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)].

The optical gradient force provides optomechanical interactions, for particle trapping and manipulation, as well as for near-field optical imaging in scanning probe microscopy. Based on recent spectroscopic experiments, its extension and use for a novel form of chemical scanning probe nanoimaging was proposed. Here, we provide the theoretical basis in terms of spectral behavior, resonant enhancement, and distance dependence of the optical gradient force from numerical simulations in a coupled nanoparticle model geometry. We predict an asymmetric line shape of the optical gradient force for molecular electronic or vibrational resonances, corresponding to the real part of the dielectric function of the sample materials. Yet the line shape can become symmetric and absorptive for collective polaritonic excitations. The corresponding magnitudes of the force range from fN to pN, respectively. The distance dependence scales considerably less steeply than simple point dipole model predictions due to multipole effects. The combination of these characteristics of the optical gradient force offers the chance to experimentally distinguish it from competing processes such as thermal expansion induced forces. In addition we provide a perspective for further resonant enhancement and control of optical forces.

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Engineering the optical magnetic field with optical antennas or metamaterials extends the ways to control light-matter interaction. The slot antenna, as the electromagnetic dual of the linear rod antenna, provides the simplest form of a magnetic resonator tunable through its length. Using combined far-and near-field spectroscopy and imaging, and theory, we identify magnetic dipole and higher order bright and dark magnetic resonances at mid-infrared frequencies. From resonant length scaling and spatial near-field distribution, we confirm the applicability of Babinetʼs principle over the mid-infrared spectral region. Babinet's principle thus provides access to spatial and spectral magnetic field properties, leading to the targeted design of magnetic optical antennas.

Scattering-type scanning near-field optical microscopy (s-SNOM) provides few nanometer optical spatial resolution and is compatible with nearly any form of linear and nonlinear optical spectroscopy. We have developed a versatile s-SNOM instrument operating under cryogenic and variable temperature (∼20-500 K) and compatible with high magnetic fields (up to 7 T). The instrument features independent tip and sample scanning and free-space light delivery with an integrated off-axis parabolic mirror for tip-illumination and signal collection with a numerical aperture of N.A. = 0.45. The optics operate from the UV to THz range allowing for continuous wave, broadband, and ultrafast s-SNOM spectroscopy, including different variants of tip-enhanced spectroscopy. We discuss the instrument design, implementation, and demonstrate its performance with mid-infrared Drude response s-SNOM probing of the domain formation associated with the metal-insulator transitions of VO2 (TMIT ≃ 340 K) and V2O3 (TMIT ≃ 150 K). This instrument enables the study of mesoscopic order and domains of competing quantum phases in correlated electron materials over a wide range of controlled electric and magnetic fields, strain, current, and temperature.

Phonon-polaritons (PhPs) in layered crystals, including hexagonal boron nitride (hBN), have been investigated by combined scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. Nevertheless, many of such s-SNOM-based FTIR spectra features remain unexplored, especially those originated from the impact of boundaries. Here we observe real-space PhP propagations in thin-layer hBN sheets either supported or suspended by s-SNOM imaging. Then with a high-power broadband IR laser source, we identify two major peaks and multiple auxiliary peaks in the near-field amplitude spectra, obtained using scattering-type near-field FTIR spectroscopy, from both supported and suspended hBN. The major PhP propagation interference peak moves toward the major in-plane phonon peak when the IR illumination moves away from the hBN edge. Specific differences between the auxiliary peaks in the near-field amplitude spectra from supported and suspended hBN sheets are investigated regarding different boundary conditions, associated with edges and substrate interfaces. The outcomes may be explored in heterostructures for advanced nanophotonic applications.

Surface polaritons comprise a wealth of light–matter interactions with deep sub-wavelength scale confinement of electromagnetic modes. However, their nanoscale localized dissipation and thermalization processes are not readily accessible experimentally. Here, we introduce photothermal force microscopy to image surface plasmon polaritons (SPPs) in monolayer graphene through their non-radiative SiO2 substrate dissipation. We demonstrate the real-space SPP imaging via photo-induced atomic force detection, and from comparison with scattering-type scanning near-field optical microscopy imaging attribute the force response to substrate dissipation-enhanced thermal expansion. This work illustrates that nano-optical tip–sample induced dissipative forces facilitate a direct mechanical detection of surface polariton interactions with monolayer sensitivity.

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