Field-free molecular orientation steered by combination of super-Gaussian and THz half-cycle laser pulses

The molecular orientation created by laser fields is important for steering chemical reactions. In this paper, we propose a theoretical scheme to manipulate field-free molecular orientation by using an intense super-Gaussian laser pulse and a time-delayed terahertz half-cycle pulse(THz HCP). It is shown that the degree of field-free orientation can be doubled by the combined pulse with respect to the super-Gaussian pulse or THz HCP alone. Moreover, different laser intensities, carrier envelop phases, shape parameters, and time delays have great influence on the positive and negative orientations, with other conditions unchanged. Furthermore, it is indicated that the maximum degree and direction of molecular orientation can be precisely controlled by half of the duration of the super-Gaussian pulse. Finally, by adjusting the laser parameters of the super-Gaussian laser pulse and THz HCP, the optimal results of negative orientation and corresponding rotational populations are obtained at different temperatures of the molecular system.

Effect of Duty Cycle on Properties of Pulse-electro-deposited SnS:Ag Thin Films

SnS∶Ag thin films were deposited on ITO glasses by pulse electro-deposition. By studying the effect of duty cycle on the properties of SnS∶Ag thin films, the optimum off-time(toff) is obtained to be 5 s, namely, the optimal duty cycle is about 67%. The primary phase of SnS∶Ag films deposited on optimum parameters condition is SnS compound with good crystallization, and the films prefer to grow towards (111) plane. The films are dense, smooth and uniform with good microstructure, and the grains in the films are densely packed together, and their direct bandgap is about 1.40 eV. In addition, the bandgap of the films first rises and then drops with the increase of the duty cycle.

Broadband Terahertz Wave Generation from Monolayer Graphene Driven by Few-Cycle Laser Pulse

We theoretically investigate the characteristics of terahertz(THz) radiation from monolayer graphene exposed to normal incident few-cycle laser pulses, by numerically solving the extended semiconductor Bloch equations. Our simulations show that the THz spectra in low frequency regions are highly dependent on the carrier envelope phase(CEP) of driving laser pulses. Using an optimal CEP of few-cycle laser pulses, we can obtain broadband strong THz waves, due to the symmetry breaking of the laser-graphene system. Our results also show that the strength of the THz spectra depend on both the intensity and central wavelength of the laser pulses. The intensity dependence of the THz wave can be described by the excitation rate of graphene, while wavelength dependence can be traced back to the band velocity and the population of graphene. We find that a near single-cycle THz pulse can be obtained from graphene driven by a mid-infrared laser pulse.

Electron acceleration by tightly focused radially polarized few-cycle laser pulses

Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.

Dynamic Alignment of D2 Enhanced by Two Few-cycle Pulses

二很少周期脉搏导致的 D 2 的动态排列被基于一个僵硬转子模型数字地解决时间依赖者 Schrodinger 方程调查。只要在二之间的时间延期搏动， D 2的排列能被二很少周期脉搏与由单个很少周期脉搏可完成的水平相比提高的结果表演适当地被选择，并且二激光的脉搏持续时间在 D 2分子的排列过程起一个重要作用。

Accurate determination of the absolute phase and temporal-pulse phase of few-cycle laser pulses

A Fourier analysis method is used to accurately determine not only the absolute phase but also the tempuralpulse phase of an isolated few-cycle （chirped） laser pulse. This method is independent of the pulse shape and can fully characterize the light wave even though only a few samples per optical cycle are available. It pavas the way for investigating the absolute phase-dependent extreme nonlinear optics, and the evolutions of the absolute phase and the temporal-pulse phase of few-cycle laser pulses.

Intensity-dependent asymmetric photoionization in few-cycle laser pulses

The asymmetric photoionization of atoms irradiated by intense, few-cycle laser pulses is studied numerically. The results show that the pulse intensity affects the asymmetric photoionization in three aspects. First, at higher intensities, the asymmetry becomes distinctive for few-cycle pulses of longer durations. Second, as the laser intensity increases, the maximal asymmetry first decreases then increases after it has reached a minimal value. Last, the value of the carrier-envelope phase corresponding to the maximal asymmetry varies with the pulse intensity. This study reveals that the increasing of pulse intensity is helpful for observing the asymmetric photoionization.

