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2003 年 8 月 31 日 ( 日 ) 応用物理学会秋季学術講演会（福岡大学）

高強度軟エックス線パルス同時照射による He + イオンからの高調波発生の飛躍的増大 Dramatically enhanced high-order harmonic generation from He + under simultaneous laser and soft x-ray pulse irradiation. 2003 年 8 月 31 日 ( 日 ) 応用物理学会秋季学術講演会（福岡大学）. 石川顕一 東京大学大学院工学系研究科 Web: http://ishiken.free.fr E-mail: ishiken@q.t.u-tokyo.ac.jp.

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2003 年 8 月 31 日 ( 日 ) 応用物理学会秋季学術講演会（福岡大学）

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  1. 高強度軟エックス線パルス同時照射によるHe+イオンからの高調波発生の飛躍的増大Dramatically enhanced high-order harmonic generation from He+ under simultaneous laser and soft x-ray pulse irradiation 2003年8月31日(日) 応用物理学会秋季学術講演会（福岡大学） 石川顕一 東京大学大学院工学系研究科 Web: http://ishiken.free.fr E-mail: ishiken@q.t.u-tokyo.ac.jp

  2. 高次高調波発生 近年の高次高調波発生技術の進歩により、高強度のコヒーレント軟エックス線・極端紫外(XUV)光パルスの発生が可能になってきている。 • 理化学研究所レーザー物理工学研究室(緑川克美) • 0.33 mJ @ l = 29.6 nm (Ti:Sapphire H27) • 1 mJ @ l = 54 nm (Ti:Sapphire H15) • 4.7 mJ @ l = 62.3 nm (Ti:Sapphire H13) • 7 mJ @ l = 72.7 nm (Ti:Sapphire H11) • CEA-Saclay, DSM/DRECAM/SPAM (P. Salieres) • 1.9 mJ @ l = 53.3 nm (Ti:Sapphire H15) • 東京大学物性研究所(渡部俊太郎) • 1.2 mJ @ l = 49.7 nm (KrF Excimer H5) Takahashi et al. Phys. Rev. A 66, 021802(2002) Opt. Lett. 27, 1920(2002) JOSA B 20, 158 (2003) Hergott et al. Phys. Rev. A 66, 021801 (2002) Yoshitomi et al. Opt, Lett. 27, 2170 (2002)

  3. 軟エックス線 極端紫外 0.33 mJ @ l = 29.6 nm (Ti:Sapphire H27) 4.7 mJ @ l = 62.3 nm (Ti:Sapphire H13) ミラーで 10mm2 に集光 パルス幅 < 30 fs 1015 W/cm2 1014 W/cm2 高次高調波発生 これまで可視光近傍に限られていた高強度場物理現象の、短波長領域(極端紫外・軟エックス線)での実現も夢ではない！

  4. Numerical experiments for He+ • Two-photon ionization of He+ by the 27th harmonic of a Ti:Sapphire laser （物理学会2002年春） • Three-photon ionization of He+ by the 13th harmonic of a Ti:Sapphire laser （応物学会2003年春） • Simultaneous laser and soft x-ray (Ti:S H27) pulse irradiation to He+ • Photoemission • Ionization

  5. Model Field of the combined harmonic and fundamental pulse Time-dependent Schrodinger equation • Numerical method • Alternating direction implicit (Peaceman-Rachford) method • He2+ Yield • evaluated as the number of electrons absorbed by the mask function at the outer radial boundary. • Harmonic intensity • obtained from the Fourier transform of the dipole acceleration

  6. E = 0 Recombination photoemission 2s, 2p 54.4 eV 40.8 eV Classical motion 1s He+ Field ionization High-order harmonic generation(Fundamental pulse alone) Pulsewidth = 10 fs • In the case of He+... • Higher energy cut-off • But… Extremely low efficiency Cut-off energy of HHG Ponderomotive energy Ionization potential P. Corkum (1993) Simpleman’s theory Ti:S H27 〜 40 eV

  7. 17桁の増大！ 基本波＋27次高調波同時照射 ヘリウムイオン 軟エックス線（高調波） • The harmonic intensity is enhanced by 17 orders of magnitude! • The cut-off energy remains high. • The efficiency is even slightly higher than [Laser → H] さらに高次の高調波 レーザー光 Pulsewidth = 10 fs

  8. Laser (3x1014W/cm2)+H27 (1012W/cm2) 基本波＋27次高調波同時照射 ヘリウムイオン 軟エックス線（高調波） • 高調波強度は、基本波波長に大きくは依存しない。 さらに高次の高調波 レーザー光 Pulsewidth = 10 fs

  9. E = 0 仮想準位 2p E = 0 40.8 eV 27次 基本波 2s 1s He+ 40.8 eV 27次 1s He+ Field ionization Field ionization メカニズム Harmonic generation from a coherent superposition of states Two-color frequency mixing 仮想準位からのOFI Watson et al., Phys. Rev. A53, R1962 (1996)

  10. Decrease of the yield !! He2+ yield (イオン化) Laser intensity • The yield by [fundamental+H27] >>>>> [fundamental alone] or [H27 alone] • H27 plays a major role in 1s → 2p • Fundamental plays a major role in 2p → continuum • The yield by [fundamental+H27] is proportional to the H27 intensity. • Saturation at the higher intensity Both are necessary for efficient ionization !

  11. 基本波強度依存性 • イオン化の収量は、基本波強度の単調増加関数ではない！

  12. E = 0 H27-fund. E = 0 2p 基本波 2s H27-2xfund. 40.8 eV 27次 H27 40.8 eV 27次 Field ionization 1s 1s He+ He+ E = 0 仮想準位 2p 40.8 eV 27次 Field ionization Field ionization 1s He+ 波長依存性 H27: 1013 W/cm2

  13. H27-fund. E = 0 基本波 H27: 1013 W/cm2 2s H27-2xfund. H27 40.8 eV 27次 1s He+ Field ionization 基本波には３つの役割がある。 波長800nmの場合 • To field-ionize from the excited levels. • To assist the transition to the excited levels through two-color photon excitation • To shift and broaden the excited levels through the dynamic Stark effect. 複雑な基本波強度依存性

  14. Summary Dramatic enhancement of harmonic photoemission ionization Combined high-order harmonic and fundamental laser pulse (H27) compared with the case of the fundamental pulse alone This might open a way to develop an intense light source with an even shorter wavelength than available today.

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