Abstract. The multi-pass photoacoustic spectrometer (PAS) is an important tool for the direct measurement of light absorption by atmospheric aerosol. Accurate PAS measurements heavily rely on accurate calibration of their signal. Ozone is often used for calibrating PAS instruments by relating the photoacoustic signal to the absorption coefficient measured by an independent method such as cavity ring down spectroscopy (CRD-S), cavity-enhanced spectroscopy (CES) or an ozone monitor. We report here a calibration method that uses measured absorption coefficients of aerosolized, light-absorbing organic materials and offer an alternative approach to calibrate photoacoustic aerosol spectrometers at 404 nm. To implement this method, we first determined the complex refractive index of nigrosin, an organic dye, using spectroscopic ellipsometry and then used this well-characterized material as a standard material for PAS calibration.
We present dielectric function e ¼ e 1 þ ie 2 spectra and critical-point energies of Cu 2 ZnSnSe 4 determined by spectroscopic ellipsometry from 0.5 to 9.0 eV. We reduce artifacts from surface overlayers to the maximum extent possible by performing chemical-mechanical polishing and wet-chemical etching of the surface of a Cu 2 ZnSnSe 4 thin film. Ellipsometric data are analyzed by the multilayer model and the e spectra are extracted. The data exhibit numerous spectral features associated with critical points, whose energies are obtained by fitting standard lineshapes to second energy derivatives of the data. The experimental results are in good agreement with the e spectra calculated within the GW quasi-particle approximation, and possible origins of the pronounced critical-point structures are identified. V
Key challenges limiting the adoption of metallic plasmonic nanostructures for practical devices include structural stability and the ease of large scale fabrication. Overcoming these issues may require novel metamaterial fabrication with potentials for improved durability under extreme conditions. In this work, we report a self-assembled growth of a hybrid plasmonic metamaterial in thin-film form, with epitaxial Ag nanopillars embedded in TiN, a mechanically strong and chemically inert matrix. One of the key achievements lies in the successful control of the tilt angle of the Ag nanopillars (from 0° to 50°), which is attributed to the interplay between the growth kinetics and thermodynamics during deposition. Such an anisotropic nature offered by the tilted Ag nanopillars in TiN matrix is crucial for achieving broadband, asymmetric optical selectivity. Optical spectra coupled with numerical simulations demonstrate strong plasmonic resonance, as well as angular selectivity in a broad UV-visible to near-infrared regime. The nanostructured metamaterials in this work, which consist of highly-conductive metallic nanopillars in a durable nitride matrix, have the potential to serve as a novel hybrid material platform for highly-tailorable nanoscale metamaterial designs, suitable for high temperature optical applications.
Vanadium dioxide (VO 2 ) is a well-studied Mott-insulator because of the very abrupt physical property switching during its semiconductor-to-metal transition (SMT) around 341 K (68 °C). In this work, through novel oxide-metal nanocomposite designs (i.e., Au:VO 2 and Pt:VO 2 ), a very broad range of SMT temperature tuning from ≈323.5 to ≈366.7 K has been achieved by varying the metallic secondary phase in the nanocomposites (i.e., Au:VO 2 and Pt:VO 2 thin films, respectively). More surprisingly, the SMT T c can be further lowered to ≈301.8 K (near room temperature) by reducing the Au particle size from 11.7 to 1.7 nm. All the VO 2 nanocomposite thin films maintain superior phase transition performance, i.e., large transition amplitude, very sharp transition, and narrow width of thermal hysteresis. Correspondingly, a twofold variation of the complex dielectric function has been demonstrated in these metal-VO 2 nanocomposites. The wide range physical property tuning is attributed to the band structure reconstruction at the metal-VO 2 phase boundaries. This demonstration paved a novel approach for tuning the phase transition property of Mott-insulating materials to near room temperature transition, which is important for sensors, electrical switches, smart windows, and actuators.
Cu 2 ZnGeSe 4 is of interest for the development of next-generation thin-film photovoltaic technologies. To understand its electronic structure and related fundamental optical properties, we performed first-principles calculations for three structural variations: kesterite, stannite, and primitive-mixed CuAu phases. The calculated data are compared with the room-temperature dielectric function ε = ε 1 + iε 2 spectrum of polycrystalline Cu 2 ZnGeSe 4 determined by vacuumultraviolet spectroscopic ellipsometry in the photon-energy range of 0.7 to 9.0 eV. Ellipsometric data are modeled with the sum of eight Tauc-Lorentz oscillators, and the best-fit model yields the bandgap and Tauc-gap energies of 1.25 and 1.19 eV, respectively. A comparison of overall peak shapes and relative intensities between experimental spectra and the calculated ε data for three structural variations suggests that the sample may not have a pure (ordered) kesterite phase. The complex refractive index N = n + ik, normal-incidence reflectivity R, and absorption coefficients α are calculated from the modeled ε spectrum, which are also compared with those of Cu 2 ZnSnSe 4. The spectral features for Cu 2 ZnGeSe 4 appear to be weaker and broader than those for Cu 2 ZnSnSe 4 , which is possibly due to more structural imperfections presented in Cu 2 ZnGeSe 4 than Cu 2 ZnSnSe 4 .
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