Research progress of two-dimensional organic topological insulators
The discovery of new materials promotes the progress in science and technique. Among these new materials, topological materials have received much attention in recent years. Topological phases represent the advances both in the fundamental understanding of materials and in the broad applications in spintronics and quantum computing. The two-dimensional (2D) topological insulator (TI), also called quantum spin Hall insulator, is a promising material which has potential applications in future electronic devices with low energy consumption. The 2D TI has a bulk energy gap and a pair of gapless metallic edge states that are protected by the time reversal symmetry. To date, most of topological insulators are inorganic materials. Organic materials have potential advantages of low cost, easy fabrications, and mechanical flexibility. Historically, inorganic materials and devices have always found their organic counterparts, such as organic superconductors, organic light emitting diodes and organic spintronics. Recently, it has been predicted that some metal-organic lattices belong in an interesting class of 2D organic topological insulator (OTI). In this review, we present the progress of OTIs mainly in two typical types of them. In the first group, metal atoms bond with three neighboring molecules to form a hexagonal lattice, while they bond with two neighboring molecules to form a Kagome lattice. The electronic properties show that the Dirac band around Fermi level mainly comes from the hexagonal sites, and the flat band around Fermi level mainly is from Kagome lattice. It has been found that some of the materials from the first group could be intrinsic OTIs. However, none of the 2D OTIs predicted in the second group with a Kagome lattice is intrinsic. To obtain intrinsic OTIs from those non-intrinsic ones, in the heavy doping of material (one or two electrons per unit cell) it is required to move the Fermi level inside the gap opened by spin-orbit coupling, which is hard to realize in experiment. Therefore, many efforts have been made to search for intrinsic OTIs. It has been reported that the first group of 2D OTIs with a hexagonal lattice is found to be more possible to be intrinsic. By performing an electron counting and analyzing the orbital hybridization, an existing experimentally synthesized Cu-dicyanoanthracene (DCA) metal-organic framework is predicted to be an intrinsic OTI. Furthermore, like Cu-DCA, the structures consisting of molecules with cyanogen groups and noble metal atoms could be intrinsic OTIs. Finally, we discuss briefly possible future research directions in experimental synthesis and computational design of topological materials. We envision that OTIs will greatly broaden the scientific and technological influence of topological insulators and become a hot research topic in condensed matter physics.
Competition between different nonlinear optical effects of GaN-based thin-film semiconductors
In recent years, new optoelectronic materials such as GaN-based thin-film semiconductors and rare-earth-ion doped luminescent materials have aroused the interest of many researchers. The GaN-based semiconductors have wide and direct energy gaps which could be adjusted to cover the whole visible light spectrum region by doping. They have been successfully applied to fabrications of blue lasers and light emitting diodes. The rare-earth-ion doped luminescent materials have exhibited many advantages in luminescent properties such as intense narrow-band emissions, high conversion efficiency, wide emission peaks ranging from ultraviolet to near infrared, long lifetime ranging from nanoseconds to milliseconds, and good thermal stability. They have been widely applied in the fields of illumination, imaging, display, and medical radiology. So far, the studies on GaN-based thin-film semiconductors and rare-earth-ion doped luminescent materials focus mainly on their growth and linear optical properties. In contrast, the investigations of the nonlinear optical properties of these materials, which have potential applications in many fields, are still lacking. In this paper, GaN-based thin-film semiconductors, such as undoped GaN, Mg-doped GaN and InGaN/GaN multiple quantum wells, are successfully grown by metal-organic chemical vapor deposition. Their nonlinear optical properties are studied by using an 800-nm femtosecond laser light. The nonlinear optical properties are different when the laser light is focused on different positions of the samples. The competition between different nonlinear optical effects reflect directly the competition in stimulated luminescence energy. And particularly, it is closely related to the density of energy states, stimulated luminescence energy, and the sample band gap energy difference. In addition, the competition between different nonlinear optical effects, such as multiphoton-induced luminescence and second harmonic generation, is clearly revealed and is manifested in the dependence of the nonlinear optical signal on excitation intensity in this investigation. And also, the competition mechanism is preliminary studied in this paper.
Nonlinear absorption, nonlinear scattering, and optical limiting properties of carbon nanotube/molybdenum diselenide organic glass
Because MoSe2 has broadband saturable absorption, and higher nonlinear refractive index. Compared with MoS2, thin-layered MoSe2 possesses very attractive properties, including narrow bandgap, low optical absorption coefficient, and large spin-splitting energy at the top of the valence band. The narrow bandgap and low optical absorption coefficient could make MoSe2 more applicable than MoS2. And the tunable excitation photoelectric effecthas great potential applications in the fields of photoluminescence, phototransistor, solar cells, nonlinear optics and other aspects. However, pure MoSe2 has high photogenerated recombination rate, thus limiting its applications in some optical fields. By designing nanocomposites of MoSe2, the photogenerated recombination rate of these materials can be reduced and their application field can be broadened. In this work, MoSe2 nanocomposites are prepared by simple methods. The two-dimensional layered MoSe2 nanosheets are combined with nanorods. By integrating the surface effect, small size effect and interfacial effect of CNT, the optical nonlinearity and optical limiting performance of MoSe2 composites are improved. The CNT/MoSe2 composite nanomaterials are first synthesized based on narrower band gap and lower light absorption coefficient of MoS2 than those of MoSe2 by growing MoSe2 nanoparticles on the surface of CNT through a solvothermal method, and then is dispersed in methyl methacrylate (MMA) to prepare an organic glass by a casting method, and the MMA is polymerized into poly (methyl methacrylate) (PMMA). The nonlinear absorption (NLA), nonlinear scattering (NLS) and optical limiting (OL) properties of the CNT/MoSe2/PMMA organic glass are studied by the modified Z-scan technique for the first time. The CNT/MoSe2/PMMA organic glass exhibits the saturable absorption (SA) and a changeover from SA to reverse saturable absorption by adjusting input energy. The experimental results show that the CNT/MoSe2/PMMA plexiglass exhibits better anti-saturation absorption and higher optical limiting properties than MoSe2/PMMA and CNT/PMMA plexiglass. Besides, the NLA and OL properties of the CNT/MoSe2/PMMA organic glass are enhanced compared with CNT/PMMA and MoSe2/PMMA organic glasses, which can be attributed to the existence of the C=C double bonds in CNTs, the layered structure of MoSe2 nanosheets, and the interfacial charge transfer between CNTs and MoSe2. And the results demonstrate that the CNT/MoSe2/PMMA organic glass is very promising for optical devices such as optical limiters and mode-locked/Q-switched lasers.
