20 April 2006 Design of apertureless tips with very high plasmon field enhancement
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Abstract
In contrast with aperture-limited Scanning Near-field Optical Microscopy, where the focusing of light is achieved only with very high attenuation, in apertureless near-field optics light is both focused and strongly amplified by the surface plasmons of the probe. Although the general feasibility of this idea and the unprecedented in optics lateral resolution of ~ 15-30 nm have already been demonstrated, the actual field enhancement has so far been well below theoretical expectations, and the useful optical signals have been weak. To bridge the gap between the "proof-of-concept" experiments and reliable optical microscopy with molecular-scale resolution, one needs to unify accurate simulation with effective measurements of the optical properties of the tips and with fabrication. We use dark-field microscopy with side collecting optics for measurements of the optical properties of the tip. The side view allows us to observe the radiation of the tip and hence to analyze its optical properties at the apex. In addition, the measured Raman signal provides an estimate of the electric field enhancement by the tip. Our simulation protocol consists of two parts: electrostatics and wave analysis. Electrostatic simulations give good qualitative predictions, are very fast and therefore conducive to multiparametric optimization. Full wave analysis is needed to evaluate the dephasing effects and far-field signals. The Finite Element Method is used for all simulations. Various tip designs with the field enhancement ranging from ~ 50 to over 250 (depending on various parameters), with the commensurate enhancement of the Raman signal by ~ 454 (for gold coating) and ~ 2704 (for silver coating), are presented and analyzed.
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F. Čajko, F. Čajko, I. Tsukerman, I. Tsukerman, A. Kisliuk, A. Kisliuk, A. P. Sokolov, A. P. Sokolov, } "Design of apertureless tips with very high plasmon field enhancement", Proc. SPIE 6195, Nanophotonics, 619510 (20 April 2006); doi: 10.1117/12.663122; https://doi.org/10.1117/12.663122
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