Search

In the archetypal lithium-rich cathode compound Li1.2Ni0.13Co0.13Mn0.54O2, a major part of the capacity is contributed from the anionic (O2−/−) reversible redox couple and is accompanied by the transition metal ions migration with a detrimental voltage fade. A better understanding of these mutual interactions demands for a new model that helps to unfold the occurrences of voltage fade in lithium-rich system. Here we present an alternative approach, a cationic reaction dominated lithium-rich material Li1.083Ni0.333Co0.083Mn0.5O2, with reduced lithium content to modify the initial band structure, hence ~80% and ~20% of capacity are contributed by cationic and anionic redox couples, individually. A 400 cycle test with 85% capacity retention depicts the capacity loss mainly arises from the metal ions dissolution. The voltage fade usually from Mn4+/Mn3+ and/or On−/O2− reduction at around 2.5/3.0 V seen in the typical lithium-rich materials is completely eliminated in the cationic dominated cathode material.

By using multidimensional particle-in-cell simulations, we demonstrate that the coherent interaction of two relativistic laser pulses in plasmas offers a degree of freedom for manipulating the electron acceleration process. It is shown that by adjusting the relative phase between two intense laser pulses, their interaction features in plasmas are well controlled, which subsequently induce different electron acceleration processes. In this case, the properties of the accelerated electron beam, including the directionality, the maximum energy, and the conversion efficiency, can be controlled though the manipulation of the energy redistribution of these intense laser pulses. In particular, compared to the cases with a single laser pulse or two out-of-phase laser pulses, two in-phase laser pulses fuse into a much stronger laser pulse, which significantly prolongs the electron acceleration length in plasmas and leads to the generation of energetic electron beam with much higher conversion efficiency.

The dynamics of electron injection in the direct laser acceleration (DLA) regime is investigated using three-dimensional particle-in-cell simulations and theoretical analyses. It is shown that, as an ultraintense laser pulse propagates into a near-critical density or relativistically transparent plasma, the longitudinal charge-separation electric field excites ion density spikes, which modulate the local electric field. The corresponding electric field acts as a series of potential wells to guide the electrons on the edge of the plasma channel into its center where the DLA can take place. On the other hand, an azimuthal magnetic field is self-generated, and it can deflect the injected electrons from the intense laser-field region. Understanding these physical processes paves the way for further optimizing the properties of direct-laser accelerated electron beams and the associated x- and $\ensuremath{\gamma}$-ray sources.

Self-shaping and propagation of intense laser beams of different radial profiles in plasmas is investigated. It is shown that when a relativistic elliptically Gaussian beam propagates through an underdense plasma, its radial profile will self-organize into a circularly symmetric self-similar smooth configuration. Such self-similar propagation can be attributed to a soliton-like structure of the laser pulse. The anisotropic electron distribution results in a circular electric field that redistributes the electrons and modulates the laser pulse to a circular radial shape.

The interactive dynamics of two relativistic laser beams copropagating in underdense plasmas is studied using a coupled model equation for the relativistic laser propagation. It is shown that the relative phase difference between the two laser pulses plays a significant role in their interaction processes. When the relative phase varies, the two laser beams display different features, such as attraction, repulsion, and energy shift. In particular, energy flow from the phase-advanced beam to the spot domain of the phase-delayed beam is observed when the relative phase difference is between zero and $\ensuremath{\pi}$. When the relative phase is larger than $\ensuremath{\pi}/2$, repulsion is dominant and the interaction gradually becomes weak. When the relative phase difference is smaller than $\ensuremath{\pi}/2$, attraction becomes dominant and, as the phase difference decreases, the phase-advanced beam shifts most of its energy into the spot domain of the phase-delayed beam. These conclusions are verified by our three-dimensional particle-in-cell simulations. This provides an efficient way to manipulate the energy distribution of relativistically intense laser pulses in plasmas by adjusting their relative phase.

In this study, a ray-mimic underwater robot was designed and implemented. The robot is propelled by its pectoral fins. The robot is equipped with IR sensors and an image sensor for obstacle avoidance and object racking, respectively. Its specific gravity is adjusted by two water storage tanks and its center of gravity is controlled by two screw-rods. The image data is processed on board using a DSP chip installed in the robot and motion control is achieved by PIC16F877 micro-controllers. Obtained data are also sent to its user via wireless communications so that real time performance can be accomplished. Experimental results show that the robot achieves its designed functions successfully.