Topological phase transitions in square-octagon lattice with Rashba spin-orbit coupling
Motivated by the square-octagon lattice which supports topological phases over a wide range of parameters and a number of interesting quantum phase transitions in the phase diagram when considering the intrinsic spin-orbit coupling, we investigate the topological phase transitions in the isotropic square-octagon lattice combining the effects of both spin-orbit couplings and exchange field. The inversion symmetry and time-reversal symmetry are broken when both Rashba spin-orbit coupling and exchange field are present. The Z2 index is not applicable for quantum spin Hall systems without time-reversal symmetry, but the spin Chern number remains valid even in the absence of time-reversal symmetry. Therefore, we use the Chern number and spin Chern number to describe the topological properties of the system. We explore that a variety of topologically nontrivial states appear with changing the exchange field, including time-reversal-symmetry-broken quantum spin Hall states and quantum anomalous Hall states. The phase transition between these topological phases is accompanied by the closing of band gaps. Interestingly, the quantum spin Hall effect described by nonzero spin Chern number is found to remain intact when the time-reversal symmetry is broken. Furthermore, the variation of the amplitude of the exchange field and filling factor drive interesting topological phase transitions from the time-reversal-symmetry-broken quantum spin Hall phase to spin-filtered quantum anomalous Hall phase. A spin-filtered quantum anomalous Hall phase is characterized by the presence of edge states with only one spin component, which provides an interesting route towards quantum spin manipulation. We also present the band structures, edge state wave functions, and spin polarizations of the different topological phases in the system. It is demonstrated that the energy spectra of edge states are in good agreement with the topological characterization based on the Chern number and spin Chern number. In particular, we observe that gapless edge states can appear in a time-reversal-symmetry-broken quantum spin Hall system, but the corresponding spin spectrum gap remains open on the edges. Recently, an important functional material ZnO with quasi square-octagon lattice has been found experimentally. Consequently, the results found in our work are helpful for understanding the property of square-octagon lattice and studying the real materials with square-octagon structure.
First-principles study on Jahn-Teller effect in Cr monolayer film
Computational physics has been used in many scientific research fields, in which first-principles calculation based on density functional theory has made brilliant achievements. Unlike three-dimensional materials, low-dimensional materials present fantastic physical effect, due to the reduction of material dimensions. With the rapid development of two-dimensional materials, people have a more in-depth understanding of them. Requirements for high performance of two-dimensional materials are raised for potential applications, so the exploration of some effects affecting the stability of two-dimensional materials becomes more and more important. Based on the pioneers' work, Jahn-Teller effect is found to have a certain influence on the stabilities of two-dimensional structure of some elements. In the present paper, we explain the stable structure of Cr monolayer film through theoretical calculation, providing a guidance for experimental synthesis. Using first-principles calculation, we study a series of two-dimensional structures (rectangular, square, hexagonal, oblique and centered rectangular) of Cr monolayer film, focusing on the structural stability and electronic properties. Firstly, the equilibrium lattice constant and cohesive energy of each structure are calculated. Then, the bond angle and lattice constant dependence of the total energy are analyzed in detail. Finally, we investigate the energy band structures, total electronic densities of states, charge densities and electron occupation numbers of orbitals. The results show that low-symmetry oblique and centered rectangular lattice are stable in the two-dimensional system of Cr, while high-symmetry square and hexagonal lattices are not stable and the adhesive energy of the rectangular lattices is very small. Two stable structures of Cr monolayer sheet are formed due to hexagonal structure distortion. The hexagonal structure can shape into a centered rectangular structure with the increase of bond angle, while it changes into an oblique structure with the decrease of bond angle. Because of Jahn-Teller effect, the degenerate energy level spontaneously splits. Then the structure deforms into two reduced-symmetry structures, resulting in a stable system. Therefore, we can infer that the Jahn-Teller effect plays a crucial role in the structural stability of monolayer sheet.
Ultralow-voltage albumen-gated electric-double-layer thin film transistors
In recent years, environment-friendly and biocompatible electronics have received extensive attention. As a kind of natural biological material with rich sources, proteins have been widely used in electronic devices. In this work, electric-double-layer (EDL) thin-film transistors (TFTs) gated by natural chicken albumen are fabricated at room temperature. The indium-tin-oxide (ITO) conductive glass is employed as a substrate. The spin coated chicken albumen film is used as the gate dielectric. The indium-zinc-oxide (IZO) is sputtered on an albumen-coated ITO glass as the channel and the source/drain electrodes with only one shadow mask. The capacitance-frequency measurements demonstrate an ultra-large specific capacitance of the albumen film at low frequencies. For the physical understanding of the capacitive coupling within the albumen film, the phase angle is characterized as a function of frequency. The results indicate that such an ultra-large capacitive coupling can be attributed to the proton migration under the electric field, which results in the EDL effect at the interface of the albumen film. By DC sweep measurements, a low leakage current is observed (<3.0 nA at Vgs=1.5 V), which indicates a good isolation of the albumen-based dielectric. By transfer and output measurements, an ultralow operation voltage of 1.5 V, a high field-effect mobility of 38.01 cm2/(V·s), a low subthreshold swing of 164 mV/decade, and a large on-off ratio of 2.4×106 are obtained for such albumen-gated TFTs. The ultra-large EDL capacitive coupling is responsible for such good electrical characteristics. The dynamic bias stress stability of the albumen-gated TFTs is also investigated. The device exhibits a good reproducibility in response to the repeatedly pulsed gate voltage. A maintainable on-to-off ratio (>106) and no obvious current loss are observed, which suggests that neither chemical doping nor chemical reaction occurs at the albumen-based dielectric/IZO channel interface when the gate potential is biased. After being aged one day in air ambient without surface passivation, the albumen-gated TFTs show a good stability of the electrical properties. Such ultralow-voltage EDL-TFTs gated by albumen electrolyte will be useful for the bioelectronic and low-energy portable electronic products. And our results will also have potential applications in biocompatible artificial neuron networks and brain-inspired neuromorphic systems.
Ultrafast polarization modulation of laser pulses at terahertz frequencies via optical Kerr effect
Polarized light has already been widely used for photography and display technologies. Magneto-optical Faraday effect, i.e., the light polarization rotates in the magnetic field applied to the material in the direction of light propagation, plays a crucial role in the interaction between light and spin. Faraday effect allow us to understand the nature of magnetization in condensed materials. As an effect opposite to the Faraday effect, the magnetization can be induced in a transparent medium exposed to a circularly polarized electromagnetic wave, which is called inverse Faraday effect. Knowledge of the mechanism provides the opportunities of modulation devices in photonics, ultrafast opto-magnetism and magnonics. In this paper, we experimentally demonstrate a proof-of-concept ultrafast polarization modulation by employing circularly polarized light to demonstrate a strengthened terahertz (THz) frequency Kerr modulation signal, at room temperature. By using the transient pumpprobe spectroscopy with the reflected geometry, we are able to demonstrate the feasibility of such an ultrafast magneto-optical polarization modulation at 0.19 THz in a paramagnetic Li:NaTb (WO4)2 crystal with a thickness of 3 mm. The time-resolved modulation signal is explained by the interaction between two counter-propagating laser pulses (central photon energy of 1.55 eV) within the crystal via the optical Kerr effect. We find that the amplitude of the modulation increases with the pump fluence increasing, while the modulation frequency is dependent neither on the pump fluence nor on polarization of pump beam. However, it can further be found that the phase and amplitude of the transient Kerr modulation are strongly dependent on the helicity of the circularly polarized pump pulses. Indeed, these oscillating signals may be mistaken for spin excitation modes. The present findings allow us to get an insight into the transient magneto-optical dynamical process in transparent medium. In addition, the polarization modulation of ultrashort laser pulse on a picosecond time scale will facilitate all-optical data processing, as well as the polarization-dependent ultrafast dynamics in various material systems, which span from condensed matter to molecular spectroscopy. In this regard, our experimental results provide a possibility for designing novel all-optical (magneto-optical) modulators operating at THz clock frequencies. The magneto-optical polarization response modulated at THz frequencies may have new possibilities for designing all-optical devices, such as ultrafast modulators.
