Microwave Absorption in Photonic Crystals Composed of SiC/Resin with a Diamond Structure

Three-dimensional photonic crystals were fabricated by infiltrating an epoxy mold with a SiC/polyester mixture. The epoxy molds with normal or inverse diamond structures were formed by stereolithography. The size of the mold was 45 mm × 18 mm × 18mm, and the lattice constant of the photonic crystals was 18 mm. The effects of the epoxy mold type, aspect ratio (the ratio of height and diameter of a diamond lattice rod), and number of sample units on the formation of photonic band gap (PBG) and microwave absorption ability along the 〈100〉 direction were studied. The attenuations of microwave transmission and reflection were measured through the photonic crystal samples at a frequency range of 3–12 GHz with a network analyzer. The results obtained suggest that the combination of the absorbing material SiC and diamond structure has a dual effect to form a PBG with a high absorption ability.     

Three‐Dimensional Stereolithography of Alumina Photonic Crystals for Terahertz Wave Localization

Using stereolithography as an additive manufacturing (AM) process, photonic crystals, with a diamond‐like structure composed of alumina microlattices, were fabricated and their electromagnetic band gap profiles, in terahertz frequency ranges, were investigated. Acrylic resins with dispersed alumina particles were fabricated by stereolithography with micrometer‐order accuracy. After dewaxing and sintering processes, alumina lattices were obtained with high relative densities that reflected the terahertz waves, through Bragg diffraction, perfectly in all directions. Twinned crystal structures with mirror symmetric diamond lattices were designed to introduce defect interfaces. Double‐cavity defects consisting of unit cells hollowed from the diamond lattices formed the coupled resonation modes.     

Terahertz Wave Control Using Ceramic Photonic Crystals with a Diamond Structure Including Plane Defects Fabricated by Microstereolithography

The fabrication and terahertz wave properties of alumina microphotonic crystals with a diamond structure were investigated. Acrylic diamond structures with alumina particles' dispersion were formed using microstereolithography. Fabricated precursors were dewaxed and sintered in air. The electromagnetic wave properties were measured by terahertz time-domain spectroscopy. A complete photonic band gap was observed from 0.40 to 0.47 THz, and showed good agreement with the simulation of a plane wave expansion method. Moreover, a localized mode was observed by introducing a plane defect between twinned diamond structures. The localized mode was analyzed using transmission line modeling simulation.     

Localization of acoustic modes in periodic porous silicon structures

The propagation of longitudinal acoustic waves in multilayer structures based on porous silicon and the experimental measurement of acoustic transmission for the structures in the gigahertz range are reported and studied theoretically. The considered structures exhibit band gaps in the transmission spectrum and these are localized modes inside the band gap, coming from defect layers introduced in periodic systems. The frequency at which the acoustic resonances appear can be tuned by changing the porosity and/or thickness of the defect layer.     

Fabrication of Three-Dimensional Micro Photonic Crystals of Resin-Incorporating TiO2 Particles and their Terahertz Wave Properties

Three-dimensional (3D) photonic crystals with a diamond structure made from 40 vol% TiO₂–acrylate dielectric composites were formed by means of a CAD/CAM micro-stereolithography system. The lattice constant of the diamond unit cell was 500 μm and the forming accuracy was 10 μm. The photonic band gap in the Γ–X 〈100〉 direction measured by terahertz-time-domain spectroscopy appeared at 280–360 GHz, which agreed fairly well with the band gap calculated by the plane wave expansion method.     

Band Gap Modification of Diamond Photonic Crystals by Changing the Volume Fraction of the Dielectric Lattice

Photonic crystals with a diamond structure of epoxy lattices in which TiO₂-based ceramic particles are dispersed were fabricated by stereolithography. The periodicity of the lattice was designed to reflect electromagnetic waves in the gigahertz range. The volume fraction (β) of the dielectric lattice medium was modified from 14% to 33% by changing the rod diameter of the lattice. The photonic band gap was observed along Γ-L 〈111〉, Γ-X 〈100〉, and Γ-K 〈110〉 directions and the complete photonic band gap was formed at over β= 20%. The width of the forbidden gap increased gradually when the β increased over 14%, and reached 2.4 GHz at β= 33%. These results agreed with the band calculation using the plane wave expansion method.     

