The dielectric function of silver is a fundamental quantity related to its electronic structure and describes its optical properties. However, results published over the past six decades are in part inconsistent and exhibit significant discrepancies. The measurement is experimentally challenging with the values of dielectric function spanning over five orders of magnitude from the mid-infrared to the visible/ultraviolet spectral range. Using broadband spectroscopic ellipsometry, we determine the complex valued dielectric function of evaporated and template stripped polycrystalline silver films from 0.05 eV (λ = 25 µm) to 4.14 eV (λ = 300 nm) with a statistical uncertainty of less than 1%. From Drude analysis of the 0.1 -3 eV range, values of the plasma frequency ω p = 8.9 ± 0.2 eV, dielectric function at infinite frequency ∞ = 5 ± 2, and relaxation time τ = 1/Γ = 17 ± 3 fs are obtained, with the absolute uncertainties estimated from systematic errors and experimental repeatability. Further analysis based on the extended Drude model reveals an increase in τ with decreasing frequency in agreement with Fermi liquid theory, and extrapolates to τ 22 fs for zero frequency. A deviation from simple Fermi liquid behavior is suggested at energies below 0.1 eV (λ = 12 µm) with the onset of a further increase in τ connecting to the DC value from transport measurements of ∼ 40 fs. The results are consistent with a wide range of optical and plasmonic experiments throughout the infrared and visible/ultraviolet spectral range. The influence of grain boundaries, defect scattering, and surface oxidation is discussed. The results are compared with our previous measurements of the dielectric function of gold [Phys. Rev. B 86, 235147 (2012)].

Reversible addition-fragmentation chain transfer polymerization is employed here to allow detector-free visualization of specific DNA sequences for which dynamic polymer growth is used in signal amplification. In particular, surface-initiated polymer growth was regulated by the immobilization of chain transfer agents on the Au surface where DNA hybridization occurred. A linear polymer growth was observed as a function of the reaction time, characteristic of "living" polymer reactions. Significant improvement in assay sensitivity was realized in comparison to the previously reported polymerization-based sensing method by enhancing polymer growth rate and reducing background noises caused by nonspecific adsorption. Direct visualization of fewer than 2,000 copies of a short oligonucleotide sequence was demonstrated in a detector-free fashion.

Optical metamaterials have unique properties which result from geometric confinement of the optical conductivity. We developed a series of infrared metasurfaces based on an array of metallic square loop antennas. The far-field absorption spectrum can be designed with resonances across the infrared by scaling the geometric dimensions. We measure the amplitude and phase of the resonant mode as standing wave patterns within the square loops using scattering-scanning near-field optical microscopy (s-SNOM). Further, using a broad-band synchrotron-based FTIR microscope and s-SNOM at the Advanced Light Source, we are able to correlate far-field spectra to near-field modes of the metasurface as the resonance is tuned between samples. The results highlight the importance of multi-modal imaging for the design and characterization of optical metamaterials.

Optical nanoantennas have been studied as a means to manipulate nanoscale fields, local field enhancements, radiative rates, and emissive directional control. However, a fundamental function of antennas, the transfer of power between a coupled load and far-field radiation, has seen limited development in optical antennas owing largely to the inherent challenges of extracting impedance parameters from fabricated designs. As the transitional element between radiating fields and loads, the impedance is the requisite information for describing, and designing optimally, both emissive (transmitting) and absorptive (receiving) nanoantennas. Here we present the first measurement of an optical nanoantenna input impedance, demonstrating impedance multiplication in folded dipoles at infrared frequencies. This quantification of optical antenna impedance provides the long sought enabling step for a systematic approach to improve collection efficiencies and control of the overall antenna response.

A metasurface consisting of an infinite array of square loops was designed for maximal absorptivity for s-polarized light at a wavelength of 10.6 µm and 60 degrees off-normal. We investigate the effects of array truncation in finite arrays of this design using far-field FTIR spectroscopy and scattering scanning near-field optical microscopy. The far-field spectra are observed to blue-shift with decreasing array size. The near-field images show a corresponding decrease in uniformity of the local electric field amplitude and phase spatial distributions. Simulations of the far-field absorption spectra and local electric field are consistent with the measured results.