1.3 μm single photon emission from InAs/GaAs quantum dots
Single-photon emitters are crucial for the applications in quantum communication, random number generation and quantum information processing. Self-assembled InAs/GaAs quantum dots (QDs) have demonstrated to have singlephoton emission with high extraction efficiency, single-photon purity, and photon indistinguishability. Thus they are considered as the promising deterministic single-photon emitters. To extend the emission wavelength of InAs/GaAs QDs to telecom band, several methods have been developed, such as the strain engineered metamorphic quantum dots, the use of strain reducing layers and the strain-coupled bilayer of QDs. In fact, it is reported on single-photon emissions based on InAs/InP QDs with an emission wavelength of 1.55μm, but it is difficult to combine such QDs with a high-quality distributed Bragg reflector (DBR) cavity because the refractive index difference between InP and InGaAsP is too small to obtain a DBR cavity with high quality factor. Here we investigate 1.3μm single-photon emissions based on selfassembled strain-coupled bilayer of InAs QDs embedded in micropillar cavities. The studied InAs/GaAs self-assembled QDs are grown by molecular beam epitaxy on a semi-insulating (100) GaAs substrate through strain-coupled bilayer of InAs QDs, where the active QDs are formed on the seed QDs capped with an InGaAs layer, and two-layer QDs are vertically coupled with each other. In such a structure the emission wavelength of QDs can be extended to 1.3μm. The QDs with a low density of about 6×108 cm-2 are embedded inside a planar 1-λ GaAs microcavity sandwiched between 20 and 8 pairs of Al0.9Ga0.1As/GaAs as the bottom and top mirror of a DBR planar cavity, respectively. Then the QD samples are etched into 3μm diameter micropillar by photolithography and dry etching. The measured quality factor of studied pillar cavity has a typical value of approximately 300. Photoluminescence (PL) spectra of QDs at a temperature of 5 K are examined by using a micro-photoluminescence setup equipped with a 300 mm monochromator and an InGaAs linear photodiode array detector. A diode laser with a continuous wave or a pulsed excitation repetition rate of 80 MHz and an excitation wavelength of 640 nm is used to excite QDs through an near-infrared objective (NA 0.5), and the PL emission is collected by the same objective. The time-resolved PL of the QDs is obtained by a time-correlated single photon counting. The second-order correlation function is checked by a Hanbury-Brown and Twiss setup through using ID 230 infrared single-photon detectors.
In summary, we find that the 1.3μm QD exciton lifetime at 5 K is measured to be approximately 1 ns, which has the same value as the 920 nm QD exciton lifetime. The second-order correlation function is measured to be 0.015, showing a good characteristic of 1.3μm single photon emission. To measure the coherence time, i.e., to perform highresolution linewidth measurements, of the QDs emitted at the wavelength of 920 and 1300 nm, we insert a Michelson interferometer in front of the spectrometer. The obtained coherence time for 1.3μm QDs is 22 ps, corresponding to a linewidth of approximately 30μeV. Whereas, the coherence time is 216 ps for 920 nm QDs, corresponding to a linewidth of approximately 3μeV. Furthermore, both emission spectral lineshapes are different. The former is of Gaussian-like type, while the latter is of Lorentzian type.
Experimental study about single event functional interrupt of ferroelectric random access memory induced by 30-90 MeV proton
Ferroelectric random access memory (FRAM) is a promising memory for space application. The performance of FRAM under irradiation environment should be investigated, especially under proton irradiation environment, which dominates the particles in the space environment. The experiments on single event effects are carried out for two types of FRAMs (FM22L16 and FM28 V100) based on the proton cyclotron of China institute of atomic energy. Both dynamic and static mode are tested for each chip under the irradiation of proton in an energy range from 30 MeV to 90 MeV. Single event upsets (SEUs) and single event functional interrupts (SEFIs) are observed only on FM22L16, where the SEFI is recorded as a significantly transient error with or without memory cell upsets. The SEFI can be subdivided into soft SEFI and hard SEFI according to whether those significantly transient errors disappear or not when the irradiation is paused. Single event effect performances of FM22L16 are accurately described, and the SEFI cross section in an energy range from 50 MeV to 90 MeV is obtained experimentally. The cross section of SEFI increases with proton energy increasing and reaches 10-3/cm2 at 90 MeV. To further study the mechanism of SEFI, the pulsed laser beam with a wavelength of 1064 nm is used to pinpoint the sensitive area of SEFI in the FRAM. Pulsed laser experiment is easy to carry out when single pulsed laser radiates on the device from the back side. Results show that a certain part in peripheral circuit is detected as a sensitive area to SEFI. The sensitive area could be a register or buffer which is vulnerable to irradiation. Only SEUs are observed when the pulsed laser radiates others area of peripheral circuit and memory cell. A hypothesis that a micro latch-up in the CMOS-based peripheral circuit leads to the SEFI is proposed to explain the test results, for the CMOS-based peripheral circuit is sensitive to irradiation. The further reason is the energy deposition in silicon substrate by protons with energies ranging from 30 MeV to 90 MeV through nuclear reaction, which triggers the silicon controlled rectifier structure in the FRAM peripheral circuit. According to the hypothesis, a transient current should be generated in the peripheral circuit when the micro latch-up happens. The transient current is observed on the output of device by using a high frequency oscilloscope which demonstrates the reasonability of the hypothesis.
Orientation relationship between ferrite and austenite and its influence on ultrasonic attenuation in cast austenitic stainless steel
Cast austenitic stainless steel (CASS) is widely used in important engineering components, which has a two-phase microstructure, i.e.austenite and ferrite. With slow cooling rate during solidification procedure, the austenite grain is coarse and the morphology of ferrite is complex. Due to the remarkable elasticity anisotropy of austenite, the resulting structural noise makes the recognition of macroscopic defects quite difficult in ultrasonic testing. To improve the signal-to-noise ratio, the ultrasonic testing frequency is generally small, about 0.5-2.0 MHz, and the ultrasonic scattering effect of ferrite is ignored. However, for submillimeter or even smaller defect and damage near the surface, the ultrasonic testing frequency should be increased to achieve a higher resolution. In these cases, how the ferrite influences the ultrasonic wave propagation behavior and the testing result is still not conclusive. Therefore, CASS Z3CN20-09M is studied as an example in this paper. Based on ultrasonic propagation modeling and “in situ” experimental design, the crystal orientation relationship between ferrite and austenite in CASS is studied and the factors influencing the ultrasonic scattering attenuation are clarified. The results would be helpful for clarifying the ultrasonic response mechanism of CASS and critical for the quantitative evaluation of small defects and early-stage damage.
