Characterization of semiconductor core-shell nanoparticles by resonant Raman scattering and photoluminescence spectroscopy

Colloidal CdSe nanoparticles (NPs), passivated with CdS and ZnS, were characterized by resonant Raman scattering and photoluminescence (PL). The effect of the passivating shell, its volume and formation procedure on optical and vibrational spectra is discussed. Analyzing the Raman peaks due to optical phonons inside the core and those related to the core-shell interface allows some understanding of the relation between the core-shell structure and its PL properties to be achieved. In particular, a compositional intermixing at the core/shell interface of the NPs was deduced from the Raman spectra, which can noticeably affect their PL intensity.     

Effective mode area and its optimization in silicon-nanocrystal waveguides

We revisit the problem of the optimization of a silicon-nanocrystal (Si-NC) waveguide, aiming to attain the maximum field confinement inside its nonlinear core and to ensure optimal waveguide performance for a given mode power. Using a Si-NC/SiO2 slot waveguide as an example, we show that the common definition of the effective mode area may lead to significant errors in estimation of optical intensity governing the nonlinear optical response and, as a result, to poor strength evaluation of the associated nonlinear effects. A simple and physically meaningful definition of the effective mode area is given to relate the total mode power to the average field intensity inside the nonlinear region and is employed to study the optimal parameters of Si-NC slot waveguides.     

A comparative study of spliced optical fibers

A fusion-spliced optical fiber is examined with a laser sheet of light. A CCD camera is used to record the transverse interference pattern from the fiber. The buckling on the fiber material in one direction of the spliced point is distinct inside the transverse interference pattern. The refractive index profile inside the fiber core, obtained at different illumination directions, is calculated using a new method showing the change in the refractive index due to fusion splicing of the fiber. A simple theoretical model is introduced to simulate the anomalous behavior in the transverse interference fringes due to a slight change of the optical parameters.     

Nonlinear waveguide modes caused by generation of excitons and biexcitons in semiconductors in a symmetrical three-layer structure

A theory of nonlinear TE-polarized waveguide modes propagating inside a symmetrical planar three-layer structure with a linear core and nonlinear coatings is developed. The coatings are assumed to be nonlinear due to the optical exciton-biexciton conversion. The dispersion laws of the propagating waves are derived and investigated.     

Suspended twin-core f iber for optical switching

A kind of novel fiber, comprising two fiber cores which are suspended in air inside the outer cladding via a central thin membrane ,is proposed for optical switching application. When a hydrostatic pressure applied on the optical fiber, the pressure-induced refractive index change of the two fiber cores will contribute to the periodical change of the intensity of guided light in the fiber core. The mode coupling of two cores under different hydrostatic pressure and influences of each structure parameter of the proposed fiber on the switching nressure have been numerically invo.stigated.     

Anomalous dielectric behavior of water in ionic newton black films

The electrostatics of two charged surfactant layers in aqueous media (surfactant/water/surfactant films) is investigated using molecular dynamics simulations. In the films studied (with a surfactant-surfactant distance from approximately 35 A to contact) we observe an anomalous dielectric response of water. The electrostatic potential phi(z) inside the aqueous core of the films (containing bulk water with rho=1 g/cm(3)) is completely different from that expected for a film containing a dielectric medium with the dielectric constant of water. In addition, our results are not consistent with a local relation between the water polarization P(z)(z) and the electric field E(z)(z). The polarization P(z)(z) is maximum at the interfaces (due to solvent molecules forming part of the structure of the surfactant layers) and decays from the interfaces inside the aqueous core with a decay length of order of approximately 10 A.     

Determination of the refractive index profile of a symmetric fiber preform by the transport of intensity equation

The refractive index profile of an axially symmetric fiber preform is determined by using the transport of intensity equation. In this method the preform is immersed in an index-matching liquid, and a collimated light beam impinges on it laterally. The intensity distributions of the transmitted light are measured on two close parallel planes inside the preform core. From the recorded intensity distributions, the deflection function is calculated by the transport of intensity equation. The refractive index profile is obtained by means of Abel inversion. Also, for comparison, the refractive index profile of the preform is measured by the focusing method and the results are in agreement with less than 3% error.     

