Surface morphology improvement of homoepitaxial GaN grown on free-standing GaN substrate by metalorganic chemical vapor deposition
Free-standing GaN is generally regarded as an ideal substrate for GaN-based devices due to its advantage of low threading dislocation density (TDD) and good thermal conductivity. However, new surface features such as hillocks and ridges appear on the GaN homoepitaxy films. In this paper, the influences of the intermediate GaN (IM-GaN) layer on the surface defects and crystal quality of GaN homoepitaxy films grown on c-plane GaN substrates by metalorganic chemical vapor deposition are investigated. It is found that hexagonal hillocks and ridges on the surface can be avoided by inserting an IM-GaN layer grown at an intermediate temperature (650—850℃), prior to the growth of GaN at 1050℃. The results based on X-ray diffraction (XRD) measurements and differential interference contrast microscopy images demonstrate that the growth temperature of the IM-GaN layer has a significant influence on GaN homoepitaxy layer, which is one of the most critical parameters determining the surface morphology and crystal quality. As the IM-GaN growth temperature decreases from 1050℃ to 650℃, thed densities of hillocks and ridges on the surface reduce gradually. While, the XRD full width at half maximum (FWHM) values of (002) and (102) peaks for the homoepitaxy films are increased rapidly, indicating the adding of the TDD in the films. The atomic force microscopy (AFM) images show that the quasi-step growth mode change into layer-layer growth mode with the growth temperature decreasing from 1050℃ to 650℃ during the IM-GaN layer growing. It is speculated that the growth mode is determined by the diffusion length of adatom on the growing surface, which is proportional to the growth temperature. In the case of IM-GaN grown at low temperature, the formation of hillocks can be suppressed by reducing the adatom diffusion length. Finally, High crystal quality GaN homoepitaxy films (2 μm) without hillocks is achieved by optimizing the growth parameters of IM-GaN layer, which is about 150 nm in thickness and grown at 850℃. The crystal quality of GaN homoepitaxy film is assessed by XRD rocking curve measured with double-crystal optics. The FWHMs of the (002) and (102) peaks are 125arcsec and 85arcsec respectively, indicating that rather low TDD is formed in the film. And well defined steps are observed on the image of AFM test, the root-mean square roughness value of the which is only about 0.23 nm for 5 μm×5 μm scan area.
Structure evolution of electron irradiated borosilicate glass simulated by molecular dynamics
Sodium borosilicate (NBS) glass is one of the candidate materials for high-level waste glass immobilization. A large number of experiments are performed to study the effect of irradiation by electrons or heavy ions on this type of glass. However, only a few researches of numerically investigating the effect of irradiated NBS glass have been reported. Furthermore those studies mainly focus on heavy-ion irradiation, and none of them is devoted to simulating the effects of electron irradiation on glass that has been irradiated by electrons, especially for structure evolution. In this paper, we propose a novel method of using molecular dynamics (MD) to simulate structure evolution of electron-irradiated NBS glass with compositions of 67.73% SiO2, 18.04% B2O3 and 14.23% Na2O, in mol.%. This method is based on the previous experimental results of Raman spectra and mechanism of structure transformation in irradiated glass. The Raman spectra confirm that the peak indicating the existence of molecular oxygen appears at 1550 cm-1 in irradiated glass. It is assumed that those oxygen atoms do not have any interactions with other adjacent atoms nor participate in the glass network recombination. This assumption is reasonable, for molecular oxygen mainly exists as dissolved oxygen instead of oxygen bubble and is located at interstice of glass network. Thus the presence of molecular oxygen does not have any effect on glass network structure. Then irradiated glass can be obtained by gradually randomly removing a certain number of oxygen atoms from the pristine glass. The glass with removed oxygen atoms is regarded as an irradiated glass which is considered as one irradiated by electrons in experiments. The results derived from MD simulation include average Si—O—Si bond angle, ring size distribution, sodium profile, evolution of [BO4] units, and [BO3] units. With the increase of removed oxygen atoms, the average bond angle of Si—O—Si decreases and the number of small rings gradually increases in irradiated glass. Besides, sodium phase separation is observed obviously after extensively removing oxygen. Moreover, in the process of removing oxygen, some [BO4] units transform into [BO3] units, and the transformation process reaches a saturation state finally. Those effects derived from MD such as decrease of Si—O—Si bond angle, increase of small rings in number, phase separation of sodium and structure change between [BO4] units and [BO3] units, are consistent with those of glass irradiated by electrons in previous experiments. Therefore, the method proposed in this paper will provide a new perspective to understand the mechanism of structure evolution in sodium borosilicate glass after being irradiated by electrons.
Study on resonance frequency of doping silicon nano-beam by theoretical model and molecular dynamics simulation
With the rapid development of nanoelectromechanical system technologies, silicon nanostructures have attracted considerable attention for the remarkable mechanical properties. A number of studies have been made on the mechanical properties through theoretical analysis, atomistic or molecular dynamics and experiments. In this paper, the resonance frequency of the doping silicon nano-beam is investigated by a theoretical model based the semi-continuum approach to achieve the goal of accurately capturing the atomistic physics and retaining the efficiency of continuum model. The temperature dependence of the resonance frequency of the nanostructure is important for application design, which is considered by the Keating anharmonic model used to describe the strain energy at finite temperature. The resonance frequencies are also simulated by the molecular dynamics at different temperatures. The studies indicate that the resonance frequency of the P doped silicon nano-beam is influenced by the size, the doping concentration and the temperature. The results show that the resonant frequency decreases with the increase of the length of the beam, and increases with the increase of the doping concentration of the silicon nano-beam. The resonant frequency of silicon nano-beam decreases with the increase of temperature, but the changes of the resonant frequency is not obvious. The doping concentration has a little effect on the resonance frequency of the silicon nano-beam. The conclusion can be drawn that neither the effect of doping concentration nor the effect of temperature on resonant frequency of the silicon nano-beam is obvious, the size is a major factor influencing the resonance frequency of the silicon nano-beam.
Rapid solidification mechanism of Fe-Al-Nb alloy droplet and its influence on microhardness under microgravity condition
High temperature Fe-Al-Nb alloys will be prospectively applied to the industrial field, i.e., aviation, gas turbine, etc. In this paper, rapid solidification of Fe67.5Al22.8Nb9.7 ternary alloy under microgravity condition is realized by using drop tube containerless processing technique. Our purpose is to investigate the microstructural transition pattern and relevant micromechanical properties, and then to reveal the influence of rapid eutectic growth on application performance. The sample of 2 g is placed in a quartz tube with an orifice at the bottom, and the quartz tube is then placed at the top of 3 m drop tube. The sample is inductively melted and further superheated to a certain temperature with the protecting mixture gas composed of argon and helium. The alloy melt is ejected through the orifice by an argon gas flow and dispersed into fine droplets. The droplets are undercooled and finally rapidly solidified during their free fall in the drop tube. The alloy droplets with the diameter sizes ranging from 40 to 1000 μm are achieved. The liquidus temperature of the alloy is 1663 K. The microstructure of the alloy consists of Nb(Fe, Al)2 and (αFe) phases. In the master alloy prepared by arc melting, the segregation along the gravity direction takes place because of the difference in cooling rate inside the master alloy. By comparison, the microstructures of the alloy droplets are homogeneous. The variations of thermodynamical parameters with droplet size are analyzed. As droplet diameter decreases, its Nusselt and Reynolds numbers rise from 3 to 8 and from 5 to 137, respectively, its undercooling and cooling rate increase from 50 to 216 K and from 1.23×103 to 5.53×105 K· s-1 respectively. This causes the corresponding microstructural transition. A small amount of primary Nb(Fe, Al)2 phase transforms from dendrite to equiaxed grain, the lamellar eutectic is replaced by the fragmented eutectic. The relationship between eutectic interlamellar spacing and undercooling satisfies an exponential equation, indicating that the eutectic is refined by three times. Consequently, mainly owing to the eutectic refinement, the microhardness of the alloy increases by 10% with the increase of undercooling according to the Hall-Petch behavior in terms of both eutectic grain size and interlamellar spacing. Compared with the microstructure of the alloy undercooled to the same level under electromagnetic levitation in our recent work, the microstructure in drop tube is more refined due to the larger cooling rate, contributing to the microhardness of the alloy increasing by 2%-6%.