The orientation relationship between ferrite and austenite and its influence on ultrasonic scattering attenuation in CASS are studied. The crystal orientations and their relationships between two phases are characterized by the EBSD technique. A two-dimension anisotropic model is built based on the morphology of ferrite, and the ultrasonic propagation is calculated by the time domain finite difference method. The influences of orientation relationship and morphology on the longitudinal wave attenuation are analyzed and verified by “in-situ” experiments. Results show that ferrite grains with bar or island shape are distributed on the austenite grains. The orientation relationship between ferrite and austenite is mainly Kurdjumov-Sachs relationship, and only a minority of ferrite and austenite satisfy the Nishiyama-Wassermann relationship. Numerical simulation of the ultrasonic propagation under a testing frequency of 15 MHz indicates that the orientation relationships between two phases and ferrite morphologies present collaborative effects on the ultrasonic scattering attenuation, which could not be ignored. The factors influencing the ultrasonic attenuation in <101> austenite grain are quantitatively analyzed. It is found that in single austenite grains of CASS, the inhomogeneity of crystal orientation, the orientation relationship between austenite and ferrite and the ferrite morphology play an important role in determining the total ultrasonic attenuation.
The results would provide supports for clarifying the ultrasonic response mechanism of CASS and developing the quantitative evaluation methods.
Nitrogen/sulfur co-doped porous carbon nanosheets and its electrochemical performance
Porous carbon materials have aroused extensive interest in the field of energy conversion and storage due to their high surface area, regulatable pore structure, high electrical conductivity and stability, and good electrochemical activity. Nevertheless, granular porous carbons usually result in the relatively long electrolyte-diffusion pathway, which seriously limits the ions transport and then damage the electrochemical performance. Two-dimensional (2D) carbon materials can solve this problem because they can provide short electrolyte-diffusion channel and realize the fast electron transport. On the other hand, dual-heteroatom codoping has been confirmed to be quite an effective approach to improving the electrochemical performance of carbon materials. Therefore, a simple and efficient synthesis of co-doped 2D porous carbon materials is highly attractive.
In this work, nitrogen/sulfur co-doped porous carbon nanosheets (NSPCNs) are prepared from methyl orange (MO) doped polypyrrole (PPy) nanotubes by a thermal-treating process in the presence of KOH under N2 atmosphere. MO-doped PPy nanotubes are prepared through a self-degraded process by using MO-FeCl3 complex as the template initiator. In the thermal process, the combination of the dedoping derived from the interaction between MO and KOH, the pyrolysis of PPy, and KOH activation results in the exfoliation of PPy nanotubes and the formation of NSPCNs. Scanning electron microscopy and transmission electron microscopy analyses demonstrate that as-prepared NSPCNs interconnect to form a hierarchical porous architecture containing micropores, mesopores, and macropores, which provides the three-dimensional interconnected channel for electrolyte diffusion with little hindrance. The N2 sorption measurements indicate that NSPCNs have a high specific area of 1744.8 m2/g and volume of 1.01 cm3/g. The X-ray photoelectron spectroscopy measurements indicate that nitrogen and sulfur have been incorporated into the framework of the as-prepared carbon sample. The doped nitrogen is present in the form of pyridinic, pyrrolic, and quaternary state, and the doped sulfur appears in the form of C-Sn-S and-SOn-configuration. The synergistic effect of co-doped nitrogen and sulfur promote the redistribution of spin and charge density, which can greatly enhance the surface wettability and increase the electrochemical active sites of carbon materials. These features endow as-prepared NSPCNs with excellent electrochemical properties. Electrochemcial impedance spectroscopic measurements indicate that the charge transfer resistance of NSPCN in polysulfide electrolyte is 11.2 Ω·cm2, suggesting a very high electrocatalytic activity of NSPCNs for regenerating the polysulfide electrolyte. Under the illumination of 100 mW/cm-2, the NSPCNs' electrode-based quantum dot-sensitized solar cell achieves a conversion efficiency of 4.30%, which is comparable to that of the PbS electrode-based cell. Furthermore, NSPCNs display excellent capacitive performance. In 6 M KOH aqueous electrolyte, NSPCNs achieve a high specific capacitance of 312.8 F/g at a current density of 0.4 A/g. Even the current density increases to 20 A/g, the NSPCNs still maintain a specific capacitance of 200.6 F/g, indicating a good rate performance. Therefore, the as-prepared NSPCNs can be used as the high-performance electrode materials for quantum-dot sensitized solar cells and supercapacitors.
Tunable microwave frequency comb generation based on double-loop mixing-frequency optoelectronic oscillator
With the development of wireless communication technology and micro-cell technology, optical-borne microwave technology, specially optical-borne multi-carrier technology has become one of the most important trends for generating high-quality sources. Therefore, the efficient generation of high-quality microwave signals has always been a requirement in wireless communication systems. Due to its low-noise and high-frequency output characteristics, photoelectric oscillator is widely used to generate high-quality microwave frequency sources in communication systems. Combining the advantages of photoelectric oscillator's low-noise output and direct-modulated laser's gain-switching state characteristics, a tunable optical-borne microwave frequency comb scheme based on dual-loop mixing-frequency photoelectric oscillator is proposed in this paper. And a direct-modulated laser operating in a gain-switching state is used to generate the original optical-borne microwave frequency comb signals. The dual-loop adjacent resonant frequencies are separated by two different high-frequency microwave bandpass filters. The beat frequency of adjacent frequencies mentioned above is injected back into laser to form photoelectric resonance, and thus enhancing the generated original optical-borne microwave frequency comb signals. To suppress the side modes caused by long resonant cavity, a polarized dual-loop structure is used in the system, and thus improving the noise characteristics of output signals. After experimental analysis, the dual-loop filtered resonant microwave signals and low-phase-noise microwave comb signals with a frequency interval of 797.4 MHz are all obtained. The microwave output side-mode suppression ratio after polarized dual-loop adjustment is improved to 47 dB. And microwave comb signal's first-order carrier phase noise is lower than-101.7 dBc/Hz at 10 kHz,-115.2 dBc/Hz at 50 kHz. In addition, higher-order carriers all come from the light multiplication of first-order carrier, they share the same low noise characteristics with first-order microwave comb signal. The output power of first-to-fourth, fifth-to-thirteenth order carriers are balanced to 10 dB by photoelectric resonance injection. And their side-mode suppression ratios are all better than 40 dB. Furthermore, theoretically, the comb interval can be adjusted to any frequencies by changing the center frequencies of two high-frequency bandpass microwave filters. Therefore, optical-borne multi-carrier microwave signals are generated efficiently and cost-effectively by this tunable optical-borne microwave frequency comb scheme, and the generated low-noise multi-carrier frequency sources meet the demand of an optical-borne microwave wireless communication system.
