SCANNING NEAR-FIELD OPTICAL MICROSCOPY FOR MEASURING MATERIALS PROPERTIES AT THE NANOSCALE

Apertureless scanning near-field optical microscopy is a valuable tool for characterization of chemical and spectroscopic properties of the materials at the nanoscale. Description of apertureless near-field microscope is provided along with the description of a homodyne detection of the near-field signal which allows enhancing of a weak scattered radiation. Experimental evidences that homodyne detection markedly improves the signal-to-noise ratio of the detected signal are presented. A model for the dependence of the near field signal, as a function of the normal distance of the tip from the surface, is discussed. Application of a model in which the tip is represented by two spherical scatterers, one large and one small, indicates the electromagnetic field enhancement is 90 fold greater at the sharp apex of metallic probe tip.Apertureless near-field scanning infrared microscopy was employed to study samples patterned with regions of DNA and hexadecanethiol. Chemical contrast imaging was achieved by examining IR absorption in the spectral region of the phosphate stretching band of DNA molecules and harmonic demodulation of the signal scattered by the oscillating probe. IR absorption maps revealed that the IR signal was not coupled to the vertical tip motion, indicating artifact-free imaging. Monolayer-sensitive chemical imaging with a lateral spatial resolution of approximately 200 nm is demonstrated.The field enhancement in very small aperture lasers was studied using apertureless near-field microscopy. The near-field optical pattern around the aperture indicates the interference of surface plasmons with incident light. A surface plasmon point-source model has been used to determine the wavelength and the decay length of surface plasmons at the Al/silicon nitride interface. Near-field measurements also confirmed a preferred orientation of the rectangular aperture waveguide for the signal enhancement in very small aperture lasers.Optical field confinement in a ridge waveguide nanostructure designed for ultrahigh-density recording was observed using an apertureless near-field scanning optical microscope. The aperture was fabricated on a commercial edge-emitting semiconductor laser as the light source. The emission patterns are in agreement with theoretical simulation of such structures. A 90 nm x 70 nm full-width-half-maximum spot size was measured and is comparable to the ridge width of the aperture