘virtual’ or ‘immaterial’ tips. Unfortunately, this technique was hindered by the need for perfectly radially polarized light beams. In this communication, we propose a simple, stable and cheap method allowing the generation of beams of any polarization and more especially of purely radially polarized light beams. We also demonstrate both theoretically and experimentally that for near‐field imaging systems polarization is a limiting factor of resolution and light confinement. Finally, we present the very first experimental results dealing with virtual tips.

Annular aperture arrays: study in the visible region of the electromagnetic spectrum

Baida and Van Labeke recently proposed a structure that exhibits a supertransmission of light through an array of nanometric coaxial apertures in a metallic film that has been named an annular aperture array (AAA) [Opt. Commun. 209, 17 (2002); Phys. Rev. B 67, 155314 (2003); J. Microsc. 213, 140 (2003)]. We present the first experimental study, to our knowledge, of an AAA structure in the visible region. For technological reasons, the structure under study does not produce a supertransmission of 80% as in Baida and Van Labeke [Opt. Commun. 209, 17 (2002)]. We built the nanostructure and experimentally recorded its far-field spectral response. This transmission shows only one broad band with a maximum around lambda = 700 nm, giving a maximum efficiency around 17%. A finite-difference time-domain simulation reproduces quite well the obtained transmission spectrum.

Excitation profiles for totally symmetric modes in resonance Raman scattering

Abstract The relation between resonance Raman excitation profiles and absorption spectra is derived for a model based on displaced harmonic oscillators and compared with experimental observations on the permanganate ion.

A theoretical model for the Inverse Scanning Tunneling Optical Microscope (ISTOM)

Abstract Recently experiments have been performed with a new kind of Scanning Near-field Microscope. The apparatus is derived from a Scanning Tunneling Optical Microscope by simply inverting the direction of light propagation: it is thus an Inverted Scanning Tunneling Optical Microscope (ISTOM) where the tip is used in emission mode and where detection can be mediated via homogeneous or evanescent waves. We propose a theoretical calculation of the detected intensity measured in ISTOM experiments. In this model, the sample is a relief on the hemisphere surface and the source is an aperture tip described within the Bethe-Bouwkamp approximation. The use of plane wave expansions of the various fields and of a perturbation method for matching the boundary conditions lead to concise analytical formulas. The discussion of the influence of the various parameters on the detected intensity is thus quite easy. We illustrate the formulas by some examples showing the variations of the detected signal versus tip-surface distance, tip radius and detection angle.

Waveguiding through a two‐dimensional metallic photonic crystal

We present a two‐dimensional (2D) finite‐difference time domain simulation of the propagation of light through linear and bent channels in metallic photonic crystals. We took as a starting point the Bozhevolnyi experiment, consisting of the scattering of surface plasmons by a 2D structure of finitely sized periodic gold dots arranged in a triangular lattice of 400‐nm period. We model injection and propagation of light through linear channels of different widths. We also study the behaviour of light in the presence of a 90° bent line defect made in the structure. We show that the confinement depends on the orientation of the input and output line defects. The two cases of ΓM and ΓK orientations are considered and a spectral study for five different wavelengths is carried out.

Theoretical study of transient phenomena in near-field optics

A time‐domain study of the propagation of a light pulse is made by the finite‐difference time‐domain method. This method is described briefly and then two applications are presented: creation and diffraction of surface plasmons in the time‐domain, and propagation of a light pulse through two tip models, a dielectric one and a metal‐coated one. Results on propagation speed of surface plasmons, spatio‐temporal shape and spectral study of the field emitted by a tip are presented.

Interaction between a dielectric tip and an ionic crystal. Application to scanning force microscopy on LiF and MgO

Abstract We present a model for calculating the interaction energy of an ionic crystal and a spherical dielectric tip, applicable to experiments in scanning force microscopy. The repulsive energy is introduced by performing a pairwise summation between the atoms of the probe and each atom of the substrate. The attractive energy is separated into a dispersive term and an inductive contribution which takes into account the local field generated by the permanent charges lying at the surface of the sample. These contributions are expanded in the (2D) reciprocal lattice associated with the planes of the crystal. This procedure allows the separation of the van der Waals force into two parts describing the continuum character and the corrugation of the surface. Numerical results are proposed for the (100) face of LiF and MgO.

Interaction between a spherical probe and an atomic lattice: implication for atomic force microscopy on graphite and diamond

Abstract This work describes a new method for calculating the van der Waals interaction between a real surface and a spherical probe placed in its neighbourhood. The essence of the method is similar to that developed in physisorption calculations since it relies on the knowledge of a reduced number of parameters characterising the probe and the surface. The long range interaction energy (dispersive plus inductive) is obtained within the scheme of a nonlocal theory expressed in terms of generalised electric susceptibilities of the two partners. Short range interaction is then included through an atom-atom empirical pair potential. The discrete structure of each plane of the solid is described with an increasing accuracy by increasing the order of the Fourier expansion in the reciprocal planar lattice. This allows us to separate the van der Waals force probe-sample in two parts describing the continuum character and the corrugation of the surface, respectively. Numerical results are proposed by varying two parameters: the radius of the probe and the distance of nearest approach. Implications for atomic force microscopy on graphite and diamond are also discussed.

Enhanced optical transmission by light coaxing: mechanism of the TEM-mode excitation.

This paper presents a theoretical study showing the mechanism of light transmission through opaque metallic films perforated with nanocoaxial apertures thanks to the excitation of their cutoff-free TEM (Transverse ElectroMagnetic) guided mode. Full three-dimensional Finite Difference Time Domain (3D-FDTD) together with a Body-Of-Revolution FDTD simulation results are presented and discussed in order to optimize this extraordinary transmission. Very promising findings are pointed out opening the path to the design of new devices for both nano-optic and photovoltaic applications.