Image enhancement based on multi-guided filtering
Image enhancement, as a basic image proicessing technique, contains much research content, such as enhance contrast, image restoration, noise reduction, image sharpening, distortion correction, etc. The purpose of image enhancement is to effectively highlight the useful information in target image and suppress noise as well. The conventional image enhancement methods are always powerless to tackle the complicated gradient distributions in natural images, and they are also difficult to retain the information about edges accurately. For improving the status of over-smoothing on boundaries, we propose an image enhancement method based on multi-guided filtering. We first synthetically analyze the property of joint filtering and propose the general image optimization model in which the variable parameter is filter kernel. Different filter kernel in the optimization model above generate different filtering method. That is to say, we can use this model to describe the image enhancement problems. The existing joint filters can be regarded as close form solutions of the optimization model above. Inspired by ensemble theory, we use multiple guided images in joint filtering instead of a single guided image to make full use of structure information. By doing so, the image enhancement based on multi-guided filtering can obtain more accurate filtering results. In order to keep the consistency among the multiple filtering outputs of multi-guided filtering method, we add a regularization term into a general image optimization model. We also take into consideration the consistency of pixels in the same image. The experimental results about the noise reduction and image enhancement show that the image enhancement based on multi-guided filtering can give rise to significant outputs. The peak-signal-to-noise ratio of output image of proposed method is higher than those from the traditional image enhancement methods. Therefore, the image enhancement based on multi-guided filtering can improve the quality of digital images efficiently and effectively. This provides a good precondition for subsequent image processing steps and has a prospect of very wide application.
Effects of interatomic potential on precipitation sequences of medium Al concentration in Ni75AlxV25-x alloys
Bénard-von Kármán vortex street in dipolar Bose-Einstein condensate trapped by square-like potential
Bi-stability in a fifth-order voltage-controlled memristor-based Chua's chaotic circuit
Electronic and optical performances of (Cu, N) codoped TiO2/MoS2 heterostructure photocatalyst: Hybrid DFT (HSE06) study
Two-color photoassociation spectra of ultra-cold Cs (60D5/2)2 Rydberg molecule
The long-range multipole interactions between ultra-cold Rydberg atoms form adiabatic potentials, one of which shows a binding potential that can be used to bind Rydberg-Rydberg molecules. Rydberg-atom molecule, known as macrodimer due to its larger size (~μm), has the properties of the abundant vibrational energy levels and large electric dipole moment and so on. Compared with Rydberg atom, the Rydberg molecule, including Rydberg-ground molecule and Rydberg-Rydberg molecule, is susceptible to manipulate by an external field and possesses potential applications in the weak-signal detection, the quantum gas correlation measurement and the vacuum fluctuation and so on.
In this paper, we investigate a (60D5/2)2 Rydberg macrodimer theoretically and experimentally. In the calculation, we take into account the multipole interaction of a Rydberg-atom pair, including dipole-dipole, dipole-quadrupole, dipole-octupole and quadrupole-quadrupole interaction and so on. The adiabatic potential of 60D5/2 Rydberg-atom pair is obtained by diagonalizing the interaction Hamiltonian on a grid of internuclear separations, R. The potential depth and binding length of the Rydberg molecular potential well are obtained. In experiment, we prepare the ultra-cold Cs (60D5/2)2 Rydberg molecules by a two-color photoassociation method in a cesium ultracold atom trap. The first-color (pulse-A) resonantly excites a seed Rydberg atom A, and the second color (pulse-B) is detuned and resonantly excites the second Rydberg atom B near to the atom A. Both pulse-A and pulse-B are two-photon excitations (852 nm + 510 nm), between which their 852-nm lasers have the same frequency, whereas the 510-nm laser frequency of the pulse-A is set to be resonant with the atomic transition and the frequency of the pulse-B is detuned by using a double-passed acousto-optic modulator. When the pulse-B is detuned to the molecular binding energy, atom-A and-B are bonded, forming an ultra-cold Cs (60D5/2)2 Rydberg molecule. The two-color photoassociation spectra of Rydberg-Rydberg molecules are detected by the field ionization of Rydberg atoms and molecules with a ramped electric field. Molecular spectra are compared with calculated adiabatic molecular potentials, which yields the binding energy and equilibrium internuclear distance. The two-color photoassociation method used in this work has a doubly resonant character that results in the enhanced excitation rate.
Optical transmission characteristics of multi-band triangular-lattice photonic crystal coupling cavity waveguide based on annular microcavity
The light localization characteristics of the near-infrared triangular-lattice photonic crystal annular microcavity are studied theoretically in this paper. The photonic crystal has a lattice constant of a=540 and it is composed of silicon rods each with a radius of r=135 immersed in air background. The two kinds of annular microcavities are obtained by removing 12 silicon rods which are located respectively at a distance of 2a and at a distance of √3a to the central rod. Five resonant wavelengths and the corresponding eigen mode profiles of the microcavity are studied. A coupled resonant optical waveguide is formed by integrating the microcavities with a periodic length of 7a in space. The group velocity of light beam propagation within multiple guiding bands are analyzed by the tight-binding approximation method. The maximum and minimum velocity of 0.0028c and 0.00082c are obtained, where c is the light velocity in vacuum. The light transmittance values and spatial steady distributions of the electric field's amplitude through the structure at several wavelengths within the guiding bands are studied by the finite-difference time-domain method. The results are consistent with that calculated by the plane wave expand method. Interleaving circular microcavities perpendicular to the direction of optical transmission at a lateral distance of 2√3a, the coupling region between the adjacent microcavities is changed, the difference in group velocity between guiding bands apparently decreases and the transmittance values of two frequency bands are enhanced.
Keeping the size of silicon rods unchanged, two kinds of microcavities are constructed by removing the six rods with the distances of 2a and √3a from the center of the central silicon rod, respectively. The resonant wavelengths supported by the above two microcavities are studied. Two coupled-resonant optical waveguides with a periodic length of 7a are proposed. Connecting these two coupled cavity optical waveguides with the W1-typed input/output waveguides, the selecting and sharing function of guiding band are finally achieved for wavelengths within different frequency bands. Keeping the group velocity slowing down, a maximum value of one guiding band reaches 0.00047c.
Effect of detection efficiency on phase sensitivity in quantum-enhanced Mach-Zehnder interferometer
Three kinds of quantum light sources:Fock state, correlated Fock-state and squeezed vacuum state, which serve as the injection end of Mach-Zehnder interferometer (MZI) are investigated. The effect of detection quantum efficiency on the sensitivity of phase measurement in MZI is analyzed by using the intensity difference detection scheme. By analyzing the MZI system, the quantitative relationship between the sensitivity of phase measurement and the detection efficiency is obtained. It is found that the phase sensitivity cannot go beyond the standard quantum limit in any case when the Fock state is injected into interferometer, that is, the Fock state does not realize quantum enhanced measurement (QEM). And the injection of correlated Fock-state or squeezed vacuum state of light can go beyond the standard quantum limit, but the conditions for realizing quantum enhancement are different, quantum enhancement can only be achieved when the detection efficiency is greater than 75% for correlated Fock-state, or the squeezed vacuum state of light is injected into interferometer. There is no limitation of the minimum detection efficiency for realizing quantum enhancement on squeezed vacuum state. In principle, quantum enhancement can be achieved as long as the squeezed vacuum state is injected. The influence of detection efficiency on the phase sensitivity is investigated when the correlated Fock-state and the squeezed vacuum state are injected into the MZI. It is found that the phase sensitivity or quantum enhancement becomes better as the quantum efficiency of the detection system turns higher. And it is the squeezed vacuum state injected into the interferometer that has better quantum enhancement effect than the correlated Fock-state. In this study, the requirements for the detection efficiency for realizing QEM in experiment are given, which is of great significance for studying the QEM, when taking the real experimental system into account. In addition, the conclusions obtained from the MZI model discussed can also be used to analyze the sensitivity of detecting the gravitational wave, it explains that the improvement of detector efficiency can indeed improve the sensitivity to gravitational wave detection, which will play an important role in exploring gravitational waves and understanding the time and space to reveal the mystery of the universe in the future.
