煤烟聚集粒子的光散射特性研究(英文)

基于分形理论,用计算机模拟了由球形基本粒子构成的煤烟聚集粒子。利用离散偶极子近似方法(Discrete Dipole Approximation)研究了煤烟聚集粒子的散射特性,讨论了分形煤烟聚集粒子的散射强度随煤烟聚集粒子的分形结构、大小、相对折射率及入射波波长变化情况。     

Computational analysis and diagonal preconditioning for the discrete dipole approximation on surface

The construction of a matrix for the discrete dipole approximation (DDA) on surface and its relationship to an iterative solver is analyzed. It is shown that the spectral characteristics of the DDA for free space and surface correlates to different convergence characteristics. Compared with the free space DDA, when a surface is introduced, both the dipole polarizability matrix and the reflection–interaction matrix contributes to the diagonal/off-diagonal element, and solvability of the iterative method is related to several physical parameters such as incident angle, polarization, and refractive indices. Finally, we propose a diagonal preconditioning technique and show the effectiveness of the preconditioned to a semiconductor pattern with isolated contaminant which is assumed to be PSL, Si₃N₄, and Si. The result shows that when there is difference in the refractive index, the diagonal preconditioning reduces the total computation time up to 27% for low refractive index cases. However the result shows limitation for the higher refractive index cases.     

Plasmon-phonon coupling in graphene-hyperbolic bilayer heterostructures

Polar dielectrics are important optical materials enabling the subwavelength manipulation of light in infrared due to their capability to excite phonon polaritons.In practice,it is highly desired to actively modify these hyperbolic phonon polaritons(HPPs) to optimize or tune the response of the device.In this work,we investigate the plasmonic material,a monolayer graphene,and study its hybrid structure with three kinds of hyperbolic thin films grown on SiO_2 substrate.The inter-mode hybridization and their tunability have been thoroughly clarified from both the band dispersions and the mode patterns numerically calculated through a transfer matrix method.Our results show that these hybrid multilayer structures are of strong potentials for applications in plasmonic waveguides,modulators and detectors in infrared.     

The far infrared optical constants of GaP

The FIR optical constants of GaP have been determined on either side of the Reststrahlen band by single pass transmission dispersive Fourier transform spectroscopy. These results extend the range of measured refractive index data for GaP, and reveal the dispersion in the refractive index in regions of weak absorption. Features in the spectrum are assigned as phonon combination bands with the aid of a critical point analysis based on phonon frequencies calculated using an 11-parameter rigid ion model.     

Interaction of oxygen and carbon impurities with the multiphonon infrared absorption bands of Silicon at low temperatures

Infrared absorptance measurements of polycrystalline silicon and single crystals of silicon having oxygen and carbon impurities were carried out at low temperatures in the wavelength range of multiphonon infrared absorption of silicon. Peak absorptance and frequencies were measured at temperatures from 10 to 130 K at intervals of 10 K. Absorptance-temperature curves were plotted for multiphonon bands and impurity bands. It has been observed that the three phonon band [2 TO (Γ) + TA (L)] exhibits a resonance-like behaviour with the anti-stretching mode of Si-O at low temperatures.     

随机取向烟幕凝聚粒子的消光特性

采用蒙特卡罗方法根据团簇-团簇凝聚(CCA)模型对由球形原始微粒凝聚而成的烟幕凝聚粒子进行模拟,用离散偶极子近似(DDA)方法研究随机取向烟幕凝聚粒子的消光特性.结果表明,凝聚粒子的消光特性受到原始微粒数量以及粒径的影响,粒子的凝聚将减弱烟幕的消光性能;当凝聚结构中原始微粒的数目一定时,存在使烟幕消光性能达到最大的原始微粒粒径.     

Two-dimensional silicon-carbon hybrids with a honeycomb lattice: New family for two-dimensional photovoltaic materials

We predict a series of new two-dimensional(2D) inorganic materials made of silicon and carbon elements(2D SixC1?x) based on density functional theory. Our calculations on optimized structure, phonon dispersion, and finite temperature molecular dynamics confirm the stability of 2D SixC1?x sheets in a two-dimensional, graphene-like, honeycomb lattice. The electronic band gaps vary from zero to 2.5 e V as the ratio x changes in 2D SixC1?x changes, suggesting a versatile electronic structure in these sheets. Interestingly, among these structures Si0.25C0.75 and Si0.75C0.25 with graphene-like superlattices are semimetals with zero band gap as their ? and ?* bands cross linearly at the Fermi level. Atomic structural searches based on particle-swarm optimization show that the ordered 2D SixC1?x structures are energetically favorable. Optical absorption calculations demonstrate that the 2D silicon-carbon hybrid materials have strong photoabsorption in visible light region, which hold promising potential in photovoltaic applications. Such unique electronic and optical properties in 2D SixC1?x have profound implications in nanoelectronic and photovoltaic device applications.     