Determination of GR-IN optical fibre parameters from transverse interferograms considering the refraction of the incident ray by the fibre

The core–cladding refractive index difference Δn and the index gradient profile parameter of the graded index (GR-IN) optical fibre has been determined. The curve representing the real path, in the core region, due to refraction of the beam and the change in the exit wavefront are considered. Multiple-beam Fizeau fringes and two-beam interference Pluta polarizing microscope, are used for this investigation. A new method, based on a derived mathematical expression, is used with a prepared computer program to estimate the fringe shift inside GR-IN of the fibre core. The estimated and experimental values of the fringe shift, along the core radius, are used to obtain the refractive index profile of the optical fibre. A comparison between the new method and a previous model considering the refraction has been shown. Microinterferograms are given for illustrations.     

一种FPGA芯片在射频干扰下的失效机理

研究了一种Xilinx公司FPGA芯片XC7A200T-2FBG676在射频干扰下的失效机理。通过对该FPGA内核供电引脚注入射频干扰发现,某些频率下,随着干扰强度的增大,FPGA会依次出现三种不同类型的失效,分别为该FPGA的内核失效、I/O失效和配置失效。测试分析和HSPICE仿真表明,内核失效是由于BRAM的逻辑层抗扰性差所致,I/O失效是由于射频干扰下输入/输出信号的同时失真所致,配置失效则是由于配置系统读取错误的配置使能信号所致。研究可为该FPGA芯片或者系统电磁兼容设计以及该FPGA抗扰性检测方案的制定提供指导。     

Improving SERS uniformity by isolating hot spots in gold rod-in-shell nanoparticles

Surface-enhanced Raman scattering (SERS) tags show ultrasensitivity and multiplexing abilities due to strong and characteristic Raman signals and therefore can be utilized as optical labeling agents similar to fluorescent dyes and quantum dots for biosensing and bioimaging. However, SERS tags have the difficulty to realize quantitative analysis due to the uniformity and reproducibility issue. In this work, we have reported on a new type of SERS tag called Au rod-in-shell (RIS) gap-enhanced Raman tag (GERT). With the high-resolution transmission electron microscopy (TEM) and optical absorbance measurements, we have demonstrated the subnanometer sized gap junctions inside the RIS GERTs. SERS measurements and FDTD calculations show that the core–shell subnanometer gap geometry in the RIS GERTs not only generates strong SERS hot spots but also isolates SERS hot spots by Au shells to avoid the influence when the particle aggregates form, therefore showing better SERS uniformity and stronger SERS intensity than normal Au nanorods. Those RIS NPs exhibit great potential as the labeling agents for SERS-based bioimaging and biosensing applications.     

Core level shake up structures of N2 adsorbed on nickel surfaces: Cluster models

Self-consistent Hartee-Fock-LCAO calculations on a linear NiN₂ cluster are used to explain the two-peak structure observed in core level photoemission from N₂ adsorbed on the Ni (100) surface. The two observed peaks are due to different N₂ core hole final states, a screened and an unscreened final state that can be clearly identified in the cluster model. The energy separation and intensity ratio (computed using the sudden approximation) of the two final states in NiN₂ are comparable with the experimental peak separation and intensity ratio for reasonable cluster geometries. A comparison with previous model studies on NiCO is used to explain the different shape of the core level spectra of N₂ and CO adsorbed on nickel surfaces.     

Method for investigation of parameters of X-ray beams diffracted from a single crystal in the presence of temperature gradient

A method for investigation of the parameters of X-ray beams pumped from the transmission direction into the reflection one depending on the value of applied temperature gradient is proposed. An increase in the integral intensity of diffracted X-ray beam is due to both the overthrowing effect inside the Darwin table and an increase in the angular width of beam. It is shown that it is possible to determine to a high accuracy the value of temperature gradient, beginning with which the angular width of the pumped X-ray beam increases.     

