Generation of single-cycle relativistic infrared pulses at wavelengths above 20 μm from density-tailored plasmas

Ultra-intense short-pulse light sources are powerful tools for a wide range of applications.However,relativistic short-pulse lasers are normally generated in the near-infrared regime.Here,we present a promising and efficient way to generate tunable relativistic ultrashort pulses with wavelengths above 20μm in a density-tailored plasma.In this approach,in the first stage,an intense drive laser first excites a nonlinear wake in an underdense plasma,and its photon frequency is then downshifted via phase modulation as it propagates in the plasma wake.Subsequently,in the second stage,the drive pulse enters a lower-density plasma region so that the wake has a larger plasma cavity in which longer-wavelength infrared pulses can be produced.Numerical simulations show that the resulting near-single-cycle pulses cover a broad spectral range of 10–40μm with a conversion efficiency of∼2.1%(∼34 mJ pulse energy).This enables the investigation of nonlinear infrared optics in the relativistic regime and offers new possibilities for the investigation of ultrafast phenomena and physics in strong fields.

Post-solitons and electron vortices generated by femtosecond intense laser interacting with uniform near-critical-density plasmas

Generation of nonlinear structures,such as stimulated Raman side scattering waves,post-solitons and electron vortices,during ultra-short intense laser pulse transportation in near-critical-density(NCD)plasmas is studied by using multidimensional particle-in-cell(PIC)simulations.In two-dimensional geometries,both P-and S-polarized laser pulses are used to drive these nonlinear structures and to check the polarization effects on them.In the S-polarized case,the scattered waves can be captured by surrounding plasmas leading to the generation of post-solitons,while the main pulse excites convective electric currents leading to the formation of electron vortices through Kelvin-Helmholtz instability(KHI).In the P-polarized case,the scattered waves dissipate their energy by heating surrounding plasmas.Electron vortices are excited due to the hosing instability of the drive laser.These polarization dependent physical processes are reproduced in two different planes perpendicular to the laser propagation direction in three-dimensional simulation with linearly polarized laser driver.The current work provides inspiration for future experiments of laser-NCD plasma interactions.

Collimated GeV attosecond electron–positron bunches from a plasma channel driven by 10 PW lasers

High-energy positrons and bright g-ray sources are of great importance both in fundamental research and for practical applications.However,collimated GeV electron–positron pair jets and g-ray flashes are still rarely produced in the laboratory.Here,we demonstrate that by irradiating a near-critical-density plasma channel with two 10 PW-scale laser pulses,highly directional GeV electron–positron pairs and bright g-ray beams can be efficiently generated.Three-dimensional particle-incell simulations show the formation of GeV positron jets with high density(8×10^(21)=cm^(3)),attosecond duration(400 as),and a divergence angle of 14°.Additionally,ultrabright[2×10^(25) photons s^(-1)1 mm^(-2) mrad^(-2) (0.1%bandwidth_(-1)]collimated attosecond(370 as)g-ray flashes with a laser energy conversion efficiency of 5.6%are emitted.These features show the significant advantage of using a plasma channel as compared with a uniformplasma and thus open up new possibilities for a wide variety of applications.

Electron Dynamics and Characteristics of Attosecond Electromagnetic Emissions in Relativistic Laser-Plasma Interactions

Generation of attosecond electromagnetic （EM） pulses and the associated electron dynamics are studied using particle-in-cell simulations of relativistic laser pulses interacting with over-dense plasma foil targets. The inter- action process is found to be so complicated even in the situation of utilizing driving laser pulses of only one cycle. Two electron bunches closely involved in the laser-driven wavebreaking process contribute to attosecond EM pulses through the coherent synchrotron emission process whose spectra are found to follow an exponential decay rule. Detailed investigations of electron dynamics indicate that the early part of the reflected EM emission is the high-harmonics produced through the relativistic oscillating mirror mechanism. High harmonics are also found to be generated through the Bremsstrahlung radiation by one electron bunch that participates in the wavebreaking process and decelerates when it experiences the local wavebreaking-generated high electrostatic field in the moving direction.

A history of high-power laser research and development in the United Kingdom

The first demonstration of laser action in ruby was made in 1960 by T.H.Maiman of Hughes Research Laboratories,USA.Many laboratories worldwide began the search for lasers using different materials,operating at different wavelengths.In the UK,academia,industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications.This historical review looks at the contribution the UK has made to the advancement of the technology,the development of systems and components and their exploitation over the last 60 years.

Efficient generation of relativistic near-single-cycle mid-infrared pulses in plasmas

Ultrashort intense optical pulses in the mid-infrared(mid-IR)region are very important for broad applications ranging from super-resolution spectroscopy to attosecond X-ray pulse generation and particle acceleration.However,currently,it is still difficult to produce few-cycle mid-IR pulses of relativistic intensities using standard optical techniques.Here,we propose and numerically demonstrate a novel scheme to produce these mid-IR pulses based on laser-driven plasma optical modulation.In this scheme,a plasma wake is first excited by an intense drive laser pulse in an underdense plasma,and a signal laser pulse initially at the same wavelength(1 micron)as that of the drive laser is subsequently injected into the plasma wake.The signal pulse is converted to a relativistic multi-millijoule near-singlecycle mid-IR pulse with a central wavelength of ~5 microns via frequency-downshifting,where the energy conversion efficiency is as high as approximately 30% when the drive and signal laser pulses are both at a few tens of millijoules at the beginning.Our scheme can be realized with terawatt-class kHz laser systems,which may bring new opportunities in high-field physics and ultrafast science.

