Ordinary Crystals Research Articles

Frequency‐Resolved Reciprocal‐Space Mapping of Visible Spontaneous Emission from 3D Photonic Crystals

Information on propagating and evanescent Bloch modes as well as on remaining disorder is provided by experiments on titania woodpile photonic crystals with a complete three-dimensional photo­nic bandgap at visible wavelengths. Such frequency-resolved reciprocal-space mapping of visible spontaneous emission from 3D photonic crystals is analogous to angle-resolved photoemission spectroscopy (ARPES) on ordinary crystals.

医用液晶ディスプレイと汎用液晶ディスプレイにおける視覚的評価─胸部CT 画像とモノクロ，カラー表示における核医学画像の模擬信号の検出能─

The purpose of this study was to evaluate the detection performance of simulated nodules in chest computed tomography (CT) images and nuclear medicine images with an ordinary liquid crystal display (LCD) and a medical LCD (grayscale standard display function: GSDF) and gamma 2.2. We collected 72 chest CT image slices obtained from an LSCT phantom with simulated signals composed of various sizes and CT values and 78 slices of monochrome and color nuclear medicine images obtained from a digital phantom with a simulated signal composed of various sizes and radiation levels. Six observers performed receiver operating characteristic (ROC) analysis using a continuous scale. The area under the ROC curve (AUC) was calculated for each monitor. The average AUC values for detection of chest CT images on a medical LCD (GSDF), medical LCD (gamma 2.2), and ordinary LCD were 0.71, 0.67, and 0.73, respectively. The average AUC values for detection of monochrome nuclear medicine images using a medical LCD (GSDF), medical LCD (gamma 2.2), and ordinary LCD were 0.81, 0.75, and 0.72, respectively. The average AUC values for detection of color nuclear medicine images on a medical LCD (GSDF), medical LCD (gamma 2.2), and ordinary LCD were 0.88, 0.86, and 0.90, respectively. Observer performance for detection of simulated nodules in chest CT images and nuclear medicine images was not significantly different between the three LCD monitors. We therefore conclude that an ordinary LCD monitor can be used to detect simulated nodules in chest CT images and nuclear medicine images.

A high efficiency highly oriented pyrolitic graphite X-ray spectrometer

A novel design of X-ray spectrometer is implemented, using a high efficiency HOPG (highly oriented pyrolitic graphite) Bragg crystal. The spectral resolution of it can reach above 350 with energy ranging from 6.891 to 9.193 keV, theoretically. And the efficiency are 3 order of magnitude higher than the ordinary X-ray crystal spectrometer. Application of this instrument is demonstrated in high intensity laser-foil interaction experiments. It is shown that HOPG spectrometer has much lower threshold of X-ray intensity than ordinary one. The spectral resolution for 8.048 keV (Cu Kup -line) can reach 40 eV, which gives the spectral resolution 200.

About the atomic structures of icosahedral quasicrystals

Abstract This paper is a survey of the crystallographic methods that have been developed these last twenty five years to decipher the atomic structures of the icosahedral stable quasicrystals since their discovery in 1982 by D. Shechtman. After a brief recall of the notion of quasiperiodicity and the natural description of Z -modules in 3-dim as projection of regular lattices in N > 3 -dim spaces, we give the basic geometrical ingredients useful to describe icosahedral quasicrystals as irrational 3-dim cuts of ordinary crystals in 6-dim space. Atoms are described by atomic surfaces (ASs) that are bounded volumes in the internal (or perpendicular) 3-dim space and the intersections of which with the physical space are the actual atomic positions. The main part of the paper is devoted to finding the major properties of quasicrystalline icosahedral structures. As experimentally demonstrated, they can be described with a surprisingly few high symmetry ASs located at high symmetry special points in 6-dim space. The atomic structures are best described by aggregations and intersections of high symmetry compact interpenetrating atomic clusters. We show here that the experimentally relevant clusters are derived from one generic cluster made of two concentric triacontahedra scaled by τ and an external icosidodecahedron. Depending on which ones of the orbits of this cluster are eventually occupied by atoms, the actual atomic clusters are of type Bergman, Mackay, Tsai and others….

Formation of Nanostructures in Brittle Materials with Transformations

Deformation and fracture of metal matrix composites under various loading schemes are studied. It is shown that in conditions far from transformation the material is deformed by dislocation glide as an ordinary crystal. In this case, yield stress is inversely proportional to the carbide spacing; in alloys with high solid phase content it is not achieved up lo fracture. Fracture of the material is catastrophically brittle. Deformation tests of composites near the phase transition temperature show that there are different transformations induced by a highly non-uniform stress state of the binding phase. Under loading the transformation scheme B2-> B2+ B19->B2 + "quasi-amorphous state" is realized in the binding phase with the formation of a microcrystalline, highly misoriented structure with the characteristic size of crystallites less than 10 nm. This structure has high plasticity and strong hardening. It governs an efficient transfer of external load to solid particles, inducing dislocation glide even in typically brittle titanium carbide particles. The physical meaning of using binders with unstable structure in composites involves a decrease in the scale of the structural level of plastic deformation due to the formation of the microcrystalline structure of the binding phase during non-uniform loading.

