Gradient metal nanoislands as a unified surface enhanced Raman scattering and surface enhanced infrared absorption platform for analytics.

In the last few decades, the use of plasmonics in vibrational spectroscopy has expanded the scope of (bio)analytical investigations. Nevertheless, there is a demand for a combined platform that can be simultaneously efficient for Surface Enhanced Raman Scattering (SERS) and Surface Enhanced Infrared Absorption (SEIRA). Here, we present a solution on the basis of a plasmonic Ag nanoparticle layer with a thickness gradient. The optical resonance along the layer varies from the visible to the infrared range offering optimal and intermediate sites for SERS and SEIRA of the analyte molecule (mercaptobenzonitrile). Enhancement factors for the same mode were determined to be ca. 104 and 170 for SERS and SEIRA, respectively. We present a full optical and vibrational characterization and demonstrate further tunability. The platform resolves reproducibility and comparability issues by a combination of the two methods. It also offers individualized solutions for different investigation conditions, i.e. a choice between excitation wavelengths and resonant Raman molecules. The multiple applicabilities of the presented unifying substrate can contribute to the expansion of the vibrational spectroscopic field and to analytics.

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.

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.

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 Wood's anomalies. The lack of angular dispersion of the anti-symmetric gap-modes does not agree with the model and requires further theoretical investigation.

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.

Optical properties of silver nanowire arrays with 35 nm periodicity.

We present highly ordered Ag nanowire arrays with 35nm periodicity grown on patterned templates. The optical properties measured using generalized ellipsometry exhibit strong anisotropy. Dielectric functions are calculated by fitting the Jones matrix elements with a biaxial layer model, accounting for both metallic behavior and localized surface plasmon resonances. The amplitude and wavelength maximum of the plasmon resonance perpendicular to the wires increase with increasing wire width and thickness. The dielectric coefficients of 10-mm-wide nanowires show a transition behavior from insulating in UV to metallic above 550nm. Their potential application as polarization-dependent plasmonic-scattering transparent conductive electrodes is discussed.