Metallic nanoparticles have drawn much interest due to their distinct plasmonic characteristics especially in imaging and
sensing applications. Surface plasmon resonance (SPR) based biosensors have evolved in many ways, among which
sensitivity enhancement towards molecular sensing capability came up with strategies to overcome the hard limit of the
intrinsic sensitivity of gold thin film. Recently adoption of signal contrast materials has proven successful in biochemical
sensing applications. This study employs gold-SiO2 core-shell nanoparticles (CSNPs) as a strong SPR signal contrast
agents. To reveal the underlying physics for the contrast mechanism, the particle characteristics were analytically
evaluated in terms of light interaction coefficients. We experimentally demonstrate the effect of the CSNPs by applying
them to acquire enhanced signal in DNA hybridization sensing scheme. We also applied gold nanowire grating structure
on conventional gold thin film to further amplify the intrinsic sensitivity, where localized surface plasmon and locally
amplified evanescent fields take parts. The results suggest that CSNPs and the grating structure cooperatively enhance
the sensitivity and the role of nanowire gratings was analyzed with numerical methods to allow optimum sensitivity
enhancement in terms of fill factor variations. The effects of field localization, amplification and enlarged signature of
CSNPs are also discussed.
DNA hybridization can be measured with enhanced sensitivity based on localized surface plasmon (LSP) induced by
surface nanowire structure. Changes made to the structure result in higher plasmon momentum, which can be coupled to
a particle plasmon induced by gold nanoparticles to which DNA molecules are adsorbed. With the insight gained from
near-field pattern via calculation, target localization effect is also experimentally shown. We expect that orders of
magnitude can be improved in terms of sensitivity if one is to combine the effect of particle-to-LSP coupling and target
localization scheme.
The presence of multiple polarized beams can limit the polarimetric discrimination capability of a wire-grid polarizer
(WGP). In this study, the effect of linearly polarized background on the polarimetric performance of a WGP has been
investigated and compared with that of a perfect polarizer. Simulation results based on rigorous coupled-wave analysis
indicate that while a WGP mimics a perfect polarizer in discrimination characteristics, the range of the object
polarization angle that can be discriminated against polarized background is fairly limited. The negative impact of
Rayleigh anomaly is also discussed. The detectability of the object polarization can be strongly enhanced by employing a
multi-cell WGP with multiple polarization orientations.
For biomedical applications, various optical techniques have been explored, all of which are inherently subject to the
limited penetration depth of observation. Nanoparticles have thus been introduced as a potential agent to overcome the
limit and to reveal more information on biomedical samples. Optical properties of nanoparticles in a turbid medium,
however, have remained difficult to measure precisely. In this paper, we investigate the scattering and absorbing
properties of gold nanoparticles of varying diameter and different concentration in absorbing liquid phantoms of silica
colloid, based on diffuse photometry. Diffuse reflectance patterns have been collected for each phantom with
nanoparticles of different size and concentrations relative to base media. The results imply that particle concentration and
size perturbs the medium properties, and the effect is dependent on the scattering properties of the particle. For the case
where the particle size stays within a range, the scattering property is dominated by the concentration.
While the effective medium theory (EMT) has been useful to explain optical characteristics of a dielectric periodic structure analytically, it has failed to describe metallic structures correctly. In this paper, a fitting-based approach is introduced to applying an effective medium theory to structures that include metallic material. The effective indices of a metallic medium were first obtained by numerically fitting to reflectance characteristics calculated with rigorous coupled wave analysis (RCWA). Searching for an effective medium has been performed through binary searches rather than a time-consuming simulated-annealing algorithm. The calculated effective medium showed results that are in good agreement with RCWA. The deviation was minimal in the long-wavelength limit when angles of incidence, grating depths, or refractive indices of a superstrate are varied. In particular, TE polarization showed more robust features against the variations while TM polarization was more sensitive to the modeling parameters. In terms of the standard deviation, the calculated effective medium was the least affected by the change of grating depths. The applicability of the fitting-based approach was investigated by applying it to a three-dimensional metallic photonic crystal. Simulation results based on the fitting-based EMT perfectly reproduced broad photonic bandgap as observed in published experimental data. Also, the fitting-based approach provided valid results in the wider wavelength range than a traditional EMT.
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