Design for beam splitting components employing silicon-on-insulator rib waveguide structures

We present a new design for beam splitting components employing a silicon-on-insulator rib waveguide structures. In the new design, a high-index thin-film layer is deposited in the rib section to reduce the wave field dispersive tails in the slab section and accordingly render the mode field a confined spot. This in turn improves the beam splitting performance of some conventional waveguide components such as y branches and multimode interference couplers (MMICs), in terms of the excess loss, fiber coupling loss, and compactness of these components. For a 1 x 2 y-branch beam splitter, the excess loss can be as small as 0.43 dB in the new design, which is much lower than that for a conventional rib waveguide structure (which is 1.28 dB). For a 1 x 2 MMIC in our example, the new rib waveguide structure presents an excess loss of 0.064 dB for the TE mode and 0.046 dB for the TM mode, with negligible nonuniformity in dimensions of 30 microm x 1040 microm, whereas its counterpart (i.e., the one with the same dimensions but without a thin-film layer) presents an excess loss of approximately 0.86 dB for both modes. A conventional MMIC must have dimensions larger than 70 microm x 5650 microm to maintain almost the same low excess loss.     

Measurement of the interaction force among particles in three-dimensional plasma clusters

The interaction forces between particles have been studied in a 3D plasma cluster under weak external confinement. A suitable combination of dc and rf applied to a small electrode provided gravity compensation, uniform over dimensions much larger than the cluster itself. The forces acting on the particles could be reconstructed due to unique three-dimensional diagnostics, which allow us to obtain coordinates and velocities of all the particles simultaneously. The measurements yield a maximum (external) confinement force of 1.4 x 10(-15)N and interparticle force that is repulsive at short distances and attractive at larger distances, with a maximum attractive force of 2.4 X 10(-14)N at particle separation 195 microm.     

Three dimensional magnetic resonance imaging by magnetic resonance force microscopy with a sharp magnetic needle

An electropolished magnetic needle made of Nd(2)Fe(14)B permanent magnet was used for obtaining better spatial resolution than that achieved in our previous work. We observed the magnetic field gradient |G(Z)|=80.0G/microm and the field strength B=1250G at Z approximately 8.8 microm from the top of the needle. The use of this needle for three dimensional magnetic resonance force microscopy at room temperature allowed us to achieve the voxel resolution to be 0.6 microm x 0.6 microm x 0.7 microm in the reconstructed image of DPPH phantom. The acquisition time spent for the whole data collection over 64 x 64 x 16 points, including an iterative signal average by six times per point, was about 10 days.     

Fast handheld two-photon fluorescence microendoscope with a 475 microm x 475 microm field of view for in vivo imaging

A fast handheld two-photon fiber-optic fluorescence endoscope for three-dimensional (3D) in vivo cellular imaging is developed. The compact handheld probe of the two-photon endoscope can simply be placed into contact with the target tissue to reveal clear 3D surface and subsurface histological images without biopsy. The new system has, to the best of our knowledge, the largest field of view (FOV) of 475 microm x 475 microm and a 3D imaging volume larger than 475 microm x 475 microm x 250 microm. A real-time two-photon fluorescence image is displayed at 0.4 mm(2)/s. The lateral and axial resolutions of the two-photon fluorescence endoscope are better than 1 and 14.5 microm, respectively.     

应用于微细流场的全息荧光粒子图像三维测速系统

利用亚微米荧光示踪粒子的离焦微细观测时的衍射图样,可以分析确定荧光粒子垂直于图像平面的轴向的位置, 实现对示踪粒子的三维定位．再结合应用于二维流畅测速领域的PIV技术,可利用全息荧光粒子图像测速技术实现流体速度分布的三维测量。     

Fresnel particle tracing in three dimensions using diffraction phase microscopy

We have developed a novel experimental technique for tracking small particles in three dimensions with nanometer accuracy. The longitudinal positioning of a micrometer-sized particle is determined by using the Fresnel approximation to describe the transverse distribution of the wavefront that originated in the particle. The method utilizes the high-sensitivity quantitative phase imaging capability of diffraction phase microscopy recently developed in our laboratory. We demonstrate the principle of the technique with experiments on Brownian particles jittering in water both in bulk and in the vicinity of a boundary. The particles are localized in space within an error cube of 20 nm x 20 nm x 20 nm for a 33 Hz acquisition rate and 20s recording time.     