Tunable synchrotron-like radiation from centimeter scale plasma channels

Synchrotron radiation(SR)sources are immensely useful tools for scientific researches and many practical applications.Currently,the state-of-the-art synchrotrons rely on conventional accelerators,where electrons are accelerated in a straight line and radiate in bending magnets or other insertion devices.However,these facilities are usually large and costly.Here,we study a compact all optical synchrotron-like radiation source based on laser-plasma acceleration either in a straight or a curved plasma channel.With the laser pulse off-axially injected,its centroid oscillates transversely in the plasma channel.This results in a wiggler motion of the whole accelerating structure and the self-trapped electrons behind the laser pulse,leading to strong synchrotron-like radiations with tunable spectra.It is further shown that a palmtop ring-shaped synchrotron is possible with current high power laser technologies.With its potential of high flexibility and tunability,such light sources once realized would find applications in wide areas and make up the shortage of large SR facilities.

Upper-limit power for self-guided propagation of intense lasers in underdense plasma

It is found that there is an upper-limit critical power for self-guided propagation of intense lasers in plasma in addition to the well-known lower-limit critical power set by the relativistic effect.Above this upper-limit critical power,the laser pulse experiences defocusing due to expulsion of local plasma electrons by the transverse ponderomotive force.Associated with the upper-limit power,a lower-limit critical plasma density is also found for a given laser spot size,below which self-focusing does not occur for any laser power.Both the upper-limit power and the lower-limit density are derived theoretically and verified by two-dimensional particle-in-cell simulations.The present study provides new guidance for experimental designs,where self-guided propagation of lasers is essential.

Simulation of High Power THz Emission from Laser Interaction with Tenuous Plasma and Gas Targets

With one-and two-dimensional particle-in-cell(PIC)codes,we simulate the generation of high power terahertz(THz)emission from the interaction of ultrashort intense lasers with tenuous plasma and gas targets.By driving high-amplitude electron plasma waves either with a laser wakefield or the beatwave of two laser pulses,powerful THz electromagnetic pulses can be produced by linear mode conversion in inhomogeneous plasma or by the transient current induced at the surfaces of a thin plasma layer of few plasma wavelengths.Even with incident lasers at moderate intensity such as 1017W/cm2,the produced emission can be at the level of tens of MW in power and capable of affording field strengths of a few MV/cm,suitable for the studies of THz nonlinear physics.With field ionization included in the PIC codes,THz emission from laser interaction with tenuous gas targets is simulated.It is found that the transient transverse current formed during the ionization processes is responsible for the THz emission.With this mechanism,onemay also obtain THz fields ofMV/cm at lower laser intensity as compared with the schemes of plasma-wave excitation.

Electromagnetic radiation from laser wakefields in underdense plasma

It is demonstrated by simulations and analysis that a wakefield driven by an ultrashort intense laser pulse in underdense plasma can emit tunable electromagnetic radiation along the laser propagation direction. The profile of such a kind of radiation is closely associated with the structure of the laser wakefield. In general, electromagnetic radiation in the terahertz range with its frequency a few times the electron plasma frequency can be generated in the moderate intensity regime. In the highly nonlinear case, a chain of radiation pulses is formed corresponding to the nonlinear structure of the wake. Study shows that the radiation is associated with the self-modulation process of the laser pulse in the wakefield and resulting transverse electron momenta from modulated asymmetric laser fields.

Upper-limit power for self-guided propagation of intense lasers in underdense plasma-CORRIGENDUM

The authors would like to apologize for an error found in the acknowledgement section on page 79.The acknowledgement should read:This work is supported in part by National Basic Research Program of China(Grant No.2013CBA01501)and

Modeling of Bound Electron Effects in Particle-in-Cell

To include the bound electron effects in particle-in-cell(PIC)simulation,we propose a model in which the response of the dipole components of partially ionized ions to external electromagnetic fields can be included.Instead of treating the macroion particle as a single particle without an internal structure,the ions are considered to have a structure composed of a central nucleus and a bounded electron cloud in our model.The two parts experience the interactions of both the external electromagnetic fields and the internal Coulomb fields.In this way,the laser scattering effects by a partially ionized medium can be modeled properly in the PIC simulation.The laser propagation in a neutral medium and the Bragg scattering of the laser in crystal structure have been simulated with a PIC code modified based on our model as the benchmark.Our model may find applications to study some interesting problems,such as the x-ray laser-driven wakefield acceleration in crystals,the x-ray laser-driven high energy density physics,and intense laser propagation in partially ionized nonlinear optical materials,etc.

Vlasov-Fokker-Planck Simulations for High-Power Laser-Plasma Interactions

A review is presented on our recent Vlasov-Fokker-Planck(VFP)simulation code development and applications for high-power laser-plasma interactions.Numerical schemes are described for solving the kinetic VFP equation with both electronelectron and electron-ion collisions in one-spatial and two-velocity(1D2V)coordinates.They are based on the positive and flux conservation method and the finite volume method,and these twomethods can insure the particle number conservation.Our simulation code can deal with problems in high-power laser/beam-plasma interactions,where highly non-Maxwellian electron distribution functions usually develop and the widely-used perturbation theories with the weak anisotropy assumption of the electron distribution function are no longer in point.We present some new results on three typical problems:firstly the plasma current generation in strong direct current electric fields beyond Spitzer-H¨arm’s transport theory,secondly the inverse bremsstrahlung absorption at high laser intensity beyond Langdon’s theory,and thirdly the heat transport with steep temperature and/or density gradients in laser-produced plasma.Finally,numerical parameters,performance,the particle number conservation,and the energy conservation in these simulations are provided.