In metal optics gold assumes a special status because of its practical importance in opto-electronic and nano-optical devices, and its role as a model system for the study of the elementary electronic excitations that underlie the interaction of electromagnetic fields with metals. However, largely inconsistent values for the frequency dependence of the dielectric function describing the optical response of gold are found in the literature [1][2][3]. We performed precise spectroscopic ellipsometry measurements on evaporated gold, template-stripped gold, and single-crystal gold to determine the optical dielectric function across a broad spectral range of 300 nm -25 µm (0.05 -4.14 eV) with high spectral resolution. We fit the data to the Drude free-electron model, with an electron relaxation time τD = 14 ± 3 fs and plasma energy ωp = 8.48 eV. We find that the variation in dielectric functions for the different types of samples is small compared to the range of values reported in the literature. Our values, however, are comparable to the aggregate mean of the collection of previous measurements from over the past six decades. This suggests that although some variation can be attributed to surface morphology, the past measurements using different approaches seem to have been plagued more by systematic errors than previously assumed.

We describe and demonstrate a new nanometer-scale broadband light source. It is based on the grating-coupled excitation of surface plasmon polaritons (SPPs) on the shaft of a sharp conical metal taper with a tip radius of few tens of nanometers. Far-field excitation of linear nanoslit gratings results in the resonant generation of SPPs traveling over more than 10 mum to the tip apex and converging to an intense radiative local light spot. Such nanofabricated tips are expected to find various applications in nanospectroscopy, overcoming problems with background illumination in apertureless microscopy.

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)].

Characterizing and ultimately controlling the heterogeneity underlying biomolecular functions, quantum behavior of complex matter, photonic materials, or catalysis requires large-scale spectroscopic imaging with simultaneous specificity to structure, phase, and chemical composition at nanometer spatial resolution. However, as with any ultrahigh spatial resolution microscopy technique, the associated demand for an increase in both spatial and spectral bandwidth often leads to a decrease in desired sensitivity. We overcome this limitation in infrared vibrational scattering-scanning probe near-field optical microscopy using synchrotron midinfrared radiation. Tip-enhanced localized light-matter interaction is induced by low-noise, broadband, and spatially coherent synchrotron light of high spectral irradiance, and the near-field signal is sensitively detected using heterodyne interferometric amplification. We achieve sub-40-nm spatially resolved, molecular, and phonon vibrational spectroscopic imaging, with rapid spectral acquisition, spanning the full midinfrared (700-5,000 cm) with few cm −1 spectral resolution. We demonstrate the performance of synchrotron infrared nanospectroscopy on semiconductor, biomineral, and protein nanostructures, providing vibrational chemical imaging with subzeptomole sensitivity.any properties and functions of natural or synthetic materials are defined by chemically or structurally distinct phases, domains, or interfaces on length scales of a few nanometers to micrometers. Characterizing these heterogeneities has long driven the development of advanced microscopy techniques, with notable advances in electron (1), photoemission (2), X-ray (3), and superresolution (4) microscopies. Although extremely effective, these techniques have strict sample requirements: either they operate in vacuum, are limited by electron or X-ray beam damage, or rely on labeling with exogenous fluorophores.In contrast, infrared (IR) vibrational spectroscopic imaging is minimally invasive, requires little sample preparation, is applicable in situ and under ambient conditions, and provides intrinsic chemical contrast and spectroscopic identification for a wide range of materials (5), including living cells (6) and tissues (7,8). The spatial resolution, however, is diffraction-limited to 2-10 μm, depending on wavelength. Moreover, high signal-to-noise spectra even at this low resolution are only possible with a source of high spectral irradiance (9). Subwavelength imaging has been achieved to some extent through point-spread function deconvolution (10, 11) and attenuated total reflection techniques (12). However, the micrometer-size wavelength of IR radiation, in general, has fundamentally limited its application for the characterization of essentially any mesoscopic, heterogeneous material where chemical information at the nanoscale is desired.Infrared scattering-scanning near-field microscopy (IR s-SNOM) overcomes the diffraction limit by scattering incident light with the typically metallic tip of an atomic for...

Despite the relatively simple stoichiometry and structure of VO(2), many questions regarding the nature of its famous metal-insulator transition (MIT) remain unresolved. This is in part due to the prevailing use of polycrystalline film samples and the limited spatial resolution in most studies, hindering access to and control of the complex phase behavior and its inevitable spatial inhomogeneities. Here, we investigate the MIT and associated nanodomain formation in individual VO(2) microcrystals subject to substrate stress. We employ symmetry-selective polarization Raman spectroscopy to identify crystals that are strain-stabilized in either the monoclinic M1 or M2 insulating phase at room-temperature. Raman measurements are further used to characterize the phase dependence on temperature, identifying the appearance of the M2 phase during the MIT. The associated formation and spatial evolution of rutile (R) metallic domains is studied with nanometer-scale spatial resolution using infrared scattering-scanning near-field optical microscopy (s-SNOM). We deduce that even for small crystals of VO(2), the MIT is influenced by the competition between the R, M1, and M2 crystal phases with their different lattice constants subjected to the external substrate-induced stress. The results have important implications for the interpretation of the investigations of conventional polycrystalline thin films where the mutual interaction of constituent crystallites may affect the nature of the MIT in VO(2).

Excitons, Coulomb-bound electron-hole pairs, are elementary photo-excitations in semiconductors that can couple to light through radiative relaxation. In contrast, dark excitons (X) show anti-parallel spin configuration with generally forbidden radiative emission. Because of their long lifetimes, these dark excitons are appealing candidates for quantum computing and optoelectronics. However, optical read-out and control of X states has remained challenging due to their decoupling from light. Here, we present a tip-enhanced nano-optical approach to induce, switch and programmably modulate the X emission at room temperature. Using a monolayer transition metal dichalcogenide (TMD) WSe on a gold substrate, we demonstrate ~6 × 10-fold enhancement in dark exciton photoluminescence quantum yield achieved through coupling of the antenna-tip to the dark exciton out-of-plane optical dipole moment, with a large Purcell factor of ≥2 × 10 of the tip-sample nano-cavity. Our approach provides a facile way to harness excitonic properties in low-dimensional semiconductors offering new strategies for quantum optoelectronics.

The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same n; m type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a lowfrequency phonon mode.

Single-molecule vibrational Raman spectroscopy of malachite green adsorbed on planar metal surfaces is achieved by means of optical local-field enhancement provided by a scanning nanoscopic metallic tip. The single-molecule signature is evident from spectral diffusion and a discretization of Raman peak intensities. The optical tip-sample coupling gives rise to a localization of the response down to a sub-10 nm length scale and a Raman enhancement up to ϳ5 ϫ 10 9 . This combines vibrational spectroscopy with high resolution scanningprobe microscopy for ultrasensitive in situ analysis of individual molecules.