Spectroscopy of Single AlInAs Quantum Dots

A system of quantum dots based on Al _x In_1?xAs/Al _y Ga_1?yAs solid solutions is investigated. The use of Al _x In_1?xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al _x In_1?xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al _x In_1?xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al _x In_1?xAs quantum dots.     

Adatom concentration distribution on an extrawide Si(111) terrace during sublimation

The formation of an adsorption layer on the Si(111) surface during sublimation at temperatures of 1000–1100 °C and subsequent quenching at T = 750 °C is studied by methods of in situ ultrahigh-vacuum reflection electron microscopy and ex situ atomic force microscopy. The adatom concentration distribution on an extrawide (~60 μm) atomically flat terrace is determined for the first time, and the diffusion length x_s = 31±2 μm at T = 1000 °C is obtained. The analysis of the temperature dependence of the equilibrium concentration of adatoms near a monatomic step allows pioneering measurements of the energy necessary for adatom detachment from the step and attachment to the terrace E _ad ≈ 0.68 eV. Based on these results, the energy parameters for some atomic processes on the Si(111) surface are estimated.     

A method for measuring spectral and time characteristics of mixed pulsed gamma-neutron fields with scintillation and Cherenkov detectors with nanosecond time resolution

A method for measuring spectral and time characteristics of pulsed mixed (n, γ) fields is described. The essence of this method is that a sequence of signals from individual (n, γ) particles registered by (n, γ) detectors is written in a computer with the subsequent amplitude-time analysis of the signal parameters. Fast scintillation and Cherenkov detectors with FWHM of pulses of ～1.5 and 2.5 ns, respectively, are used as (n, γ) detectors. A TDS-3054 broadband digital oscilloscope records signals and transmits them to the computer through a GPIB-USB interface. The equipment used ensures efficient detection of (n, γ) particles at detector counting rates of up to ～2 × 10⁸ pulses/s. The efficiency of this method has been tested in measurements of characteristics of (n, γ) radiation fields from an ИНГ-031 pulsed neutron generator and from a copper target irradiated with a beam of carbon nuclei with an energy of 200 MeV/amu from the TVN-ITEF acceleration-storage complex.     

Experimental study of laser recording and heterodyne detecting of microstructures in volume media

Results of an experimental study of a two-beam laser method of recording and heterodyne detection of elementary microstructures (microgratings) in “thick” recording media are described. The study is performed on iron-doped lithium niobate crystals and on thick photopolymer materials in the single-photon absorption mode. The size of the volume micrograting is demonstrated to depend on the operating wavelength, numerical aperture of the focusing microobjective, and refractive index of the medium. The experiments are performed with microgratings 1.0 × 1.4 × 5.7 μm in the X, Y, and Z directions, respectively. Characteristics of individual microgratings and matrices of microgratings recorded at different depths of recording media are given.     

Segmented high-purity germanium detectors

Segmented planar high-purity germanium detectors were developed. Segmentation of p-n junction was performed by implantation of boron (for the n-type Ge) and mechanical grooving of the Li diffusion layer (for the p-type Ge). The electric and spectrometric characteristics of each individual segment and interactions between segments were investigated. The reliability and stability of the detectors were improved by using special passivation of the crystal surfaces.     

Radiation detectors based on PbSnTe:In films,sensitive in the terahertz range of the spectrum

This paper presents a review of studies of the photoelectric properties of PbSnTe:In films obtained by molecular beam epitaxy and photosensitive structures in the far infrared and submillimeter ranges based on these films. The parameters of photodetector arrays of this type and detectors based on doped semiconductors and superconductors are compared. One-dimensional (2×128 elements) and two-dimensional (128 × 128 elements) PbSnTe:In based arrays with a sensitivity threshold of ~22 μm and an operating temperature of T ≤ 16 K are implemented. Under background-free conditions, the noise equivalent power (NEP) was NEP ≤ ^10?18 W/Hz^0.5 at T = 7 K for a black body radiation source at T_BB = 77 K. In the submillimeter range of the spectrum, sensitivity to laser radiation with a wavelength λ ≤ 205 μm and a value NEP ≤ 10^?12 W/Hz^0.5 was observed without optimization of the design of the photosensitive element and minimization of the measurement circuit noise. The directions of the development of PbSnTe:In based radiation detectors are considered..     

