ABSTRACT
Light diffraction through a subwavelength aperture located at the apex of a metallic screen with conical geometry is investigated theoretically. A method based on a multipole field expansion is developed to solve Maxwell's equations analytically using boundary conditions adapted both for the conical geometry and for the finite conductivity of a real metal. The topological properties of the diffracted field are discussed in detail and compared to those of the field diffracted through a small aperture in a flat screen, i.e., the Bethe problem. The model is applied to coated, conically tapered optical fiber tips that are used in near-field scanning optical microscopy. It is demonstrated that such tips behave over a large portion of space like a simple combination of two effective dipoles located in the apex plane (an electric dipole and a magnetic dipole parallel to the incident fields at the apex) whose exact expressions are determined. However, the large "backward" emission in the P plane--a salient experimental fact that has remained unexplained so far--is recovered in our analysis, which goes beyond the two-dipole approximation.
ABSTRACT
The far-field transmission pattern of a tapered optical tip with small aperture (radius approximately < 40 nm) is modelled by solving Maxwell's equations in the radiation zone with boundary conditions appropriate to the conical geometry. The model is able to reproduce the large differences between the S and P polarizations observed previously in the emission profile of such a tip [Obermüller and Karrai. Appl. Phys. Lett. (1995) 67, 3408].
ABSTRACT
A near-field optical microscope has been developed for operation at low temperature. This microscope is used to study the photoluminescence of CdTe-based quantum dots. Spectra collected upon approaching the optical tip into the near-field region of the sample reveal the evolution from a broad far-field luminescence band - that is typical for a large ensemble of dots - to a near-field structure made up of a few sharp peaks originating from individual dots. Experiments carried out in the excitation-collection mode through the optical tip allow study of the effect of an increase in excitation power on the near-field spectra. It is found that upon increasing the excitation by two orders of magnitude, a spatially resolved spectrum progressively transforms back into a broad 'far-field-like' spectrum. Photoluminescence images taken by scanning the sample under the tip are used to discriminate various contributions coming from individual dots.
