I- Development of Vibrational Raman Near-Field Scanning Optical Microscopy.

This new project starts in the fall 2004. Near-field scanning optical microscopy (NSOM) is a considerable recent breakthrough in optical microscopy since it permits the characterisation of nanometric objects with an incomparable resolution of about 20 nm. Similarly to AFM, NSOM gives detailed information on the topography of a surface, but unlike AFM it presents the advantage that it can also be coupled to numerous optical measurements in the linear or nonlinear optical regimes. Despite a very weak scattering cross section, recent coupling with vibrational Raman spectroscopy has been demonstrated by L.Novotny on isolated single walled carbon nanotubes. This result is very promising and foreshadows future Raman measurements on other nanostructured materials and single molecules. The idea of near-field optical microscopy is based on a reduced numerical aperture, the diameter of which is smaller than the wavelength of visible light (l). As suggested by E.Synge in 1928, near-field optics has emerged as a logical continuation of the progress with scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) and was developed to overcome the resolution limit of optical microscopy which is limited by the diffraction limit to roughly l/2. If it is possible to use a microscope in many configurations and we can distinguish between two different kinds of near-field microscopes which are either using an optical fiber (with numerical aperture) or using a sharp metallic tip (the meaningless “apertureless” is also commonly used to describe such tips!?!). Using a sharp metallic tip is probably the most well-adapted approach for Raman measurements since it is based on an enhancement of the electromagnetic field at the tip-sample interface. To benefit from this surface enhancement, the tip must be coated with gold or silver that acts as a scatter centers. The photons are then collected with a far-field objective and analyzed. Our proposed project involves the spectral analysis of the signal coming from nano-structures. The goal is to detect photons scattered at wavelengths different than that of the input photons. Effort will be focused on the Raman-Stokes signal knowing that this is a challenge because the Raman signal is much weaker than the elastic Rayleigh scattering by 10 to 12 order of magnitude. The Raman-Stokes signal must therefore be enhanced under resonant conditions using electromagnetic field enhancement or electronic Raman resonance effect.