Diffraction-limited performance and focusing of high harmonics from relativistic plasmas

Abstract

When a pulse of light reflects from a mirror that is travelling close to the speed of light, Einstein's theory of relativity predicts that it will be up-shifted to a substantially higher frequency and compressed to a much shorter duration. This scenario is realized by the relativistically oscillating plasma surface generated by an ultraintense laser focused onto a solid target. Until now, it has been unclear whether the conditions necessary to exploit such phenomena can survive such an extreme interaction with increasing laser intensity. Here, we provide the first quantitative evidence to suggest that they can. We show that the occurrence of surface smoothing on the scale of the wavelength of the generated harmonics, and plasma denting of the irradiated surface, enables the production of high-quality X-ray beams focused down to the diffraction limit. These results improve the outlook for generating extreme X-ray fields, which could in principle extend to the Schwinger limit

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.…”

“…It appears that for the lowest energy value, the pre-pulse is at a low enough level as not to form a pre-plasma before the arrival of the main pulse. This suggests that the interaction occurs in a plasma with a relatively steep density gradient similar to the conditions encountered when a double plasma mirror is used to reduce the pre-pulse level [17,19]. In contrast, the observed spectral structure of the harmonics for the intermediate and maximum energies appears to be a signature of the interaction taking place in a preformed plasma of increasing scalelength.…”

“…Under these circumstances, it has been recently demonstrated that the harmonic emission leads to temporal bunching with attosecond pulse durations [13,14]. At higher intensities, the relativistic oscillating mirror (ROM) mechanism becomes dominant [15][16][17]. The delimitation between the two mechanisms is usually based on the normalized vector potential a L value, which in terms of the focused laser intensity I L and laser wavelength L is given by…”

“…More importantly, instabilities occurring at the plasma-vacuum interface can cause rippling of the critical surface resulting in the scattering of the emission into a large solid angle up to 2π [21][22][23]. The generation of ROM harmonics with divergence corresponding to the harmonic wavelength, i.e., less than the original laser beam was accomplished only since the availability of high-contrast ultrashort laser pulses using double plasma mirrors [17]. It is therefore essential that the efficiency measurement is accompanied by a simultaneous determination of the beam divergence.…”

Attosecond Lighthouses: How To Use Spatiotemporally Coupled Light Fields To Generate Isolated Attosecond Pulses

“…The left panels shows the far-field intensity distribution of the attosecond pulses (harmonic orders 15 to 30) generated when an intense laser field (a0 = 6, w f = 8λL, τ f = 22 f s, θL = 56 mrad), with a WFR velocity vr = v max r = 1.3 mrad/f s, reflects on a plasma mirror where it induces the ROM effect. These physical parameters are experimentally realistic, and the obtained ratio of harmonic and laser divergences consistent with experimental data [15]. The right panel shows the angularly-integrated temporal intensity profile of this field (red curve), after filtering by the diaphragm displayed in red on the left panel, which selects ≈ 60 % of the main pulse energy.…”