Numerical simulation of light propagation in metal-coated SNOM tips

Presented are the results of numerical simulations accomplished to investigate the propagation of electromagnetic excitations in certain types of metal-coated tapered tips terminating SiO2 multimode optical fibres with a subwavelength output aperture. The numerical simulations were initiated in order to enable better interpretation of previously reported experimental results concerning some features of the mesoscopic effect of spectral modulation observed for a broadband light transmitted by such tips. This effect occurs due to the interference between a small number of waveguide modes exiting a metal-coated tip, and the experimental results indicate a possible mode-selective photon-plasmon coupling in the studied tips. To match the experimental conditions, the tips were modelled for the light wavelength of 800 nm as three-layer systems (with the intermediate adhesion Cr layer and the outer layer of Al or Au). However, due to computational restrictions the end of a tip, only 18 mu m long (most significant), was modelled. Numerical simulations yielded the dependences of propagation and attenuation constants on the fibre core radius for the most intensive (both photonic and plasmonic) output modes. The pairs of modes most probably contributing to the observed spectral modulation were identified. Although the simulations did not reveal any explicit mode coupling, the imperfections of real tips can cause mode transformations implying possible involvement of more than two modes. The thin (20 nm) Cr layer plays the main role for plasmonic modes generated on its SiO2 interface, which explains the small outer metal layer influence on the observed modal dispersion.