The hosing processes of a relativistic laser pulse, electron acceleration, and betatron radiation in a parabolic plasma channel are investigated in the direct laser acceleration regime. It is shown that the laser hosing instability would result in the generation of a randomly directed off-axis electron beam and radiation source with a large divergence angle. While employing a preformed parabolic plasma channel, the restoring force provided by the plasma channel would correct the perturbed laser wave front and thus suppress the hosing instability. As a result, the accelerated electron beam and the emitted photons are well guided and concentrated along the channel axis. The employment of a proper plasma density channel can stably guide the relativistically intense laser pulse and greatly improve the properties of the electron beam and radiation source. This scheme is of great interest for the generation of high quality electron beams and radiation sources.

It was shown that in the interactions of ultra-intense circularly polarized laser pulse with the near-critical plasmas, the angular momentum can be transferred efficiently from the laser beam to electrons through the resonance acceleration process. The transferred angular momentum increases almost linearly with the acceleration time t_{a} when the electrons are resonantly accelerated by the laser field. In addition, it is shown analytically that the averaged angular momentum of electrons is proportional to the laser amplitude a_{L}, and the total angular momentum of the accelerated electron beam is proportional to the square of the laser amplitude a_{L}^{2} for a fixed parameter of n_{e}/n_{c}a_{L}. These results are verified by three-dimensional particle-in-cell simulations. This regime provides an efficient and compact alternative for the production of high angular momentum electron beams, which may have many potential applications in condensed-matter spectroscopy, new electron microscopes, and bright x-ray vortex generation.

Energetic electron acceleration processes in a plasma hollow tube irradiated by an ultraintense laser pulse are investigated. It is found that the longitudinal component of the laser field is much enhanced when a linear polarized Gaussian laser pulse propagates through the plasma tube. This longitudinal field is of $\ensuremath{\pi}/2$ phase shift relative to the transverse electric field and has a $\ensuremath{\pi}$ phase interval between its upper and lower parts. The electrons in the plasma tube are first pulled out by the transverse electric field and then trapped by the longitudinal electric field. The trapped electrons can further be accelerated to higher energy in the presence of the longitudinal electric field. This acceleration mechanism is clearly illustrated by both particle-in-cell simulations and single particle modelings.

Dimensional effects in particle-in-cell (PIC) simulation of target normal sheath acceleration (TNSA) of protons are considered. As the spatial divergence of the laser-accelerated hot sheath electrons and the resulting space-charge electric field on the target backside depend on the spatial dimension, the maximum energy of the accelerated protons obtained from three-dimensional (3D) simulations is usually much less that from two-dimensional (2D) simulations. By closely examining the TNSA of protons in 2D and 3D PIC simulations, we deduce an empirical ratio between the maximum proton energies obtained from the 2D and 3D simulations. This ratio may be useful for estimating the maximum proton energy in realistic (3D) TNSA from the results of the corresponding 2D simulation. It is also shown that the scaling law also applies to TNSA from structured targets., Comment: 20 pages, 7 figures, 3 tables, 45 references

In this paper, we demonstrate the use of photonic crystal (PC) directional couplers to separate light of wavelengths at 1.31 and 1.55 m. The PC structure consists of InAlGaAs nanorods arranged in hexagonal lattice. The simulation of our device is implemented by the finite-difference time-domain method. The devices are fabricated by e-beam lithography and conventional photolithography. We use the strong inverse method (SIM) of e-beam lithography to make the pattern smoother. The measurement results confirm that 1.31/1.55-m wavelength splitter can be realized in PC structures formed by nanorods.

It is shown that the filamentation instability of relativistically intense laser pulses in plasmas can be mitigated in the case where the laser beam has an elliptically distributed beam profile. A high-power elliptical Gaussian laser beam would break up into a regular filamentation pattern-in contrast to the randomly distributed filaments of a circularly distributed laser beam-and much more laser power would be concentrated in the central region. A highly elliptically distributed laser beam experiences anisotropic self-focusing and diffraction processes in the plasma channel ensuring that the unstable diffractive rings of the circular case cannot be produced. The azimuthal modulational instability is thereby suppressed. These findings are verified by three-dimensional particle-in-cell simulations.

Silver(I) N-heterocyclic carbene complexes (Ag(I)−NHCs) with long N-alkyl side chains formulated as (a) ion pairs, (b) halide-bridged neutral complexes, and (c) disilver tetrahalide anion bridged tetranuclear complexes were obtained by the reaction of benzimidazolium and imidazolium halide salts with Ag2O. Interestingly, when excess Ag2O was used, Ag nanoparticles were formed along with Ag(I)−NHCs. Crystal structures of Ag(I)−NHC complexes including both long and short N-alkyl chains were determined. The diversity of the solid-state structures depended on the halide ions, chain length, carbene cores, and crystallization conditions. The Ag(I)−NHCs of long alkyl chains did not show liquid crystal properties but did so when mixed with their corresponding imidazolium salts. These organic−inorganic hybrids were good single-source precursors for silver nanomaterials.