Effect of imperfect experimental condition on generation of Schrödinger cat state
Schrödinger cat state is an important non-classical state, and it can be used in quantum teleportation, quantum computation and quantum repeater. Schrödinger cat state is usually obtained experimentally by subtracting one photon from a squeezed-vacuum state. The fidelity between a photon-subtracted squeezed state and a cat state can be very high under suitable parameters. However, the quality of the generated state will be affected by the imperfect experimental conditions. In this paper, the effect of imperfect experimental conditions on the generation of cat state is theoretically calculated and analyzed.
The input squeezed-vacuum field is represented by Weyl characteristic function, which contains the fluctuation variance of the squeezed and amplified noises. The characteristic function of generated state is obtained by using the transmission matrix of beam splitter and the measurement operator of single-photon detector. We acquire the expression of Wigner function of generated state by the Fourier transform of the Weyl characteristic function. The fidelity is calculated by using the formula F=1/π∫d2ζC1(ζ)C|cat->(ζ), where C1(ζ) and C|cat->(ζ) represent Weyl characteristic function of the generated state and the Schrodinger cat state, respectively. The imperfection of the input squeezed state, the imperfection of the single-photon detector and the loss of the balanced homodyne detection are included in our theoretical model. We calculate the Wigner function at the phase-space origin W(0) and the fidelity in terms of different experimental parameters.
The results show that the fidelity and negativity of W(0) decrease with squeezing purity decreasing. A pure squeezed-vacuum state is composed of even photon number states. In the case of impure squeezing, some odd photon number states appear in the photon number distribution. After subtracting one photon from the impure squeezing state, the generated state consists of not only odd photon number state but also even photon states, which degrades the fidelity of the generated state. The lower squeezing purity is required to meet the demand for W(0)<0 under the condition of higher squeezing degree. There is an optimal squeezing degree to maximize the fidelity of generated state with impure squeezing. The use of inefficient on-ff single-photon detector and the loss of the balanced homodyne detection will further reduce the fidelity of the generated state. Under the practical experimental condition:squeezing degree s=-3 dB, the squeezing purity μ=99% and the quantum efficiency of balanced homodyne detection η=98%, the fidelity of generated state can reach 0.88 with using a commercially available on-off single-photon detector. This work can provide theoretical guidance for generating a high-quality Schrödinger cat state.
Improvement on reflectivity of tunable photonic band gap with spontaneous generated coherence
The photonic band gap is a spectral range which cannot propagate in a periodic optical nanostructure, that is, the structure itself has a “forbidden band”. It has been successfully applied to the filters, amplifiers, mixers, etc. As is well known, dynamically tunable photonic band gaps in cold atomic lattices are of great importance in various research fields. However, the photonic band gaps of a traditional photonic crystal are non-tunable because the periodic structure is determined once the photonic crystal is grown. On the other hand, a majority of previous researches focused on improving the reflectivity of photonic band gap, which can only keep approaching to 1. Due to the action of the vacuum of the radiation field, near-degenerate lower level has an additional coherence term, the spontaneously generated coherence term. In this paper, we consider a three-level ∧-type atomic system driven by a strong coherent field, a weak coherent field and an incoherent pump, in which the two ground states are of hyperfine structure. The one-dimensional photonic band gaps are formed by cold atoms trapped in a one-dimensional-ordered optical lattice and this system may create two photonic band gaps (PBGs). The trapped cold atoms have a Gaussian density distribution in each period as determined by the optical potential depth and the average atomic temperature. We investigate in detail how the reflectivities of the two PBGs are influenced by the coherent effect of spontaneously generated coherence. Then, we find that the reflectivities of the two band gaps can be significantly improved by the spontaneously generated coherence. The reflectivities of such two band gaps can be dynamically manipulated by varying the intensity of incoherent driving field and the relative phase between the probe field and the coupling field, which cannot be realized in a conventional ∧-type atomic system. Besides, by adjusting the parameters appropriately, the reflectivities of these two band gaps can be higher than 1, which is because probe field gain stems from the spontaneously generated coherence. In the future, photonic transport properties can be investigated in the three-dimensional atomic lattices and this work is meaningful for the optical routing, photodiode and transistor.
Simulation of multiband imaging technology of large aperture spatial heterodyne imaging spectroscopy
A multiband imaging technology based on large aperture spatial heterodyne imaging spectroscopy (LASHIS) is proposed in this paper. It retains the advantages of high spectral resolution, high stability and high detection sensitivity of LASHIS. In addition, by using the multistage diffraction gratings, several spectral bands can be detected simultaneously in this system, thus the spectral range is broadened. The basic principle of this multiband imaging technology based on LASHIS is described. The difference between optical path differences produced by the Sagnac lateral shearing interferometer and the parallel gratings is calculated. The mathematical expressions, the interferogram calculation procedures, and the spectrum reconstruction method are presented. As a pair of multistage diffraction gratings is introduced into the Sagnac interferometer, the rays of different diffraction orders corresponding to different spectral bands are mixed together in the interferometer. The spectral bands should be separated before they are imaged on the detector. Two separation methods are proposed:introducing a filter array in front of the detector and introducing dichroic mirrors to assign different spectral bands to different detectors. Finally, a design example is given and an optical model is setup in ZEMAX. In this example, a pair of parallel echelon gratings with 316 lines/mm is introduced into the Sagnac interferometer. Two dichroic mirrors and three detectors are used to separate and detect three spectral bands simultaneously. The three spectral ranges are from 529.2 nm to 532.96 nm, from 588 nm to 592.18 nm, and from 661.5 nm to 666.20 nm. The average spectral resolutions are 0.015 nm, 0.016 nm, and 0.018 nm respectively. Two kinds of sources are analyzed:one is a sodium lamp with two emission peaks at 589 nm and 589.6 nm, and the other is a source with three monochromatic wavelengths at 530 nm, 589 nm, and 662 nm. The interferograms of these two sources traced in the optical model are consistent with the theoretical results. The recovered spectra show good agreement with the input spectra. These verified the correctness of the principle and the spectrum reconstruction method. The multiband imaging technology based on LASHIS with the advantages of high spectral resolution, high detection sensitivity, and multiband detection capability, is especially suitable for multiband hyperspectral highstability and high-sensitivity detection, such as the detection of greenhouse gases.
