Terahertz-driven positron acceleration assisted by ultra-intense lasers.

Generation and acceleration of energetic positrons based on laser plasma have attracted intense attention due to their potential applications in medical physics, high energy physics, astrophysics and nuclear physics. However, such compact positron sources face a series of challenges including the beam dispersion, dephasing and unstability. Here, we propose a scheme that couples the all-optical generation of electron-positron pairs and rapid acceleration of copious positrons in the terahertz (THz) field. In the scheme, nanocoulomb-scale electrons are first captured in the wakefield and accelerated to 2.5 GeV. Then these energetic electrons emit strong THz radiation when they go through an aluminum foil. Subsequently, abundant γ photons and positrons are generated during the collision of GeV electron beam and the scattering laser. Due to the strong longitudinal acceleration field and the transvers confining field of the emitted THz wave, the positrons can be efficiently accelerated to 800 MeV, with the peak beam brilliance of 2.26 × 1012s-1mm-2mrad-2eV-1. This can arouse potential research interests from PW-class laser facilities together with a GeV electron beamline.

Ultra-intense laser field amplification from a petawatt-class laser focusing in moderate density plasma.

The rapid development of laser technologies promises a significant growth of peak laser intensity from 1022 W/cm2 to >1023 W/cm2, allowing the experimental studies of strong field quantum-electrodynamics physics and laser nuclear physics. Here, we propose a method to realize the ultra-intense laser field amplification of petawatt-class laser pulse in moderate density plasma via relativistic self-focusing and tapered-channel focusing. Three-dimensional particle-in-cell simulations demonstrate that almost an order of magnitude enhancement of laser intensity is possible even though the γ-ray radiation results in massive laser energy loss. In particular, with a seed laser intensity of ∼1023 W/cm2, duration of 82.5 fs and power of 31 petawatt, one can obtain ∼1024 W/cm2 intensity and up to ∼60% energy conversion efficiency from the initial seed laser to the focused laser in plasma with density of 3.3 × 1022/cm3. This may pave the way to the new research field of ultra-intense laser plasma interaction in the upcoming laser facilities.

Ultra-brilliant GeV betatronlike radiation from energetic electrons oscillating in frequency-downshifted laser pulses.

Electrons can be accelerated to GeV energies with high collimation via laser wakefield acceleration in the bubble regime and emit bright betatron radiation in a table-top size. However, the radiation brightness is usually limited to the third-generation synchrotron radiation facilities operating at similar photon energies. Using a two-stage plasma configuration, we propose a novel scheme for generating betatronlike radiation with an extremely high brilliance. In this scheme, the relativistic electrons inside the bubble injected from the first stage can catch up with the frequency-downshifted laser pulse formed in the second stage. The laser red shift originates from the phase modulation, together with the group velocity dispersion, which enables more energy to be transfered from the laser pulse to γ-photons, giving rise to ultra-brilliant betatronlike radiation. Multi-dimensional particle-in-cell simulations indicate that the radiated γ-photons have the cut-off energy of GeV and a peak brilliance of 1026 photons s-1 mm-2 mrad-2 per 0.1%BW at 1 MeV, which may have diverse applications in various fields.

Betatron radiation polarization control by using an off-axis ionization injection in a laser wakefield acceleration.

Tunable X-ray sources from a laser-driven wakefield have wide applications. However, due to the difficulty of electron dynamics control, currently the tunability of laser wakefield-based X-ray sources is still difficult. By using three-dimensional particle-in-cell simulations, we propose a scheme to realize controllable electron dynamics and X-ray radiation. In the scheme, a long wavelength drive pulse excites a plasma wake and an off-axis laser pulse with a short wavelength co-propagates with the drive pulse and ionizes the K-shell electrons of the background high-Z gas. The electrons can be injected in the wakefield with controllable transverse positions and residual momenta. These injected electrons experience controllable oscillations in the wake, leading to tunable radiations both in intensity and polarization.

Copious positron production by femto-second laser via absorption enhancement in a microstructured surface target.

Laser-driven positron production is expected to provide a non-radioactive, controllable, radiation tunable positron source in laboratories. We propose a novel approach of positron production by using a femto-second laser irradiating a microstructured surface target combined with a high-Z converter. By numerical simulations, it is shown that both the temperature and the maximum kinetic energy of electrons can be greatly enhanced by using a microstructured surface target instead of a planar target. When these energetic electrons shoot into a high Z converter, copious positrons are produced via Bethe-Heitler mechanism. With a laser (wavelength λ = 1 µm) with duration ~36 fs, intensity ~5.5 × 1020 W/cm2 and energy ~6 Joule, ~109 positrons can be obtained.

Dense relativistic electron mirrors from a Laguerre-Gaussian laser-irradiated micro-droplet.

We investigate dense relativistic electron mirror generation from a micro-droplet driven by circularly polarized Laguerre-Gaussian lasers. The surface electrons are expelled from the droplet by the laser's radial electric field and evolve into dense sheets after leaving the droplet. These electrons are trapped in the potential well of the laser's transverse ponderomotive force and are steadily accelerated to about 100 MeV by the longitudinal electric field. Particle-in-cell simulations indicate that the relativistic electron mirrors are characterized by high beam charge, narrow energy spread, and large angular momentum, which can be utilized for bright X/γ-ray emission and photon vortex formation.

Attosecond electron bunches from a nanofiber driven by Laguerre-Gaussian laser pulses.