Confinement limit of a Dirac particle in two and three dimensions

Consider a particle that is in a stationary state described by the Dirac equation with a finite-range potential. In two and three dimensions the particle can be confined to an arbitrarily small spatial region. This is in contrast to the one-dimensional case in which the confinement region cannot be much narrower than the Compton wavelength.     

Three-dimensional particle imaging by wavefront sensing

We present two methods for three-dimensional particle metrology from a single two-dimensional view. The techniques are based on wavefront sensing where the three-dimensional location of a particle is encoded into a single image plane. The first technique is based on multiplanar imaging, and the second produces three-dimensional location information via anamorphic distortion of the recorded images. Preliminary results show that an uncertainty of 8 microm in depth can be obtained for low-particle density over a thin plane, and an uncertainty of 30 microm for higher particle density over a 10 mm deep volume.     

In situ measurement of three-dimensional ion densities in focused femtosecond pulses

We image spatial distributions of Xeq+ ions in the focus of a laser beam of ultrashort, intense pulses in all three dimensions, with a resolution of approximately 3 microm and approximately 12 microm in the two transverse directions. This allows for studying ionization processes without spatially averaging ion yields. Our in situ ion imaging is also useful to analyze focal intensity profiles and to investigate the transverse modal purity of tightly focused beams of complex light. As an example, the intensity profile of a Hermite-Gaussian beam mode HG1,0 recorded with ions is found to be in good agreement with optical images.     

非球形烟尘粒子后向散射场的光谱分析

在利用后向散射法测量烟尘浓度和粒径的过程中,对烟尘粒子模型的后向散射光谱特性进了计算,确定影响后向散射光谱强度的主要因素并进行分析。对实际排放的烟尘进行显微观察表明,利用椭球、圆柱和广义切比雪夫3种非球模型可以较好地模拟烟尘粒子,其等效直径约1μm。通过"T矩阵法"对这3种非球形粒子模型后向散射场的光谱特性进行了分析,结果表明:非球形粒子的可见/红外波段后向散射现象较球形粒子明显,特别是广义切比雪夫粒子的后向散射光强最高可达到前向的3.5倍;对于吸收性非球形粒子(复折射率m=1.57-0.56i),后向散射光强远大于非吸收性非球形粒子(复折射率m=1.57-0.001i);随着粒子等效半径的增大,光源波长也应随之增加。这为在实际测量时光源及方位的选择提供了理论依据。     

Numerical Simulation of Random Close Packings in Particle Deformation from Spheres to Cubes

Variation of packing density in particle deforming from spheres to cubes is studied. A new model is presented to describe particle deformation between different particle shapes. Deformation is simulated by relative motion of component spheres in the sphere assembly model of a particle. Random close packings of particles in deformation form spheres to cubes are simulated with an improved relaxation algorithm. Packings in both 2D and 3D cases are simulated. With the simulations, we find that the packing density increases while the particle sphericity decreases in the deformation. Spheres and cubes give the minimum (0.6404) and maximum (0.7755) of packing density in the deformation respectively. In each deforming step, packings starting from a random configuration and from the final packing of last deforming step are both simulated. The packing density in the latter case is larger than the former in two dimensions, but is smaller in three dimensions. The deformation model can be applied to other particle shapes as well.     

