Near-field probing of slow Bloch modes on photonic crystals with a nanoantenna.

We study the near-field probing of the slow Bloch laser mode of a photonic crystal by a bowtie nano-aperture (BNA) positioned at the end of a metal-coated fiber probe. We show that the BNA acts as a polarizing nanoprobe allowing us to extract information about the polarization of the near-field of the slow-light mode, without causing any significant perturbation of the lasing process. Near-field experiments reveal a spatial resolution better than λ/20 and a polarization ratio as strong as 110. We also demonstrate that the collection efficiency is two orders of magnitude larger for the BNA than for a 200 nm large circular aperture opened at the apex of the same metal-coated fiber tip. The BNA allows for overcoming one of the main limitations of SNOM linked to the well-known trade off between resolution and signal-to-noise ratio.

Bowtie nano-aperture as interface between near-fields and a single-mode fiber.

We present the development and study of a single bowtie nano-aperture (BNA) at the end of a monomode optical fiber as an interface between near-fields/nano-optical objects and the fiber mode. To optimize energy conversion between BNA and the single fiber mode, the BNA is opened at the apex of a specially designed polymer fiber tip which acts as an efficient mediator (like a horn optical antenna) between the two systems. As a first application, we propose to use our device as polarizing electric-field nanocollector for scanning near-field optical microscopy (SNOM). However, this BNA-on-fiber probe may also find applications in nanolithography, addressing and telecommunications as well as in situ biological and chemical probing and trapping.

Full vectorial imaging of electromagnetic light at subwavelength scale.

We propose a concept of near-field imaging for the complete experimental description of the structure of light in three dimensions around nanodevices. It is based on a near-field microscope able to simultaneously map the distributions of two orthogonal electric-field components at the sample surface. From a single 2D acquisition of these two components, the complementary electric and magnetic field lines and Poynting vector distributions are reconstructed in a volume beneath the sample using rigorous numerical methods. The experimental analysis of localized electric and magnetic optical effects as well as energy flows at the subwavelength scale becomes possible. This work paves the way toward the development of a complete electromagnetic diagnostic of nano-optical devices and metamaterials.

Annular nanoantenna on fibre micro-axicon.

In this paper, we propose to extend the concept of loop antenna to the optical domain. The aim is to develop a new generation of optical nanocollectors that are sensitive to specific electric or magnetic vectorial field components. For validating our approach, a preliminary one-micron-diameter gold nanoring is micromachined on the apex of a cone lens obtained from a tapered optical fibre. It is shown that such a nano-object behaves as a nano-antenna able to detect the longitudinal electric field from a Bessel beam in radial polarization and the longitudinal magnetic component from a Bessel beam in azimuthal polarization. In the latter case, the annular nano-antenna exhibits the properties of an optical inductance.

Linear to radial polarization conversion in the THz domain using a passive system.

This paper addresses a passive system capable of converting a linearly polarized THz beam into a radially polarized one. This is obtained by extending to THz frequencies and waveguides an already proven concept based on mode selection in optical fibers. The approach is validated at 0.1 THz owing to the realization of a prototype involving a circular waveguide and two tapers that exhibits a radially polarized beam at its output. By a simple homothetic size reduction, the system can be easily adapted to higher THz frequencies.

Early steps in layer-by-layer construction of polyelectrolyte films: the transition from surface/polymer to polymer/polymer determining interactions.

The layer-by-layer deposition of two polyelectrolytes, quaternized poly(dimethylaminoethyl methacrylate chloride) (MADQUAT) and poly(acrylic acid) (PAA) on a silica substrate was investigated using optical reflectometry, as a function of pH (pH 4, 5.5 and 9), ionic strength (10(-3) to 10(-1) M) and type of salt. Attention was given to the successive deposited weights and to the corresponding deposited charge densities within the ten first deposited layers. Results show a change of growth regime between an early stage where the substrate had a dominating influence in the build-up and a second stage where the polymer uptake was ruled essentially by polymer-polymer interactions. The pH was seen to influence the growth via the charge densities of silica (first stage) and PAA (first and second stages). The increase of NaCl concentration induced an increase of the film weight between 10(-3) and 10(-2) M, but the trend was more sophisticated between 10(-2) and 10(-1) M where the polymer uptake increased in the first stage of the growth and decreased in subsequent layers. The film weight increased in accordance with the rank of ions in the Hofmeister series. AFM images revealed a heterogeneous film morphology with bumps and valleys, which was explained by a growth mechanism made of the successive formation and growth of polymer complexes.

Shaping the reflection near-field optical probe: finite domain time difference modelling and fabrication using a focused ion beam.

An optical fibre ending in a trihedral tip is proposed as a convenient probe for reflection near-field optical microscopy in emission/collection mode. Its shape is obtained by ion milling. A first example of manufacturing and numerical models using the bi-dimensional FDTD method is presented. It confirms the strong influence of the facet angle on the intensity reflected by the probe, which is predicted by a rough analysis. This method can help us to optimize the 'reflection probe' by reducing this offset signal.

Analysis of optical near-field images by Karhunen-Loève transformation.

With regard to classical image transformations, the Karhunen-Loève transform is based on the statistical analysis of signal or image data. We propose using it to analyze and recognize the content of images generated in near-field microscopy. It is shown that such a transform is an efficient tool for separating actual information from noise without reducing the spatial-frequency band.