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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 modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of background artifacts due to propagating fields through the use of higher harmonic detection. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the absorbance measurement of submicrometer-size silver spheroids formed on a graphite substrate. Attempt at correlating the surface-plasmon spectrum of the films with theoretical properties the substrate upon which they reside; Response of the particles in time-varying field; Substrate effects on the surface-plasmon spectrum; Absorption spectrum in normal incidence for heat treated silver particles on a transparent quartz substrate.

Using an apertureless scanning near-field optical microscope, we obtained wavelength dependent images of an isolated gold nanometric particle laid on a quartz substrate. The images show a sharp resonance around λ=488 nm. The far-field surface plasmon resonance of the spherical particles (diameter is 40 nm) in solution is centered at λ=530 nm. Atomic force microscope images give evidence that the particles spread on the substrate. The near-field blueshift of the resonance is thus explained by excitation of a surface plasmon resonance along the minor axis of an oblate spheroid shaped particle. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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.

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.