Efficient guiding and focusing of intense laser pulse using periodic thin slits

Slits have been widely used in laser-plasma interactions as plasma optical components for generating high-harmonic light and controlling laser-driven particle beams.Here,we propose and demonstrate that periodic thin slits can be regarded as a new breed of optical elements for efficient focusing and guiding of intense laser pulse.The fundamental physics of intense laser interaction with thin slits is studied,and it is revealed that relativistic effects can lead to enhanced laser focusing far beyond the pure diffractive focusing regime.In addition,the interaction of an intense laser pulse with periodic thin slits makes it feasible to achieve multifold enhancement in both laser intensity and energy transfer efficiency compared with conventional waveguides.These results provide a novel method for manipulating ultra-intense laser pulses and should be of interest for many laser-based applications.

Nonlinear branched flow of intense laser light in randomly uneven media

Branched flow is an interesting phenomenon that can occur in diverse systems.It is usually linear in the sense that the flow does not alter the properties of the medium.Branched flow of light on thin films has recently been discovered.It is therefore of interest to know whether nonlinear light branching can also occur.Here,using particle-in-cell simulations,we find that in the case of an intense laser propagating through a randomly uneven medium,cascading local photoionization by the incident laser,together with the response of freed electrons in the strong laser fields,triggers space–time-dependent optical unevenness.The resulting branching pattern depends dramatically on the laser intensity.That is,the branching here is distinct from the existing linear ones.The observed branching properties agree well with theoretical analyses based on the Helmholtz equation.Nonlinear branched propagation of intense lasers potentially opens up a new area for laser–matter interaction and may be relevant to other branching phenomena of a nonlinear nature.

Proton acceleration from picosecond-laser interaction with a hydrocarbon target

As an intense picosecond laser pulse irradiates a hydrocarbon target,the protons therein can be accelerated by the radiation pressure as well as the sheath field behind the target.We investigate the effect of the laser and hydrocarbon target parameters on proton acceleration with two/threedimensional particle-in-cell simulations.It is found that the resulting two-ion species plasma can generate a multiple peaked charge-separation field that accelerates the protons.In particular,a smaller carbon-to-hydrogen ratio,as well as the thinner and/or lower density of the target,leads to a larger sheath field and thus proton beams with a larger cutoff energy and smoother energy spectrum.These results may be useful in achieving high-flux quasi-monoenergetic proton beams by properly designing the hydrocarbon target.

Particle-in-cell simulations of low-β magnetic reconnection driven by laser interaction with a capacitor–coil target

The dynamics of low-β magnetic reconnection(MR) driven by laser interaction with a capacitor–coil target are reexamined by simulations in this paper. We compare two cases MR and non-MR(also referred as AP-case and P-case standing for the anti-parallel and parallel magnetic field lines, respectively) to distinguish the different characteristics between them.We find that only in the AP-case the reconnection electric field shows up around the X line and the electron jet is directed toward the X line. The quadruple magnetic fields exist in both cases, however, they distribute in the current sheet area in the AP-case, and out of the squeezing area in the P-case, because electrons are demagnetized in the electron diffusion region in the MR process, which is absent in the P-case. The electron acceleration is dominant by the Fermi-like mechanism before the MR process, and by the reconnection electric field when the MR occurs. A power-law electron energy spectrum with an index of 1.8 is found in the AP-case. This work proves the significant potential of this experimental platform to be applied in the studies of low-β astronomy phenomena.

Local wavelength evolution and Landau damping of electrostatic plasma wave driven by an ultra-relativistic electron beam in dense inhomogeneous plasma

Evolution of an electrostatic plasma wave driven by a low-density ultra-relativistic electron beam in dense inhomogeneous plasma is considered. In particular, the wavelength variation as observed at fixed locations in the plasma is analyzed in terms of the wave characteristics. It is shown that for a negative density gradient, the observed local wavelength decreases monotonically with time, but for a positive density gradient, it first increases and then decreases with time, accompanied by reversal of the wave phase. However, in both cases the local wavelength eventually decreases with time since Landau damping becomes significant as the wavelength becomes of the order of the plasma Debye length. Results from particle-in-cell simulations agree well with theoretical analyses of the wavelength variation.