Experimental prediction of multi-nozzle spray characteristics for optimal design in a lead frame etching process

The objective of this paper is to provide quantitative information of uniform impact forces on the sprayed surface in order to optimize the multi-nozzle spray etching system. Spray characteristics obtained from optical non-intrusive measurements using particle image velocimetry (PIV) and particle motion analysis system (PMAS) are measured in single- and twin-nozzle sprays, and then the multi-nozzle spray characteristics is simulated based on those of measurement data. The influences of the multi-nozzle arrangement, nozzle pitch, and pipe pitch on the spray characteristics such as droplets’ velocity, diameter, number density, impact force and their uniformity are properly evaluated. The experimental cases E1 and E2 represent single-spray nozzle A and B, respectively. For twin-spray tests, three nozzle combinations, namely E3 (nozzle A-A), E4 (nozzle A-B) and E5 (nozzle B-B) are considered with different nozzle pitches. The multi-spray simulation cases S1 and S6 represent the multi-spray cases with a homo-nozzle arrays which is consisted in all nozzles of nozzle A or B. For cases from case S2 to S5, the multi-spray cases with a hybrid-nozzle arrays which is consisted in all nozzles of nozzle A and B. The results show that the impact force increases approximately twice as much for changing of experimental test cases from E1 to E5 owing to the differences in nozzle characteristics of single-sprays and the overlap region between two adjacent nozzles. For the multi-nozzle spray simulation, the uniformity of impact force (UI) is increased with increasing the number of nozzle B which has larger orifice diameter and a wider spray angle. The optimum multi-nozzle spray arrangement is case S4 with more than 90 % UI, based on the fact that the UI is quite stable with increasing the nozzle pitch ranging from 90 mm to 145 mm.     

Experimental study of effects of water-cement ratio on the acoustic emission rate “<Emphasis Type="Italic">a</Emphasis>” values in concrete

The critical microcracks make durability of the concrete seriously weakened, and the water-cement (w/c) ratio plays a key role in the durability of concrete, acoustic emission (AE) rate “a” value is an effective parameter to evaluate the amount of critical microcracks quantitatively, so the study on the influence factors and the distribution of w/c ratio on the value “a” can provide a promising reference value for the practical engineering. A total of 90 specimens in different w/c ratio are tested under axial compression in the experiment. The whole damage evolution processes are monitored by fulldigital AE acquisition system. The differences of the amount of critical microcracks in different w/c ratio are explored based on the AE rate process theory, and the rule of the effects of w/c ratio on the rate “a” value is revealed through the analysis of fracture energy and cumulative AE energy in the failure process. The results show that a linear relationship was observed between cumulative AE energy and rate “a” value, the amount of critical microcracks and the rate “a” value increase with the increase of w/c ratio, while the cumulative AE energy and cumulative AE hits are on the contrary.     

A system for detecting weak light signals with nanosecond resolution

A simple method for detecting time-resolved weak light signals with a digital oscilloscope, which allows detection of several photons per one excitation period, is described. A system for detecting low-temperature photoluminescence with a resolution of 15 ns has been implemented on the basis of this technique and a method for attaining a resolution of 1–3 ns has been developed. The luminescence kinetics of Si/Ge _x Si_{1 ? x} /Si quantum wells was measured in the near-IR region.     