Generation of attosecond bunches of energetic electrons offers significant potential from ultrafast physics to novel radiation sources. However, it is still a great challenge to stably produce such electron beams with lasers, since the typical subfemtosecond electron bunches from laser-plasma interactions either carry low beam charge, or propagate for only several tens of femtoseconds. Here we propose an all-optical scheme for generating dense attosecond electron bunches via the interaction of an intense Laguerre-Gaussian (LG) laser pulse with a nanofiber. The dense bunch train results from the unique field structure of a circularly polarized LG laser pulse, enabling each bunch to be phase-locked and accelerated forward with low divergence, high beam charge and large beam-angular-momentum. This paves the way for wide applications in various fields, e.g., ultrabrilliant attosecond x/γ-ray emission.

Ultra-bright γ-ray emission and dense positron production from two laser-driven colliding foils.

Matter can be transferred into energy and the opposite transformation is also possible by use of high-power lasers. A laser pulse in plasma can convert its energy into γ-rays and then e - e + pairs via the multi-photon Breit-Wheeler process. Production of dense positrons at GeV energies is very challenging since extremely high laser intensity ~1024 Wcm-2 is required. Here we propose an all-optical scheme for ultra-bright γ-ray emission and dense positron production with lasers at intensity of 1022-23 Wcm-2. By irradiating two colliding elliptically-polarized lasers onto two diamondlike carbon foils, electrons in the focal region of one foil are rapidly accelerated by the laser radiation pressure and interact with the other intense laser pulse which penetrates through the second foil due to relativistically induced foil transparency. This symmetric configuration enables efficient Compton back-scattering and results in ultra-bright γ-photon emission with brightness of ~1025 photons/s/mm2/mrad2/0.1%BW at 15 MeV and intensity of 5 × 1023 Wcm-2. Our first three-dimensional simulation with quantum-electrodynamics incorporated shows that a GeV positron beam with density of 2.5 × 1022 cm-3 and flux of 1.6 × 1010/shot is achieved. Collective effects of the pair plasma may be also triggered, offering a window on investigating laboratory astrophysics at PW laser facilities.

Directional enhancement of selected high-order-harmonics from intense laser irradiated blazed grating targets.

Relativistically intense laser solid target interaction has been proved to be a promising way to generate high-order harmonics, which can be used to diagnose ultrafast phenomena. However, their emission direction and spectra still lack tunability. Based upon two-dimensional particle-in-cell simulations, we show that directional enhancement of selected high-order-harmonics can be realized using blazed grating targets. Such targets can select harmonics with frequencies being integer times of the grating frequency. Meanwhile, the radiation intensity and emission area of the harmonics are increased. The emission direction is controlled by tailoring the local blazed structure. Theoretical and electron dynamics analysis for harmonics generation, selection and directional enhancement from the interaction between multi-cycle laser and grating target are carried out. These studies will benefit the generation and application of laser plasma-based high order harmonics.

Ultra-bright γ-ray flashes and dense attosecond positron bunches from two counter-propagating laser pulses irradiating a micro-wire target.

We propose a novel scheme to generate ultra-bright ultra-short γ-ray flashes and high-energy-density attosecond positron bunches by using multi-dimensional particle-in-cell simulations with quantum electrodynamics effects incorporated. By irradiating a 10 PW laser pulse with an intensity of 1023 W/cm2 onto a micro-wire target, surface electrons are dragged-out of the micro-wire and are effectively accelerated to several GeV energies by the laser ponderomotive force, forming relativistic attosecond electron bunches. When these electrons interact with the probe pulse from the other side, ultra-short γ-ray flashes are emitted with an ultra-high peak brightness of 1.8 × 1024 photons s-1mm-2mrad-2 per 0.1%BW at 24 MeV. These photons propagate with a low divergence and collide with the probe pulse, triggering the Breit-Wheeler process. Dense attosecond e-e+ pair bunches are produced with the positron energy density as high as 1017 J/m3 and number of 109. Such ultra-bright ultra-short γ-ray flashes and secondary positron beams may have potential applications in fundamental physics, high-energy-density physics, applied science and laboratory astrophysics.

All-optical bright γ-ray and dense positron source by laser driven plasmas-filled cone.

An all-optical scheme for bright γ-rays and dense e-e+ pair source is proposed by irradiating a 1022 W/cm2 laser onto a near-critical-density plasmas filled Al cone. Two-dimensional (2D) QED particle-in-cell (PIC) simulations show that, a dense electron bunch is confined in the laser field due to the radiation reaction and the trapped electrons oscillate transversely, emitting bright γ-rays forward in two ways: (1) nonlinear Compton scattering due to oscillation of electrons in the laser field, and (2) Compton backwardscattering resulting from the bunch colliding with the reflected laser by the cone tip. Finally, the multi-photon Breit-Wheeler process is initiated, producing abundant e-e+ pairs with a density of ∼ 1027m-3. The scheme is further demonstrated by full 3D PIC simulations, which indicates a positron number up to 2 × 109. This compact γ-rays and e-e+ pair source may have many potential applications, such as the laboratory study of astrophysics and nuclear physics.

[Experimental research on remote sensing spectrometer's instrument response function].

Remote sensing spectrometers were widely used in the fields such as spectral measure and trace gas supervision in atmosphere. The instrument response functions should be measured to eliminate the effect of the spectrometer. The instrument response functions of MCT detector and InSb detector in BLUKER TENSOR 37 passive FTIR spectrometer were measured in the present paper. The instrument response functions and background functions were offered with different temperature interval. The instrument response functions of MCT detector increased with temperature, but it was the reverse for InSb detector. The background functions of InSb detector increased with temperature, and the reverse for MCT detector.