Effects of anisotropic interface kinetics and surface tension on deep cellular crystal growth in directional solidification
In this paper, we study the effects of anisotropic interface kinetics and surface tension on deep cellular crystal growth in directional solidification. The following assumptions are made: the process of solidification is viewed as a two-dimensional problem; the minor species in this binary mixture system is considered as an impurity; the solute diffusion in the solid phase is negligible; the thermodynamic properties other than the diffusivities are the same for both solid and liquid phases; there is no convection in the system; the anisotropic interface kinetics and the anisotropic surface tension are a four-fold symmetry function each; neither the preferred directions of the anisotropic interface kinetics nor the anisotropic surface tensions are necessarily the same as their counterparts for the solid and liquid phases respectively; the angle between the preferred directions of the two anisotropies is θ0. By using the matched asymptotic expansion method and the multiple variable expansion method, we obtain the diagram of interface morphology for a deep cellular crystal in directional solidification. The results show that there exists a discrete set of the steady-state solutions subject to the quantization condition (35). The quantization condition yields the eigenvalue ???106801-20170033???* as a function of parameter ε and other parameters of the system, which determines the interface morphology of the cell. The results also show the variation of the minimum eigenvalue ???106801-20170033???*(0) with parameter ε. It is seen that when the preferred directions of the two anisotropies are the same, i.e., θ0 = 0, the minimum eigenvalue ???106801-20170033???*(0) reduces with the increase of anisotropic surface-tension coefficient α 4 , increases with the augment of parameter ε, and is unrelated to anisotropic interface kinetic coefficient β4 in the low order; when the angle 0 ≤ θ0 ≤ π/4, as the θ0 increases, the minimum eigenvalue ???106801-20170033???*(0) increases; when the angle π/4 ≤ θ0 ≤ π/2, as the θ 0 increases, the minimum eigenvalue ???106801-20170033???*(0) decreases. In addition, the results show the composite solution for the interface shape function ηB described on (X, Y) plane. It is seen that both of the anisotropy and the angle θ0 have a significant effect on the total length and the root of deep cellular crystal, however, have little influence on the other solid-liquid interface, such as the top of deep cellular crystal. When the angle θ0 is 0, as anisotropic coefficient increases, the total length of the finger increases, the curvature of the interface near the root increases or the curvature radius decreases. It is found that the influence of the anisotropic surface-tension coefficient on interface morphology is more remarkable than that of the anisotropic interface kinetics coefficient. when the angle 0 ≤ θ0 ≤ π/4, as the θ0 increases, the total length of the finger decreases, the curvature of the interface near the root decreases or the curvature radius increases; when the angle π/4 ≤ θ0 ≤ π/2, as θ0 increases, the total length of the finger increases, the curvature of the interface near the root increases or the curvature radius decreases.
Coherent perfect absorption and transmission of a generalized three-mode cavity optico-mechanical system
With the rapid development of nano-physics and quantum optics, optico-mechanical coupling system is developing toward the miniaturization and lightweight. The physical characteristics of optical cavity and applications of optic-mechanical devices have received much attention. In this paper, a generalized three-mode cavity optico-mechanical system is presented, the steady-state responses of the system to the characteristics of weak detection of light absorption and dispersion in several different coherent driving modes are studied. Situated in the middle of system is a portable total reflection mechanical oscillator with a reflectance of 100%, and located on each side is a fixed optical cavity mirror with partial transmittance, Three-mode cavity optical mechanical system consists of “fixed-mirror, removable-vibrator, fixed-mirror” structure. in which the two optical cavities are coupled by coupling a stronger control field and weak probe light with the same mechanical oscillator. Analysis and numerical results show that under the mechanism with different parameters, due to nonlinear effect of pressure, in the three-mode cavity optical mechanical system, there appear some interesting quantum coherent phenomena such as coherent perfect absorption, coherent perfect transmission and coherent perfect synthesis. When coherent perfect absorption occurs, the mutual conversion between input signal power full-field energies and oscillator vibration of internal coherence can be realized, and the law of conservation of energy is satisfied. When relaxation rate due to mechanical oscillator is very small, the coherent perfect transmission is completely transmitted from the system side of the input field to the other side in the case of no loss of energy. And mechanical relaxation rate of the oscillator approaches to zero in the middle, which can ensure that the perfect transmission of the detection field takes place on one side, and the field total reflection and coherent perfect synthesis happen on the other side of. In addition, we alsofind that the adjustment of coupling between cavity and cavity can change the intensity of the probe field of quantum coherent control thereby realizing that the output of the detection field is transformed between coherent perfect absorption and coherence transmission; through simple phase modulation the output direction and input direction of detection field for left cavity-right cavity can swap mutually. So, these dynamic controls in quantum information networks can be used to construct some optical devices with special functions, such as photon switch, photo router, photon exchange machine, etc.
First-principle study of structure stability and electronic structures of γ graphyne derivatives
A new carbon allotrope—graphyne has attracted a lot of attention in the field of material sciences and condensed-matter physics due to its unique structure and excellent electronic, optical and mechanical properties. First-principles calculations based on the density functional theory (DFT) are performed to investigate the structures, energetic stabilities and electronic structures of γ -graphyne derivatives (γ -N). The studied γ -graphyne derivative consists of hexagon carbon rings connected by onedimensional carbon chains with various numbers of carbon atoms (N=1-6) on the chain. The calculation results show that the parity of number of carbon atoms on the carbon chains has a great influence on the structural configuration, the structural stability and the electronic property of the system. The γ -graphyne derivatives with odd-numbered carbon chains possess continuous C—C double bonds, energetically less stable than those with even-numbered carbon chains which have alternating single and triple C—C bonds. The electronic structure calculations indicate that γ-graphyne derivatives can be either metallic (when N is odd) or direct band gap semiconducting (when N is even). The existence of direct band gap can promote the efficient conversion of photoelectric energy, which indicates the advantage of γ -graphyne in the optoelectronic device. The band gaps of γ -2, 4, 6 are between 0.94 eV and 0.84 eV, the gap decreases with the number of triple C—C bonds increasing, and increases with the augment of length of carbon chains in γ -2, 4, 6. Our first-principles studies show that introducing carbon chains between the hexagon carbon rings of graphene gives us a method to switch between metallic and semiconducting electronic structures by tuning the number of carbon atoms on the chains and provides a theoretical basis for designing and preparing the tunable s-p hybridized two-dimensional materials and nanoelectronic devices based on carbon atoms.
Modeling plastic deformation effect on the hysteresis loops of ferromagnetic materials based on modified Jiles-Atherton model
Plastic deformation is one of the most important features that affect the hysteresis magnetic properties of steels, because it changes the dislocation density and affects domain-wall movement and pinning. In order to model the effect of plastic deformation on the magnetic properties, the prevailing Jiles-Atherton (J-A) theory is extensively used. However, the J-A models in a series of papers published by Jiles et al. are not completely consistent. As a result, there exists no uniform formula of magneto-plastic model established by different researchers, based on different J-A models, and various versions given by different mathematic expressions of magneto-plastic model often create difficulty in discriminating the accuracies and effectivenesses of the analyzed results. Therefore, it is necessary to establish an accurate and reasonable magneto-plastic model. In this paper, on the basis of magnetization mechanism of ferrimagnet and plastic deformation model, the effects of plastic deformation on the magnetic characteristic parameters adopted in magneto-plastic model, such as dislocation density, pinning coefficient and scaling constant, are analyzed and the relationship between them is first established. Then, by contrasting the fitting formula of the anhysteretic magnetization curve, the energy conservation equation and the effective magnetic field equation established by different researchers, several queries are proposed, and the irrationality and inaccuracy of the existing magneto-plastic model are elucidated, such as the mixing of anhysteresis magnetization and magnetization, the unreasonably regarding the irreversible magnetization energy as actual total magnetization energy. Thus, the energy conservation equation, the effective magnetic field equation and the anhysteretic magnetization equation are modified, and the differential expression of the magneto-plastic model is re-derived finally. Comparing the results calculated by the existing magneto-plastic models with the experimental results, it is seen indeed that a more sharp change of magnetization appears at small plastic deformation, then, the values of magnetization decrease more slowly with the increase of plastic deformation than those from the models respectively proposed by Li Jian-Wei, Leng Jian-Cheng and Wang Zheng-Dao; the saturation magnetization and residual magnetization decrease with the increase of plastic deformation, the coercive force is increased oppositely and the trend to reach the saturation magnetization becomes gentler, which is more exactly consonant with experiment observation than that calculated by the Sablik's model; additionally, the hysteresis loops of the plastically deformed carbon-steel samples calculated by the modified magneto-plastic model are also in better agreement with the experimental results than those from the existing models. Consequently, the modification is effective, and the modified magneto-plastic model is more accurate to simulate the plastic deformation effect on the magnetic property of ferromagnetic material.