Enhanced fluorescence emission using a photonic crystal coupled to an optical cavity

All fluorescent assays would benefit from greater signal-to-noise ratios (SNRs), which enable detection of disease biomarkers at lower concentrations for earlier disease diagnosis and detection of genes that are expressed at the lowest levels. Here, we report an approach to enhance fluorescence in which surface adsorbed fluorophore-tagged biomolecules are excited on a photonic crystal surface that is coupled to an underlying Fabry-Perot type cavity through a gold mirror reflector beneath the photonic crystal. This approach leads to 6× increase in signal-to-noise ratio of a dye labeled polypeptide compared to ordinary photonic crystal enhanced fluorescence.

Invisible defects in complex crystals

We show that invisible localized defects, i.e. defects that cannot be detected by an outside observer, can be realized in a crystal with an engineered imaginary potential at the defect site. The invisible defects are synthesized by means of supersymmetric (Darboux) transformations of an ordinary crystal using band-edge wavefunctions to construct the superpotential. The complex crystal has an entire real-valued energy spectrum and Bragg scattering is not influenced by the defects. An example of complex crystal synthesis is presented for the Mathieu potential.

Coupled external cavity photonic crystal enhanced fluorescence.

We report a fundamentally new approach to enhance fluorescence in which surface adsorbed fluorophore-tagged biomolecules are excited on a photonic crystal surface that functions as a narrow bandwidth and tunable mirror of an external cavity laser. This scheme leads to ∼10× increase in the electromagnetic enhancement factor compared to ordinary photonic crystal enhanced fluorescence. In our experiments, the cavity automatically tunes its lasing wavelength to the resonance wavelength of the photonic crystal, ensuring optimal on-resonance coupling even in the presence of variable device parameters and variations in the density of surface-adsorbed capture molecules. We achieve ∼10(5) × improvement in the limit of detection of a fluorophore-tagged protein compared to its detection on an unpatterned glass substrate. The enhanced fluorescence signal and easy optical alignment make cavity-coupled photonic crystals a viable approach for further reducing detection limits of optically-excited light emitters that are used in biological assays.

Array of Virtual Frisch-Grid CZT Detectors With Common Cathode Readout for Correcting Charge Signals and Rejection of Incomplete Charge-Collection Events

New results from testing an array of 6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\,\times\,$</tex></formula> 6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\,\times\,$</tex></formula> 15 mm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{3}$</tex></formula> virtual Frisch-grid CdZnTe (CZT) detectors with common-cathode readout for charge signals correction and rejection of incomplete charge collection events (ICC) are presented. The array employs parallelepiped-shaped crystals of a large geometrical aspect ratio with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode placed on the sides to create the virtual Frisch-grid effect. The detectors are arranged in 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\,\times\,$</tex></formula> 2 or 3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\,\times\,$</tex></formula> 3 detector modules with the common cathode readout by a single electronic channel. Because of the common cathode, the length of the shielding electrode can be further reduced with no adverse effects on the device performance. By implementing a novel technique for rejecting ICC events caused by the extended defects, we can achieve good spectral responses from ordinary CZT crystals, which can be produced with higher yield and at lower cost. For such crystals, the resolution of individual detectors is expected to be in the range of 0.8–1.5% FWHM at 662 keV with an average value of 1.3%. Arrays of virtual Frisch-grid detectors offer a robust and low-cost approach for making large-area detection modules that can potentially substitute for more advanced, but also more expensive and less available, pixel detectors in applications with slightly relaxed requirements on position- and energy-resolution (e.g., for coded aperture telescopes). In addition, such virtual Frisch-grid arrays will require a comparably smaller number of readout channels, which allows for lower power consumption.

