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Near-field infrared nanoscopy of phonon resonant silica and silica-coated goldnoparticles

Y. Abate
California State University Long Beach, US

Keywords: near-field


Single silica capped and uncapped gold nanoparticles are imaged using apertureless near-field optical microscopy (ANSOM) in the visible (λ=632 nm) and infrared frequencies (λ=10.7 ┬Ám). Identification of capped and uncapped particles with nanoscale spatial resolution is demonstrated at both in the visible and infrared frequencies. It is shown that the optical contrast depends strongly on the vertical composition of the sample. Experimental evidence is provided that the near-field probing depths improves with increasing tapping amplitude of the probing tip. Previous works have shown that ANSOM can image structures beneath surfaces. The work here demonstrates for the first time that by using nanoparticles as small as 30 nm in diameter that ANSOM can not only distinquish between bare silica and silica capped-gold core nanoparticles, it can also image the polarization states of the metal core nanoparticle below the silica capping layer. Experimental results are in very good agreement with dielectric function model calculations which includes vertical composition of the nanoparticles in the extended dipole approximation. From these calculations the thickness of the capping layer is determined.
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