Performance optimization of scintillator neutron detectors for EMD in CSNS

Chinese Spallation Neutron Source(CSNS) has successfully produced its first neutron beam in 28th August 2017. It has been running steadily from March, 2018. According to the construction plan, the engineering materials diffractometer(EMD) will be installed between 2019–2023. This instrument requires the neutron detectors with the cover area near3 m2in two 90° neutron diffraction angle positions, the neutron detecting efficiency is better than 40%@1A, and the spatial resolution is better than 4 mm×200 mm in horizontal and vertical directions respectively. We have developed a onedimensional position-sensitive neutron detector based on the oblique6Li F/Zn S(Ag) scintillators, wavelength shifting fibers,and Si PMs(silicon photomultipliers) readout. The inhomogeneity of the neutron detection efficiency between each pixel and each detector module, which caused by the inconsistency of the wave-length shifting fibers in collecting scintillation photons, needs to be mitigated before the installation. A performance optimization experiment of the detector modules was carried out on the BL20(beam line 20) of CSNS. Using water sample, the neutron beam with Φ5 mm exit hole was dispersed related evenly into the forward space. According to the neutron counts of each pixel of the detector module, the readout electronics threshold of each pixel is adjusted. Compared with the unadjusted detector module, the inhomogeneity of the detection efficiency for the adjusted one has been improved from 69% to 90%. The test result of the diffraction peak of the standard sample Si showed that the adjusted detector module works well.

Nonadiabatic geometric phase in a doubly driven two-level system

We study theoretically the nonadiabatic geometric phase of a doubly driven two-level system with an additional relative phase between the two driving modes introduced in. It is shown that the time evolution of the system strongly depends on this relative phase. The condition for the system returning to its initial state after a single period is given by the means of the Landau–Zener–Stückelberg–Majorana destructive interference. The nonadiabatic geometric phase accompanying a cyclic evolution is shown to be related to the Stokes phase as well as this relative phase. By controlling the relative phase, the geometric phase can characterize two distinct phases in the adiabatic limit.

Synthesis of Nanostructured TiO₂Photocatalyst with Ultrasonication at Low Temperature

A thin film TiO2 in hierarchical nano-structure with high photocatalytic activities was synthesized in simple steps with ultrasonication. The crystal structure and morphology of the photocatalyst were investigated by X-ray diffraction (XRD) and high-resolution field emission scanning electron microscope (FE-SEM). In the present work, nanostructured TiO2 was directly formed onto a Ti substrate via a solution approach. This nanostructured TiO2 photocatalyst can be reused and will not generate secondary contamination to treated water. The photocatalytic activity of the synthesized TiO2 photocatalyst was evaluated by the degradation of phenol under UVC irradiation in water and was compared with the general sol-gel derived TiO2 films as well as a commercial DP-25 TiO2 coating. It was found that the synthesized nanostructured TiO2has significantly high and stable photocatalytic activity.

A 61-mJ,1-kHz cryogenic Yb:YAG laser amplifier

We report a diode-pumped rod-type Yb:YAG laser amplifier operating at 1 kHz.Cryogenic cooling method was adopted to make the Yb:YAG crystal work with four-level behavior.A single-frequency fiber laser acts as the seed in an actively Q-switched Yb:YAG oscillator.The resonator delivers 5.75-mJ pulses at 1 kHz with a pulse duration of approximately 40 ns.The pulses were amplified to 61 mJ in a four-pass rod-type Yb:YAG amplifier with optical-to-optical efficiency of 24%in the main amplifier.The M^(2)parameter of the output laser is<1.4.

Manipulating Nonsequential Double Ionization of Argon Atoms via Orthogonal Two-Color Field

Using a three-dimensional classical ensemble model,we investigate the dependence of relative frequency and relative initial phase for nonsequential double ionization(NSDI)of atoms driven by orthogonal two-color(OTC)fields.Our findings reveal that the NSDI probability is clearly dependent on the relative initial phase of OTC fields at different relative frequencies.The inversion analysis results indicate that adjusting the relative frequency of OTC fields helps control returning probability and flight time of the first electron.Furthermore,manipulating the relative frequency at the same relative initial phases can vary the revisit time of the recolliding electron,leading that the emission direction of Ar^(2+)ions is explicitly dependent on the relative frequency.

