Bessel-like beams modulated by arbitrary radial functions

An approximate method for determining the radial and axial intensity of a Bessel-like beam is presented for the general case in which a radial Bessel distribution of any order is modulated by an arbitrary function. For Bessel-Gauss, generalized Bessel-Gauss, and Bessel-super-Gauss beams, this simple approximation yields results that are very close to the exact values, while they are exact for Bessel beams. A practical beam that can be generated with a combination of simple lenses is also analyzed and illustrated.     

Single-photon generation by electron beams

We propose a drastically new method for generating single photons in a deterministic way by interaction of electron beams with optical waveguides. We find a single swift electron to produce a guided photon with large probability. The change in energy and propagation direction of the electron reveals the creation of a photon, with the photon energy directly read from the energy-loss spectrum or the beam displacement. Our study demonstrates the viability of deterministically creating single guided photons using electron beams with better than picosecond time uncertainty, thus opening a new avenue for making room temperature, heralded frequency-tunable sources affordable for scientific and commercial developments.     

Fiber composite plates for strengthening bridge beams

Although the use of Glass Fiber Reinforced Plastics (GFRP) has increased significantly in recent years, its application in civil engineering has been limited at best. The high strength of GFRP and its resistance to corrosion makes it a suitable candidate in many applications where steel has been predominantly used in the past. In this study, five reinforced concrete beams were strengthened by epoxy-bonding GFRP plates along the tension flange of the beams. The load versus deflection curves to failure and the behavior of each specimen under static loading is presented. It is shown that the method presents great potential for solving some of the global problems facing the aging infrastructure.     

Optical element for generation of accelerating Airy beams

We demonstrate an optical element for generation of accelerating Airy beams. The element is conveniently constructed by combination of positive and negative cylindrical lenses of matching radii of curvature. With proper choice of lens curvatures, the resulting surface profile closely follows a cubic polynomial. Passing a gaussian beam through this element and performing optical Fourier transform yields beam profiles close to the Airy function. Our experiments demonstrate parabolic propagation, or acceleration, of the resulting focal spots.     

Thickness distribution of films fabricated by the molecular beam epitaxy technique

The thickness distribution of the film deposited on a rotating substrate holder depends on both the geometrical parameters of the molecular-beam-epitaxy apparatus and the angular distribution of molecular flux from the evaporation furnace. First, Deppisch's formulation for calculation of the thickness distribution is generalized so that it can be applied to a more practical case when the furnace is inclined relative to the axis of rotation of the holder. Next, the angular distributions are calculated for several types of furnaces of practical use by means of the test-particle Monte Carlo method. They are expressed in simple analytical forms. Lastly, example calculations are made to show how to determine such geometrical parameters as are optimum for yielding uniformly thick films.     

Using mosaic crystals for the generation of intense X-ray beams

The parametric X-ray (PXR) yield due to 500-MeV electrons in a 2-mm-thick diamond crystal with a mosaicity angle of ～0.2 mrad has been studied. It is shown that the mosaic crystal structure leads to a significant (about fourfold) increase in the PXR yield doe to the contribution of diffracted bremsstrahlung radiation. Advantages of using mosaic crystals for the generation of intense X-ray beams are discussed.     

Silicon nano-crystalline structures fabricated by a sequential plasma hydrogenation and annealing technique

Silicon nano-crystalline structures have been prepared from amorphous silicon films on silicon substrates using direct-current plasma hydrogenation and annealing at temperatures about 450 °C. Plasma power densities about 5.5 W/cm² were found to be suitable for the creation of nano-porous layers. The nano-porous structures produced visible luminescence at room temperature as confirmed by photoluminescence spectroscopy. The effects of plasma power and annealing temperature on the grain size and luminescence properties of these layers have been investigated by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and photoluminescence. Lowering the temperature during the hydrogenation step led to an increase in the diameter of the grains. In addition, lowering the plasma power density caused the distribution of the porous surface structures to become less widely distributed and the formation of more packed structures resulted.     

