Within the framework of the direct moment method, a theory for calculating the optical transmission of nanometric circular aperture on top of a metal coated optical fiber tip is developed. The primary purpose of the study is to estimate transmission efficiency of near field probes widely used in near field scanning optical microscopy. Field distributions immediately after the aperture in the core and cladding of the fiber tip can be accurately calculated in three dimensions. It turns out that the transmission is a function of many variables, such as the radii of the aperture and fiber core, the thickness of the metallic coat, the material responses in the core and cladding area of the fiber, and finally the light frequency.
SummaryA theory for calculating the optical transmission of nanometric circular apertures in a thick and perfectly conducting screen coated upon an optical fibre has been developed. The theory is intended for the study of near-field probes and differs from other well known theories of radiation transmission through subwavelength apertures in the fact that it includes an optical fibre, making possible to distinguish which part of energy passing through the aperture is effectively coupled in guided modes. In a scanning near-field optical microscope tip, only the guided modes will reach the photodetector, and will contribute to the final readout. A numerical calculation for different fibre parameters, to show the dependence of the guided transmission coefficient for the guided modes of the fibre, is presented. The agreement of the theory with earlier calculations where the optical fibre is not included is emphasized.
We present a quantum mechanical calculation of the diamagnetic optical response of metallic ultrathin films. The study shows that in the optical response of ultrathin films (less than 100 Å in thickness), there exists an oscillatory dependence on the film thickness, and the period of the oscillation corresponds to one or a few monolayers. We show that the oscillation can be attributed to the intraband fluctuations of the valence electrons in discrete energy states. For comparison, we present experimental results on the infrared (
= 9.2 µm) optical reflectance of Al ultrathin films of thickness 5-112 Å, which exhibit experimentally the predicted oscillations.
We present theoretical calculations of the optical transmission through subwavelength apertures in a conducting screen of finite thickness. The results are employed to study the origin of the enhanced transmission observed in experiments where randomly distributed small holes of various subwavelength sizes are fabricated in a thin gold film. In the wavelength spectrum from 350 nm to 650 nm, a number of strongly enhanced transmission peaks are observed. These transmission peaks can only be observed in the near field. We discuss the experimental results in the light of the model calculations.
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