Abstract
A systematic study of the influence of the excitation angle, the light polarization and the coating thickness of commercial SPM tips on the field enhancement in an apertureless scanning near-field optical microscope is presented. A new method to optimize the alignment of the electric field vector along the major tip axis by measuring the resonance frequency was developed. The simulations were performed with a MNPBEM toolbox based on the Boundary Element Method (BEM). The influence of the coating thickness was investigated for the first time. Coatings below 40 nm showed a drastic influence both on the resonance wavelength and the enhancement. A shift to higher angles of incidence for the maximum enhancement could be observed for greater tip radii.
Highlights
In optical nanolithography, the dependence of the Abbe diffraction limit on the laser wavelength can be overcome by the application of apertureless near-field microscopes[1,2,3,4,5,6,7] where a laser-illuminated tip is located at a nanometre distance from a substrate
The simulations were performed with a MNPBEM toolbox based on the Boundary Element Method (BEM)
The highest field enhancement is achieved for an excitation wavelength of 650 nm, which corresponds to the resonance case for the investigated parameters and a coating thickness of 55 nm (Fig. 12a), which is exactly two times the skin depth in Au for this wavelength
Summary
The dependence of the Abbe diffraction limit on the laser wavelength can be overcome by the application of apertureless near-field microscopes[1,2,3,4,5,6,7] where a laser-illuminated tip is located at a nanometre distance from a substrate. A systematic study on the influence of geometrical and illumination parameters of SPM tips is presented. The original motivation was the design and development of an apertureless scanning near-field microscope based on a highpower fs-laser oscillator as a beam source, which can be used for surface modification beyond the diffraction limit of light. The influence of various experimental parameters like excitation angle, light polarization and coating thickness was evaluated, theoretically and experimentally. The influence of the laser polarization was studied with a newly developed method adaptive for the fine tuning of the polarization angle for light lever based SPM-systems. It could be shown that the metal coating thickness on SPM-tips strongly determines high field enhancements
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