溶胶-凝胶法制备BiFeO3铁电薄膜的结构和特性

采用 sol gel 方法在 Pt/Ti/SiO2/Si 衬底上制备出纯相铁酸铋薄膜。采用热分析方法研究了凝胶的化学变化和析晶过程。分析讨论了退火温度对薄膜的结构和形貌的影响。并用 XRD、SEM 等手段对样品在不同温度条件退火处理后的薄膜相和形貌进行了分析。在800℃时采用层层退火方式,有效抑制 Fe价态转化,从而降低了电子波动引发的氧空位数目,制备出纯铁相高电阻率的BiFeO3 铁电薄膜,并观测到饱和电滞回线,其 Ps 和 Pr 分别为 6.9μC/cm2 和 2.8μC/cm2。     

Direct generation of Bessel beams

Two implementations are identified to create a Bessel beam directly, i.e. without the spatial filtering of an initially Gaussian beam. The first implementation is based on a resonator configuration whose lowest-loss transverse mode is a Bessel beam. Numerical simulation to corroborate the geometrical optical arguments is presented. The second implementation is based on the theorem that the angular-plane wave spectrum of a Bessel beam is composed of a cone of wave vectors. This cone is also generated through a phase-matching condition in a four-wave mixing process. This leads to the conclusion that anti-Stokes radiation generated in a nonlinear material will leave the substrate under the form of a Bessel beam.     

Efficiency of the generation of characteristic radiation by stopping low-energy electron beams

The Monte Carlo method is used to calculate the generation of characteristic radiation by stopping low-energy electrons in converters made of low-Z materials and it is shown that this method of generating characteristic radiation can compete with Z-pinches in terms of conversion efficiency. Pis’ma Zh. Tekh. Fiz. 24, 77–79 (February 26, 1998)     

Laser ablation of silicon using a Bessel-like beam generated by a subwavelength annular aperture structure

Using a femtosecond laser incident to an oxide-metal-oxide film engraved with a subwavelength annular aperture (SAA) structure, we generated a Bessel-like beam to ablate silicon. Experimental results show that the silicon can be ablated with a 0.05 J/cm(2) input ablation threshold at 120 fs pulse duration. We obtained a surface hole possessing a diameter less than 1 μm. Optical performance, including depth-of-focus and focal spot of the SAA structure, were simulated using finite-different time-domain calculations. We found that a far-field laser beam propagating through a SAA structure possesses a submicrometer focal spot and high focus intensity. Our method can be easily adopted for surface machining in microfabrication applications.     

Automatic three-dimensional mesh generation by the modified-octree technique

An approach for the fully automatic generation of three-dimensional finite element meshes is presented. The method is specifically designed for use with solid modelling systems which provide a complete and unique definition of the geometry of a part. The method follows from the basic concepts of the octree encoding technique with specific modifications made to produce valid, user controllable finite element meshes. Example problems are included to demonstrate the technique.     

Micrograting-array beam-shaping technique for asymmetrical laser beams

A beam-shaping technique is presented for asymmetrical laser beams with different beam waists and divergences in both vertical and horizontal directions. We utilize a pair of two-dimensional micrograting arrays to equalize the beam parameter products of an asymmetrical beam in orthogonal directions by deflecting the appointed parts of the beam on the longer side of the beam and by recombining the parts on the shorter side. When combined with divergent transformation by means of collimating optical components, the beam-shaping system can produce a symmetrical beam in orthogonal directions with optimized beam waists and divergences. A beam-equalization system is designed and demonstrated for a typical asymmetrical beam of a laser diode bar. In the experiment an emission beam with dimensions of 1 microm x 10 mm and half-divergences of 148 mrad x 576 mrad in the far field is transformed into an almost-square distribution with dimensions of -12 mm x 12 mm and half-divergences of -2 mrad x 2 mrad, which confirm the effectiveness of the proposed technique.     

