会議発表用資料 Point-like coherent soft x-ray sources and challenges for optics

ピロジコフ, アレキサンダー  ,  エシロケポフ, ティムル  ,  匂坂, 明人  ,  小倉, 浩一  ,  A. Pikuz, Tatiana  ,  Ya. Faenov, Anatoly  ,  コーガ, ジェームズ  ,  桐山, 博光  ,  N. Ragozin, Eugene  ,  Neely, David  ,  大道, 博行  ,  加藤, 義章  ,  ブラノフ, セルゲイ  ,  近藤, 公伯  ,  河内, 哲哉  ,  神門, 正城

2017-11-29
内容記述
1.Experimental discovery of new high-order harmonic generation regimeWe discovered [1],[2] a new regime of bright high-order harmonic generation by multi-terawatt femtosecond lasers [3],[4] focused to micrometer spots in supersonic gas jets, Fig. 1(a). The focused laser irradiance exceeded 10^18 W/cm^2 rendering the interaction relativistically intense. Using either linearly or circularly polarized laser pulses, we observed coherent x-rays emitted from the gas jet. The obtained spectra were comb-like, with hundreds of even and odd harmonic orders reaching the 'water window', up to the photon energy of 360 eV. We found that the photon number scaled quadratically with the laser power. With a 120 TW laser, we obtained 4x10^9 photons (90 nJ pulse energy) in a single harmonic at 120 eV. Our experimental results were surprising and could not be explained by existing theories.2.Simulations and catastrophe theory modelWe performed Particle-In-Cell (PIC) simulations and found that the harmonics were generated by extremely high-density, sharp electron density spikes; the peak density was orders of magnitude larger than the density of the unperturbed plasma. The emission was coherent due to the constructive interference of waves emitted by a strongly localized source.We developed a model based on the catastrophe theory explaining the formation and robustness of the density spikes. The laser pulse pushed the electrons aside (away from the laser axis), creating an almost void cavity in the electron density and an outgoing bow wave [5]. The electrostatic potential of the remaining ions pulled background electrons transversely toward the laser axis. This created multi-stream plasma motion, which, in accordance with catastrophe theory, resulted in the formation of several types of density catastrophes (singularities). Sharp outlines of the cavity wall and bow wave corresponded to fold catastrophes. The density spike corresponded to a higher-order cusp catastrophe at the joint of two folds. Thus, the model predicted that the harmonics were generated by extremely localized, nanometer-scale sources. In particular, for linearly polarized lasers, the model predicted two singularities (point-like sources) situated in the laser polarization plane. Because of the small source size, the angular distribution of the x-ray emission was relatively broad, up to ~20-30°.3.Experimental confirmation of theory predictions: point-like double x-ray source and off-axis harmonicsWe performed dedicated experiments and confirmed these cornerstone theory predictions. In particular, we detected coherent off-axis XUV radiation [6] and recorded bright soft x-ray harmonics within the off-axis observation angle range from 8 to 18° [7]. Employing a high-resolution LiF detector [6], we imaged the emitters and obtained two point-like sources situated along the laser polarization direction [7]. The direct observation gave a 0.8 um source size, which was limited by geometrical aberrations of the spherical mirror. The presence of fine fringes indicated that the actual source size was much smaller, of the order of 100 nm or less.4.Challenges for imaging x-ray opticsIt is extremely important to measure the source size with higher resolution, as the simulations predict that the source can be as small as ~10 nm. Together with wide angular distribution, this represents a great challenge for imaging x-ray optics. Additional difficulty comes from the multi-TW laser, which must be filtered out. We are now preparing experiments with high-resolution normal-incidence imaging multilayer mirrors [8]; another option could be a variation of the coherent diffractive imaging.5.Ultrabright compact coherent x-ray sourceOur findings open a way to a new ultra-bright, spatially and temporally coherent x-ray source driven by a compact high-power laser, available for university- and industry labs. The photon energy can reach a few keV with petawatt driving lasers. Moreover, the peak spectral brightness of the new source can approach the record held by free-electron lasers in the photon energy range from 10 eV to ~ 1 keV.[1]A. S. Pirozhkov, M. Kando, T. Zh. Esirkepov, P. Gallegos, H. Ahmed, E. N. Ragozin, A. Y. Faenov, T. A. Pikuz, et al., "Soft-X-Ray Harmonic Comb from Relativistic Electron Spikes," Phys. Rev. Lett. 108, 135004-5 (2012).[2]A. S. Pirozhkov, M. Kando, T. Zh. Esirkepov, P. Gallegos, H. Ahmed, E. N. Ragozin, A. Y. Faenov, T. A. Pikuz, T. Kawachi, et al. "High order harmonics from relativistic electron spikes" New J. Phys. 16, 093003-30 (2014).[3]H. Kiriyama, M. Mori, Y. Nakai, T. Shimomura, M. Tanoue, et al., "High-contrast, high-intensity laser pulse generation using a nonlinear preamplifier in a Ti : sapphire laser system", Opt. Lett. 33, 645-647 (2008).[4]C. J. Hooker, J. L. Collier, O. Chekhlov, R. Clarke, E. Divall, K. Ertel, B. Fell, P. Foster, et al., "The Astra Gemini project -- A dual-beam petawatt Ti:Sapphire laser system", J. Phys. IV France 133, 673-677 (2006).[5]T.Esirkepov, Y.Kato, S.Bulanov, "Bow Wave from Ultraintense Electromagnetic Pulses in Plasmas" PRL 101 265001 (2008).[6]T. Pikuz, A. Faenov, A. Pirozhkov, A. Astapov, G. Klushin, S. Pikuz, et al., "High performance imaging of relativistic soft X-ray harmonics by sub-micron resolution LiF film detectors," Physica Status Solidi C 9, 2331 (2012).[7]A. S. Pirozhkov, T. Zh. Esirkepov, T. A. Pikuz, A. Ya. Faenov, K. Ogura, Y. Hayashi, H. Kotaki, E. N. Ragozin, et al., "Burst intensification by singularity emitting radiation in multi-stream flows," arXiv:1611.05547 (2016).[8]A.S.Pirozhkov, E.N.Ragozin "Aperiodic multilayer structures in soft X-ray optics" Phys. Usp. 58 1095 (2015).
The 14th Symposium on X-ray Imaging Optics

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