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


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.

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Figure 1: Characteristics of HOHG emitted from plasma surfaces.
Figure 2: Diffraction-limited performance of harmonic emission from relativistically oscillating plasma surfaces.
Figure 3: Comparison of harmonic emission under conditions of varying target root-mean-square roughness over sub-λLaser scale (φr.m.s.).
Figure 4: Background-subtracted lineouts of harmonic spectra from targets with φr.m.s.<1 nm (Fig. 3a) and φr.m.s.18 nm (Fig. 3b).
Figure 5: Angular distribution of harmonic radiation emitted from targets with φr.m.s.<1 nm and φr.m.s.18 nm.
Figure 6: Expected level of denting from PIC code simulations for conditions similar to those used in experiments.

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M.Z. acknowledges support from the Royal Society. This work was supported by EPSRC and EU COST action MP0601.

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This experiment was carried out in the main by B.D., D.A., R.H. and Y.N. The simulations were carried out by S.G.R., with support from M.G. Experimental planning and data analysis was carried out by B.D. and M.Z.

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Correspondence to M. Zepf.

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Dromey, B., Adams, D., Hörlein, R. et al. Diffraction-limited performance and focusing of high harmonics from relativistic plasmas. Nature Phys 5, 146–152 (2009).

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