Theory of Attosecond Pulses from Relativistic Surface Plasmas

Abstract

High harmonic generation by relativistically intense laser pulses from overdense plasma layers is surveyed. High harmonics are generated in form of (sub-)attosecond pulses when the plasma surface rebounds towards the observer with relativistic velocity. Different cases are considered. The "relativistically oscillating mirror" (ROM) model, describing the most typical case, is analyzed in detail. The resulting harmonic spectrum is usually a power law with the exponent -8/3 [1], but possible exceptions due to "higher order γ-spikes" are considered. It is shown that under certain conditions, ultra-dense electron nanobunches can be formed at plasma surface that emit coherent synchrotron radiation. The resulting spectrum is much flatter and leads to the formation of a giant attosecond pulse in the reflected radiation. The harmonics radiation is also considered in time domain, where they form a train of attosecond pulses. It is characterized and a possibility to select a single attosecond pulse via polarization gating is described. Further, the line structure in relativistic harmonic spectra is analyzed. It is shown that the harmonics have an intrinsic chirp and it can be responsible for experimentally observed spectral modulations. Finally, high harmonic generation is considered in realistic three-dimensional geometry. It is shown that free space diffraction can act as a high pass filter, altering the spectrum and the reflected field structure. The high harmonics tend to be self-focused by the reflecting surface. This leads to a natural angular divergence as well as to field boost at the focal position. Coherently focusing the harmonics using an optimized geometry may result in a significantly higher field than the field of the driving laser.

Attospiral generation upon interaction of circularly polarized intense laser pulses with conelike targets

“…2 and 3 are composed by these higher harmonics, which resemble the spectrum derived in the regime of CSE in Ref. [15].…”

“…In that case the linearly polarized intense laser light can be efficiently converted into attopulses with the same polarization. In the present work we study this attopulse generation process in 3D in temporal and spatial domains and we show that the measured frequency spectrum is well described by numerically evaluated CSE * zsolt.lecz@eli-alps.hu spectra in the regime of higher order gamma-spikes [15]. Instead of a flat foil and linearly polarized pulse we use cylindrical targets and circularly polarized (CP) pulse in order to ensure conditions for p-polarized incidence at each point of the inner target surface.…”

“…In our case higher values of n are also permitted if the electron bunch is accelerated fast enough to nearly the speed of light and propagates together with the laser pulse for a relatively long time. The final expression for the harmonic spectrum can be formulated with generalized Airy functions [15], or using the Lienard-Wiechert potentials the time dependent electric field can be derived [12,24], then transformed into the frequency domain to obtain the curves presented in Fig. 5a.…”

“…The color code is the same in both pictures. cal integration method described in [15], where ω rs ≈ ( √ 2γ) (2n+1)/n (α 1 ) 1/2n is a fitting parameter. The red dashed line is the Fourier transform of the electric field derived in [24], which shows a better agreement with the simulations.…”

Wavebreaking-associated transmitted emission of attosecond extreme-ultraviolet pulses from laser-driven overdense plasmas

“…A promising approach to generate such XUV sources is high-order harmonics generation (HHG) from relativistic laser-irradiated overdense plasma surfaces [5], which holds the potential and advantage to obtain XUV pulses with high brilliance. Several radiation mechanisms have been identified, such as coherent wake emission (CWE) [6,7,8,9], relativistic oscillating mirror (ROM) [10,11,12,13,14,15], and coherent synchrotron emission (CSE) [16,17,18]. While CWE predominates at moderate laser intensities, i.e., the laser normalized vector potential a ≤ 1, ROM and CSE are more efficient for highly relativistic laser intensities with a ≫ 1.…”