RESUMO
On the basis of Maxwell's equations a light scattering system of axial symmetry is considered, which consists of a nanoparticle, a dipole and a metal film (covering a dielectric support). Nanoparticle (NP) and dipole are situated on an axis of symmetry and the dipole is oriented along the axis and placed between film and nanoparticle. The field enhancement factor F and dipole energy flux D are calculated by the Green's function method: the initial system of Maxwell's equations is reduced to a system of boundary integral equations, and solutions are obtained by the boundary element method. Illumination of the scattering system by a radially polarized Bessel light beam causes a field enhancement in the vicinity of the film surface. The metallic NP closely placed at the film surface acts as nano-antenna. Surface plasmons excited in the particle and film convert the incident propagating EM field into non-propagating evanescent near-field. Then the field is confined and strongly enhanced in a particle/film gap. The enhancement of Raman radiation depends on many factors: size and shape of NP, permittivities of all materials, light wavelength, film thickness, angle of light beam, and - very strongly - on the gap distance. The field enhancement in a gap ~1 nm can be 10(3) and more and the Raman radiation enhancement factor can reach huge values ~10(10)-10(12). Whereas for small nanoparticles the field enhancement factor F and the dipole energy flux D do not depend on the direction of the exciting beam and on the angle of emission, a strong influence is found for extended particles. This influence is plausibly explained by a larger overlap between the electric field of the exciting beam or the emitted radiation field with the near field distribution of the nanoparticle leading to higher F and D values, respectively.
RESUMO
This article considers the possibility to use tips, which are functionalised by Raman active molecules, as new Raman probes for near-field optics in such a way that the Raman spectrum can be recorded of such a tip. If the Raman spectrum of the probe molecules is sensitive to their immediate environment, the probe can be used to map a surface by its local influence on the Raman spectrum of the probe. This new concept may be very promising for the investigation of specific interactions at the nanoscale by an optical response. Examples of the sensitivity of such a probe to the local environment are presented as a basis for further development of such a probe.
RESUMO
It is shown experimentally that a Gaussian beam focused from within a finite wedge onto its edge at an angle of total reflection excites a leaky waveguide mode propagating along the edge. This newly described phenomenon is interpreted in terms of waveguide modes of a tapered dielectric slab of a mode index, which is known to decrease with slab thickness. This decrease in the refractive index leads to a gradual refraction of the incident beam parallel to the edge. A numerical simulation of a focused Gaussian beam incident on a finite dielectric wedge gives a full account of this phenomenon.
RESUMO
Transmission spectra of bilayers of a strongly absorbing dye molecule on thin semitransparent metallic films show a pronounced variation of the shape as a function of the thickness of the metal film. The shape changes with increasing thickness of the metal film from the form of an absorption spectrum as determined by the imaginary part of the dielectric function to an antisymmetric shape characteristic of the dispersion of the real part of the dielectric function in the vicinity of a resonance. These different spectra shapes were exploited to derive the complex dielectric function of a dye layer from transmission spectra of the layer on metal films of a different thickness. This method proved to be a simple alternative to determination of the dielectric function of a thin film of a dye by spectroscopic ellipsometry.