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Spin-polarized positron beams have widely been utilized in applications ranging from fundamental physical studies to material processing. Preparing highly polarized positron beams for accurate probing is a long-standing issue. Here, we put forward a method to produce ultra-relativistic polarized positrons with unprecedented purity in a femtosecond timescale employing a few-PW circularly polarized laser pulse. The fully spin-resolved QED Monte Carlo method is used for simulating the two successive QED processes during the interaction, i.e., nonlinear Compton scattering and nonlinear Breit–Wheeler pair production. As the photons emitted in a circularly polarized laser field are symmetrically polarized, the polarization of the intermediate gamma photon beam averages out to zero, which is advantageous for improving the polarization of positrons. Meanwhile, the moderate laser intensity suppresses the depolarization of the new-born positrons induced by radiation reaction effect. As a result, the polarization of the positrons can reach up to ≳ 90%, the highest among the laser-driven polarization schemes conceived hitherto. Furthermore, our method relaxes the requirement on laser intensity to few-PW level, offering a promising way of preparing polarized positrons with current-generation laser facilities. [ABSTRACT FROM AUTHOR]

The broadband laser induced white emission (LIWE) spectra of NdAlO3 nanocrystals in vacuum, air, and liquid N2 were explored. In particular, their dependence on excitation laser power was investigated. It was found that the LIWE spectra intensity increased exponentially the with laser power above the excitation threshold in all three cases. The process for NdAlO3 nanocrystals was discussed in terms of multiphoton ionization assisted by ejection of hot electrons. Two differences were observed depending on the conditions: the number of photons involved in the emission was higher in liquid N2 due to additional multiphoton ionization contribution of N2. Also, threshold value for LIWE showed an increasing behavior in air and liquid N2, discussed in terms of relative permittivity and thermal dissipation. [ABSTRACT FROM AUTHOR]

We have proposed a scheme to increase the ultra-intense femtosecond laser absorption efficiency by using carbon aerogel targets. Particle-in-cell simulations show that, compared to a homogeneous target of the same density, the carbon aerogel target can significantly enhance the laser absorption efficiency, and the maximum laser absorption efficiency can reach 88%. Such high absorption efficiency can be attributed to the porous structure and larger surface area of carbon aerogel than ordinary materials, which allows the laser to enter more of the target and interact more fully with the particles inside rather than being reflected off the surface. By studying the interaction between the laser and the carbon aerogel targets, we found that both the nanoparticle diameter and porosity have great effects on the laser absorption efficiency, which is enhanced significantly as the nanoparticle diameter and porosity increase. Such carbon aerogel targets should have significant applications in the fields of laser generation of x rays and high energy density physics. [ABSTRACT FROM AUTHOR]

Millijoule-energy few-cycle laser pulses excite the plasma wakefield and accelerate electrons at kilohertz repetition rate, generating mega-electron volt (MeV) electron bunches with femtosecond temporal duration for ultrafast electron diffraction applications. By simulating few-cycle laser pulses interacting with the underdense nitrogen plasma, we have studied the mechanism of periodic electron self-injection, which manifests the laser carrier envelop phase (CEP) effect in few-cycle laser wakefield acceleration. A few-cycle laser pulse experiencing a significant wavelength redshift induces the transverse oscillation of the plasma bubble at the same frequency as the laser CEP changes by 2π. The oscillation of the plasma bubble periodically injects free electrons into the bubble at a doubled frequency, broadening the electron energy spread. [ABSTRACT FROM AUTHOR]