Fast and accurate shape measurement system utilizing the fringe projection method with a ferroelectric liquid-crystal-on-silicon microdisplay

Fringe projection methods using the phase-shifting technique have the advantages of fast 3-D shape measurement and high accuracy. The performance of the fringe projection system, including image quality, nonlinearity of the projected intensity, stability, and switching time for multiple phase-shifted patterns, is essential for fast and accurate shape measurement. A fast and accurate measurement system using a ferroelectric liquid-crystal-on-silicon microdisplay and a high-powered light emitting diode light source is developed. Our results indicate that the nonlinearity of the projected intensity and the stability of the fringe projection were dramatically improved compared with an ordinary commercial liquid crystal display projector. The rapid measurement of the warpage distribution of a flip chip ball grid array electronic package was performed by using the developed system. Nine phase-shifted fringe images with a resolution of 1280×960 pixels were recorded in 1.6 s. In addition, the measurement results obtained by our system agreed well with the results obtained from a micrometer and laser focus sensor. The average error was 2.6 µm, and the standard deviation was less than 10 µm with a 6-mm measurement range.

Band structures of two dimensional solid/air hierarchical phononic crystals

The hierarchical phononic crystals to be considered show a two-order “hierarchical” feature, which consists of square array arranged macroscopic periodic unit cells with each unit cell itself including four sub-units. Propagation of acoustic wave in such two dimensional solid/air phononic crystals is investigated by the finite element method (FEM) with the Bloch theory. Their band structure, wave filtering property, and the physical mechanism responsible for the broadened band gap are explored. The corresponding ordinary phononic crystal without hierarchical feature is used for comparison. Obtained results show that the solid/air hierarchical phononic crystals possess tunable outstanding band gap features, which are favorable for applications such as sound insulation and vibration attenuation.

Design and computer modeling of the supracrystals

The possibility of solid state crystalline structures, which we called supracrystals, is shown. Unlike ordinary crystals, there are no separate atoms or ions in the nodes of crystalline lattice. They are replaced by symmetric atomic associates. Symmetry classes of 2D- and 3D-supracrystals are determined. The bond length, energy per one atom, bond energy, and gap energy of 2D- and 3D-supracrystals consist of C, Si, B, N,S are calculated by the program ABINIT-5.8.4 in Hartree–Fock approximation. Unusual properties of supracrystals are of interest for their production and practical use.Energy stability and electrical properties of the nanotubes produced from 2D-sypracrystals suggested by the authors before are researched by computers modeling technic. Supracrystaline nanotubes from the atoms of carbon, silicon, boron and nitrogen, suphfur are considered. It is shown that the electrical properties of supracrystals nanotubes may be controlled in large range (from metallic to dielectric) changing their chemical composition, structure, diameter and chirality.

Dislocations in icosahedral quasicrystals

Dislocations in quasicrystals, as a direct result of the lack of translational symmetry in these materials, possess various salient features. The Burgers vector of a dislocation in an icosahedral quasicrystal is a 6-dimensional vector, which reflects the fact that the dislocation, besides the phonon-type strain field analogous to dislocations in ordinary crystals, is associated inseparably with a further type of defect, the phasons. Phasons are critically involved in the formation and motion of dislocations in quasicrystals and govern the macroscopic plastic behaviour of these materials. In this article the properties of dislocations in icosahedral quasicrystals are comprehensively reviewed, starting from a continuum-mechanical description, via core-structure simulation, to their full experimental characterization. The experimental results presented address the icosahedral phases in the well explored systems Al-Pd-Mn and Zn-Mg-Dy.

A scanning filtering method for enhancing the signal-to-noise ratio of chirped laser pulse

Based on the principle of scanning filtering, a new optical scanning filtering method for improving signal-to-noise ratio(SNR) of the chirped pulse is proposed. For the scanning filtering scheme of Fabry-Perot (F-P) etalon including an electro-optic crystal, the spectral characteristics of the scanning filter are analyzed quantitatively. Furthermore, the effects of the reflectivity of the parallel-plates of F-P etalon and the types of the electro-optic crystals on the output SNR are discussed in detail, and the influence of the variation of the controlling voltage applied to the electro-optic crystal on the filtering effect is also studied. The results show that the narrower the bandwidth of scanning filter transmission window is, the better the improved SNR will be. In order to ensure the filtering effect, the reflectivity of the parallel-plates of F-P etalon should be more than 0.9. Compared with the scheme of scanning filter built-in ordinary linear electro-optic crystal, the scheme of F-P etalon including an secondary standard electro-optic crystal, i.e., KTa1-xNbxO3 crystal, requires a low voltage, and it is easy to control. The differences of KTa1-xNbxO3 crystal components have little influence on the filtering effect, whereas the variations of controlling voltage and chirp rate of the signal exhibit a greater influence on the filtering effect.

Goos-Hänchen shift in complex crystals

The Goos-H\"anchen (GH) effect for wave scattering from complex $\mathcal{PT}$-symmetric periodic potentials (complex crystals) is theoretically investigated, with specific reference to optical GH shift in photonic crystal slabs with a sinusoidal periodic modulation of both real and imaginary parts of the dielectric constant. The analysis highlights some distinct and rather unique features as compared to the GH shift found in ordinary crystals. In particular, as opposed to GH shift in ordinary crystals, which is large at the band gap edges, in complex crystals the GH shift can be large inside the reflection (amplification) band and becomes extremely large as the $\mathcal{PT}$ symmetry-breaking threshold is approached.