Near-infrared luminescence properties of small-sized homogeneous/heterogeneous core/shell structured NaGdF4:Nd3+ nanoparticles
In recent years, considerable researches have focused on the upconversion phosphor nanoparticles in the application of biomedical imaging, which emit visible light. Nevertheless, these kinds of nanoparticles limit the light penetration depth and imaging quality. The Nd3+ doped nanoparticles excited and emitted in a spectral range of 700-1100~nm can overcome those shortcomings. Furthermore, considering the applications of rare earth nanoparticles in biomedical imaging, smaller particle size is needed. However, the luminescence efficiencies of nano-structured materials are lower due to the inherent drawback of high sensitivity of Nd3+ ions to the surface defects. So, it is of vital importance for introducing a shell with low phonon energy to be overgrown on the surface of nanoparticles. According to the ratio of core material to the shell, core/shell structured nanoparticles are separated into “homogeneous” and “homogeneous”nanoparticles. And the shell material may influence the luminescence performance. In few reports there have been made the comparisons of luminescence performance of Nd3+ between heterogeneous and homogeneous core/shell nanoparticles. In the present work, small-sized hexagonal NaGdF4:3%Nd3+ nanoparticles with an average size of sub-5~nm are synthesized by a coprecipitation method. To overcome the nanosize-induced surface defects and improve the luminous performance, the NaGdF4:3%Nd3+ nanoparticles are coated with homogeneous and heterogeneous shells, respectively. Core/shell structured nanoparticles with different values of shell thickness are synthesized by using the core/shell ratios of 1:2, 1:4 and 1:6. The luminescence properties of the prepared nanoparticles are characterized by photoluminescence spectra and fluorescence lifetimes. Under 808~nm excitation, the NaGdF4:3%Nd3+ nanoparticles exhibit nearinfrared emissions with sharp bands at ~866 nm, ~893 nm, ~1060 nm, which can be assigned to the transitions of 4F3/2 to 4I9/2, 4F2/3 to 4I11/2, respectively. The locations of emission peaks of the core/shell nanoparticles are in accordance with the those of cores while the fluorescence intensity increases significantly. In addition, the average lifetimes of Nd3+ ions at 866 nm of core/shell nanoparticles are longer than those of the cores, which indicates that the undoped shell can minimize the occurrence of unwanted surfac-related deactivations. Notably, comparing with the homogeneous NaGdF4:3%Nd3+@NaGdF4 nanoparticles, the fluorescence intensity of heterogeneous NaGdF4:3%Nd3+@NaYF4 nanoparticles is enhanced and their lifetimes become longer. It is due to the low stability of hexagonal NaYF4, which suppresses the nucleation of the shell precursor and makes the shell able to be fully coated on the core. The decrease of electron charge density on the surface of core/shell nanoparticles is also beneficial to shell growth and crystallization. The high crystallinity of heterogeneous core/shell structured nanoparticles can eliminate negative influence of surface effect more efficiently. In addition, the phonon energy of NaYF4 is lower than that of NaGdF4, which leads to low possibility of non-radiative cross-relaxation between Nd3+ ions, thereby improving the luminescence efficiency in the near in frared emission.
Thermoluminescence and optical stimulated luminescence trap parameters of LiMgPO4: Tm, Tb
In recent years, the preparation and luminescent properties of LiMgPO4 as a matrix have received much attention, but most of the studies are limited to the trap parameters of thermoluminescence (TL), which do not involve the trap parameters of optical stimulated luminescence (OSL). In this paper, LiMgPO4:Tm, Tb powder samples are synthesized by solid-state reaction at high temperature. All the experiments reported here are measured by Riso TL/OSL-15-B/C reader after being irradiated by beta-rays. The TL glow curves obtained show that the high temperature peak at 300℃ belongs to the first-order kinetic peak because the peak temperature does not change as irradiation dose increases. Based on the first-order kinetics, the TL trap depth E = 1.72 eV and the frequency factor s= 3.97× 1014 are determined by the methods of various heating rates.However, LiMgPO4:Tm, Tb is also an OSL material, the analysis of its OSL trap kinetic parameters would help to understand the OSL mechanism and to know the relationship between TL and OSL traps. The pulse annealing method is suitable for OSL trap parameter analysis. For low sensitivity samples, the fluctuation of the pulse annealing method is relatively large. And this method only records the OSL signal of fast decay component, which is suitable for measuring the samples with high sensitivity and fast fading OSL signals. In order to study the OSL signal of slow decay component, the multiple annealing method is proposed based on the pulse annealing method. The multiple annealing procedure is as follows. Firstly, the sample is annealed from room temperature to 500℃ which lasts 30 s. The heating rate is 5℃/s. Secondly, the sample is irradiated with 90Sr beta radiation doses of 1 Gy. Thirdly, the sample is preheated to 150℃ with a heating rate of 0.2 ℃/s. And then OSL measurement lasts 500 s after cooling to room temperature. The above steps are repeated in preheating temperature steps of 10℃. Four repetitive measurements are made for each preheating rate. The preheating rates are 0.2, 0.5, 1, and 2℃/s.Finally, the OSL trap parameters E = 1.69 eV and s = 1.05× 1014 are determined by the multiple annealing method. The correlation between TL and OSL trap parameters shows that the TL and OSL signals are likely to come from the same traps. Besides, the trap depth of the main peak of the phosphor shows that the sample has better thermal stability than those of the other phosphors of LiMgPO4 as the matrix.
Physical mechanism and optimal design of silicon heterojunction solar cells
Silicon heterojunction (SHJ) solar cells are crystalline silicon wafer-based photovoltaic devices fabricated with thin-film deposition technology. The SHJ solar cells hold great potential for large-scale deployment for high conversion efficiencies with low-cost manufacturing. Recently Kaneka Corporation has fabricated an interdigitated-back-contact (IBC) SHJ solar cell with a certified 26.33% conversion efficiency in a large area (180.4 cm2), which is a world record for any 1-sun crystalline silicon wafer-based solar cell. The key feature of SHJ solar cells is the impressive highopen-circuit voltages (Voc) achieved by the excellent amorphous/crystalline silicon interface passivation. Generally, in SHJ solar cells, the boron doped hydrogenated amorphous silicon [(p)a-Si:H] serves as hole collector and the phosphorus doped hydrogenated amorphous silicon [(n) a-Si:H] functions as electron collector. In order to improve the lateral carrier transport of these layers, transparent conductive oxides (TCOs) are usually deposited on both sides of the solar cell. Therefore the parameters such as the heterointerface passivation quality, doping concentration and thickness of the a-Si:H doped layer, and work function of the transparent conductive oxide layer are the key factors that affect the performances of SHJ solar cells. Enormous research efforts have been devoted to studying the effects of the aforementioned influencing parameters on the photovoltaic characteristics of SHJ solar cells. Some research groups have addressed the physical mechanism behind the limitation of the solar cell efficiency. Owing to the insight into the physical mechanism some guidelines for optimally designing the high-performance solar cells in future are obtained. It seems therefore important to summarize the research efforts devoted to the physical mechanism and optimal design of SHJ solar cells.In the present review, we mainly discuss three important issues: 1) the amorphous/crystalline silicon interface passivation; 2) the Schottky barrier resulting from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer; 3) the screening length that is required to efficiently shield the parasitic opposing band from bending originating from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer. The numerical simulation and optimal design of SHJ solar cells are analyzed, and three strategies that may improve the solar cell performances are presented: 1) a hybrid SHJ solar cell structure with a rear heterojunction emitter and a phosphorus-diffused homojunction front surface field; 2) replacing the (p)a-Si:H doped layer by higher doping efficiency microcrystalline silicon alloys such as μc-Si:H, μc-SiOx:H or μc-SiCx:H; 3) replacing the (p)a-Si:H doped layer by higher work function transition metal oxides such as MoOx, WOx or VOx. Finally, the research progress and future development of SHJ solar cells are also described.