Generation of few-cycle laser pulses:Comparison between atomic and molecular gases in a hollow-core fiber

We numerically study the pulse compression approaches based on atomic or molecular gases in a hollow-core fiber.From the perspective of self-phase modulation（SPM）, we give the extensive study of the SPM influence on a probe pulse with molecular phase modulation（MPM） effect. By comparing the two compression methods, we summarize their advantages and drawbacks to obtain the few-cycle pulses with micro- or millijoule energies. It is also shown that the double pump-probe approach can be used as a tunable dual-color source by adjusting the time delay between pump and probe pulses to proper values.

The pulsed microwave damage trend of a bipolar transistor as a function of pulse parameters

In the present paper we conduct a theoretical study of the thermal accumulation effect of a typical bipolar transistor caused by high power pulsed microwaves（HPMs）,and investigate the thermal accumulation effect as a function of pulse repetition frequency（PRF） and duty cycle.A study of the damage mechanism of the device is carried out from the variation analysis of the distribution of the electric field and the current density.The result shows that the accumulation temperature increases with PRF increasing and the threshold for the transistor is about 2 kHz.The response of the peak temperature induced by the injected single pulses indicates that the falling time is much longer than the rising time.Adopting the fitting method,the relationship between the peak temperature and the time during the rising edge and that between the peak temperature and the time during the falling edge are obtained.Moreover,the accumulation temperature decreases with duty cycle increasing for a certain mean power.

Modeling for V–O_2 reactive sputtering process using a pulsed power supply

In this article, we present a time-dependent model that enables us to describe the dynamic behavior of pulsed DC reactive sputtering and predict the film compositions of VOx prepared by this process. In this modeling, the average current J is replaced by a new parameter of Jeff. Meanwhile, the four species states of V, V2O3, VO2, and V2O5 in the vanadium oxide films are taken into consideration. Based on this work, the influences of the oxygen gas supply and the pulsed power parameters including the duty cycle and frequency on film compositions are discussed. The model suggests that the time to reach process equilibrium may vary substantially depending on these parameters. It is also indicated that the compositions of VOx films are quite sensitive to both the reactive gas supply and the duty cycle when the power supply works in pulse mode. The ＇steady-state＇ balance values obtained by these simulations show excellent agreement with the experimental data, which indicates that the experimentally obtained dynamic behavior of the film composition can be explained by this time-dependent modeling for pulsed DC reactive sputtering process. Moreover, the computer simulation results indicate that the curves will essentially yield oscillations around the average value of the film compositions with lower pulse frequency.

An improved constant volume cycle model for performance analysis and shape design of PDRE nozzle

An improved constant volume cycle （CVC） model is developed to analyze the nozzle effects on the thrust and specific impulse of pulse detonation rocket engine （PDRE）. Theoretically, this model shows that the thrust coefficient/specific impulse of PDRE is a function of the nozzle contraction/expansion ratio and the operating frequency. The relationship between the nozzle contraction ratio and the operation frequency is obtained by introducing the duty ratio, by which the key problem in the theoretical design can be solved. Therefore, the performance of PDRE can be accessed to guide the preliminary shape design of nozzle conveniently and quickly. The higher the operating frequency of PDRE is, the smaller the nozzle contraction ratio should be. Besides, the lower the ambient pressure is, the larger the expansion ratio of the nozzle should be. When the ambient pressure is 1.013 × 105 Pa, the optimal expansion ratio will be less than 2.26. When the ambient pressure is reduced to vacuum, the extremum of the optimal thrust coefficient is 2.236 9, and the extremum of the specific impulse is 321.01 s. The results of the improved model are verified by numerical simulation.