We use three-dimensional particle-in-cell simulations to demonstrate that a plasma density channel can stably guide the petawatt laser pulse in near critical plasmas. In this regime, a directed, collimated, and micro-sized gamma photon beam is emitted by the direct-laser accelerated electrons along the channel axis. While in the case without the plasma density channel, the laser tilting behavior leads to the generation of randomly deflected gamma photon beams with a large divergence angle and transverse source size. In addition, in the plasma density channels, the divergence angle of the gamma photon beams can be much reduced by using a smaller value of n0/a0nc. The energy conversion efficiency can also be improved by increasing the laser power or the plasma density. This regime provides an efficient and compact approach for the production of high quality gamma photon beams.

This paper presents the design, fabrication, and test results of a three-stage 155-GHz monolithic low-noise amplifier (LNA) fabricated with the 0.1-/spl mu/m pseudomorphic (PM) InAlAs-InGaAs-InP HEMT technology. With this amplifier in a test fixture, a small-signal gain of 12 dB was measured at 155 GHz, and more than 10-dB gain from 151 to 156 GHz. When the amplifier was biased for a low noise figure (NF), an NF of 5.1 dB with an associated gain of 10.1 dB was achieved at 155 GHz. All the results above are referred to the monolithic millimetre-wave integrated circuit (MIMIC) chip with the input and output waveguide-to-microstrip line transition losses corrected.

Plasma defocusing and higher-order Kerr effects on multiple filamentation and pattern formation of ultrashort laser pulse propagation in air are investigated. Linear analyses and numerical results show that these two saturable nonlinear effects can destroy the coherent evolution of the laser field, and small-scale spatial turbulent structures rapidly appear. For the two-dimensional case, numerical simulations show that blow-up-like solutions, spatial chaos, and pseudorecurrence can appear at higher laser intensities if only plasma defocusing is included. These complex patterns result from the stochastic evolution of the higher- or shorter-wavelength modes of the laser light spectrum. From the viewpoint of nonlinear dynamics, filamentation can be attributed to the modulational instability of these spatial incoherent localized structures. Furthermore, filament patterns associated with multiphoton ionization of the air molecules with and without higher-order Kerr effects are compared.

Laser-driven ion acceleration potentially offers a compact, cost-effective alternative to conventional accelerators for scientific, technological, and health-care applications. A novel scheme for heavy ion acceleration in near-critical plasmas via staged shock waves driven by intense laser pulses is proposed, where, in front of the heavy ion target, a light ion layer is used for launching a high-speed electrostatic shock wave. This shock is enhanced at the interface before it is transmitted into the heavy ion plasmas. Monoenergetic heavy ion beam with much higher energy can be generated by the transmitted shock, comparing to the shock wave acceleration in pure heavy ion target. Two-dimensional particle-in-cell simulations show that quasi-monoenergetic ion beams with peak energy 168 MeV and considerable particle number are obtained by laser pulses at intensity of in such staged shock wave acceleration scheme. Similarly a high-quality ion beam with a well-defined peak with energy 250 MeV and spread can also be obtained in this scheme.

Ion acceleration in near-critical plasmas driven by intense laser pulses is investigated theoretically and numerically. A theoretical model has been given for clarification of the ion acceleration dynamics in relation to different laser and target parameters. Two distinct regimes have been identified, where ions are accelerated by, respectively, the laser-induced shock wave in the weakly driven regime (comparatively low laser intensity) and the nonlinear solitary wave in the strongly driven regime (comparatively high laser intensity). Two-dimensional particle-in-cell simulations show that quasi-monoenergetic proton beams with a peak energy of 94.6 MeV and an energy spread 15.8% are obtained by intense laser pulses at intensity I0 = 3 × 1020 W/cm2 and pulse duration τ = 0.5 ps in the strongly driven regime, which is more advantageous than that got in the weakly driven regime. In addition, 233 MeV proton beams with narrow spread can be produced by extending τ to 1.0 ps in the strongly driven regime.

Laser driven proton acceleration is proposed to be greatly enhanced by using a cone-tube target, which can be easily manufactured by current 3D-print technology. It is observed that energetic electron bunches are generated along the tube and accelerated to a much higher temperature by the combination of ponderomotive force and longitudinal electric field which is induced by the optical confinement of the laser field. As a result, a localized and enhanced sheath field is produced at the rear of the target and the maximum proton energy is about three-fold increased based on the two-dimentional particle-in-cell simulation results. It is demonstrated that by employing this advanced target scheme, the scaling of the proton energy versus the laser intensity is much beyond the normal target normal sheath acceleration (TNSA) case.