Experimental optimal design on BaY2ZnO5: Tm3+/Yb3+ phosphor and its up-conversion luminescence property
To obtain a maximal blue up-conversion luminescence of Tm3+/Yb3+ co-doped BaY2ZnO5 phosphors, orthogonal experimental design combined with quadratic general rotary unitized design method is employed to optimize the Tm3+ and Yb3+ ions doping concentration. Two sets of BaY2ZnO5:Tm3+/Yb3+ phosphors are synthesized by the traditional high temperature solid reaction method. The doping concentration ranges of Tm3+ and Yb3+ are first narrowed by orthogonal experimental design, and then quadratic general rotary unitized design is performed and one regression equation is established based on the experimental results from the latter design. The theoretical maximum value of the blue up-conversion luminescence intensity and the optimal Tm3+ and Yb3+ doping concentrations are obtained by genetic algorithm. The optimal sample is synthesized and its crystal structure and up-conversion luminescence properties are investigated. It is found that the blue up-conversion luminescence originates from three photon processes under 980 nm excitation. Temperature dependent up-conversion luminescence spectra of the optimal sample show that the blue up-conversion luminescence intensity declines with increasing temperature, implying the occurrence of thermal quenching of up-conversion luminescence. The calculated excitation energy is about 0.602 eV.
Wide band electromagnetically induced transparency in graphene metasurface of composite structure
The electromagnetic induction transparency (EIT) is a phenomenon in which the originally opaque medium becomes transparent under certain resonant electromagnetic fields. It has been seen in applications ranging from nonlinear optics, slow light and optical storage. From the viewpoint of single-frequency, researchers have paid much attention to the realization of broadband electromagnetic induction transparency in recent years. In this paper, a broadband electromagnetic induction transparency effect is investigated theoretically by the finite difference time-domain method. A composite structure based on graphene metasurface which consists of graphene strip with air groove, gallium nitride, silica and titanium dioxide is designed in infrared range. A broadband electromagnetically induced transparency effect could be found in the designed composite structure compared with those in several similar structure. The electromagnetically induced transparency window can be tuned gently by the width of air groove and gallium nitride dielectric slabs. The results show that a wideband electromagnetically induced transparency window of 4 terahertz is found in the infrared frequency range. By comparison with the existing research results, a wider band of electromagnetically induced transparency is found in our structure. We study the physical mechanism of broadband electromagnetically induced transparency from the aspects of structural parameters and electromagnetic field distribution. The thickness w1 of gallium nitride, the width wa and depth h of air groove on graphene strip are discussed in this article. The smaller the length wa or depth h, the wider the EIT band is. The peak of high frequency at which the transmission is near to zero is blue-shifted as h increases. However, red-shift is found as width wa increases. It is found that graphene strip exists as a bright mode. coupling action acts as air groove and gallium nitride slabs function as dark mode, resulting in broadband electromagnetic induced transparency. That is to say, the principle of broadband electromagnetically induced transparency is due to a bright mode coupling in two different forms of dark mode, thus widening the transmission band. This work provides a kind of structure and a design way, to gain the broadband of electromagnetically induced transparency effect. Moreover, it is found that changing the refractive index of background medium, the frequency of high frequency band has a red-shift, the greater change of the refractive index can lead to smaller frequency range. It can be seen that the values of group index ng of three frequency peaks exceeding 25 are observed. The results also show that the slow-light effect and the sensing effect in several frequency ranges are obtained in the proposed structure and potential applications in the optical storage and highly sensitive infrared-band sensor, infrared optical switching, etc.
Vibrational resonance in a periodic potential system with α stable noise
A periodic potential system excited by multi-low frequency weak signals, the high frequency signal and additive α stable noise is constructed. Based on this model, the vibrational resonance phenomenon under α stable noise is investigated by taking the mean signal-noise-ratio gain (MSNRI) of output as a performance index. Then the influences of stability index α (0 < α ≤ 2), the skewness parameter β (-1 ≤ β ≤ 1) of α stable noise, the amplification factor D and the high frequency signal amplitude B, and frequency Ω on the resonant output effect are explored. The results show that under the different distributions of α stable noise, the multi-low frequency weak signals detection can be realized by adjusting the high frequency signal parameter B or Ω to induce vibrational resonance within a certain range. When α (or β ) is given different values, the curve of MSNRI-B has multiple peaks with the increase of B for a certain frequency Ω, and the values of MSNRI corresponding to peaks of the curve of MSNRI-B are equal. So the intervals of B which can induce vibrational resonances are multiple, and the multiple resonance phenomenon turns periodic with the increase of B. Similarly, the curve of MSNRI-Ω also has multiple peaks with the increase of Ω for a certain amplitude B, so the intervals of Ω which can induce vibrational resonances are also multiple. The difference is that the multiple resonance phenomenon becomes irregular with the increase of Ω. Besides, the resonance intervals of B and Ω do not change with α nor β. Under the different values of amplitude factor D, the resonance intervals of B (or Ω ) do not change with the increase of D, indicating that only the energy of the high frequency signal transfers toward the signals to be measured, and the energy of α stable noise does not transfer toward the signals to be measured. Besides, when B and Ω are fixed, it can still be realized to detect the weak signal with the increase of D, which shows that the weak signal detection method based on vibrational resonance can overcome the shortcoming that noise intensity in industrial sites cannot be regulated and controlled. The results provide a new method of detecting the weak signal, and have potential application value in signal processing.
Defect induced asymmetric soliton transmission in the nonlinear circuit
Electrical diode, the first device to rectify the current flux, has significantly revolutionized fundamental science and advanced technology in various aspects of our routine life. Motivated by the one-way rectification effect, considerable effort has been dedicated to the study of the unidirectional transmission in other physical systems for the potential applications, such as the acoustic diode, thermal diode, etc. The nonlinear LC circuit, which has unique advantages in the measurement of energy with which the voltage and current can be achieved by digital oscilloscope conveniently, provides a simple and effective way of studying the nonlinear wave propagation in a dispersive medium. In this paper, we design a defective LC nonlinear circuit deliberately to realize asymmetric transmission of energy, and the energy carrier is nonlinear wave which is so-called soliton, instead of the linear wave in the pass band. The defect-induced localized wave is a kind of intrinsic bound-state wave mode that is evanescent away from the defect site but vibrates around the site with an intrinsic frequency fr. In the LC circuit, when the defect is close to the driver, with the frequency of driven signal in the forbidden band of system approaching to the intrinsic resonance frequency fr of the defect, the resonance induced by the defect enables the circuit to turn on, which is relevant to but somewhat different from what was uncovered by Leon et al. about the intrinsic instability of evanescent waves stirred up directly by a boundary drive. On the other hand, the system acts like an insulator, for the defect is far away from the drive. The defect changes the homogeneity of the line, which allows the soliton to be released in one direction by the local resonance, with the driver being at a lower amplitude. As a result, the introducing of defects significantly improves conversion efficiency from the driver energy into the soliton. To further understand this phenomenon in the defective LC nonlinear circuit, we numerically investigate the relationship among transmission energy, defect coefficient and driver amplitude. Finally, the combined defects are also considered to further adjust the LC nonlinear circuit.