Ion channeling studies of hydrogen in homoepitaxially grown CVD diamond

Using ion channeling measurements in conjunction with the resonant ¹H(¹⁵N,αγ)¹²C nuclear reaction or with proton Rutherford backscattering spectrometry we studied the incorporation of hydrogen from the plasma into homoepitaxial (100) and (111) CVD diamond films. The measured hydrogen concentration and the structural defect density of the diamond lattice derived from proton channeling yields were observed to be correlated. Hydrogen lattice location studies for different incident ion channeling directions showed no dominant fraction of H occupying either one of the two theoretically predicted sites. These results suggest that in CVD films with quite high dislocation densities and relative H concentrations above 10⁻³, hydrogen is predominantly incorporated at structural defect sites uncorrelated with the lattice symmetry.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

二维磁振子晶体点缺陷模耦合特性的研究

利用平面波展开法和超原胞计算,我们研究了存在点缺陷耦合态的二维磁振子晶体(由一种铁磁材料的圆柱体构成正方排列插入另外一种铁磁材料中形成)中自旋波的传播,结果表明该结构中的缺陷使带隙中产生局域态。局域磁振子的这些耦合特性在应用于设计自旋波滤波器方面将有很大的应用价值。     

Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO₂ ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO₂ paste (before sintering) was around 15 μm. After the SiO₂-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO₂ ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO₂ photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X 〈100〉 direction, between 490 and 630 GHz in the Γ– K 〈110〉 direction, and between 400 and 510 GHz in the Γ– L 〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.     

Band Structure Calculations of the High-Temperature Electronic Structure of Magnesium Oxide

The effects of high temperature on the electronic structure of a material consist of two major contributions, thermal lattice expansion and the electron-phonon interaction. These can produce dramatic changes in the electronic structure and play a critical role in the high-temperature properties and behavior of ceramics. We have used ab initio pseudofunction band structure methods to calculate the temperature dependence of the electronic structure of MgO from 300 to 1300 K modeling the independent effects of thermal lattice expansion and the electron-phonon interaction. The band structure calculations were performed self-consistently on a (MgO)₄ supercell using experimental values obtained from high-temperature X-ray diffraction to determine the lattice constants up to 1300 K and the root mean square amplitude of phonon displacements. Lattice thermal expansion contributed -0.15 meV/K to the band gap temperature dependence. Individual phonon modes, with displacements in the 〈111〉, 〈110〉, and 〈100〉 directions, were modeled using distorted lattice calculations. The electron-phonon coupling was found to be strongest for the 〈100〉 mode modeled, with strong coupling seen for modes which lead to the smallest decrease in the Mg-O bond length. The overall magnitude of the electron-phonon contribution to the band gap temperature dependence for the phonon modes modeled was −0.95 meV/K. The theoretical results account for a band gap temperature dependence in MgO of −1.1 meV/K, which compares well with the temperature dependence of approximately −1 meV/K determined experimentally using vacuum ultraviolet spectroscopy.     

Vibrations of crystallographic defects associated with a single chain in polyethylene

The vibrational behaviour of crystallographic defects associated with a single chain were investigated for a dispiration, disclination, and dislocation in polyethylene. An approximate longitudinal modulus for the defects was determined by using conformational calculations to estimate the energy changes associated with changes in the length of a defect. This modulus, combined with the mass per unit length of the defect, was used to estimate the lowest longitudinal frequency of the defect, which was found to be around 100 cm^?1 for all the defects considered. Normal mode vibrational calculations for oligomers containing defects showed that the predicted lowest longitudinal modes could be identified by examination of the displacements associated with modes occurring in the estimated frequency range. It was shown that the defects could be considered as localized oscillators embedded in the crystal and coupled to the vibrational modes of the crystal. The presence of defects provides special mechanisms for coupling light waves and lattice vibrations in the crystal which may affect the Raman spectrum.     

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.     

Local structure analysis of heavily boron-doped diamond by soft x-ray spectroscopy

To clarify the electronic structure of metallic heavily boron (B)-doped diamonds, the discrete variational (DV)-Xα method was used to analyze the continuous X-ray absorption near edge structure (XANES) in the BK and CK regions of a 71,000-ppm B-doped diamond. The continuous XANES profiles are well reproduced by the unoccupied B2p- and C2p-DOS of the caged B-cluster models, in which an octahedron B₆ cluster, a cuboctahedron B₁₂ cluster, or an icosahedron B₁₂ cluster is inserted into the defect space of the diamond lattice. The delocalized conduction band structure can be understood from the hybridization of the B atoms in B-clusters with the surrounding C atoms in the diamond lattice. The results indicate that the B atoms in heavily B-doped diamonds form caged B-clusters in the defect space of the diamond lattice.     

Surface photo-voltage effect of hydrogenated diamond subjected to low energy electron irradiation

In this paper we report on the influence of sub-band gap photon illumination on the electron emission of hydrogenated diamond surfaces subjected to continuous low energy electron irradiation. Hydrogenated diamond surface traps charge by resonance electron attachment (REA) and formation of C–H_(ads)⁻ anionic transient surface species with a maximum cross section at an incident electron energy of ∼ 9 eV. The steady state population of these anionic surface species results in up-wards surface band bending and consequent decrease of low energy electron emission. It is demonstrated that concurrent low energy electron bombardment and illumination of the diamond surface with sub-band gap photons enhances the secondary electron emission yield of diamond. It is argued that these effects are associated to a surface photo-voltage effect, which results in unpinning of the surface band.     

