Harmonic Generation from Relativistic Plasma Surfaces in Ultrasteep Plasma Density Gradients

Abstract

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...

Multi-μJ harmonic emission energy from laser-driven plasma

“…Although the observed beamed harmonics show an elliptical distribution, the observed structure was reproducible exhibiting a FWHM beam divergence of Θ XUV = 17 ± 3 mrad along the minor axis of the elliptical shape. This is in excellent agreement with previous observations under similar conditions [17,27]. Note that the divergence of the laser beam Θ L ≃ 370 mrad is an order of magnitude higher, and at the same distance from the source, it would produce a footprint almost four times the size of the MCP detector.…”

“…This is attributed to the laser pulse characteristics that determine to a great extent the steepness of the density ramp at the plasma-vacuum interface. As has been already reported [19,27,29], the initial density gradient scalelength L has a profound influence on the spectral and spatial properties of the harmonic emission. In our experiment, we were able to observe "beaming" of the harmonic emission and a reproducible spectrum, only when we attenuated the laser beam energy to the level that the pre-pulse effects were switchedoff.…”

“…The solid angle of the emission in these reports was mostly estimated without concurrent determination of the actual emission cone. More recent work has addressed the issues of divergence and XUV efficiency and their dependence on the initial density scalelength prevailing at the plasma-vacuum interface [19,27,28].…”

Influence of laser contrast on high-order harmonic generation from solid-density plasma surfaces

“…However, the peak laser intensity when the PM is used in the experiment is W cm with corresponding a , which is beyond the relativistic intensity threshold. Second, it is verified both by numerical simulations [17,24] and experiments [25,26] that there is an optimum plasma scale length range L for the CWE or the ROM harmonic generation. When the plasma density is steplike L~0, both the CWE and the ROM could not be efficiently excited.…”

Nanometer-scale characterization of laser-driven compression, shocks, and phase transitions, by x-ray scattering using free electron lasers

“…These include X-ray absorption spectroscopy and phase contrast imaging (PCI) [37], [38], X-ray Thomson scattering [39]- [41], X-ray diffraction or wide angle X-ray scattering (WAXS) [42], [43], self-emission X-ray spectroscopy and x-ray imaging [44], optical probing [45], proton radiography [46], transient imaging [47] and deflectometry [48] and less direct probes such as harmonic generation [49], [50], and particle acceleration or nuclear reactions [51].…”