It is shown that when an few-cycle, relativistically intense, p-polarized laser pulse is obliquely incident on overdense plasma, the surface electrons may form ultra-thin, highly compressed layers, with a width of a few nanometers. These electron "nanobunches" emit synchrotron radiation coherently. We calculate the one-dimensional synchrotron spectrum analytically and obtain a slowly decaying power-law with an exponent of 4/3 or 6/5. This is much flatter than the 8/3 power of the BGP (Baeva-Gordienko-Pukhov) spectrum, produced by a relativistically oscillating bulk skin layer. The synchrotron spectrum cut-off frequency is defined either by the electron relativistic $\gamma$-factor, or by the thickness of the emitting layer. In the numerically demonstrated, locally optimal case, the radiation is emitted in the form of a single attosecond pulse, which contains almost the entire energy of the full optical cycle.Comment: to appear in Physics of Plasma

Harmonic generation in the limit of ultra-steep density gradients is studied experimentally. Observations demonstrate that while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale-lengths (Lp/λ < 1) the absolute efficiency of the harmonics declines for the steepest plasma density scale-length Lp → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the Relativistic Oscillating Mirror (ROM) was estimated to be in the range of 10 −4 − 10 −6 of the laser pulse energy for photon energies ranging from 20 − 40 eV, with the best results being obtained for an intermediate density scale-length.PACS numbers: 52.59.Ye, 52.38.-r Keywords: surface high-harmonic generation, relativistic laser plasma interaction, attosecond pulse generation Ultrashort XUV pulses are a promising tool for a wide range of applications including attosecond laser physics and seeding of free-electron X-ray lasers. Typically, they are created by the nonlinear frequency up-conversion of an intense femtosecond driving laser field in a gaseous medium. Remarkable progress has been made to the present date with efficiencies reaching the level of 10 −4 at 20 nm wavelengths [1,2]. Such efficiencies are not yet available at shorter wavelengths or for attosecond pulse generation and the low intensities at which harmonic conversion takes place in gaseous media, makes harnessing the high peak power in the 0.1−1PW regime challenging. High-harmonic generation at a sharp plasma-vacuum interface via the Relativistically Oscillating Mirror (ROM) mechanism [3] is predicted to overcome these limitations and result in attosecond pulses of extreme peak power [4,5].While other mechanisms such as Coherent Wake Emission (CWE) can also emit XUV harmonics [6], the ROM mechanism is generally reported to dominate in the limit of highly relativistic intensities, where the normalized vector potential a 2 0 = Iλ 2 /(1.37 · 10 18 µm 2 W/cm 2 ) 1. The efficiency of ROM harmonics is predicted to converge to a power law for ultra-relativistic intensities [7], such that the conversion efficiency is given by η ≈ (ω/ω 0 ) −8/3 up to a threshold frequency ω t ∼ γ 3 , beyond which the spectrum decays exponentially. Here, γ is the maximum value of the Lorentz-factor associated with the reflection point of the ROM process. While these predictions correspond well with the observations made in experiments using pulse durations of the order of picoseconds in terms of highest photon energy up to keV [8,9] and the slope of the harmonic efficiency [10], no absolute efficiency measurements have been reported to date.The plasma density scale-length plays a critical role in determining the response of the plasma to the incident laser radiation. In the picosecond regime, the balance between the laser pre...

When a laser pulse hits a solid surface with relativistic intensities, XUV attosecond pulses are generated in the reflected light. We present an experimental and theoretical study of the temporal properties of attosecond pulse trains in this regime. The recorded harmonic spectra show distinct fine structures which can be explained by a varying temporal pulse spacing that can be controlled by the laser contrast. The pulse spacing is directly related to the cycle-averaged motion of the reflecting surface. Thus the harmonic spectrum contains information on the relativistic plasma dynamics.

We consider here a few options to use relativistic laser plasmas for novel sources of short wavelength radiation. Electrons accelerated in underdense plasmas in the bubble regime wiggle in an ion channel. This leads to broadband incoherent synchrotron-like radiation bursts which are of femtosecond duration. The photon energies are in the kiloelectronvolt to megaelectronvolt energy range. However, this radiation is not coherent. To reach coherency, the electron bunch must have structure at the wavelength of the emitted x-rays. This can be achieved in principle by sending the laser-accelerated electron bunch through an external wiggler. However, to reach free electron lasing in the x-ray regime, the energy spread of the laser-accelerated electrons must be reduced dramatically. Another option is to use high harmonic generation at overdense plasma boundaries. The high harmonics are emitted in coherent subfemtosecond flashes. The harmonic spectra decay as a power of the harmonic number, with an exponent that can be as low as −6/5. This can make the high harmonics potentially the brightest laser-driven short wavelength sources with unique properties.

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.