Super-cell calculation of the nitrogen defect in diamond and cubic silicon carbide

The nitrogen point defect in diamond and cubic SiC was studied using an ab-initio plane wave pseudopotential approach that highlights the differences in the conduction bands of the two materials. Each N defect energy level is located relative to the various energy bands. The N defect level in diamond shows very little dispersion through the bands, which is indicative of its localized character. On the contrary, in cubic SiC the defect is far less localized, with the level essentially depending on the occupancy of either the C or Si site. Both sites may give n-type behavior, but N on the C site is preferred, with a defect level substantially pinned to the conduction band.     

First-principles studies of diamond polytypes

Using first-principles methods, we have performed a systematic investigation of the cubic, rhombohedral and hexagonal diamond polytypes, both in equilibrium and under hydrostatic pressure. For each of the diamond polytypes (2H, 3C, 4H, 6H, 8H, 9R, 10H-SiC form, 10H-ZnS form, 15R and 21R), the structural, energetic, and electronic properties are investigated. The calculated lattice parameters, mass densities and ground-state energies of the diamond polytypes show clear trends with their hexagonality, while bulk modulus and elastic parameters are rather insensitive to the hexagonality. It is found that there are no phase transitions between these diamond polytypes studied in this work, at a pressure range from 0 to 500 GPa. For the series of diamond polytypes studied, the calculated results indicate that the relationship between ground-state electronic band gap and hexagonality at zero pressure is different from that observed at the hydrostatic pressure of 500 GPa.     

Structural and vibrational properties of the 6H diamond: First-principles study

The 6H diamond is the first successfully synthesized diamond polytype, which are not found in nature. We have performed the investigations on structural properties and vibrational properties of the 6H diamond with a first-principles method and we also have identified the vibration normal modes of the 6H diamond with group theory. The structural properties of the 6H diamond are quite similar to the cubic diamond (3C diamond) and the lonsdaleite (2H diamond) as expected. The vibrational properties of the 6H diamond are more complicated than the 3C and 2H diamonds. At Γ point of the first Brillouin zone of the 6H diamond, there are 15 Raman active modes with nine different frequencies, and six IR active modes with four different frequencies. However, there are no IR active modes in the 3C and 2H diamonds. Therefore, the IR active modes of the 6H diamond can provide the indication modes for distinguishing the 6H diamond from the 2H and 3C diamonds. For Raman active modes, the frequencies of the E_1g² and E_2g² modes of the 6H diamond are much smaller than the Raman frequencies of the 2H and 3C diamonds; consequently, it also can contribute to distinguishing the 6H diamond from the 2H and 3C diamonds. The frequencies of two Raman active modes of the 6H diamond–the A_1g² mode and the E_1g¹ mode–agree very well with the experimental results.     

Fabrication and Microwave Properties of Three-Dimensional Photonic Crystals With a Diamond Structure Composed of Ceramic Spheres in Resin

Three-dimensional photonic crystals with a diamond structure composed of YSZ (3 mol% Y₂O₃-stabilized ZrO₂) spheres in a resin matrix were fabricated by using stereolithography. The lattice constant was 12 mm and the diameter of the spheres was 5 mm. These photonic crystals made of ceramic spheres showed complete photonic band gaps at around 12 GHz between the eighth and ninth bands. The propagation characteristics of microwaves agreed well with the calculated results using the plane wave expansion method.     

Fabrication of Metallodielectric Photonic Crystals with a Diamond Structure and their Microwave Properties

Three-dimensional (3D) metallodielectric photonic crystals with a diamond structure were fabricated in order to investigate the formation of stop bands and the absorption ability for microwaves with the dielectric absorbing media embedded into the 3D metal lattice. First, the metallic photonic crystals were prepared by filling the epoxy molds formed by stereolithography with a metal alloy having a low melting point of 70°C, followed by removal of the molds. The metallodielectric photonic crystals were then fabricated by infiltrating the porous metal crystal with a SiC/polyester mixture. The lattice constant of photonic crystals was 15 mm. The effects of different aspect ratios of diamond lattice rods, number of metallic lattice units along Γ-L 〈1 1 1〉, Γ-X 〈1 0 0〉, and Γ-K 〈1 1 0〉 directions, and metallodielectric samples along the Γ-X 〈1 0 0〉 direction on the formation of stop band and microwave absorption ability were investigated in the frequency range from 3 to 30 GHz. Metallodielectric photonic crystals formed showed good absorption ability. The measured transmission spectra of the metallic and metallodielectric crystals agreed well with the simulation of the transmission line modeling method.