Rapid imaging of fluid flow patterns in a narrow packed bed using MRI

A gradient echo rapid velocity and acceleration imaging sequence (GERVAIS) has been developed and implemented to image liquid flow within a narrow packed bed. Two-dimensional velocity images have been acquired with an in-plane pixel size of 781 microm x 781 microm, with a data acquisition time of 20 ms for a single velocity component. Images of the x, y and z velocity vectors are reported. Data are reported for Reynolds numbers (based on particle diameter) of 200 and 300. In each case, GERVAIS images are compared with the results of a standard spin-echo phase-encoding velocity measurement. At Re = 200, steady-state flow is expected and the velocity images acquired using both techniques are consistent. At Re = 300, the GERVAIS sequence is able to image the unsteady-state flow field within this system. In contrast, the standard phase-encoding velocity measurement contains significant artefacts.     

Three-dimensional cylindrical X-band ESR imaging by a combined pulsed gradient Fourier and static gradient projection reconstruction method

Static gradient electron spin echo projection reconstruction imaging is favourable for X-band material science applications requiring temperature variation with a metal cryostat. To prevent imaging artefacts due to the high conduction electron diffusion coefficient in the preferred conduction direction of quasi-one-dimensional conductors, only pulsed gradient phase encoding for that direction can be tolerated. We present results of an appropriate cylindrical imaging scheme combining both methods. Conduction electron spin density images with 13 x 13 x 17 microm(3) volume element size or spin-lattice relaxation time images with inversion recovery sequence and 13 x 13 x 68 microm(3) volume element size are presented for fluoranthene radical cation salt single crystals of typical sizes of 0.4 x 0.4 x 1 mm(3).     

The equilibrium shape of anisotropic interfacial particles

The Wulff construction yields the equilibrium shape of a particle of fixed volume embedded in a single phase at constant temperature. When the particle attaches to an interface (boundary) between two distinct phases or grains of the same phase, the Wulff construction must be modified to account for the abutment of Wulff shapes at the boundary as well as the boundary energy that is replaced by the particle. If the boundary is non-deformable, a portion of the particle interface is constrained to replace the image (i.e. the exact position) of the boundary that is removed, and the Winterbottom construction yields the equilibrium particle shape. If the boundary can deform, the particle is not constrained to replace the image of removed boundary, and the particle shape is determined by a more general modification of the Wulff construction. In two dimensions, the particle attaches to an initially flat boundary and creates two disjoint segments that remain flat at equilibrium. In three dimensions, the boundary surrounding the particle is contiguous, and numerical calculations show that such a boundary is not necessarily flat but maintains a constant mean curvature of zero at equilibrium.     

Measurement of long-range repulsive forces between charged particles at an oil-water interface

Using a laser tweezers method, we have determined the long-range repulsive force as a function of separation between two charged, spherical polystyrene particles (2.7 microm diameter) present at a nonpolar oil-water interface. At large separations (6 to 12 microm between particle centers) the force is found to decay with distance to the power -4 and is insensitive to the ionic strength of the aqueous phase. The results are consistent with a model in which the repulsion arises primarily from the presence of a very small residual electric charge at the particle-oil interface. This charge corresponds to a fractional dissociation of the total ionizable (sulfate) groups present at the particle-oil surface of approximately 3 x 10(-4).     

Diffusion of dimethyl sulfoxide in tissue engineered collagen scaffolds visualized by computer tomography

Cryopreservation is a convenient method for long-term preservation of natural and engineered tissues in regenerative medicine. Homogeneous loading of tissues with CPAs, however, forms one of the major hurdles in tissue cryopreservation. In this study, computer tomography (CT) as a non-invasive imaging method was used to determine the effective diffusion of Me2SO in tissue-engineered collagen scaffolds. The dimensions of the scaffolds were 30 x 30 x 10 mm3 with a homogeneous pore size of 100 microm and a porosity of 98%. CT images were acquired after equilibrating the scaffolds in phosphate buffered saline (PBS) and transferring them directly in 10% (v/v)Me2SO. The Me2SO loading process of the scaffold could thus be measured and visualized in real time. The experimental data were fitted using a diffusion equation. The calculated effective diffusion constant for Me2SO in the PBS loaded scaffold was determined from experimental diffusion studies to be 2.4 x 10(-6) cm2/s at 20 degrees C.     