Numerical and experimental investigation into the dynamic response of a floating wind turbine spar array platform

The station keeping ability of a platform is the most fundamental guarantee to securing the dynamic stability of a Floating offshore wind turbine (FOWT) when being subjected to complex marine environment. These are usually evaluated in terms of rigid body dynamic response of a floating platform which supports whole FOWT. To investigate the dynamic response of array of FOWT, we put forward the concept of a large FOWT farm which consists of nine spar-type wind turbine array and meanwhile shares mooring line system by connecting those FOWT to one another. The whole FOWT is established based on OC3-Hywind spar buoy platform with NREL 5MW wind turbine. Considering effects of wind, wave and current loads, the dynamic response of the proposed FOWT was conducted based on FEM software ANSYS AQWA under both operational condition and survival condition. Besides, comparisons of dynamic response between the original single FOWT and the proposed FOWT farm were also made. Several interesting conclusions were gained through the investigation. For the original single FOWT, RAO in surge, heave and pitch are concentrated in low frequency, while the peak value of frequency is about 0.2 rad/s. In addition, FOWT (P1, P3, P7 and P9) located on the four vertices of 3×3 have higher sway motion which range from about -0.15 m~ 0.15 m, while sway motion of the rest can be neglected. With the worsening of environmental condition, surge motion of the original single FOWT increases significantly, while it’s not significant for the proposed FOWT farm. At the same time, statistic values of pitch motion of the proposed FOWT farm are quite close to those of the original single FOWT. While, under the survival condition, range of pitch motion of the proposed FOWT farm is significantly smaller than that of the original single FOWT. In all, the results verify the effectiveness of the proposed FOWT farm.     

Integrated ANN-LWPA for cutting parameter optimization in WEDM

In this paper, we present a new approach to determinate cutting parameters in wire electrical discharge machining (WEDM), integrated artificial neuron network (ANN), and wolf pack algorithm based on the strategy of the leader (LWPA). The cutting parameters considered in this paper are pulse-on, current, water pressure, and cutting feed rate. Models of the effects of the four parameters on machining time (T_p), machining cost (C_p), and surface roughness (Ra) are mathematically constructed. An ANN-LWPA integration system with multiple fitness functions is proposed to solve the modelling problem. By using the proposed approach, this study demonstrates that T_p, C_p, and Ra can be estimated at 164.1852 min, 239.5442 RMB, and 1.0223 μm in single objective optimization, respectively. For example, as for Ra, integrated ANN-LWPA has optimized the Ra value by the reduction of 0.1337 μm (11.6 %), 0.3377 μm (24.8 %), and 0.105 μm (10.3 %) compared to experimental data, regression model, and ANN model, respectively. Consequently, the ANN-LWPA integration system boasts some advantages over decreasing the value of fitness functions by comparison with the experimental regression model, ANN model, and conventional LWPA result. Moreover, the proposed integration system can be also utilized to obtain multiple solutions by uniform design-based exploration. Therefore, in order to solve complex machining optimization problems, an intelligent process scheme could be integrated into the numeric control system of WEDM.     

Improvement of AA5052 sheet properties by electromagnetic twin-roll casting

Although servo scanning 3D micro electro discharge machining (SS-3D MEDM) can achieve a high discharge ratio, the processing efficiency is still lower than expected because the discharge area at micro-electrode tip is much smaller than the area to machine. In particular, for 3D micro cavities, the processing efficiency and the machining accuracy inherently contradict each other. In this paper, an on-machine process of rough-and-finishing SS-3D MEDM is proposed with consideration that most cavity material cannot affect the dimensional accuracy. In the rough machining process, technological measures such as high discharge energy and large-diameter tool electrodes are applied to maximize processing efficiency. In the finishing machining process, a small amount of material is removed for dimensional accuracy, smooth surface, and clear edges-and-corners by changing multi-factors of machining parameters. The research is concentrated on two key techniques: rough-and-finishing border strategy and micro tool-electrode precision measurement for the process transformation from rough to finishing. Moreover, an online measurement method is proposed by the point electric contact between a micro electrode and a standard thin-rod, and the measurement accuracy was up to ±1 µm in our experimental system. Machining experiments of 3D micro cavities < 800 µm verified the proposed methods and the processes including 3D model design, rough-machining, micro-electrode measurement and fabrication, and finishing machining. The experimental results were successfully achieved as follows: the dimensional accuracy < 5 µm, surface roughness Sa0.38 µm, and the processing efficiency being improved to 2.4 times.     

