176 research outputs found
Initial carrier-envelope phase of few-cycle pulses determined by THz emission from air plasma
The evolution of THz waveform generated in air plasma provides a sensitive
probe to the variation of the carrier envelope phase (CEP) of propagating
intense few-cycle pulses. Our experimental observation and calculation reveal
that the number and positions of the inversion of THz waveform are dependent on
the initial CEP, which is near 0.5{\pi} constantly under varied input pulse
energies when two inversions of THz waveform in air plasma become one. This
provides a method of measuring the initial CEP in an accuracy that is only
limited by the stability of the driving few-cycle pulses.Comment: 13 pages, 4 figure
Pure Even Harmonic Generation from Oriented CO in Linearly Polarized Laser Fields
The first high harmonic spectrum, containing only the odd orders, was
observed in experiments 30 years ago (1987). However, a spectrum containing
pure even harmonics has never been observed. We investigate the generation of
pure even harmonics from oriented CO molecules in linearly polarized laser
fields employing the time-dependent density-functional theory. We find that the
even harmonics, with no odd orders, are generated with the polarization
perpendicular to the laser polarization, when the molecular axis of CO is
perpendicular to the laser polarization. Generation of pure even harmonics
reveals a type of dipole acceleration originating from the permanent dipole
moment. This phenomenon exists in all system with permanent dipole moments,
including bulk crystal and polyatomic molecules
Driving positron beam acceleration with coherent transition radiation
Positron acceleration in plasma wakefield faces significant challenges since
the positron beam must be pre-generated and precisely coupled into the
wakefield, and most critically, suffers from defocusing issues. Here we propose
a scheme that utilizes laser-driven electrons to produce, inject and accelerate
positrons in a single set-up. The high-charge electron beam from wakefield
acceleration creates copious electron-positron pairs via the Bethe-Heitler
process, followed by enormous coherent transition radiation due to the
electrons' exiting from the metallic foil. Simulation results show that the
coherent transition radiation field reaches up to 10's GV m-1, which captures
and accelerates the positrons to cut-off energy of 1.5 GeV with energy peak of
500 MeV and energy spread is about 24.3%. An external longitudinal magnetic
field of 30 T is also applied to guide the electrons and positrons during the
acceleration process. This proposed method offers a promising way to obtain GeV
fast positron sources
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