Three-dimensional GaN-Ga2O3 core shell structure revealed by x-ray diffraction microscopy

In combination of direct phase retrieval of coherent x-ray diffraction patterns with a novel tomographic reconstruction algorithm, we, for the first time, carried out quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to 17 x 17 x 17 nm3. We observed the platelet structure of GaN and the formation of small islands on the surface of the platelets, and successfully captured the internal GaN-Ga2O3 core shell structure in three dimensions. This work opens the door for nondestructive and quantitative imaging of 3D morphology and 3D internal structure of a wide range of materials at the nanometer scale resolution that are amorphous or possess only short-range atomic organization.     

Exact Stationary States of a Two-Dimensional Transport Model

Previous calculations of transport coefficients of particle systems in two or more dimensions have used computer simulations or various approximations, like those implicit in the Boltzmann equation or in the standard probability theory of diffusion. Here an exactly solved model of crystal growth in three dimensions is transformed into a 2D model of steady particle transport. Then the exact, many-body stationary distribution and exact transport coefficients can be found as well as correlation functions. The model is highly asymmetric. In the transverse direction, particles are strongly attracted, so that long transverse rows of particles tend to form at low temperature. Kinks in these rows are rare, so the transverse positions of kinks effectively move continuously on a large distance scale.     

Dispersion tailoring and soliton propagation in silicon waveguides

The dispersive properties of silicon-on-insulator (SOI) waveguides are studied by using the effective-index method. Extensive calculations indicate that an SOI waveguide can be designed to have its zero-dispersion wavelength near 1.5 microm with reasonable device dimensions. Numerical simulations show that soliton-like pulse propagation is achievable in such a waveguide in the spectral region at approximately 1.55 microm. The concept of path-averaged solitons is used to minimize the impact of linear loss and two-photon absorption.     

Spatial and temporal resolution effects on dynamic contrast-enhanced magnetic resonance mammography

We tested the hypothesis that partial volume effects due to poor in-plane resolution and/or low temporal resolution used in clinical dynamic contrast-enhanced magnetic resonance imaging results in erroneous diagnostic information based on inaccurate estimates of tumor contrast agent extravasation and tested whether reduced encoding techniques can correct for dynamic data volume averaging. Image spatial resolution was reduced from 469 x 469 microm2 to those reported below by selecting a subset of k-space data. We then compared the top five K(trans)/V(T) "hot spots" obtained from the original data set, 469 x 469-microm in-plane spatial resolution and an 18-s temporal resolution processed by fast Fourier transform (FFT), with values obtained from data sets having in-plane spatial resolutions of 938 x 938, 1875 x 1875 and 2500 x 2500 microm2 and a temporal resolution of 18 s, or data sets with temporal resolutions of 36, 54 and 72 and a spatial resolution of 469 x 469 microm2, and found them to statistically differ from the parent data sets. We then tested four different post processing methods for improving the spatial resolution without sacrificing temporal resolution: zero-filled FFT, keyhole, reduced-encoding imaging by generalized-series reconstruction (RIGR) and two-reference RIGR (TRIGR). The top five values of K(trans)/V(T) obtained from data sets, the in-plane spatial resolutions of which were improved to 469 x 469 microm2 by zero-filling FFT, Keyhole and RIGR, statistically differed from those obtained from the original 469 x 469 microm2 FFT parent image data set. Only the 938 x 938 and 1875 x 1875 microm2 data sets reconstructed to 469 x 469 microm2 with TRIGR reconstruction method yielded values of the top five K(trans)/V(T) hot spots statistically the same as the original parent data set, 469 x 469 microm2 in-plane spatial and 18-s temporal-resolution FFT. That is, partial volume effects from data sets of different in-plane spatial resolution resulted in statistically different values of the top five K(trans)/V(T) hot spots relative to a high spatial and temporal resolution data set, and TRIGR reconstruction of these low resolution data sets to high resolution images provided statistically similar values with a savings in temporal resolution of 2 to 4 times.