Optimal Periodic Pulse Jamming Signal Design for QPSK Systems

The problem of optimal periodic pulse jamming design for a quadrature phase shift keying（QPSK）communication system is investigated.First a closed-form bit-error-rate（BER）of QPSK system under the jamming of pulse signal is derived.Then the asymptotic performance of the derived BER is analyzed as the signal-to-noise ratio（SNR）grows to infinity.In order to maximize the BER of the QPSK system,the optimal parameters of periodic pulse jamming signal,including the duty cycle and signal-tojamming power ratio（SJR）,are found out.Numerical results are presented to verify our analytical results and the optimality of our design.

Exact Evolution of Ultrashort Hollow-Airy Pulses in the Spatiotemporal Domain

An exact analytic expression for an ultrashort hollow-Airy wave packet is presented beyond the slowly varying envelope approximation. The hollow-Airy wave packet combines the hollow-Gaussian beam in the spatial domain and the Airy pulse in the temporal domain. The spatiotemporal propagation dynamics of the ultrashort hollow- Airy pulse are analyzed by the numerical simulations. During the propagation in free space, the spatial intensity profile evolves from hollow-Gaussian to Gaussian shape; the temporal intensity profile retains Airy shape over several Rayleigh ranges. The acceleration property of the ultrashort Airy pulse is also demonstrated.

A Scaling Law of Photoionization in Ultrashort Pulses

By means of the numerical solution of time-dependent Schr6dinger equation, we verify a scaling law of photoionization in ultrashort pulses. We find that for a given carrier-envelope phase and duration of the pulse, identical photoionizations are obtained provided that when the central frequency of the pulse is enlarged by k times, the atomic binding potential is enlarged by k times, and the laser intensity is enlarged by ka times. The scaling law allows us to reach a significant control over direction of photoemission and offers exciting prospects of reaching similar physical processes in different interacting systems which constitutes a novel kind of coherent control.

Pulse Compression by Filamentation in Argon with an Acoustic Optical Programmable Dispersive Filter for Predispersion Compensation

We have experimentally demonstrated pulses 0.4 mJ in duration smaller than 12 fs with an excellent spatial beam profile by self-guided propagation in argon. The original 52 fs pulses from the chirped pulsed amplification laser system are first precompressed to 32 fs by inserting an acoustic optical programmable dispersive filter instrument into the laser system for spectrum reshaping and dispersion compensation, and the pulse spectrum is subsequently broadened by filamentation in an argon cell. By using chirped mirrors for post-dispersion compensation, the pulses are successfully compressed to smaller than 12 fs.

High-order harmonic generation in sub-one-cycle regime

We theoretically investigate the high-order harmonic generation from the hydrogen atom driven by the laser pulses with the durations less than the optical cycle. It is found that the switching term of the laser field may have an obvious influence on the cutoff, intensity or plateau structure of the high-order harmonic spectrum. Generally speaking, the switching term can shorten the cutoff of the high-order harmonic spectrum for a relatively longer pulse and extend the cutoff for a relatively shorter pulse.

电流体喷印脉冲电压参数配置对沉积液滴体积的影响

为了精准控制电流体喷印液滴的最小体积并提高电流体喷印加工的分辨率,研究了电流体喷印的脉冲电压参数配置规律。首先,基于流体力学和电动力学基本原理,分析了喷印过程中泰勒锥的受力情况,并基于此建立了锥射流的体积分数仿真模型,探究了初始脉冲电压参数窗口。然后,搭建了电流体喷印平台,通过实验分析发现沉积液滴体积随脉冲电压幅值增大而减小,随脉冲频率的变化不明显,随着脉冲占空比增大,液滴体积先增大后减小。对比了参数配置前后液滴阵列的沉积液滴体积,参数配置前沉积液滴体积为40.89~45.25 nL,参数配置后平均液滴沉积体积为39.54 nL,验证了脉冲电压参数配置规律的有效性,可为节约生产过程中的材料和人工成本,进一步提高微纳电子产品的使用寿命提供参考。

