Plasmonic nanostructures enable field confinement which is locally amplified within sub-diffraction limited volume. The localized near-field can be useful in many biomedical sensing and imaging applications. In this research, we present the near-field characteristics localized by plasmonic nano-post arrays for biomedical spectroscopy. Circular gold nano-post arrays were modeled on gold and chrome films fabricated on a glass substrate whose thickness was 50, 20 and 2 nm, respectively. The nano-post arrays were fabricated with an e-beam lithography and a diameter of the post was 250 nm with periods varied as 500, 700, and 900 nm. The field localization produced by nano-posts was induced by angled illumination with a total internal reflection fluorescence microscope objective lens and measured by a near-field scanning optical microscope (NSOM). The NSOM has a tapered fiber probe with a 70-nm aperture and was a continuous-wave laser whose wavelength is 532 nm as light source. Incident TM-polarized light exhibited field localization on one side of an individual gold nano-post. When the direction of light incidence was changed opposite, localized field was switched to the opposite edge of the circular nano-post. We performed 3D finite difference time domain s for the field calculation and confirmed the localized field distribution at given illumination angles. We also discuss the potential applications of plasmonic field localization for analysis of biomolecules, cells, and tissues.
Locally amplified near-fields can be induced with nanostructures within a sub-diffraction-limited volume, which is useful for biomedical imaging and sensing applications. Employment of field localization in the biomedical applications where the pulsed light is used necessitates the spatial and temporal characteristics of fields near nanostructures. We considered the gold nano-post arrays of three different shapes to localize the near-fields which are circular, rhombic, and triangular. They were modeled to be located on an ITO film and a quartz substrate with periods changing from 300 to 900 nm by 200 nm. Their size changes from 50 to 250 nm which corresponds to the radius for the case of circular nanoposts and the distance between the center and the vertices for equilateral rhombic and triangular nanoposts. Numerical calculation of near-fields at the top of nanoposts was performed with finite difference time domain method when the Gaussian pulses at center wavelengths of 532, 633, and 850 nm were normally incident. Near-fields localization occurred mainly at vertices of the nanoposts, which makes the triangular nanoposts of primary interest with an observation of the strongest field intensity within a diffraction limited field-of-view. The observed fields on the triangular vertices were enhanced by 7.85, 51.54, and 7268 when the center wavelengths were 532, 633, and 850 nm respectively. Their temporal peaks were delayed by 2.05, 4.03, and 14.49 fs, which indicates the correlation between field enhancement and time delay associated with electron damping process. It was shown that with rhombic and triangular nanoposts fields can be localized below 10 nm on vertices and their signal-to-noise ratio increased with a larger period.