碳碳双键法研究萝卜不同部位的β-胡萝卜素含量分布

β-胡萝卜素广泛存在于植物体中,是典型的线性多稀分子,具有重要的生物功能。由于β-胡萝卜素是碳碳单、双键(C-C,C=C)交替的短链共轭多稀分子,含有大量离域的π电子,具有重要的光电特性。根据Andreas等对拉曼散射强度的研究,当激发光波长落在分子的电子吸收带时,会产生共振拉曼效应,能使拉曼光谱强度提高106倍。利用共振拉曼光谱技术,测量了β胡萝卜素分子及胡萝卜、青萝卜、白萝卜肉质直根不同部位其拉曼光谱,发现含β-胡萝卜素较高的胡萝卜的拉曼光谱与β-胡萝卜素的吻合很好。Gellerman等研究表明,样品浓度与拉曼峰强成正比关系,从拉曼光谱中容易发现三种萝卜的光谱强度纵向根头到主根及横向表皮到根芯逐渐降低,且青萝卜和白萝卜拉曼光谱强度都很低,并在碳碳单键的振动峰处发生峰劈裂。分别计算了碳碳单键和碳碳双键与碳氢键拉曼强度比,三种萝卜的I_CC/I_C-H随着测量部位(横向和纵向)的不同变化幅度接近：胡萝卜的表皮和根芯纵向的变化率分别为A₁=0.213 3和A₂=0.215 9,青萝卜表皮外和里的变化率分别为B₁=0.219 1和B₂=0.211 4,白萝卜表皮外和里分别为D₁=0.223 9和D₂=0.224 1;而对于I_C-C/I_C-H随着测量部位不同其变化率相差很大：胡萝卜的变化率a₁=0.212 1和a₂=0.232 4,青萝卜的变化率b₁=0.263 5和b₂=0.268 7,白萝卜的变化率d₁=0.369 0和d₂=0.304 9。对比发现三种萝卜的碳碳单键与碳氢键振动强度比随着测量部位的不同变化幅度相差很大,而从碳碳双键与碳氢键振动强度比发现三种萝卜中不同部位的β胡萝卜含量有相似的分布。这是由于青萝卜和白萝卜中β-胡萝卜素的含量少, 随着测量部位的不同C-C伸缩振动峰发生峰劈裂, 即在1 130和1 156 cm-1处出现两个振动峰, 经过计算和分析这两个峰都属于碳碳单键的伸缩振动峰, 且随着β-胡萝卜素含量的减少C-C整体的强度降低, 劈裂的新峰峰强度却有增加的趋势, 这使得原峰位的峰强度大幅度降低, 这与计算I_C-C/I_C-H的结果一致,不同品种的萝卜中β-胡萝卜素含量随测量部位的不同变化幅度截然不同。因此, 当样品中β-胡萝卜含量较少时,利用C=C振动峰峰强度同时分析样品不同部位的β-胡萝卜素含量分布变化会更准确。同时,研究和了解萝卜中不同部位β-胡萝卜素的含量为日常消费和膳食营养提供了很好的理论依据。     

Length-dependent effects due to leaky modes on multimode graded-index optical fibres

The radiation losses of tunnelling leaky modes in graded-index optical fibres are calculated theoretically, and it is shown that the near-field intensity profile has a length dependence. Consequently measurements of the near-field intensity distribution do not give the refractive index profile directly, and a correction factor must be applied. We have investigated this factor and find that it depends only on a single normalisation parameter involving fibre length, core radius and normalised frequency. A further use of the correction factor is to determine the total power attenuation due to the loss of leaky modes.     

Asymmetric evolution of eddies in rotating shallow water

It is demonstrated that cyclones evolve in a way different from that of anticyclones in rotating shallow water. The anticyclones merge and eventually take circular coherent forms, but cyclones are elongated with active enstrophy cascading. This asymmetric evolution is strengthened with increasing surface displacements. When the initial surface displacement exceeds a certain critical value, the initial elongation of a cyclonic ellipse ends up with splitting in two cyclones. This splitting of the cyclonic ellipse is always associated with the first appearance of a saddle point inside the core, due to irrotational, ageostrophic motion. The appearance of the saddle point inside the core seems to be a necessary condition for splitting of the core of the cyclonic ellipse with surface displacements. The linear stability analysis of the elliptical vortex is consistent qualitatively with both of the simulation results and the kinematic axisymmetrization/elongation principle.     

