Thomas W. H. Oates

Highly anisotropic effective dielectric functions of silver nanoparticle arrays

Variable-angle and Mueller matrix spectroscopic ellipsometry are used to determine the effective dielectric tensors of random and aligned silver nanoparticles and nanorods thin films. Randomly arranged particles are uniaxially anisotropic while aligned particles are biaxially anisotropic, with the anisotropy predominantly at the plasmonic resonances. The strong resonances in nanorod arrays result in the real part of the effective in-plane permittivities being opposite in sign over a significant range in the visible, suggesting the potential to design materials that display tunable negative-refraction. A structural tilt in the particle arrays results in monoclinic dielectric properties.

Self-organized metallic nanoparticle and nanowire arrays from ion-sputtered silicon templates

We demonstrate a production method for self-organized arrays of metal nanoparticles and aligned nanowires. Ion beam-sputtered Si∕SiO2 substrates are used as templates for metallic vapor deposition, forming aligned arrays of 5–20nm silver and cobalt nanoparticles with a period of 35nm. The 20nm diameter cobalt nanowires with lengths in excess of a micrometer are produced under appropriate conditions. All processing steps can be integrated into a single vacuum chamber and performed in a matter of minutes at mild temperatures. This inherently scalable technique can be extended to a range of substrate materials, array patterns, and nanoparticle materials.

Optical activity in sub-wavelength metallic grids and fishnet metamaterials in the conical mount

We report on measurements of optical activity in reflection in the conical mount from two plasmonically resonant nanostructures; a sub-wavelength silver meshed grid and a fishnet metamaterial. The square-centimeter size of the materials, formed by nano-imprint lithography, allows reliable investigation of such materials by plane-wave techniques with minimal focusing. For both materials we observe strong polarization conversion (s- to p-polarization, and vice versa) in generalized ellipsometry measurements. We compared the spectra to analytical predictions using surface plasmon polariton (SPP) theory and find good agreement for the meshed grid. The spectra for the meshed grid are also well modeled using the rigorous coupled wave analysis (RCWA) technique. Simulated results for the more complicated fishnet layer showing qualitative agreement are also presented. We then probe the validity of describing the observations using homogenous parameters such as dichroism and birefringence, by examining the calculated reflection of nominally polarized incident light using simulated and measured Mueller matrices. The results show that the cross-polarization that we observe is primarily related to linear birefringence and dichroism, although circular effects are indeed present.

Oblique incidence ellipsometric characterization and the substrate dependence of visible frequency fishnet metamaterials

We use spectroscopic ellipsometry to investigate the angular-dependent optical modes of fishnet metamaterials fabricated by nanoimprint lithography. Spectroscopic ellipsometry is demonstrated as a fast and efficient method for metamaterial characterization and the measured polarization ratios significantly simplify the calibration procedures compared to reflectance and transmittance measurements. We show that the modes can be well identified by a combination of comparing different substrates and considering the angular dependence of the Woods anomalies. The lack of angular dispersion of the anti-symmetric gap-modes does not agree with the model and requires further theoretical investigation.

Thickness-gradient dependent Raman enhancement in silver island films

We show evidence of a dependence of the enhancement of the Raman scattering cross section on the length of the gradient in graded silver island films. A factor-of-three increase in the Raman signal is observed for gradients with length of the order of 0.5 mm when compared to gradients of the order of 9 mm. Scanning electron microscopy reveals the nanostructure of the two films to be statistically similar. We attribute the observation to differences in plasmon hybridization in the gradients arising from long range structural differences.We show evidence of a dependence of the enhancement of the Raman scattering cross section on the length of the gradient in graded silver island films. A factor-of-three increase in the Raman signal is observed for gradients with length of the order of 0.5 mm when compared to gradients of the order of 9 mm. Scanning electron microscopy reveals the nanostructure of the two films to be statistically similar. We attribute the observation to differences in plasmon hybridization in the gradients arising from long range structural differences.

Mid-infrared gyrotropy in split-ring resonators measured by Mueller matrix ellipsometry

Understanding gyrotropic effects in the mid-infrared (MIR) spectral range is of high technological interest because it is the fingerprint region of many metamaterials and organic compounds. We present experimental and simulated ellipsometric measurements of gyrotropy in the MIR region upon reflection from an ordered array of split-ring resonators, which are inherently non-chiral. We use the symmetry properties of the Mueller matrix for interpretation of the ellipsometric measurements and to identify gyrotropy. When the plane of incidence coincides with the low-symmetry optical axis, we observe gyrotropy at oblique incidence. The origin of the gyrotropy is explained by considering not only the electric dipole–magnetic dipole interaction, as in natural optical activity, but by retaining the electric dipole–electric quadrupole contributions. In isotropic systems these average to zero, however they are significant in ordered systems such as crystals and metamaterials.

Gyrotropy in Achiral Materials: the Coupled Oscillator Model

A coupled oscillator model is developed to explain the observation of gyrotropy in achiral metamaterials. By the action of distinct excitation modes, which only combine under oblique incidence, the measurement of circular birefringence in a split-ring resonator (SRR) array is explained. The symmetry of the SRR resembles the water molecule, and parallels between the systems are drawn.

Nanoliter Sensing for Infrared Bioanalytics

Nondestructive label-free bioanalytics of microliter to nanoliter sample volumes with low analyte concentrations requires novel analytic approaches. For this purpose, we present an optofluidic platform that combines surface-enhanced in situ infrared spectroscopy with microfluidics for sensing of surface-immobilized ultrathin biomolecular films in liquid analytes. Submonolayer sensitivity down to surface densities of few ng/cm2 is demonstrated for the adsorption of the thiolate tripeptide glutathione and for the recognition of streptavidin on a biotinylated enhancement substrate. Nonfunctionalized and functionalized metal island films on planar oxidized silicon substrates are used for signal enhancement with quantifiable enhancement properties. A single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is used with ordinary planar, IR-transparent windows. The geometry circumvents the strong IR absorption of common polymer materials and of aqueous environments in the IR fingerprint region. This practice enables straightforward quantitative analyses of, e.g., adsorption kinetics as well as chemical and structural properties in dependence of external stimuli.