Ultrasonic sacttering unified theory for polycrystal material with grain sizes distribution
The existing unified theory of ultrasonic scattering can model the attenuation and phase velocity in the frequency domain by using the microstructure and mechanical properties of polycrystalline materials. However, this theory does not consider the influence of grain size distribution, thus degrading the calculation accuracy in the forward modeling. A new unified theory, which is mainly corrected by considering the grain size distribution, is developed. First, the second-order Keller approximation and the full-field Green's function are used to calculate the wave equation of inhomogeneous medium and derive the average wave in the medium, respectively. Second, the method of the truncated lognormal distribution is used to describe the grain size distribution and construct the weighted spatial correlation function. Finally, the new unified theory of ultrasonic scattering is established to reveal the influence of grain distribution on ultrasonic scattering.
Using the new unified model, the effects of the grain distribution widening on the ultrasonic scattering while the average grain size is unchanged, are analyzed for the longitudinal wave and the shear wave. The attenuation increases in the Rayleigh scattering region and the geometric scattering region, while there is less attenuation variation in the stochastic scattering region and two adjacent transition regions. The phase velocity varies strongly in the stochastic-geometric transition region, while the variation is relatively small in other scattering zones. Experiments are conducted by using a 304 stainless steel specimen. The results show that when the grain distribution characteristics are considered, the discrepancy between the longitudinal wave attenuation spectrum and experimental results, and that between the phase velocity spectrum and experimental results are reduced by 49% and 64%, respectively; for the shear wave, these discrepancies are reduced by 12% and 4%, respectively.
From all above aspects, the accuracy of the new model is higher than that of the traditional model. The new unified theory proposed in this paper can effectively correct the discrepancy of the attenuation spectrum and phase velocity spectrum caused by the grain size distribution and provide a theoretical basis for inverse problem of grain distribution. Also, the theory can be extended to materials containing elongated grains, macroscopic texture or multiple phases.
Micro-perforated acoustic metamaterial with honeycomb-corrugation hybrid core for broadband low frequency sound absorption
A novel acoustic metamaterial is proposed by making micro-perforations on both the top facesheet and the corrugate plates of sandwich plate with honeycomb-corrugation hybrid core. The hybrid-cored metamaterial is ultra-lightweight, occupies a small volume, and exhibits excellent mechanical properties and good low-frequency sound absorption property. Based on the classical Maa theory of thin plates with micro-perforations, a theoretical model of sound absorption is established for the proposed metamaterial. The method of finite elements is subsequently used to validate the model, showing that their good agreement is achieved. Physical mechanism behind the energy dissipation in each sub-structure of the metamaterial is explored. It is found that the main route of energy dissipation is via viscous effect at the micro-perforation, and thermal dissipation is negligible. The influence of key geometrical parameters, such as upper facesheet thickness, perforation diameter and corrugated plate thickness, on sound absorption is systematically investigated. The present results are helpful for designing multifunctional lightweight materials/structures for simultaneous load-bearing, energy absorption and noise control.
Compressional-shear wave coupling induced by velocity gradient in elastic medium
In the real ocean environment, the compressional and shear wave velocities in an elastic sediment layer vary with depth, leading to the coupling between compressional and shear waves. As the coupling will affect the underwater sound field, in this paper, a typical sound velocity distribution (where the compression wave velocity has an n2 linear distribution and the square of shear wave velocity has a linear distribution) is analyzed. Based on the wave equation in inhomogeneous elastic medium, coupled equations of wavenumber kernels of scalar and vector potential functions are established. Based on the perturbation method, approximate analytical solutions of integration kernels are acquired by successive differentiation. The comparison between theoretical prediction and experimental data, which are from the pressure sensor of ocean-bottom seismometer (OBS) consisting of three orthogonal hydrophones and one hydrophone, located at the bottom of the sea near Qingdao City, shows that the coupling between shear wave and compression wave has little effect on near-field sound propagation, while the prediction of long-range sound propagation needs to consider the influence of eigenvalue change caused by coupling. Theoretic analysis shows that there will be coupling between the two waves only if the gradient, σ, of the square of the shear wave velocity is nonzero. When α, the gradient of the reciprocal of the square of the compression wave velocity, becomes larger, and σ remains unchanged, the simulation results show that the change of the eigenvalue is very small when considering the coupling effect. Thus, transmission loss curves calculated by the coupled and uncoupled algorithm are almost the same. When σ becomes larger while α remains unchanged, the simulation results show that eigenvalues are changed to some extent if considering the coupling effect, and that the difference between transmission loss calculated by the coupled and uncoupled algorithms increases. That means the effect of σ value on coupling is greater than that of α value. In addition, the coupling between the compression wave and shear wave can lead the eigenfunctions and derivative eigenfunctions in the sediment to change. The horizontal displacement and vertical displacement are the Fourier-Bessel integral functions of eigenfunctions and derivative eigenfunctions. So the displacement field of particle in the sediment layer is different in the coupled case from that in the uncoupled cases. By comparing the transmission loss of sound pressure simulated by COMSOL software and that obtained from our proposed method, the correctness of the proposed method is verified. And the calculation time is much shorter than the calculation time by using COMSOL software.
Laser ablation mechanism of contamination on surface of single crystal iron
The laser induced damage in high-power laser system has received much attention in the area of laser engineering. Optical components with contaminants, which are installed in the final optical assembly (FOA), can be severely damaged under the action of extremely high laser energy. So the ultra-high cleanliness inside the high-energy laser system is required for both optical and mechanical components. Research shows that a large part of the metal particulate contaminants inside the device come from the mechanical components. The metal particulate contaminants are produced when mechanical structure surface is damaged under the irradiation of stray light. However the research about the cleanliness inside the device is mostly concentrated on the surfaces of optical components currently. The laser ablation of the mechanical components absorbing contaminants is studied little, so it is quite important to investigate the ablation mechanism of mechanical components under laser irradiation. Due to the presence of contaminants on the surfaces of mechanical components, laser ablation of monocrystalline iron absorbing contaminants is investigated by using molecular dynamics simulation. The ablation process of iron material under laser irradiation is presented. The influences of loading mode and energy density of laser as well as contamination on the surface are analyzed in the ablation process of monocrystalline iron. The results indicate that the surface atoms of monocrystalline iron show different motion states under the violent collision of contaminants atoms after laser loading. Ablated iron can be divided into ablation zone, melting zone and crystal zone according to the variation of the temperature and mass density of the atoms in each region of the ablated material. The atoms in each region show macroscopic characteristics of gaseous, liquid and solid atoms respectively. Iron is damaged more easily when laser energy is instantaneously loaded. Contaminants on the surface of iron can be removed, and iron cannot be damaged when laser energy density is below 0.0064 J/cm2. The result of the analysis shows that the presence of contaminants makes the ablation of iron easier. Different energy loading modes affect the heat transfer mode directly. Monocrystalline iron materials are more likely to be damaged in the mode of adiabatic laser ablation in the case of short laser pulse. Thermal effect can be thought as a dominant factor for the ablation in the case of long laser pulse. The research results of this paper are helpful for providing the theoretical basis for improving the cleanliness of high-power laser system.
