Scanning Near-Field Optical Microscopy Study of Biomolecules, Elaboration of New SNOM Probes and Detection Methods

Abstract

The combination of the topographic resolution of Scanning Probe Microscopy with the subwavelength information of light/matter interaction leads to a new instrument called Scanning Near-field Optical Microscope (SNOM). This new instrument is an ideal tool to study physico-chemical properties of nano-sized objects. Most of the biomolecular processes require exactly the spatial resolution of SNOM. However, to routinely achieve a nanometric scale topographic and optical resolution, further improvements of the technique are needed. These improvements concern mainly the shear-force tip-sample gap control, that defines the interaction force, and the geometrical as well as the mechanical properties of the probes. This thesis mainly focuses on the instrumental aspect of a SNOM dedicated to the study of biological molecules. A SNOM was designed and implemented with an emphasis on the near-field optical probes elaboration and characterization. Two novel SNOM's probes were reported: the ultrasharp carbon whisker probe that combines a well defined ultrasharp tip with the subwavelength aperture of conventional SNOM probes, and polymethylmethacrylate optical fiber probes whose chemical properties could permit a wide range of chemical and/or biological functionalizations. Additionally, 'double-resonance' transducers were built and using these probes, high resolution topographic images of double-stranded and single-stranded DNA were successfully recorded revealing the performances of the instrument. Finally, a modification of the method to study the dynamic of receptor-ligand complexes at the single molecule level and with a sub-millisecond time resolution was proposed. Beside the SNOM's development, we elaborated an innovative 'optical beam deflection method' to increase the force sensitivity of the Atomic Force Microscope (AFM). This technique is based on the reflection from a diffraction grating etched on the backside of an AFM's cantilever. It was demonstrated that the force sensitivity of the technique can be improve by a factor 10-20 and potentially much more.