不同占空比的脉冲光源对工业大麻生长和生理生化的影响

试验研究室外栽培条件下,利用不同占空比的脉冲光源对工业大麻进行补光,探究其对工业大麻生长发育、生理生化以及主要大麻素含量的影响。结果表明:占空比100%(LED1)和占空比70%(LED2)的光源促进了工业大麻株高、茎粗、SPAD值以及花叶干重和鲜重的增加,但LED2与不补光(CK)差异不显著,而占空比50%(LED3)光源显著降低了工业大麻的株高、茎粗、SPAD值以及花叶干重和鲜重;LED1、LED2和LED3提高了工业大麻叶片中可溶性糖、可溶性淀粉以及游离氨基酸的含量,LED1与CK差异显著,LED2和LED3与CK差异不显著;LED1和LED2显著降低了超氧化物歧化酶(SOD)、过氧化物酶(POD)和过氧化氢酶(CTA)活性,而LED3显著促进抗氧化物酶(SOD、POD和CTA)的活性;LED1显著降低了四氢大麻酚(THC)和大麻二酚(CBD)的含量,而LED1、LED2和LED3显著提高了大麻萜酚(CBG)的含量。LED1和LED2光源均可以促进工业大麻的生长发育,增加可溶性渗透调节物质含量,降低抗氧化物活性,增加CBG含量,为今后利用节能(占空比)脉冲光源提高工业大麻品质和产量提供理论参考。

脉冲束流下车载加速器中子源靶传热特性

为了研究不同脉冲束流频率下车载加速器中子源靶的传热特性及其热动态响应,通过调整脉冲束流的频率或脉宽对背部冷却微通道靶的传热性能进行模拟,并通过分析靶内部结构的热沉积分布,探究车载加速器中子源靶的热负载上限以及制约束流的因素。选用能量为2.5 MeV、峰值流强为10 mA的束流开展模拟,结果表明:当束流占空比保持在3%不变时,靶的最高温度呈周期波动,且存在上下包络;随着束流频率的上升,每个脉冲周期内靶的上包络温度逐渐下降,这是由于每个脉冲期间的热冲击下降所导致的;频率上升时,每个周期内加热时间与冷却时间同时缩短,导致其下包络温度在最开始增大后基本保持不变;靶的最高温度随着束流频率增加出现从锂层到中间层钽层的转移。该研究对于车载加速器中子源的束流参数选择和靶冷却设计具有一定的参考价值。

中重度牙周炎患者维护期应用PDCA循环管理模式联合脉冲式冲牙器的临床疗效

目的:观察中重度牙周炎患者维护期应用质量控制(PDCA)循环管理模式联合脉冲式冲牙器的临床效果,为PDCA循环管理模式在牙周炎患者中的应用提供理论依据。方法:通过预定的试验纳入标准、排除标准和剔除标准选取50例中重度牙周炎患者,随机分为实验组(n=25)和对照组(n=25),实验组患者通过脉冲式冲牙器联合巴氏(BASS)刷牙法维护,对照组患者单纯通过BASS刷牙法维护。2组患者均采用PDCA循环管理模式,在自我维护2、4、8和12周分别要求患者复诊,通过患者菌斑堆积量进行个性化纠正和指导,在自我维护4和12周时观察并记录2组患者牙周临床检查指标,包括菌斑指数(PLI)、探诊深度(PD)和出血指数(BI),检测龈沟液中肿瘤坏死因子α(TNF-α)和白细胞介素17(IL-17)水平。结果:经过4和12周自我维护,与对照组比较,实验组患者PLI、PD和BI明显降低(P<0.01);与基线时比较,自我维护4和12周时2组患者PLI、PD和BI均明显升高(P<0.05或P<0.01);与自我维护4周时比较,自我维护12周时2组患者PLI、PD和BI值明显升高(P<0.01)。经过4和12周自我维护,与对照组比较,实验组患者龈沟液中TNF-α和IL-17水平明显降低(P<0.01);与基线时比较,自我维护4和12周时2组患者龈沟液中TNF-α和IL-17水平均明显升高(P<0.01)。结论:在PDCA循环管理模式下应用脉冲式冲牙器可明显降低中重度牙周炎患者的PLI、PD、BI和龈沟液中炎症因子水平,有效抑制菌斑的形成并控制牙龈炎症状态,有利于中重度牙周炎患者治疗效果的维护。