Lattice Boltzmann simulations of rising bubble driven by buoyancy in a complex microchannel
The movement of bubbles in the viscous fluid is a typical process in many industrial applications, such as in evaporators of refrigeration cycles, petroleum refining, boiling process, steam bubble rising in boiler tubes and heat exchangers, etc. It is an important research problem in engineering and physics. Although this kind of problem has been extensively studied, their flow details are largely unknown due to the complexity of the interface dynamics, which hinders the understanding of the physical mechanism. In order to further study underlying physics of the issue, a gas bubble rising under buoyancy in a complex micro-channel is investigated by using a gas-liquid two-phase flow lattice Boltzmann method. Initially, the model as well as a classical problem of bubble rising in a smooth vertical microchannel is tested by Laplace law. Then it is then applied to the study of a bubble rising in a complex micro-channel. Specially, the dynamic behaviors of the bubble deformation, breaking up, coalescence, and the following movement in the micro-channel are presented. The rising velocity, terminal velocity and residual mass of the bubble under the influence of micro-channel surface wettability, buoyancy force, obstacle size and the initial position of bubble are examined. The simulation results show that the surface wettability of the obstacle has a significant influence on the bubble motion. For smaller values of the contact angle, the whole bubble passes through the channel with obstacles successfully. For higher values of contact angle, the bubble is attracted to the obstacle surface of the micro-channel in the movement process. In this case, an appreciable deformation of the bubble is observed. After detachment, part of the bubble is attached by the obstacle surface, so only the rest of the bubble can go through the micro-channel, which leads the the bubble residual mass to decrease. Correspondingly, the rising velocity and terminal velocity of the bubble decrease with the wettability of the micro-channel obstacle increasing. On the other hand, with the increase of buoyancy force the detachment and coalescence phenomenon happen easily, and the bubble residual mass and terminal velocity increase logarithmically. Furthermore, as the radius of the obstacle structure increases, the bubble clings more tightly to the obstacle surface when it rises in the micro-channel. And the bubble residual mass decreases first slowly and then rapidly, while the bubble terminal velocity approximately decreases linearly. Finally, the numerical results also show that when the bubble is located at the sidewall initially, the variation trend of bubble rising velocity, terminal velocity and residual mass are consistent with that of initial position placed in the middle of the micro-channel, however all of the corresponding values decrease and the bubble deformation is more significant in the rising process.
Molecular dynamics study of bubble nucleation on a nanoscale
With the rapid development of nanotechnology, nucleate boiling has been widely applied to the thermal management of nanoelectronics, owing to its highly-efficient heat transfer characteristics. Considering the scale effects, such as temperature jump at solid-liquid interface, a further study of nucleation boiling mechanism at a microscopic level is needed. At present, extensive studies have been carried out for providing a significant insight into the formation of nano-bubbles in a nanoscale thermal system, but the effect of heat transfer efficiency affected by the surface wettability on bubble nucleation over solid substrate is rarely available in the literature. Therefore, in this paper, the effect of surface wettability on the initial nucleation process and growth rate of bubbles are investigated and the mechanism of bubble nucleation on a nanoscale is analyzed, by the molecular dynamics simulation. The modified Lennard-Jones potential is used for investigating the solid-liquid interaction. Changing the potential parameters α and β can obtain different surface wettability. The atomic sites, liquid density profiles and bubble nucleus volumes are computed to compare the processes of bubble nucleation on different surfaces. The variation of liquid temperature, potential and absorbed heat flux with heating time are evaluated to explore the mechanism of bubble nucleation. The simulation results show that the surface wettability influences the bubble nucleation and heat transfer at liquid-solid interface significantly. On the one hand, the bubble nucleation is promoted by properly increasing the liquid-solid interaction, which is distinctly different from the existing classical theory related to nano-bubble preferably formed on a hydrophobic surface. This is because the thermal resistance of the solid-liquid interface on a nanoscale cannot be neglected. The interface thermal resistance will decrease with the increase of wettability. Therefore, the heat transfer efficiency is higher for a stronger liquid-solid interaction so that the liquid over the hot wall obtains more energy to make bubble nucleus generated earlier. On the other hand, the surface wettability also influences the bubble growth rate. The stronger the liquid-solid interaction, the faster the bubble grows. When the volume of bubble reaches a certain value, a vapor film is formed on the substrate, leading to film boiling. Furthermore, it also illustrates that initial heat flux increases with time. In this stage, the heat flux curve shows two kinds of slopes, corresponding to the occurrence of evaporation and bubble nucleation, respectively. Then, after a certain time, the heat flux profile presents a declining trend, indicating a change into film boiling.
Observation of hydrodynamic phenomena of plasma interaction in hohlraums
In indirect-drive inertial confinement fusion (ICF), laser beams are injected into a high-Z hohlraum and the laser energy is converted into intense X-ray radiation, which ablates a capsule located in the center of the hohlraum, and thus making it implode. To achieve high implosion efficiency, it is required that the hohlraum inner wall plasma movement, which will block further laser injection through the laser entrance hole (LEH), be suppressed. Evolution of hohlraum radiation nonuniformity caused by the plasma movement will result in implosion asymmetry which will prevent the ignition from happening. Therefore it is very important to study the hydrodynamic movement of high-Z plasma in ICF experiment.
In ICF hohlraum, various plasmas of laser spots, corona, radiation ablation and jets move in different ways driven by laser ablation and X-ray radiation ablation, which is hard to observe and study. An X-ray dual spectral band time-resolved imaging method is developed to clearly observe the motion of various plasmas in hohlraum. Based on the time-resolved X-ray framing camera, using the typical gold plasma emission spectrum, the gold microstrip MCP response spectrum, and the 1.5 μm Al or 3 μm Ti filter transmittance spectrum, the two narrow-band X-ray peaks at 0.8 keV and 2.5 keV are highlighted. The 0.8 keV X-ray shows the Planck spectrum of gold plasma, and 2.5 keV X-ray indicates the M-band of gold plasma.
In the vacuum hohlraum, jets are observed clearly, which are verified to be 4 times the sound speed experimentally. The generation mechanism of gold plasma jets in the ICF hohlraum is mainly due to collision rather than magnetic field, because it is estimated that thermal pressure is much bigger than magnetic pressure. In the gas-filled hohlraum, low-Z C5H12 gas can effectively eliminate high-Z gold jets and suppress the high-Z gold coronal plasma movement. The interface between the low-Z and high-Z substance is observed clearly, and gold plasma is accumulated obviously in the later period at the interface. Moreover, spike and filamentous structure occur at the interface between the two substances, which is probably caused by the hydrodynamic instability. The 0.8 keV rather than 2.5 keV X-ray is observed around inner wall, which originates from the low-temperature plasma driven by radiation ablation and is predicted by simulation code. Furthermore, the pressure balance between the two substances and the density steepness at the interface are also analyzed.