Detectors on the Basis of High-Purity Epitaxial GaAs Layers for Spectrometry of X and Gamma Rays

The characteristics of detectors of soft-X and γ rays based on high-purity epitaxial GaAs layers are discussed. The characteristics of detectors with different rectifying contacts are compared, that is, those with a Schottky barrier and a p–n junction. The spectral characteristics of the manufactured detectors that were obtained under the irradiation by ⁵⁷Co and ²⁴¹Am sources at different bias voltages and in a photovoltaic mode and the simulation results using the Geant 3.21 software package are presented.     

Development of a high-voltage waveguide photodetector comprised of Schottky diodes and based on the Ge–Si structure with Ge quantum dots for portable thermophotovoltaic converters

This paper demontstrates the possibility of developing a high-voltage waveguide photodetector comprised of Schottky diodes and based on a Au/Ge — Si structure with Ge quantum dots pseudomorphic to a silicon matrix, which ensures an increase in the external quantum yield and open-circuit voltage. It is shown on this photodetector that there is a great increase and broadening in sensitivity up to λ = 2.1 μm, which coincides with the main radiation range of a black (gray) body at the emitter temperatures from 1200 to 1700 °C, practically used in thermophotovoltaic converters. This state of the ensemble of Ge quantum dots by means of molecular beam epitaxy can be obtained only under the condition of low growth temperature (250–300 °C). It is established that, below the Si absorption edge, photoresponse on the photodetectors under consideration is determined by two main mechanisms: absorption on the ensemble of Ge quantum dots and Fowler emission. It is shown by the analysis of the Raman scattering spectra on the optical photons of Ge–Si structures that the quantum efficiency of photodetectors based on them in the first case is due to the degree of nonuniform stress relaxation in the array of Ge quantum dots. The photoresponse directly associated with the Ge quantum dots is manifested on Schottky diodes with a superthin intermediate oxide layer SiO₂, which eliminates the second mechanism. In further development, the proposed photodetector architecture with pseudomorphic Ge quantum dots can be used both for portable thermophotovoltaic converters and fiber-optic data transmission systems at wavelengths corresponding to basic telecommunication standards (λ = 0.85, 1.3 and 1.55, 1.3, and 1.55 μm) on the basis of silicon technologies.     

Formation of Thick High-Aspect-Ratio Resistive Masks by the Contact Photolithography Method

A method of fabrication of thick (~100 μm and more) resistive masks is described. These masks can be used for solving various engineering problems, e.g., for fabricating x-ray-absorbing topological patterns for LIGA masks, stamp microrelief, cast moulds, etc. Specific features of the contact photolithography method, which is used to design and fabricate the research device, are described. A source of exposure radiation in this device is a light-emitting diode. A possibility of obtaining individual elements of the resistive mask (in particular, with the lateral size ~5 μm, height of ~70 μm, and aspect ratio of ~14) and also the titanium stamp microrelief (with the height up to ~40 μm) generated by means of reactive ion-beam etching through the resistive mask, is experimentally demonstrated.     

High-power sources of ultra-wideband radiation with subnanosecond pulse lengths

Sources of high-power ultra-wideband electromagnetic pulses of nanosecond duration are described. A bipolar voltage pulse with a length of 0.5 ns and an amplitude of ～200 kV is fed to the input of a radiating system (a 16-element array antenna or a single antenna). The values of the effective radiation potential E _p R = 260 kV for a source with a single antenna and E _p R = 690 kV for a source with a 16-element antenna array were obtained at a pulse repetition frequency of 100 Hz.     