Full field of view polarization effect measurement and error analysis of non-polarized channels of spaceborne directional polarimetric camera
The optical system of spaceborne directional polarimetric camera that employs an ultra wide-angle lens for its multiangle, a filter wheel for its multispectral and also its multipolarization capability, a CCD itself for its imaging has a certain polarization effect, which can affect the radiometric accuracy of the non-polarized channels of the instrument. The transmittance of the oblique light rays that are incident on the optical element surfaces is sensitive to the orientation of the electric field, contributing to the linear polarization effect of optical system. The precise polarization measurement and calibration of the passive imaging polarimetry are in urgent need to eliminate the instrumental polarization effect and to improve its radiometric accuracy for observation scenes. The non-polarized channel radiometric model considering the linear polarization effect is deduced in detail by analyzing the instrumental principle and optical structures. Moreover, the reasonably simplified model is established based on the actual lens characteristics. A calibration method in which completely linearly polarized light with different kinds of polarization angles irradiates sparsely the instrument within full field of view and subsequently fits the response in the least square sense, is proposed and simulated. In addition, the measured relative errors of the intensity of incident light with different kinds of polarization states caused by the calibration deviations of instrumental principal physical parameters are analyzed and calculated, such as the azimuthal angle of single pixels, explicit optic polarization rate and low frequency spatial relative transmittance. The actual instrumental parameter values and their calibration deviation amounts are acquired by carrying out the laboratory calibration experiment for instrument and combining the least square fitting. Furthermore, the maximum radiometric calibration relative error caused by the deviation of the physical parameter called explicit optic polarization rate is calculated to be 0.4%, fulfilling completely the requirement of radiometric relative accuracy 5% and retaining abundant tolerance. The study provides a theoretical basis and an experimental guidance in high accurately measuring radiation, calibrating and processing data for the instrumental non-polarized channels with full field of view.
High sensitive scheme for methane remote sensor based on tunable diode laser absorption spectroscopy
Methane is an important raw material for various petrochemicals in industrial fields and as also a clean fuel in daily life. However, as an inflammable and explosive material, methane leak can lead to disastrous consequences such as fire and explosion. Furthermore, as a kind of greenhouse gas, methane has stronger influence on global warming than carbon dioxide. In this paper, we present a new high sensitive scheme for methane remote sensing, which can facilitate detection and location of methane leakage. And the 2v3 band (near 1653.7 nm) of methane is chosen as the target transition which is free from the absorption of the other molecule in atmosphere. A tunable distributed-feedback diode laser is adapted to scan across the target transition. A Fresnel lens with a diameter of 150 mm is employed to collect the ambient backscattering light from natural features such as the buildings. The first harmonic signal is used to normalize the second harmonic signal to remove the influence introduced by the unknown reflectance factor of the actual target, therefore no retro-reflector is needed. Traditional tunable diode laser absorption spectroscopy (TDLAS) method has difficulty in locating the second harmonic signal peak position in low concentration conditions because of low signal-noise-ratio (SNR). To improve the SNR especially in low concentration environment, a scheme named “baseline-offset” TDLAS is presented in the paper, in which a reference cell filled with standard methane sample is inserted into the measuring optical path. The reference cell can also be used to calibrate the sensor. Furthermore, the reference cell can be used to lock the central frequency of the diode laser to the absorption peak position to monitor concentration fluctuation continuously. In the peak-locking mode, the sensor demodulates the third harmonic signal as error signal to control the injection current of the laser source with PID control. Moreover, one advantage of peak-locking mode is that the measurement frequency is about two orders of magnitude higher than the traditional TDLAS method. With “baseline-offset” TDLAS, the remote sensor described in this paper obtains SNRs as high as 19 and 16 at a stand-off distance of 10 m and 20 m, respectively. With such a high SNR, there is no necessity for complex algorithm in absorption peak position location. By defining the standard deviation of the measuring concentration as the detection limit, experimental results show that the proposed methane remote sensor has detection limits of 5 ppm · m at a distance of 10 m and 16 ppm·m for 20 m, respectively, while measuring the ambient methane. In peak-locked mode, the experimental system has a detection limit of 22 ppm·m at a distance up to 37 m and can monitor rapid concentration fluctuation in.
Experiment on similarity between wake flow field and electromagnetic scattering characteristic of the hypersonic model
The plasma sheath and wake flow of the hypersonic vehicle can affect the electromagnetic scattering characteristics of the reentry targets when they pass through the earth atmosphere at high speed. In order to study the similarity between the wake and the characteristic of the model launched at high velocity, the simulation experiments on the electromagnetic scattering characteristics of the spherical models made of Al2O3 and their wakes are carried out under the same binary scaling parameters in the ballistic range. The models are launched by the two-stage light-gas gun. The diameters of the models are 8 mm, 10 mm, 12 mm and 15 mm, respectively, while the pressures of the target chamber are 6.3 kPa, 5.0 kPa, 4.2 kPa and 3.3 kPa, respectively. The shock standoff distance is obtained by the shadow graph system. The electron density distribution of the wake is measured by the electron density measurement system. The RCS distribution of the wake and the model are acquired by X band monostatic radars, whose visual angle is 40°. The results show that the shock standoff distance gradually increases with the increasing of the model dimension under the conditions of the same velocity and binary scaling parameters. The wake electron densities of different models are similar in their variation trends and orders of magnitude. The wake flow field of the different models with high velocity are the same as the results predicted by the double scale laws. The RCS distributions and total RCS of the wake of the models are different from each other. The electromagnetic scattering properties of the wake flow field of the various models do not conform with the predicted results obtained from the double scale law. The electromagnetic scattering energy is distributed over the regions of the models made up of aluminium oxide and the wake zones. There appears to be one center of the electromagnetic scattering energy in the area of the model coated with flow field, while several centers emerge in the region of the wake. The measuring signals of the RCS of the models show a random distribution, because the amplitude variation of the RCS and the frequency change of the RCS are random. The total RCS of the model increases with the increase of the model dimension, but the variation range of ripple frequency decreases with the increase of the model dimension.
Ab initio calculation of the potential curve of B3Σu- state of O2
The B3Σu- state of O2 molecule is an upper state of the most strongly allowed triplet-triplet (B3Σu-←X3Σg-) absorption, the Schumann-Runge (SR) transition, which plays a crucial role in protecting the earth from suffering UV radiation. Photo-dissociation of O2 molecule in the SR transition is the major source of odd oxygen (O and O3) in the stratosphere. Comprehensive knowledge of the electronic states, especially their potential energy curves (PECs), is necessary to understand those phenomena. In this paper, we calculate the PEC of B3Σu- state of O2 by using the internally contracted multi-reference configuration interaction including Davison correction method, which is denoted by icMRCI+Q, and utilize the complete active space self-consistent field (CASSCF) function as a reference function. The calculation is implemented in the MOLPRO suite of codes. Firstly, we carry out the state-averaged (SA) calculation on the four lowest states, A'3Δu, B3Σu-, 23Δu and 23Σu- states, which are in the same irreducible representation of symmetric group. The active space of CASSCF consists of full valence space. The augmented correlation-consistent aug-cc-pV5Z basis set is used. The results show that the PEC of B3Σu- state does not displays double well structure, which is contradictory to Liu's result (Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2014 124 216). By analyzing the PEC structure, we find that the double well of Liu's result comes from the “root flipping”, that is, the PEC interchange from B3Σu- state into 23Δu state. In our case the “root flipping” is avoided by the SA calculation. Secondly, in order to ensure that the most important configurations are included in the reference function, we calculate the PEC of B3Σu- state of O2 molecule at CASSCF/aug-cc-pVTZ level by changing the active space. We find that the bound well of the PEC will not appear unless the active space includes 2πu orbital which is beyond the full valence space. That means that the Rydberg configurations including 2πu orbital play a crucial role in forming the bound well. And the result is further improved by adding into the active space another two orbitals 4σg and 4σg whose orbital energies are both less than 2πu. Finally, we add the Rydberg configurations into the multi-reference configuration function by putting 2πu, 4σg, 4σu into the active space and then carry out the calculation at an icMRCI+Q/aug-cc-pVTZ level. The obtained B3Σu- state PEC and its spectroscopic constants are in good agreement with the experimental data compared with previous results. Moreover, the process we determine the reference configurations is useful for making accurate calculation at an MRCI level on other species.