We present the highest frequency performance of any solid-state monolithic microwave integrated circuit (MMIC) amplifier. A 2-stage 80-nm gate length InGaAs/InAlAs/InP HEMT MMIC balanced amplifier has a measured on-wafer peak gain of 7.2 dB at 190 GHz and greater than 5 dB gain from 170 to 194 GHz. The circuit was fabricated using a pseudomorphic 20-nm In/sub 0.65/Ga/sub 0.35/As channel HEMT structure grown on a 3-in InP substrate by MBE. Based on the measured circuit results, the intrinsic exhibits an F/sub max/ greater than 400 GHz.

We have developed W-band high-power monolithic microwave integrated circuit (MMIC) amplifiers using passivated 0.15 /spl mu/m gate length InGaAs/InAlAs/InP HEMT's. A 640 /spl mu/m single-stage MMIC amplifier demonstrated an output power of 130 mW with 13% power-added efficiency and 4 dB associated gain at 94 GHz. This result represents the best output power to date measured from a single fixtured InP-based HEMT MMIC at this frequency.

This paper presents the results of the first W-band InP-based high electron mobility transistor-heterojunction bipolar transistor (HEMT-HBT) monolithic microwave integrated circuit (MMIC). The InP-based HBT and HEMT devices are monolithically integrated using selective molecular beam epitaxy (MBE). The amplifier demonstrates the highest frequency performance MMIC so far obtained with this technology. A single-stage HBT op amp current regulator is integrated with a single-stage HEMT amplifier in order to regulate and self-bias the MEMT device over process, temperature, and age variations. The HBT regulates the HEMT bias to within 3% of the bias current while consuming only a small fraction of the total dc power. The HEMT W-band amplifier achieves a radio frequency (RF) gain of 8.25 and 5.9 dB at 77 and 94 GHz, respectively. A minimum noise figure of 4.2 dB was also recorded at 93.5 GHz. The RF performance achieved from the HEMT amplifier using the InP-based HEMT-HBT integrated technology is comparable to that of InP-based single-technology HEMT performance.

It is shown that the intense quasistatic electric and magnetic fields self-generated near the axis of the laser-driven channel in an appropriately profiled preplasma during ultraintense laser interaction with a thin target can create dense relativistic electron bunches. The latter easily penetrate through the target and can greatly enhance the sheath field at the rear, resulting in significant increase in the laser-to-ion energy conversion efficiency and the maximum energy of the target normal sheath accelerated ions. Particle-in-cell simulations show that with a hydrogen targets a proton beam of peak energy ∼38 MeV and energy conversion efficiency ≥6.5% can be produced by a linearly polarized 5 × 1019 W/cm2 laser. An analytical model is also proposed and its results agree well with those of the simulations.

Propagation characteristic of femtosecond terawatt laser pulses in N2 gas with higher-order Kerr effect (HOKE) is investigated. Theoretical analysis shows that HOKE acting as Hamiltonian perturbation can destroy the coherent structure of a laser field and result in the appearance of incoherent patterns. Numerical simulations show that in this case two different types of complex structures can appear. It is found that the high-order focusing terms in HOKE can cause continuous phase shift and off-axis evolution of the laser fields when irregular homoclinic orbit crossings of the field in phase space take place. As the laser propagates, small-scale spatial structures rapidly appear and the evolution of the laser field becomes chaotic. The two complex patterns can switch between each other quasi-periodically. Numerical results show that the two complex patterns are associated with the stochastic evolution of the energy contained in the higher-order shorter-wavelength Fourier modes. Such complex patterns, associated with small-scale filaments, may be typical for laser propagation in a HOKE medium.

The reproducibility ofTc in superconductive Bi2Sr2CaCu2O8 is known to be very poor. In our study, the reproducibility was found to depend greatly on preparation conditions. DTA, TGA and powder X-ray diffraction methods were used to study the Bi2Sr2CaCu2O8 superconductors prepared by different routes. Resistivity and diamagnetic susceptibility were taken as the measure of superconductivity. It was found that those superconductors prepared from the oxides/carbonates (one-step) process resulted in scatteredTc data which were less reproducible. The two-step synthesis from precalcined precursors of Bi-Sr-O and Sr-Ca-Cu-0 containing mixtures, resulted in a much improved reproducibility with the predominantTc = 80 K superconducting phase (more than 95%). It was also found that the amount ofTc = 115 K phase tends to decrease after repeated pulverization and sintering, leading to a single 80 K phase state. Compaction of the pulverized powder at a pressure > 4.5 ton cm−2 was found to induce a preferred (00L) orientation during sintering.