Generation and observation of multiple solitons from a mid-infrared ultrafast fiber laser

We demonstrate the generation of a unique regime of multiple solitons in a Tm-doped ultrafast fiber laser at~1938.72 nm.The temporal pulse-to-pulse separation among the multiple solitons,10 in a single-pulse bunch,increases from 0.89 ns to1.85 ns per round trip.In addition,with the increasing pump power,the number of bunched solitons increases from 3 up to 24linearly,while the average time separation in the soliton bunch varies irregularly between~0.80 and~1.52 ns.These results contribute to a more profound comprehension of nonlinear pulse dynamics in ultrafast fiber lasers.

Performance measurement and evaluation of an ionic liquid electrospray thruster

As a novel micro-propulsion system for small satellites(from micro to nano),the ionic liquid electro spray propulsion system is a promising candidate.However,performance measurement and evaluation of the Ionic Liquid Electrospray Thruster(ILET)is one of the most challenging issues for practical application,due to the difficulties in the development of a prototype and direct measurements of micro-thrust and small flow rate.To address this issue,a Modular Ionic Liquid Electrospray Thruster(MILET)prototype is constructed,and a diagnostic system for thrust and mass flow rate is specially developed based on an analytical balance method.With the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate as the propellant,a series of experiments is carried out on the MILET prototype under a wide operating condition through changing the applied voltage to control the thrust.Under different applied voltages,the thrust and the mass flow rate of the propellant are directly measured.The propulsive performance parameters of the thruster,such as thrust,specific impulse,thrust-to-power ratio,thruster efficiency,etc.,are comprehensively analyzed.Then,a performance comparison is made between the MILET and other representative ILETs.With a relatively low applied voltage ranging from 1550 V to 2000 V,the MILET achieves a quasi-constant specific impulse of 1263 s with the averaged thrust-to-power ratio of 65.2μN/W and thruster efficiency of 40.7%.The performance of ILET is also compared with other typical electric propulsions.The results demonstrate that the ILET exhibits an excellent ability of minimalization with high specific impulse and thruster efficiency,which guarantees a great superiority in micro propulsions.Finally,the ways to further improve the performance of ILET are discussed,which further confirms the potential prospect of ILET.The present result helps to advance the development and application of ILET.

Femtosecond laser writing of low-loss three-dimensional waveguide coupler in LiNbO_(3) crystal

With different interactions between material and femtosecond lasers,two-dimensional(2D)and three-dimensional(3D)waveguide couplers,whose separation distances are fabricated in z-cut lithium niobate crystal by femtosecond laser writing,are reported.Experimentally and numerically,it is shown from results that the guidance is only propagating along TM polarization due to the Type I modification and holds equal splitting ratios,which are the same as power splitters at632.8 nm.The propagation losses of 2D and 3D waveguide couplers exhibit better transmission properties than those of the previously reported Type I Y-junction waveguide splitters.

Ultrafast Hole Deformation Revealed by Molecular Attosecond Interferometry

Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes.Subjected to strong laser fields,electronic holes are formed upon ionization and evolve in the attosecond timescale.It is crucial to probe the electronic dynamics in real time with attosecond-temporal and atomic-spatial precision.Here,we present molecular attosecond interferometry that enables the in situ manipulation of holes in carbon dioxide molecules via the interferometry of the phase-locked electrons(propagating in opposite directions)of a laser-triggered rotational wave packet.The joint measurement on high-harmonic and terahertz spectroscopy(HATS)provides a unique tool for understanding electron dynamics from picoseconds to attoseconds.The optimum phases of two-color pulses for controlling the electron wave packet are precisely determined owing to the robust reference provided with the terahertz pulse generation.It is noteworthy that the contribution of HOMO-1 and HOMO-2 increases reflecting the deformation of the hole as the harmonic order increases.Our method can be applied to study hole dynamics of complex molecules and electron correlations during the strong-field process.The threefold control through molecular alignment,laser polarization,and the two-color pulse phase delay allows the precise manipulation of the transient hole paving the way for new advances in attochemistry.

High Energy Yb∶YAG Regenerative Amplifier

A diode pumped high energy Yb∶YAG rod regenerative amplifier was demonstrated with a maximum energy of22.3 m J,excellent energy stability(~0.8%root mean square),and beam quality(M2<1.2)at 10 Hz repetition rate.To the best of our knowledge,this is the highest energy so far obtained by a Yb∶YAG rod regenerative amplifier.