Ground state properties and spectral properties of borospherene B40 under different external electric fields
The recent discovery of borospherene B40 marks the onset of a new class of all-boron fullerenes. External electric field can influence the structure and property of molecule. It is necessary to understand the electrostatic field effect in the borospherene B40. In this work, density functional theory method at the PBE0 level with the 6-31G* basis set is used to investigate the ground state structures, mulliken atomic charges, the highest occupied molecular orbital (HOMO) energy levels, the lowest unoccupied molecular orbital (LUMO) energy levels, energy gaps, electric dipole moments, infrared spectra and Raman spectra of borospherene B40 under the external electric field within the range of values F=0-0.06 a.u.. The electronic spectra (the first 18 excited states contain excited energies, excited wavelengths and oscillator strengths) of borospherene B40 are calculated by the time-dependent density functional theory method (TD-PBE0) with the 6-31G* basis set under the same external electric field. The results show that borospherene B40 can be elongated in the direction of electric field and B40 molecule is polarized under the external electric field. Meanwhile, the addition of external electric field results in lower symmetry (C2v), however, electronic state of borospherene B40 is not changed under the external electric field. Moreover, the calculated results show that the electric dipole moment is proved to be increasing with the increase of the external field intensity, but the total energy and energy gap are proved to decrease with the increase of external field intensity. The addition of external electric field can modify the infrared and Raman spectra, such as the shift of vibrational frequency and the strengthening of infrared and Raman peaks. Furthermore, the calculated results indicate that the external electric field has a significant effect on the electronic spectrum of borospherene B40. The increase of the electric field intensity can lead to the redshift of electronic spectrum. With the change of the electric field intensity, the strongest excited state (with the biggest oscillator strength) can become very weak (with the small oscillator strength) or optically inactive (with the oscillator strength of zero). Meanwhile, the weak excited state can become the strongest excited state by the external field. The ground state properties and spectral properties of borospherene B40 can be modified by the external electric field. Our findings can provide theoretical guidance for the application of borospherene B40 in the future.
Determination of the hyperfine coupling constants of the 5D5/2 state of 85Rb atoms by using high signal-to-noise ratio electromagnetically-induced transparency spectra
We report the hyperfine splitting measurement of the 85Rb 5D5/2 state by electromagnetically induced transparency spectroscopy with high signal-to-noise ratio in the 85Rb 5S1/2-5P3/2-5D5/2 ladder-type system (m 780 nm + 776 nm). The frequency calibration is performed by employing a phase-type electro-optic modulator with a confocal Fabry-Perot cavity. From the measured hyperfine splittings among the manifolds of (F"=5), (F"=4) and (F"=3) of the 85Rb 5D5/2 state, we determine the magnetic dipole hyperfine coupling constant (A= (-2.222 ± 0.019) MHz) and the quadrupole coupling constant (B= (2.664 ± 0.130) MHz) of 5D5/2 state of 85Rb atoms.
Vibration states and entropy of adsorbed hydrogen molecules
The entropy and enthalpy changes upon absorption determine the equilibrium adsorption states, the adsorption/desorption kinetics, and the surface reaction rates. However, it is difficult to measure experimentally or calculate theoretically the entropy of adsorption state. Hydrogen is considered as the most promising candidate to solve the global energy problems, and the storage by adsorption on light porous solids constitutes a main avenue to research field. An ideal storage system should be able to operate under ambient conditions with high recycling capacity and suitable uptake-release kinetics. The entropy of adsorbed H2 molecules is of great significance for determining the optimum conditions for hydrogen storage and for designing the storage materials. To the best of our knowledge, however, the only report on the entropy of the adsorbed H2 molecules is that adsorbed on alkali-metal exchanged zeolites at temperatures around 100 K. Due to different assumptions of the entropy changes, the values of the optimum enthalpy ΔH reported in the publications cover a wide range. In this paper, the adsorption states, vibrational modes, and the entropies of H2 molecules adsorbed on (MgO)9 and (AlN)12 clusters are studied by using first principal method. The computation is performed by the second-order perturbation theory (MP2) with the triple zeta basis set including polarization functions 6-311G(d, p). The very-tight convergence criterion is used to obtain reliable vibration frequencies. Analysis shows that six vibrational modes of the adsorption complexes can be attributed to the vibration of H2 molecule. For these normal modes, the amplitudes of the displacements of cluster atoms are usually two orders smaller than those of the hydrogen atoms. As the vibrational frequency is inversely proportional to the square root of the mass, the zero-point energy has an important influence on the adsorption energy. The ZPE correction exceeds half of the adsorption energy, and the adsorption on the anions is not stable after including the correction. Under the harmonic approximation, the normal vibration modes are independent, so the entropy of adsorbed H2 molecules can be calculated by using the vibrational partition function based on the vibrational frequencies. The results indicate that the entropy values depend mainly on the two lowest in-phase vibrational frequencies and it is not directly related to the adsorption strength but determined by the shape of the potential energy surface. In a temperature range of 70—350 K and at a pressure of 0.1 MPa, there is a good linear correlation between the entropy of adsorbed H2 and the entropy of gas-phase. The entropy of H2 decreases about 10.2R after adsorption.
Quantum statistical properties of phase-type three-headed Schrodinger cat state
Quantum superposition is a fundamental principle of quantum mechanics, which provides a crucial basis to observe phenomena beyond the predictions of classical physics. For example, a quantum entangled state can exhibit stronger correlation than classically possible one. In quantum state engineering, many new quantum states can be obtained from the superposition of many known states.
In recent decades, the superposition of coherent states (CSs) with the same amplitude but two different phases has been a subject of great interest. This superposition state was often called Schrodinger cat state (here, we also name it 2-headed cat state (2HCS)), which becomes an important tool to study a lot of fundamental issues. Surprisingly, some studies have extended the quantum superposition to involving more than two component coherent states. In order to produce the superposition of three photons, people have considered the superposition of coherent states with three different phases (here, we also name it 3-headed cat state (3HCS)). Furthermore, in microwave cavity quantum electrodynamics of bang-bang quantum Zeno dynamics control, people have proposed the superposition of coherent states with four different phases (here, we also name it 4-headed cat state (4HCS)).
In this paper, we make a detailed investigation on the quantum statistical properties of a phase-type 3HCS. These properties include photon number distribution, average photon number, sub-Poissionian distribution, squeezing effect, and Wigner function, etc. We derive their analytical expressions and make numerical simulations for these properties. The results are compared with the counterparts of the CS, the 2HCS and the 4HCS.
The conclusions are obtained as follows. 1) The CS, the 2HCS, the 3HCS and the 4HCS have k, 2k, 3k and 4k photon number components, respectively (k is an integer); 2) small difference in average photon number among these quantum states in small-amplitude range can be observed, while their average photon numbers become almost equal in large-amplitude range; 3) the CS exhibits Poisson distribution, and the 2HCS, the 3HCS and the 4HCS exhibit super-Poisson distributions in most amplitude ranges, however, sub-Poisson distribution can be seen for the 3HCS and the 4HCS in some specific amplitude ranges; 4) except for the 2HCS that may have the squeezing property, no squeezing properties can be found in the CS, the 3HCS and the 4HCS; 5) negative values can exist in the Wigner functions for the 2HCS, the 3HCS and the 4HCS, while it is not found in the CS.
Similar to the 2HCS and 4HCS, the Wigner function of the 3HCS has negative component, which implies its nonclassicality. Different from the 2HCS, the 3HCS exhibits sub-Poisson photon number distribution in a certain amplitude range, it is weaker than that of the 4HCS. At the same time, no squeezing is found in the 3 or 4HCS, which is another difference from the 2HCS.
Efficiency analysis of 808 nm laser diode array under different operating temperatures
The 808 nm high-efficiency laser diodes have many advantages, such as high output power, high reliabilities, compact sizes, which are widely used in many areas, such as industry, communication, science, medicine and biology. In order to improve the power conversion efficiencies of 808 nm laser diodes, the following requirements must be considered, such as loss of joule heating, loss by the carrier leakage, spontaneous radiation loss below the threshold current, loss by interface voltage defect, internal losses including free-carrier absorption loss and scattering loss. These losses above are closely related to the operating temperature of laser diode. In this paper, power conversion efficiency analysis is demonstrated from the aspects of the output power, threshold current, slope efficiency, voltage, and series resistance at different temperatures.. This is the first time that the detailed study has been carried out under various temperatures (up to the lowest temperature of -40℃). And the detailed study above can be of benefit to designing the wafer epitaxial structure.