Convergent-beam electron diffraction

This paper reviews the convergent-beam electron diffraction (CBED) technique. Point- and space-group determination methods of ordinary crystals are described, along with an example of the determination method for Sr₃Ru₂O₇. The symmetry determination of one-dimensionally incommensurate crystals and quasicrystals is explained. The large-angle CBED technique, which is indispensable for lattice defect and lattice strain analysis, is also described. A real procedure for lattice strain analysis is provided, using an example of a multilayer Si₁-xGe(x)/Si material. A nanometer-scale crystal structure refinement method and charge density and crystal potential determination method by CBED are briefly described.

Effective permittivity of a metamaterial from coated wires

We present a method to determine the effective parameters of a periodic structure with small spacing. This technique is analogous to the averaged boundary condition method applied to planar periodic structures. The effective permittivity of a structure from parallel wires with dielectric and magnetic coating was obtained. It contains an effective plasma frequency and some parameter that is responsible for the spatial dispersion. The dependences of these parameters on electromagnetic and geometric characteristics of structure were analysed. Particularly, it was shown that the role of spatial dispersion decreases if we increase both the coating permittivity and the coating permeability. The magnetic coating is more effective for the taming of spatial dispersion. However, the dielectric coating provides essential taming as well, and it is simpler to fabricate than the magnetic coating. Further, different types of plane waves in the metamaterial under consideration were analysed. In particular, it was shown that, in contrast to the ordinary crystal, there are three types of waves: an ordinary wave and two types of extraordinary waves.

Analytical single-mode model for subwavelength metallic Bragg waveguides

We develop a theoretical formalism that incorporates the method of moment with the analytical eigenmode expansion to investigate the dispersion relation of light transport in subwavelength metallic Bragg waveguide (WG) with each unit cell composed of a wide and a narrow segment of metallic gap. The approach fully accounts for the light scattering at the interface between two consecutive discontinuous segments. A simple single-mode analytical model is derived for both the fundamental even and odd guided modes. The model shows that the band structure of light transport in the structure resembles that of an ordinary dielectric one-dimensional photonic crystal with appropriate physical and geometric parameters that can be analytically derived. Numerical simulations by the finite-difference time-domain method on the optical transmission spectra and band diagrams for these metallic Bragg WGs agree well with the analytical results of band diagrams. In addition, the analytical model can handle structures working in both the microwave and infrared regimes. This indicates that the simple analytical model is effective and efficient in handling various light transport problems for subwavelength metallic Bragg WGs.

Acoustic Zitterbewegung in ordinary sonic crystals: A general classical description

Based on a classical description, we demonstrate in a direct way that the acoustic Zitterbewegung could occur in an ordinary sonic crystal without involving Dirac point, similar to the case in electron systems. This study may provide a guide line for the further experimental realization of Zitterbewegung in acoustic systems.

Catalytic properties of Al13Co4 studied by ab initio methods

While the surfaces of ordinary crystals provide only a few inequivalent adsorption sites, the complex landscape of the surfaces of quasicrystals and their approximants provides a rich variety of different adsorption sites. Recently, Armbrüster et al. reported that Al13Co4, whose structure is closely related to decagonal Al–Ni–Co quasicrystals, is an efficient and selective hydrogenation catalyst for alkynes. In the present work, the hydrogenation of acetylene to ethylene on the (100) surface of Al13Co4 has been studied using ab initio density-functional simulations. Surprisingly, the stable cleavage surface of Al13Co4 is strongly corrugated. The surface is covered by zig-zag chains of edge-sharing Al pentagons, each centered on a Co atom and separated from neighboring chains by wide troughs. The binding energies for adsorption and co-adsorption of H2 and C2H2 molecules at various surface sites have been calculated. Surprisingly, in the energetically most favorable configuration, acetylene is bound in a di-σ configuration to two Al atoms, not to the Co atom. We have searched for the optimal reaction pathway for the dissociatible adsorption of hydrogen and for the hydrogenation of acetylene to vinyl, ethylene by a Langmuir–Hinshelwood mechanism. The energetic barriers for all reaction steps were calculated by the nudged-elastic-band method. It was found that the energetic barrier of any reaction step does not exceed 0.65 eV (63 kJ/mol). This value is lower than the activation energies for acetylene to ethylene hydrogenation over a Pd catalyst where barriers of 78 kJ/mol and 85 kJ/mol were reported for the rate-determining steps.