Thermal-image generation by line-scanning technique: a new computer model

A new computer model for the generation of thermal images of a flat terrain by the line-scanning technique from a moving platform is presented. The model is based on a bottom-up simulation process and the pixel-by-pixel generation of infrared thermal images. The terrain is modeled by a periodic repetition of an arbitrary two-dimensional temperature distribution over a flat area. The double discrete Fourier series of the terrain temperature, including correlation length and the transfer function of the scanning optics, is used. Several computer-generated thermal images are produced to illustrate the model. The geometrical deformations, the space filtration, and the gray-level variation with the scanning angle on generated images are in agreement with theoretical predictions.     

Eu-doped indium tin oxide thin films fabricated by sol-gel technique

We have fabricated Eu-doped indium tin oxide thin films via the conventional sol-gel technique, and confirmed that the doped Eu atoms were chemically incorporated into the indium tin oxide lattice by substituting the In sites. Optical spectra indicated that the Eu-doped films were free of any impurities leading to additional vibrational effects. Valence states of Eu ions in our Eu-doped indium tin oxide films were discussed in connection with Eu concentration.     

Dimethyl methylphosphonate detection with a single-walled carbon nanotube capacitive sensor fabricated by airbrush technique

Single-walled carbon nanotube (SWNT) films were prepared on interdigitated electrodes by airbrush technique, and their sensing properties to dimethyl methylphosphonate (DMMP) were studied. The SWNT films were characterized by field-emission scanning electron microscope. The response to different concentrations of DMMP vapors were investigated at room temperature. The results showed that the capacitance of airbrush SWNT sensor decreased rapidly in varying concentrations ranging from 12 to 60 mg/m³ (2.4–12 ppm). The capacitance sensitivity was about 12.5 % when exposed to 12 mg/m³ DMMP vapor. The capacitance sensitivity was higher when the initial capacitance and loss tangent were higher and the SWNT film was denser. It was found that the capacitance sensitivity was nearly 10 times to the resistance sensitivity. The airbrush SWNT sensor exhibited highly and fast capacitance response, good repeatability and selectivity for DMMP vapor.     

ZnO/Si solar cell fabricated by spray pyrolysis technique

The ZnO/Si heterojunctions have been prepared by depositing n-ZnO films doped with aluminium on p-Si by spray pyrolysis method. Heterojunction solar cells were fabricated using the configuration Al/ZnO/Si/In. The electrical properties of the heterojunction are investigated by means of current–voltage measurements in the temperature range 295–375 K. The cells show the rectifying behaviour characterized by the current–voltage (I–V) measurement under a dark condition, while photoelectric effects have been exhibited under the illumination. As a result, the conversion efficiency of the fabricated cell of about 6.6% was obtained.     

Utilizing asymmetric laser pulses for the generation of high-quality wakefield-accelerated electron beams

Employing temporally asymmetric laser pulses in the interaction with plasma has been recently proposed for controlling the pointing angle of an electron beam produced by a laser wakefield acceleration at low plasma density and moderate laser intensity. In this paper, results on the electron beam parameters for both symmetric and asymmetric laser pulses are presented. These results show that the highest-quality (well-pointed, well-collimated and bright) electron beams are generated in the current regime only using asymmetric laser pulses, which are longer than the plasma wave’s acceleration period, τ>λ_p/2c. The interaction between the laser pulse and the accelerated electron beam in the first plasma-wave period is extracted from the experimental results and observed in preliminary two-dimensional particle-in-cell simulation.     

Bulk wave membrane quartz resonators fabricated by a hollow cathode RF plasma etching technique

A hollow cathode maskless plasma etching method for fabrication of thin quartz membranes is presented. A special geometric arrangement of electrodes and substrates allows the complete plasma structure (plasma sheath, bulk plasma) to be transferred to the substrate area during the etching process. The process has successfully been used in preparing thin quartz membranes with plane-convex and plane-parallel shape, and thicknesses of less than 5 mum. Vibration modes in these thin quartz membranes are calculated using the method of equivalent resonant radius. The membranes are used for realization of bulk acoustic wave resonators at fundamental frequencies above 60 MHz. Good agreement between theoretical and experimental characteristics is achieved.