High-power 808 nm laser diode arrays are mounted on conduction cooled heatsinks. And the laser chips have 47 emitters with 50% in fill factor, 100 μm stripe in width and 1.5 mm in cavity length. The asymmetric broad waveguide epitaxial structure with lower absorption loss in p-type waveguide and cladding layer is designed in order to reduce the internal losses. The device performances are measured under operating temperatures ranging from -40℃ to 25℃ including the output power, threshold current, slope efficiency, series resistance, voltage, etc. Then the power conversion efficiency of 808 nm laser diode arrays are demonstrated from the output characteristics at different operating temperatures.
With temperature decreasing, the series resistance gradually increases. The loss of joule heating ratio rises from 7.8% to 10.3%. In that case, the high series resistance is the major factor to prevent the efficiency from further improving at a low temperature of -40℃. As temperature decreases from 25℃ to -40℃, the carrier leakage ratio is reduced from 16.6% to 3.1%, the carrier leakage is the dominant factor for increasing efficiency, which means that it is necessary to optimize the epitaxial structure in order to reduce the carrier leakage at the room temperature. Comparing the two different work temperatures from -30℃ to -40℃, the carrier leakage ratio only changes 0.1%, which implies that the carrier leakage could be ignored under the low temperature. Meanwhile, as temperature decreases from 25℃ to -40℃, the power conversion efficiency increases from 56.7% to 66.8%.
Finite element analysis and experimental study on electrical damage of silicon photodiode induced by millisecond pulse laser
Experimental study on two-photon fluorescence and coherent anti-Stokes Raman scattering microscopy
Image restoration and enhancement based on phase conjugation of optical parametric amplification
It is well known that the weak optical image can be amplified based on the optical parametric amplification (OPA), and the distorted wave-front can be recovered by the optical phase conjugation (OPC) method. In this paper, weak infrared images, which are barely recognizable after the propagation through the milk emulsion, are restored and optically amplified based on phase conjugation of OPA.The OPC property of OPA is demonstrated with a type-II phase matched nonlinear optical crystal KTiOPO4 (KTP). The near-infrared image at 1064 nm is the input of OPA as the signal beam, and a 10 Hz, mJ-level, 21 ps 532 nm laser is used as the pump beam. When the spatial and temporal overlap are achieved, the attenuated optical image is amplified. Due to the difference in polarization, the idler beam of the OPA is selected and detected with the CCD and the blurred image is restored by the re-entry of the turbid media.The resolution of restored image is 12 lines/mm, which has achieved a theoretical limit. Moreover, by combining the optical gain of the OPA process, over 17 dB image amplification is obtained, which is the highest for the OPC-based image restoration in turbid media to our knowledge. The significant improvement in image quality is also demonstrated by 160% increase of the peak signal-to-noise ratio. By taking advantage of tunability of the OPA, the operational wavelength of this technique can be extended to an optical therapeutic window, which is suitable for noninvasive image restoration, enhancement and detection.
Measurement of absorption spectrum around 1.572 μm
Simulation research and theoretical study on measurement of atmospheric optical turbulence and wind profile using the correlation of gradient-tilt
In this article, a theoretical method based on the fluctuation of gradient tilt (G-tilt) of active light source is proposed to estimate the horizontal profiles of atmospheric optical turbulence (Cn2) and transverse wind. The G-tilt, related to the average phase gradient, is in the same direction as the average ray direction. And G-tilt angle is considered to be equal to the ratio between the centroid position offset and the focal length. In this method, a theoretical model based on lidar system is set up, in which forward scatter light beams at different distances are taken as beacons. These beacons are detected by a two-aperture telescope. And two light columns, from which we can obtain the information about G-tilt angle, are imaged by these beacons. In order to obtain the turbulence intensity and wind velocity from G-tilt angle with our theoretical model, the differential cross-correlation expressions of G-tilt angle and its derivative are derived in detail. These two expressions are based on the spatial cross-correlation function obtained from Rytov approximation and Taylor's frozen-flow hypothesis for Kolmogorov turbulence. Simultaneously, path weighting functions of Cn2 and wind velocity are derived, and the effects of path weighting functions on the calculation of our method are analyzed. Based on such an analysis, to realize the inversion of turbulence intensity and transverse wind, the matrix transformation algorithm is proposed. We ignore some minimal values of the path weighting functions in our algorithm so that the ill-conditioned matrix is avoided. Besides, numerical simulation is used for preliminarily validating this method. In our simulation, Cn2 varies randomly between 10-15 m-2/3 and 10-14 m-2/3 while wind velocity ranges from -5 m/s to 10 m/s. The sign of the wind velocity represents the direction of wind. According to the simulation results, the horizontal profiles of atmospheric optical turbulence and transverse wind calculated are consistent with their theoretical values no matter whether there exists Gaussian noise. When the ratio between the standard deviation of Gaussian noise we added and the original signal is 0.2, the maximum relative error of logarithmic Cn2 is 3.4%, and the correlation coefficient between the calculated results and theoretical values for Cn2 is 0.8. Besides, the maximum absolute error of wind velocity is 1.82 m/s, and the correlation coefficient between the calculated results and theoretical values for wind velocity is 0.9. Even if the horizontal profiles of atmospheric optical turbulence and transverse wind vary largely, the calculation results of our method remain stable. Therefore, a new idea is provided for measuring atmospheric turbulence and wind.
Fluorescence radiation characteristics based on evanescent wave pumping in a microfluidic chip
10 mJ femtosecond Ti: Sapphire regenerative amplifier with large mode size
With advent of chirped-pulse amplification, the peak power of femtosecond laser pulse was reached to petawatt (PW) or hundreds of terawatt (TW). Many progresses of high-field physics and ultrafast dynamics in matter are achieved using TW or PW laser. Pre-amplifier is an exponential growth amplifier which is also a bridge between oscillator and power amplifier. The best choice of pre-amplifier is amplification in regenerative cavity, due to its high stability and beam quality. The quality of pre-amplified laser pulse is significant to efficiency and beam quality of the successive power amplifier. High energy pre-amplifier with high beam quality will reduce the requirement of pump laser in final power amplifier. But typical regenerative amplifier only support low output energy of few millijoule. Higher energy from only one regenerative amplifier is crucial to whole laser system.
High energy regenerative amplifier can be achieved by increasing the size of TEM00 in cavity. A new femtosecond Ti:sapphire regenerative amplifier with large mode size was demonstrated in this letter. The regenerative cavity is designed as stable linear resonator in which end mirrors are planar, the diameter of beam waist in Ti:sapphire crystal is larger than 2 mm, which can support high energy pulse amplified in cavity. By matching the focal spot of pump laser with the size of mode and optimization of cavity, the output laser energy up to 17.4 mJ was achieved under the pump energy of 60 mJ at repetition rate of 10 Hz, which corresponds to the efficiency of 29%. The amplified laser pulse from regenerative amplifier was compressed in a grating-pair compressor. By carefully alignment of incident angle and distance between the two gratings of compressor, the shortest pulses duration of 40.6 fs and energy of 13.9 mJ are obtained, which is a little bit longer than Fourier-transform limit based on spectrum of laser. The dispersion in the CPA laser system was also analyzed, after optimization of compressor, there are still high order dispersions uncompensated, which results in the duration of compressed pulses longer than Fourier-transform limit.
Based on this large mode size regenerative amplifier, peak power of 1.9 TW laser pulses which compressed pulse energy of 81.4 mJ in 43 fs were also further realized by following only one stage of multipass amplifier. The beam quality (M2) was measured to be 1.6 and 1.5 in X and Y directions respectively, and the energy stability is 2.15% (rms). The results show that this large mode size regenerative amplifier is an ideal choice of pre-amplifier in TW laser system.
