3,294 research outputs found
Tunable high-index photonic glasses
Materials with extreme photonic properties such as maximum diffuse
reflectance, high albedo, or tunable band gaps are essential in many current
and future photonic devices and coatings. While photonic crystals, periodic
anisotropic structures, are well established, their disordered counterparts,
photonic glasses (PGs), are less understood despite their most interesting
isotropic photonic properties. Here, we introduce a controlled high index model
PG system. It is made of monodisperse spherical TiO colloids to exploit
strongly resonant Mie scattering for optimal turbidity. We report spectrally
resolved combined measurements of turbidity and light energy velocity from
large monolithic crack-free samples. This material class reveals pronounced
resonances enabled by the possibility to tune both the refractive index of the
extremely low polydisperse constituents and their radius. All our results are
rationalized by a model based on the energy coherent potential approximation,
which is free of any fitting parameter. Surprisingly good quantitative
agreement is found even at high index and elevated packing fraction. This class
of PGs may be the key to optimized tunable photonic materials and also central
to understand fundamental questions such as isotropic structural colors, random
lasing or strong light localization in 3D.Comment: Main text: 8 pages, 4 figures; Supporting Information: 5 pages, 5
figure
Three-dimensional femtosecond laser nanolithography of crystals
Nanostructuring hard optical crystals has so far been exclusively feasible at
their surface, as stress induced crack formation and propagation has rendered
high precision volume processes ineffective. We show that the inner chemical
etching reactivity of a crystal can be enhanced at the nanoscale by more than
five orders of magnitude by means of direct laser writing. The process allows
to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm
feature sizes inside large crystals in absence of brittle fracture. To showcase
the unique potential of the technique, we fabricate photonic structures such as
sub-wavelength diffraction gratings and nanostructured optical waveguides
capable of sustaining sub-wavelength propagating modes inside yttrium aluminum
garnet crystals. This technique could enable the transfer of concepts from
nanophotonics to the fields of solid state lasers and crystal optics.Comment: Submitted Manuscript and Supplementary Informatio
Contrast and Resolution of Secondary Electron Images in a Scanning Transmission Electron Microscope
Recent advances in the improvement of secondary electron image resolution to the subnanometer level demand further knowledge of the origin of secondary electron emission to interpret the experimental results. The generally accepted estimation of the non-localized range of the inelastic scattering of incident electrons and the subsequent generation of secondary electrons in a solid cannot explain the 0.7 nm resolution of secondary electron images obtained in a scanning transmission electron microscope operated at 100 kV. Resolution and contrast of secondary electron images are interrelated. High contrast as well as high resolution can be obtained at the same time. Contrast mechanisms are also complicated due to the origin of the generation of secondary electrons by incident electrons or by other energetic secondary electrons. Surface adsorption and thin layer contamination will change the collected secondary electron signal dramatically which makes the image interpretation difficult. Surface defects might give observable secondary electron image contrast due to the change of total secondary electron yield caused by the defects or by the adsorbed species at the defect. Ultra-high resolution secondary electron imaging provides important information in the study of surface reactions and related surface problems in a scanning transmission electron microscopy instrument
Application and Challenges of Signal Processing Techniques for Lamb Waves Structural Integrity Evaluation: Part B-Defects Imaging and Recognition Techniques
The wavefield of Lamb waves is yielded by the feature of plate-like structures. And many defects imaging techniques and intelligent recognition algorithms have been developed for defects location, sizing and recognition through analyzing the parameters of received Lamb waves signals including the arrival time, attenuation, amplitude and phase, etc. In this chapter, we give a briefly review about the defects imaging techniques and the intelligent recognition algorithms. Considering the available parameters of Lamb waves signals and the setting of detection/monitoring systems, we roughly divide the defect location and sizing techniques into four categories, including the sparse array imaging techniques, the tomography techniques, the compact array techniques, and full wavefield imaging techniques. The principle of them is introduced. Meanwhile, the intelligent recognition techniques based on various of intelligent recognition algorithms that have been widely used to analyze Lamb waves signals in the research of defect recognition are reviewed, including the support vector machine, Bayesian methodology, and the neural networks
Microseismic event location and passive seismic imaging of crustal structures
This research is aimed to develop a migration-based earthquake location algorithm and a passive seismic imaging approach to investigate microseismicity and image steep crustal structures in the mining area Schlema-Alberoda. The dataset includes single-component records of several microseismic events which occurred between 1998 and 2012 in the area. Through some tests and comparisons, the accuracy of the localization algorithm is proved. An expected extension of pre-existing faults within the granitic body and the connection between some of the structures is comfirmed by the location results. The passive imaging procedure is conducted using only P-wave secondary arrivals from the relocated events and by applying 3-D coherency migration. The reliability of the passive imaging results is verified by comparing the final image with the results of a 3-D active reflection seismic survey in the same area. In addition, the passive image complements the 3-D active image and reveals new structures that have not been imaged previously
First-principles study, fabrication and characterization of (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy ceramic
The formation possibility of a new (Zr0.25Nb0.25Ti0.25V0.25)C high-entropy
ceramic (ZHC-1) was first analyzed by the first-principles calculations and
thermodynamical analysis and then it was successfully fabricated by hot
pressing sintering technique. The first-principles calculation results showed
that the mixing enthalpy of ZHC-1 was 5.526 kJ/mol and the mixing entropy of
ZHC-1 was in the range of 0.693R-1.040R. The thermodynamical analysis results
showed that ZHC-1 was thermodynamically stable above 959 K owing to its
negative mixing Gibbs free energy. The experimental results showed that the
as-prepared ZHC-1 (95.1% relative density) possessed a single rock-salt crystal
structure, some interesting nanoplate-like structures and high compositional
uniformity from nanoscale to microscale. By taking advantage of these unique
features, compared with the initial metal carbides (ZrC, NbC, TiC and VC), it
showed a relatively low thermal conductivity of 15.3 + - 0.3 W/(m.K) at room
temperature, which was due to the presence of solid solution effects,
nanoplates and porosity. Meanwhile, it exhibited the relatively high
nanohardness of 30.3 + - 0.7 GPa and elastic modulus of 460.4 + - 19.2 GPa and
the higher fracture toughness of 4.7 + - 0.5 MPa.m1/2, which were attributed to
the solid solution strengthening mechanism and nanoplate pullout and microcrack
deflection toughening mechanism.Comment: 49 pages,6 figures, 4 table
ALERT Doctoral School 2012: advanced experimental techniques in geomechanics
The twenty-second session of the European Graduate School 2012 (called usually ALERT Doctoral School) entitled Advanced experimental techniques in geomechanics is organized by Cino Viggiani, Steve Hall and Enrique Romero.Postprint (published version
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