Phonon considerations in the reduction of thermal conductivity in phononic crystals

Periodic porous structures offer unique material solutions to thermoelectric applications. With recent interest in phonon band gap engineering, these periodic structures can result in reduction of the phonon thermal conductivity due to coherent destruction of phonon modes characteristic in phononic crystals. In this paper, we numerically study phonon transport in periodic porous silicon phononic crystal structures. We develop a model for the thermal conductivity of phononic crystal that accounts for both coherent and incoherent phonon effects, and show that the phonon thermal conductivity is reduced to less than 4% of the bulk value for Si at room temperature. This has substantial impact on thermoelectric applications, where the efficiency of thermoelectric materials is inversely proportional to the thermal conductivity.     

Low frequency phonon spectra of mixed crystal CdxHg1−xTe

Far infrared absorption spectra for the mixed crystal Cd_xHg_1?xTe with x=0.18 to 0.45 in the wavenumber region of 10 to 400cm^?1 and temperature region of 4.5 to 300K are reported. A low frequency absorption band has been found between 20 and 50 cm^?1 for all the samples which have been measured in addition to the two-phonon bands in both sides of the reststrahlen absorption bands of the material. By comparison with the phonon densities of states estimated from the phonon frequencies of CdTe, we have attributed this band to TA band modes induced by the disorder and “impurity” effects.     

Scattering of Polarized and Natural Light by a Monolayer of Spherical Homogeneous Spatially Ordered Particles under Normal Illumination

We consider distortions of the shapes of absorption bands of strongly scattering samples in the spectra of molecules adsorbed on the surface of dispersed solids. We show that the influence of the scattering on IR spectra of dispersed samples is not reduced merely to a weakening of the intensity of the transmitted light, but, rather, the scattering affects the contour of a spectral band due to changes in the refractive index of the substance in the range of the absorption band. Using carbon dioxide adsorbed on a NaX zeolite as an example, we demonstrate two methods for taking into account the contribution of the scattering to the spectra of surface compounds: the registration of the “diffuse-transmission” spectrum and placing a sample into an immersion liquid, for which liquid oxygen is used. The obtained spectra indicate that, if the zeolite is saturated with the adsorbate, the band of the antisymmetric stretching vibration of CO₂ molecules broadens and reveals a complex structure as a result of the resonant dipole–dipole interaction between adsorbed molecules.     

红外区金属氧化物反蛋白石光子晶体的安德森定域化研究

根据Mie散射理论,在低浓度近似下,对中红外区由金属氧化物构成的反蛋白石光子晶体的禁带的存在区域,即光子定域化区进行了研究。发现在浓度为10%,此类晶体在中红外区将出现多个光子禁带区域,并且计算了影响定域化区域的各种因素。由此可知利用材料在剩余射线带内折射率小于1的特性,我们可以制备出中红外区和远红外区存在禁带的光子晶体。这一结果为反蛋白石晶体的研究提供了理论方法。     

Fano Resonance Enabled Infrared Nano-Imaging of Local Strain in Bilayer Graphene

Detection of local strain at the nanometer scale with high sensitivity remains challenging. Here we report near-field infrared nano-imaging of local strains in bilayer graphene by probing strain-induced shifts of phonon frequency.As a non-polar crystal, intrinsic bilayer graphene possesses little infrared response at its transverse optical phonon frequency. The reported optical detection of local strain is enabled by applying a vertical electrical field that breaks the symmetry of the two graphene layers and introduces finite electrical dipole moment to graphene phonon. The activated phonon further interacts with continuum electronic transitions, and generates a strong Fano resonance. The resulted Fano resonance features a very sharp near-field infrared scattering peak, which leads to an extraordinary sensitivity of ~0.002% for the strain detection. Our results demonstrate the first nano-scale near-field Fano resonance, provide a new way to probe local strains with high sensitivity in non-polar crystals,and open exciting possibilities for studying strain-induced rich phenomena.     

Effective zero index in locally resonant acoustic material

Here in locally resonant acoustic material, it is shown that effective zero refractive index can be constructed by the resonant unit-cells with coherent degenerate monopole–dipole momenta. Due to strong local resonances, the material layers with effective zero refractive index can function as a resonant cavity of high Q factor, where a subtle deviation from the resonant frequency may result in distinct increase of reflection. Full-wave simulations are performed to demonstrate some unusual wave transport properties such as invisibility cloaking, super-reflection, local field enhancement, and wavefronts rotation.     

