Jean-Louis Bijeon

Optimization of SERS-active substrates for near-field Raman spectroscopy

As a first step towards near-field Raman, we chose to study surface enhanced Raman scattering (SERS)-active substrates to cope with the weakness of Raman scattering (small cross-section and low concentration). We concentrated our work on localized surface plasmon (LSP) since they turned out to play a great part in SERS and we put forward the relation between LSP resonance and Raman enhancement. Roughness of our samples is controlled either by annealing process or electron-beam lithography (EBL); this latter technique proved to best suit to our study. Substrates are characterized by extinction spectroscopy which determines the LSP resonance and then Raman spectrum of a probe molecule, trans-1,2-bis(4-pyridyl)ethylene (BPE) is recorded. We show that maximum of enhancement is obtained when the LSP resonance is red-shifted (50 nm) compared to the excitation laser line (632.8 nm).

Influence of tip modulation on image formation in scanning near-field optical microscopy

Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and separation of the near-field signal from a background. Detection of higher harmonic modulated signals has also been used to obtain an improvement in resolution, the elimination of background, or artifacts in the signal. This article presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and waveform is shown. Some aspects of the application of this theory in a general experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modula...

Biological and chemical gold nanosensors based on localized surface plasmon resonance

In this paper, we discuss the performances of gold nanosensors based on Localized Surface Plasmon Resonance (LSPR) designed by Electron Beam Lithography (EBL) in the context of biological and chemical sensing. We demonstrate the sensitivity of our gold nanosensors by studying the influence of the concentration of 11-mercaptoundecanoic acid (MUA) on the shift of LSPR wavelength. Additionally, to study the selectivity of our nanosensors, the system Biotin/Streptavidin was used to detect very weak concentration of biomolecules. These results represent new steps for applications in chemical research and medical diagnostics.

Analysis of the influence of the tip vibration in the formation of images in apertureless scanning near-field optical microscopy

The theoretical influence of the vertical vibration of the tip is investigated to calculate the images of a quartz grating in an apertureless scanning near-field optical microscope configuration, a realistic model is used for the simulation of the lock-in detection. It is shown that the output signal issued from the lock-in detection cannot be simply approximated by a mere difference between the intensities of the field between the top and bottom positions of the tip oscillation. Several modes of the lock-in detection, amplitude and phase, X and Y channels, at the fundamental frequency or for the following harmonics, though all leading to images containing high spatial frequencies of the grating, show significant differences that can induce misinterpretations in specific cases.

Intense Bessel-like beams arising from pyramid-shaped microtips

We show both numerically and experimentally that intense, narrow, and low-divergence beams of light are produced at the apex of dielectric pyramid-shaped microtips. These beams exhibit a Bessel transverse profile but are narrower than the usual Bessel beam, allowing for a significant enhancement of the light intensity inside the beam. They are generated by axicon-like structures with submicrometric height imprinted in glass by combining optical lithography and chemical etching. The resulting beams are experimentally imaged using fluorescence microscopy, in remarkable agreement with numerical computations.

Fluorescence imaging of submicrometric lattices of colour centres in LiF by an apertureless scanning near-field optical microscope

We report fluorescence imaging of colour centres in Lithium Fluoride (LiF) using an apertureless Scanning Near Field Optical Microscope (SNOM). The sample consists of periodically spaced submicrometric coloured areas F2 laser-active colour centres produced by low-energy electron beam lithography on the surface of a LiF thin film. A silicon Atomic Force Microscope (AFM) tip is used as an apertureless optical probe. AFM images show a uniform surface roughness with a RMS of 7.2 nm. The SNOM images of the red fluorescence of colour centres excited at lambda = 458 nm with an argon ion laser show that the local photon emission is unambiguously related to the coloured areas and that topographic artefacts can be excluded.

How nanoparticles encapsulating fluorophores allow a double detection of biomolecules by localized surface plasmon resonance and luminescence

The paper shows how polysiloxane particles encapsulating fluorophores can be successfully used to detect biotin-streptavidin binding by two types of technique. After functionalization of the particles by streptavidin, the fixation of the biomolecule can indeed be detected by a shift of the localized surface plasmon resonance of the biotinylated gold dots used as substrate and by the luminescence of the fluorophores evidenced by scanning near-field optical microscopy. The development of particles allowing such a double detection opens a route for increasing the reliability of biological detection and for multi-labelling strategies crossing both detection principles.

Apertureless near-field optical microscopy: influence of the illumination conditions on the image contrast

We report a hybrid microscope composed of an apertureless scanning near-field optical microscope and a commercial atomic force microscope. We discuss the optical origin of the near-field images of a test sample. We show that the optical images have a sharp contrast that depends on the illumination parameters: the state of polarization and the angle of incidence of the incident light.

Detection in near-field domain of biomolecules adsorbed on a single metallic nanoparticle

In this paper, we study the performances of nanosensors based on Localized Surface Plasmon Resonance in the context of biological sensing. We demonstrate the sensitivity and the selectivity of our designed nanosensors by studying the influence of the concentration of Streptavidin on the shift of Localized Surface Plasmon Resonance wavelength. In addition, to study the detection of biomolecules on a single Au nanoparticle, we used a Scanning Near‐field Optical Microscope. These results represent new steps for applications in biological research and medical diagnostics.

Core/shell nanoparticles for multiple biological detection with enhanced sensitivity and kinetics

The paper shows the different methods to attach a molecule to detect streptavidin to a dielectric particle made of a rare-earth oxide core and a polysiloxane shell containing fluorescein. First, the detection of streptavidin binding on a biotinylated gold substrate can be achieved in three ways: the shift of the surface plasmon resonance of the substrate and the double luminescence (organic and inorganic) of the core/shell particle. Second, these detections are efficient even after elimination upon thermal annealing of all the undesired molecules that skew the assays. Finally, the particle that ballasts the protein enhances its binding kinetics and increases the localized surface plasmon resonance shift that detects the binding.