Design and research of three-dimensional thermal cloak with arbitrary shape based on the transformation thermodynamics
Janssen ratio in granular solid measured by photoelastic method
In this work Janssen ratio is measured in a dense granular pack. The pressure on the side walls as a function of the depth of the pack with top load under gravity is measured by photoelastic method. The samples are prepared by “point source” method with and without tapping. A non-monotonic distribution of the side pressure along the depth is found. Numerical simulation is performed and shows qualitative consistency with the experimental finding. The apparent weight of the sample is measured for different filling heights and for different top loads. Comparing with the normal stresses on the silo wall for different heights, we obtain the Janssen ratio J =σxx/σzz as a function of height. We find that although uJ = 0.11 is a constant as is expected, the Janssen Ratio is height dependent. It becomes height independent only when the top load is large enough.
Dynamical behaviors of Sn micro-sphere particles under shock wave action
Fine flow structure and mixing characteristic in supersonic flow induced by a lobed mixer
Formation of periodic ripples on silicon surface ablated by femtosecond laser
The formation mechanism of low-spatial-frequency laser-induced periodic surface structure (LSFL) on single-crystalline silicon irradiated by single femtosecond-laser pulse (pulse duration τ=150 fs and central wavelength λ=800 nm) in air is investigated theoretically based on the interference theory of Sipe-Drude model and surface plasmon polariton (SPP). In order to account for transient intrapulse changes in the optical properties of the material due to the excitation of a dense electron-hole plasma, we model the maximum of the electron density as a function of laser fluence by solving the generally accepted two-temperature equation and Drude model. The results show that both theories are applicable to explaining the LSFL formation on the high-excited silicon. In the Sipe-Drude theory, the factor η(k) is used to describe the efficacy with which the surface roughness at position k leads to inhomogeneous absorption of radiation. We find that the value of η(k) first increases until reaching a maximum at an electron density of 6×1021 cm-3 and then decreases with the laser fluence increasing. When the incident laser fluence is 0.38 J/cm2, which is the threshold for excited plasma, the period reaches a minimum value in a small range of the top. Besides, the law of period is calculated according to the relationship between the η(k) and period. In the SPP theory, the ripple period on the highly excited silicon increases with the laser fluence increasing. Comparing the scopes of application of two theories, the Sipe-Drude theory is found to be suitable for the analysis of more extensive periodic surface structures, while the SPP theory is applicable only for the case that laser fluence is close to the damage threshold. Moreover, our results are capable of explaining that the delay direction of periodic ripples are always perpendicular to the incident laser polarization direction by using the Sipe-Drude theory. When laser fluence approaches to the damage threshold, the LIPSS period is calculated sightly to be below the laser wavelength. It also reveals that the periodic surface structures are approximately the same in the different polarization directions with the increase of incident angle. Taking into account a single pulse, the corrugation period decreases with the increase of angle of incidence in the S polarization direction. And under different polarizations, with the increase of incident angle, the changes of the ripple period show an opposite trend. The obtained dependence provides a way to better control the properties of the periodic structures induced by femtosecond laser on the surface of a semiconductor material, which is of great significance for understanding the formation of periodic structure of silicon surface, caused by femtosecond laser, and its application in the field of silicon materials processing.
Uniform irradiation of a direct drive target by optimizing the beam parameters
Laser driven fusion requires a high-degree uniformity in laser energy deposition in order to achieve the high-density compression required for sustaining a thermonuclear burn. Nowadays, uniform irradiation of capsule is still a key issue in direct drive inertial confinement fusion. The direct drive approach is to drive the target with laser light, by irradiating it with a large number of overlapping laser beams. In the direct drive scheme, the laser deposition pattern on the target can be decomposed into a series of Legendre spherical harmonic modes. The high mode (shorter wavelength) nonuniformity can lead to Rayleigh-Taylor instability, which may result in the failure of target compression. This nonuniformity can be suppressed by thermal conduction and beam conditioning technologies, such as continuous phase plate, smoothing by spectral dispersion and polarization smoothing. The low mode (longer wavelength) nonuniformity is related to the number, orientation and power balance of laser beams, which is hard to suppress by thermal conduction and beam conditioning technologies. Generally, the nonuniformity of laser irradiation on a directly driven target should be less than 1% (root mean square, RMS), to meet the requirement for symmetric compression. Several methods have been proposed to optimize the irradiation configuration in direct drive laser fusion, such as truncated icosahedron with beams at the 20 faces and 12 vertices of an icosaherdron, dodecahedron-based irradiation configurations, self-organizing electrodynamic method, etc. However, limited by the different parameters of incident beams, the irradiation uniformity is often not satisfactory. Therefore, it is necessary to find new way to improve the irradiation uniformity and make it more robust. According to the analytical result, the irradiation nonuniformity can be decomposed into the single beam factor and the geometric factor. Simulation results show that the single beam factor is mainly determined by the parameters of the incident beams, including beam pattern, beam width and beam wavelength. By analyzing and simulating the single beam factor with different incident beam parameters, and comparing the single beam factor with the geometric factor, a matching relationship between them is found by using the optimized parameters. Based on the simulation results, a method to optimize the incident beam parameters is proposed, which is applied to the 32-beam and 48-beam irradiation configurations. The results show that there is a set of optimal incident beam parameters which can attain the highest irradiation uniformity for a given configuration. The feasibility to achieve more uniform irradiation by optimizing the incident beam parameters is proved. When the single beam factor is optimized in a directly driven inertial confinement fusion system, the restrictions on the beam pointing error and power imbalance between incident beams can be relaxed. The results provide an effective method of designing and optimizing the uniform irradiation system of direct drive laser facility.
Basic dynamic and scale study of quasi-spherical Z-pinch implosion
Properties of temporal X-ray in nanosecond-pulse discharges with a tube-to-plane gap at atmospheric pressure
Nanosecond-pulse discharge can produce low-temperature plasma with high electron energy and power density in atmospheric air, thus it has been widely used in the fields of biomedical science, surface treatment, chemical deposition, flow control, plasma combustion and gas diode. However, some phenomena in nanosecond-pulse discharge cannot be explained by traditional discharge theories (Townsend theory and streamer theory), thus the mechanism of pulsed gas discharge based on runaway breakdown of high-energy electrons has been proposed. Generally, the generation and propagation of runaway electrons are accompanied by the generation of X-ray. Therefore, the properties of X-ray can indirectly reveal the characteristics of high-energy runaway electrons in nanosecond-pulse discharges. In this paper, in order to explore the characteristics of runaway electrons and the mechanism of nanosecond-pulse discharge, the temporal properties of X-ray in nanosecond-pulse discharge are investigated. A nanosecond power supply VPG-30-200 (with peak voltage 0——200 kV, rising time 1.2-1.6 ns, and full width at half maximum 3-5 ns) is used to produce nanosecond-pulse discharge. The discharge is generated in a tube-to-plane electrode at atmospheric pressure. Effects of the inter-electrode gap, anode thickness and position on the characteristics of X-ray are investigated by measuring the temporal X-ray via a diamond photoconductive device. The experimental results show that X-ray in nanosecond-pulse discharge has a rising time of 1 ns, a pulse width of about 2 ns and a calculated energy of about 2.3×10-3 J. The detected X-ray energy decreases with the increase of inter-electrode gap, because the longer discharge gap reduces the electric field and the number of runaway electrons, weakening the bremsstrahlung at the anode. When the inter-electrode gap is 50 mm, the discharge mode is converted from a diffuse into a corona, resulting in a rapid decrease in X-ray energy. Furthermore, both X-ray energies measured behind the anode and on the side of discharge chamber decrease as anode thickness increases. The X-ray energy measured on the side of the discharge chamber is one order of magnitude higher than that measured behind the anode, which is because the anode foil absorbs some X-rays when they cross the foil. In addition, the X-ray energy behind the anode significantly decreases with the increase of the thickness of anode aluminum foil. It indicates that the X-ray in nanosecond-pulse discharge mainly comes from the bremsstrahlung caused by the collision between the high-energy runaway electrons and inner surface of the anode foil. Therefore, increasing the thickness of the anode foil will reduce the X-ray energy across the anode film.
Video saliency detection algorithm based on biological visual feature and visual psychology theory