Heat transfer in a turbulent jet impinging on a moving plate considering high plate-to-jet velocity ratios

In this paper, heat transfer characteristics of a turbulent slot jet impinging orthogonally on an isothermal moving hot plate is studied numerically. The governing equations were discretized using the finite volume method and the υ ² — f turbulence model was employed for turbulence modeling. The effect of the jet Reynolds number and the plate-to-jet velocity ratio (R) on the Nusselt were investigated. Despite of most previous studies, which have been restricted to R≤2, in the present research higher values of R, also were considered (0≤R≤6). Range of studied jet Reynolds number was between 3000 and 60000. The results indicate that at a fixed plate-to-jet velocity ratio increment of the Reynolds number leads to the enhancement of the average Nusselt number. For each Reynolds number, the average Nusselt number reduces with increasing the plate-to-jet velocity ratio until it becomes minimum at R = 1.25. For R>1.25 trend changes so that these parameters increase. In addition, it was found that only for R>2.5 the average Nusselt number is improved due to the plate motion in comparison with the stationary jet. The results are validated against available experimental data, showing good agreement.     

Experimental investigations on effects of tool flank wear on surface integrity during orthogonal dry cutting of Ti-6Al-4V

Machining titanium alloy Ti-6Al-4V is a challenging task since tool flank wear adversely affects surface integrity. Quantitative effects of predetermined tool flank wear values (VB) on the surface integrity were investigated through the orthogonal dry cutting of Ti-6Al-4V. Experimental results indicated that three-dimensional (3D) average surface roughness increased with the VB ranging from 0 to 0.2 mm but decreased at VB = 0.3 mm. Given the effects of rubbing and ironing enhanced, surface material burning and plastic flows emerged on the machined surface at VB = 0.3 mm. Not only the plastic deformation layer became deeper but also the grains were greatly distorted with the increase of tool flank wear. When machined by using the tool at VB = 0.3 mm, the β phase of Ti-6Al-4V decreased near the machined surface layer than that of using the fresh tool. Besides, the depth of work-harden layer increased from 20 to 60 μm with the VB increasing from 0 to 0.3 mm. The softened layer was generated near the machined surface by using the tool at VB = 0.3 mm. In addition, the residual compressive stresses of the machined surface had the trend of decreasing. Experimental results indicated that the VB less than 0.2 mm was the most suitable condition for better surface integrity during orthogonal dry cutting of Ti-6Al-4V. This study aims at providing experimental data for optimizing the processing parameters and improving the surface integrity of Ti-6Al-4V.     

Numerical investigation on the influence of surface tension and viscous force on the bubble dynamics with a CLSVOF method

We numerically investigated the rising of bubbles in a quiescent liquid layer. The numerical simulation is performed by solving the incompressible, multiphase Navier-Stokes equations via computational code in axisymmetric coordinates using a Coupled level-set and volume-of-fluid (CLSVOF) method. The numerical results show that the CLSVOF method with a novel algebraic relation between F and ? for axisymmetric two-phase flows not only can predict the bubble surface accurately, but also overcome the deficiency in preserving volume conservation. The effects of the Reynolds number Re and the Bond number Bo on the bubble deformation and its motion are investigated. The results show that with the increasing of Re (10 < Re < 150), the bubble shape transfers from oblate ellipsoidal cap to toroidal when Bo = 116. With the increasing of Bo (10 < Bo < 700), the bubble shape transfers from oblate ellipsoidal to toroidal when Re = 30. Although the toroidal bubble shapes are reached in these two cases, the transition modes are different. For the case Bo = 116, the bubble front is pierced by an upward jet from the rear of the bubble. While for the case Re = 30, the rear of the bubble is pierced by a downward jet from the front part.     

Nonuniform magnetization of an article around a flaw during magnetic inspection

Measurements of the induction in a ferromagnetic article directly around a surface flaw in the form of a rectangular slot have shown that there is a substantial magnetization nonuniformity of the article owing to the discontinuity. The nonuniformity smooths out as magnetizing field H ₀ increases. The increase in magnetizing field H ₀ increases the nonuniformity of the field distribution over the article surface, thereby changing the topography of the flaw field. In particular, in low magnetizing fields, this change leads to the absence of negative extrema in the curve of the tangential component of the flaw field, H _x (x). The discrepancy between the calculated and experimental values of the fields of flaws characterized by different geometric parameters is indicative of a considerable effect of the flaw opening on calculated data. These data should be improved through proper corrections obtained experimentally.