A Raman laser intensity sensor induced by the cooperative Rayleigh scattering in a ring configuration

In this work, a laser sensor is described that uses the multipath interference produced inside a ring cavity to measure the power loss induced by a moving taper intensity sensor. The laser is created due to the virtual distributed mirror formed by the Rayleigh scattering produced in a dispersion compensating fiber when pumped by a Raman laser. Two laser peaks were formed, one of them is obtained by the Raman gain (1555 nm) inside the ring and the second is created by the combination of the Raman gain and the Rayleigh scattering (1565 nm). A taper sensor is used as displacement sensor and when the losses is applied in the taper the second laser peak is reduced and the first peak is maintained constant and can be used as reference level.     

Radiation heat transfer in axisymmetric quartz glass tubes

A mathematical model is proposed to describe the heat transfer in quartz glass axisymmetric tubes. Heat is transferred inside the glass by radiation and conduction. Scattering of thermal radiation inside the glass is ignored. At the boundaries of the tube the radiative intensity is specularly reflected. The spectral dependent radiative intensity and the temperature distribution inside the tube are determined. The model is applied to simulate the cooling process of a quartz glass tube. The calculated temperature is in agreement with that obtained from an experiment. Furthermore, steady-state temperature distributions in quartz glass tubes of different lengths have been determined.     

微槽式光学谐振腔超声传感效应验证*

摘 要：光学微腔的高灵敏度主要源于其结构在时间和空间上对光场的局域增强作用和频率选择作用。其结构在垂直于波导方向上形成了高反射的边界,形成了一种回声腔,使得光在波导内来回反射,从而增强了波导内部的光场强度。当外界存在微小的压力波动时,它将引起波导内部的介电常数和压力场的变化,从而改变了谐振腔内的模式场分布和传输特性,据此可以实现对微小的压力波动进行高灵敏度检测。本文设计了一种高品质因子（Q）的光波导微槽式环形谐振腔超声传感器,完成器件制备并搭建了测试系统,依据倏逝波效应实现了超声探测。测试结果表明,该传感器的品质因子为1.38×107,在800 kHz ~1 MHz范围内响应平坦,在900 kHz的信噪比可以达到27 dB,灵敏度达到 -168 dB。本文设计的传感器可以为水声探测等领域的研究提供关键技术支持。     

Effect of intrinsic optical feedback on the intersubband optical properties of spherical quantum dots

Optical feedback due to mutual relation between local field effect and intersubband transition in quantum dots is investigated for the first time. In this regard, dielectric function of quantum dots is considered up to the third order of nonlinearity. It is found that near the resonance, the intensity inside the dot is a function of frequency, which is determined by the optical feedback. This effect changes the magnitude of optical nonlinearity and its symmetry around the transition energy. The results indicate that the magnitude of the dielectric function decreases at frequencies below the transition frequency because of concentration of electric field inside the dot and vice versa. It is also shown that this effect is enhanced by increasing the intensity and resonance contribution in the dielectric function.     

Optimized light-matter interaction and defect hole placement in photonic crystal cavity sensors

Photonic crystal slab cavities were investigated for increased light-matter interaction based on selective placement of sublattice hole sized defect holes inside L3 cavities. A multiple-hole defect (MHD) consisting of three defect holes placed in the regions of highest cavity mode field intensity were demonstrated through finite-difference time-domain simulations and experiments to exhibit the strongest light-matter interaction without introducing significant scattering losses. Compared to an L3 cavity without defect holes, these strategically designed three-hole MHD cavities presented higher quality factor and more than double the resonance wavelength shift upon exposure to a thin oxide and two small chemical molecules.