On the coupling efficiency of metal-coated near-field probes

A theory for calculating the optical transmission of nanometric circular ap ertures in a thick and perfectly conducting screen coated upon an optical f ibre has been developed. The theory is intended for the study of near-field probes and differs from other well known theories of radiation transmissio n through subwavelength apertures in the fact that it includes an optical f ibre, making possible to distinguish which part of energy passing through t he aperture is effectively coupled in guided modes. In a scanning near-fiel d optical microscope tip, only the guided modes will reach the photodetecto r, and will contribute to the final readout. A numerical calculation for di fferent 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 inc luded is emphasized.