界面折射率不一致对显微成像穿透深度的限制

在运用共焦显微术及低相干显微成像术等进行光学断层成像时,要将光束聚焦到样品内部以便实现光学断层成像,然而由于生物组织等的折射率与盖玻璃以及浸液不同,因而会引入很大的球差,从而使入射电磁波发生畸变.分析了由于多层折射率不一致而引入的球差的大小,并运用所得的公式计算了折射率不一致对光在样品中穿透深度及焦面附近光场分布的影响.     

由球形原子构成的多种周期性介电结构的光子能带计算

采用矢量平面波方法计算了由球形空气原子构成的体心立方结构和六角密堆结构的光子能带，两种结构下都看到了贯通整个布里渊区的禁带。本文预言周期性介电结构的最大折射率调制度和最大禁带宽度对应于同样的原子体积分数，而且这个最佳的原子体积分数是结构无关的。文中还计算了由各向异性介电小球构成的面心立方结构的光子能带并看到了禁带，这与各向同性介电小球构成的面心立方结构很不相同。     

Antireflection coating on germanium for dual channel (3–5 and 7.5–10.6 μm) thermal imagers

The dual channel thermal imager, operating in the 3–5 and 7.5–10.6 μm wavelength bands, is one of the latest achievements in instrumentation for target recognition and acquisition. While the 3–5 μm band is utilised for detecting hot objects such as engine exhausts of vehicles and fighter planes, the 7.5–10.6 μm band is employed for human bodies and objects at ambient temperatures. Many substrates are available which transmit in both these wavelength bands and their transmission can be enhanced by providing a suitable antireflection coating. In this paper, a broad band antireflection coating on germanium substrate is reported. The design approach involves achieving a continuously varying refractive index from that of the incident medium to the substrate. The continuously varying refractive index profile may be generated by using a sequence of thin layers of high and low refractive index materials. In this design a continuous refractive index profile is approximated by using a 13-layer stack of thorium fluoride and germanium as low and high index coating materials respectively. This coating conforms to environmental stability standards and shows an average transmission of 91% in 3–5 μm band and 94.5% in 7.5–10.6 μm band with a peak of 97% at 9 μm on 10 mm thick germanium substrate. Polycrystalline germanium has 2.5% absorption for a 10 mm thick substrate.     

Light propagation in linear arrays of spherical particles

A propagation of dipolar radiation in a finite length linear chain of identical dielectric spheres is investigated using the multisphere Mie scattering formalism (MSMS). A frequency pass band is shown to be formed near every Mie resonances inherent in the spheres. The manifestation of the pass band depends on the polarization of the travelling radiation. To prove this effect, a point dipole placed by the end of the chain is used as an external source of radiation. It is found that, if this dipole is directed parallel to the chain axis, the frequency pass bands exist if the refractive index of dielectric spheres is sufficiently large nr>1.9. For the dipole normal to the chain axis, the pass band can always be formed if the chain is sufficiently long. Such a distinction is due to different behavior of the far-field dipolar interaction between the spheres induced by the external source. The edges of the pass bands are defined by the guiding wave criterion based on the light-cone constraint. The criterion of creation of the pass bands correlate with condition of formation of high quality factor modes in these systems found in our previous papers. A comparison with the results available for infinite chains is made. In particular, we clarify the nature of braking down the band structure for small enough wavevectors.     

Photonic crystals and microlasers fabricated with low refractive index material

Although the investigation on photonic band gap materials has been done more than two decades, it is still a big challenge to fabricate three-dimensional photonic crystal (PC) possessing wide band gaps in visible range. In this article, we have reviewed recent progresses on fabricating the PC with low refractive index material in visible range. In contrast to the material with large refractive index, it is cheap to use low refractive index material in fabricating the PC and will be greatly beneficial for future industrial productions. The holographic method to fabricate such a PC has been introduced, applying it to the design of the microlaser has also been discussed.     

随机取向纳米石墨椭球粒子的红外消光特性

利用离散偶极子近似(DDA)方法研究了椭球形的纳米石墨粒子的红外消光特性。计算了不同形状的椭球粒子红外消光截面随波长变化规律,并与等效球形粒子的消光性能进行了比较,分析了不同入射波长时,椭球粒子形状和粒径对消光的影响。结果表明,椭球状的纳米石墨粒子红外消光性能好于等效球粒子的消光性能,椭球粒子的最佳消光等效粒径与轴长比和入射波长有关。     

Photonic crystals and microlasers fabricated with low refractive index material

Although the investigation on photonic band gap materials has been done more than two decades, it is still a big challenge to fabricate three-dimensional photonic crystal (PC) possessing wide band gaps in visible range. In this article, we have reviewed recent progresses on fabricating the PC with low refractive index material in visible range. In contrast to the material with large refractive index, it is cheap to use low refractive index material in fabricating the PC and will be greatly beneficial for future industrial productions. The holographic method to fabricate such a PC has been introduced, applying it to the design of the microlaser has also been discussed.