Modern nanotechnology allows one to scale down various important devices (sensors, chips, fibers, etc.) and thus opens up new horizons for their applications. The efficiency of most of them is based on fundamental physical phenomena, such as transport of wave excitations and resonances. Short propagation distances make phase-coherent processes of waves important. Often the scattering of waves involves propagation along different paths and, as a consequence, results in interference phenomena, where constructive interference corresponds to resonant enhancement and destructive interference to resonant suppression of the transmission. Recently, a variety of experimental and theoretical work has revealed such patterns in different physical settings. The purpose of this review is to relate resonant scattering to Fano resonances, known from atomic physics. One of the main features of the Fano resonance is its asymmetric line profile. The asymmetry originates from a close coexistence of resonant transmission and resonant reflection and can be reduced to the interaction of a discrete (localized) state with a continuum of propagation modes. The basic concepts of Fano resonances are introduced, their geometrical and/or dynamical origin are explained, and theoretical and experimental studies of light propagation in photonic devices, charge transport through quantum dots, plasmon scattering in Josephson-junction networks, and matter-wave scattering in ultracold atom systems, among others are reviewed.

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Fano resonances in nanoscale structures

The metal 2p x-ray-absorption spectra (or ${\mathit{L}}_{2,3}$ edges) of 3d transition-metal compounds are calculated, using atomic multiplet theory with inclusion of the cubic crystal field. A general overview of the effect of the cubic crystal field on the shape of the 3${\mathit{d}}^{\mathit{N}}$ to 2${\mathit{p}}^{5}$3${\mathit{d}}^{\mathit{N}+1}$ excitation spectrum is given for 14 common valencies of 3d transition-metal ions. Comparison to some high-resolution 2p x-ray-absorption spectra shows excellent agreement, which confirms the validity of the approach. Possible refinements of the theory, including lower-symmetry calculations and the inclusion of configuration interaction, are discussed.

2p x-ray absorption of 3d transition-metal compounds - an atomic multiplet description including the crystal-field

X-ray absorption and dichroism of transition metals and their compounds

NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds

The ${L}_{2}$,3 x-ray-absorption edges of 3${d}^{0}$ compounds are calculated with use of an atomic description of the 2${p}^{6}$3${d}^{0}$ to 2${p}^{5}$3${d}^{1}$ excitation, with the inclusion of the crystal field. For reasons of clarity, we confine ourselves to ${d}^{0}$ compounds in octahedral symmetry, but the same approach is applicable to all other ${d}^{N}$ compounds in any point-group symmetry. The experimental spectra of ${\mathrm{FeTiO}}_{3}$, ${\mathrm{Sc}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{ScF}}_{3}$, ${\mathrm{CaF}}_{2}$, and the potassium halides are well reproduced by the present calculations, including the previously misinterpreted small leading peaks. The splitting between the two main peaks in both the ${L}_{3}$ and ${L}_{2}$ edge are related, though not equal, to the crystal-field splitting. Comparison to experiment showed that the broadening of the main multiplet lines is different. This can be related to Coster-Kronig Auger processes for the ${L}_{2}$ edge and to a solid-state broadening which is a combination of vibrational (phononic) and dispersional broadenings. With the full treatment of the atomic multiplets, the atomic effects can be separated from solid-state effects, which offers a better description of the latter. This includes vibrational broadenings, the covalent screening of the intra-atomic Coulomb and exchange interactions, via the position of small leading peaks, and surface effects. The same general framework can be used to discuss crystal-field effects in both lower symmetries, with the possibility of polarization-dependent spectra (e.g., ${\mathrm{TiO}}_{2}$), and partly filled d bands.

L2,3 x-ray-absorption edges of d0 compounds: K+, Ca2+, Sc3+, and Ti4+ in Oh (octahedral) symmetry.

A new design method effective for a nontransparent system has been developed for soft-x-ray multilayers with the aid of graphic representation of the complex amplitude reflectance in a Gaussian plane. The method provides an effective means of attaining the absolute maximum reflectance on a layer-by-layer basis and also gives clear insight into the evolution of the amplitude reflectance on a multilayer as it builds up. An optical criterion is derived for the selection of a proper pair of materials needed for designing a high-reflectance multilayer. Some examples are given to illustrate the usefulness of this design method.

Layer-by-layer design method for soft-x-ray multilayers.

A method has been developed for analysis of a series of ellipsometric data taken in situ during deposition. With this method the optical constants n and k and thickness d of growing ion-beam-sputtered films of C, Si, Ni, Nb, Mo, Ru, Rh, Pd, Ag, W, Re, and Au have been determined as a function of deposition time t. The minimum critical thickness dc needed for a film to become optically isotropic has been determined from the n, k, d-versus-t curves. Anomalous behavior of these curves appears in the region d ≲ dc resulting from a breakdown of the isotropic film model employed. This is shown, by transmission electron micrographs, to have a correlation with a transition region from island/anisotropic to continuous/isotropic.

In situ ellipsometric study of optical properties of ultrathin films.

A high-resolution spectroscopic facility, consisting of a 6.65-m vertical dispersion off-plane Eagle spectrograph/monochromator and a unique predisperser system of zero-dispersion type, has been designed and constructed at the Photon Factory. A description is given of design principle, optical system, mechanical arrangement, vacuum system, control system, and performance in the spectrograph mode. The resolving power was estimated from the separation between two closely lying lines of Ar I at ∼79 nm in the seventh order to be >2.5 × 105 —the highest resolving power ever demonstrated for this spectral region.

High-resolution VUV spectroscopic facility at the photon factory.

Following a brief introductory remark on the holographic grating, the possibility of designing concave gratings for a specific application is discussed, giving basic ideas of the grating design, actual design procedures, and feasible ways of improving the existing design methods. As a proof of the usefulness of the design method developed by the authors, spot diagrams and experimental results are given for aberration-reduced holographic concave gratings that have been designed and produced specifically for a Seya-Namioka monochromator. The efficiency, stray light, and polarization characteristics of holographic and conventional concave gratings are compared for the purpose of giving some idea about the relative merits of the two kinds of gratings. The design method is also applied to the determination of the ruling parameters needed for the production of mechanically ruled stigmatic concave gratings.

https://iopscience.iop.org/article/10.1143/JJAP.15.1181/pdf

Design and Performance of Holographic Concave Gratings

A soft X-ray reflectometer with a laser-produced plasma source driven by a Q-switched Nd: YAG laser or a XeCl excimer laser was constructed. By comparing the reflectances of a multilayer mirror with the data obtained using synchrotron radiation, its usefulness for reflectance measurements in the soft X-ray region (40-130 eV) is demonstrated.

Takeshi Namioka

Papers

Layer-by-layer design method for soft-x-ray multilayers.

In situ ellipsometric study of optical properties of ultrathin films.

High-resolution VUV spectroscopic facility at the photon factory.

Design and Performance of Holographic Concave Gratings

Soft X-ray reflectometer with a laser-produced plasma source