1. CLEO2004 K. L. Ishikawa No. 0 Enhancement in photoemission from He + by simultaneous irradiation of laser and soft x-ray pulses Kenichi L. Ishikawa Department of Quantum Engineering & Systems Science, Graduate School of Engineering, University of Tokyo Web: http://ishiken.free.fr E-mail: ishiken@q.t.u-tokyo.ac.jp K. Ishikawa, Phys. Rev. Lett. 91, 043002 (2003) JMA3

2. CLEO2004 K. L. Ishikawa No. 1 High-order harmonic generation The recent progress in the high-order harmonic generation (HHG) technique has enabled the production of high-power coherent soft x-ray and extreme ultraviolet (XUV) pulses ■RIKEN, Laser Technology Laboratory (K. Midorikawa) 25 nJ@ = 13.5 nm (Ti:Sapphire H59) 0.33  J @ = 29.6 nm (Ti:Sapphire H27) 1  J @ = 54 nm (Ti:Sapphire H15) 4.7  J @ = 62.3 nm (Ti:Sapphire H13) 7  J @ = 72.7 nm (Ti:Sapphire H11) ■CEA-Saclay, DSM/DRECAM/SPAM (P. Salieres) 1.9  J @ = 53.3 nm (Ti:Sapphire H15) ■University of Tokyo, ISSP (S. Watanabe) 1.2  J @ = 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) Appl. Phys. Lett. 84, 4 (2004) Hergott et al. Phys. Rev. A 66, 021801 (2002) Yoshitomi et al. Opt, Lett. 27, 2170 (2002)

3. CLEO2004 K. L. Ishikawa No. 2 4.7  J @ = 62.3 nm (Ti:Sapphire H13) 0.33  J @ = 29.6 nm (Ti:Sapphire H27) 10 14 W/cm 2 10 15 W/cm 2 Soft x-ray XUV High-order harmonic generation focused to an area of 10  m 2 by a mirror Assuming the pulse duration < 30 fs High-field physics in the soft x-ray ranges may be within experimental reach !

4. CLEO2004 K. L. Ishikawa No. 3 Numerical experiments for He + ■ Two-photon ionization of He + by the 27th harmonic of a Ti:Sapphire laser K. Ishikawa and K. Midorikawa, Phys. Rev. A 65, 043405 (2002) ■ Simultaneous laser and soft x-ray (Ti:S H27) pulse irradiation to He + Photoemission Ionization He + ion Soft x-ray (Ti:S H27) Laser (Ti:S) Photoemission Ionization

5. CLEO2004 K. L. Ishikawa No. 4 Simulation model ■Numerical method Alternating direction implicit (Peaceman-Rachford) method ■He 2+ 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 Time-dependent Schrodinger equation Field of the combined harmonic and fundamental pulse

6. CLEO2004 K. L. Ishikawa No. 5 He + Ti:S H27 〜 40 eV Cut-off energy of HHG Ionization potential Ponderomotive energy 40.8 eV 1s1s1s1s 2s, 2p E = 0 54.4 eV Field ionization Classical motion Recombination photoemission P. Corkum (1993) ■In the case of He +... Higher energy cut-off But… Extremely low efficiency Pulsewidth = 10 fs High-order harmonic generation (Fundamental pulse alone) Three-step model

7. CLEO2004 K. L. Ishikawa No. 6 Fundamental + H27 ■ 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] 17 orders of magnitude ! Pulsewidth = 10 fs He + ion Soft x-ray (Ti:S H27) Laser (Ti:S) HHG of even higher orders

8. CLEO2004 K. L. Ishikawa No. 7 Dependence of ionization on fundamental wavelength H27 H27-fund. H27-2xfund. H27: 10 13 W/cm 2 H27 40.8 eV 1s1s1s1s 2p2p2p2p E = 0 He + Field ionization 40.8 eV 1s1s1s1s 2s2s2s2s E = 0 He + H27 Field ionization fundamental 2s excitation (8%)

9. CLEO2004 K. L. Ishikawa No. 8 Harmonic generation from a coherent superposition of states Fundamental wavelength = 800 nm ■92% 1s, 8% 2s + laser (800nm) 3x10 14 W/cm 2 ■100% 1s + laser (800nm) 3x10 14 W/cm 2 + H27 10 13 W/cm 2 The two spectra are strikingly similar to each other both in peak heights and positions !

10. CLEO2004 K. L. Ishikawa No. 9 Dependence of ionization on fundamental wavelength H27 H27-fund. H27-2xfund. H27: 10 13 W/cm 2 Laser (3x10 14 W/cm 2 )+H27 (10 12 W/cm 2 ) Population after the pulse 2s : 1.0 x 10 -4 2p : 7.7 x 10 -6 ■The harmonic intensity does not depend much on the fundamental wavelength. Photoemission spectra

11. CLEO2004 K. L. Ishikawa No. 10 Two-color frequency mixing Superposition of 1s, 2s, and 2p 1s Laser Laser + H27 Dominant contribution: direct process from the ground state (two-color frequency mixing) From the superposition of states Large discrepancy between the two spectra Fundamental wavelength = 785 nm

12. CLEO2004 K. L. Ishikawa No. 11 Mechanism of the enhancement 40.8 eV 1s1s1s1s 2s2s2s2s E = 0 He + H27 Field ionization fundamental 40.8 eV 1s1s1s1s 2p2p2p2p E = 0 He + H27 Field ionization Virtual state Two-color frequency mixing Optical field ionization (OFI) from a virtual state Harmonic generation from a coherent superposition of states Watson et al., Phys. Rev. A53, R1962 (1996) 800 nm 785 nm

13. CLEO2004 K. L. Ishikawa No. 12 With an even shorter wavelength The higher the photon energy, ■the weaker the photoemission ■the lower the cutoff energy Three-step model with a finite initial electron velocity

14. CLEO2004 K. L. Ishikawa No. 13 Figure 3 in the Technical Digest … r max = 125 a.u. r max = 250 a.u. Artifact due to the reflection from the calculation boundary ! Sorry for this …

15. CLEO2004 K. L. Ishikawa No. 14 Conclusions Dramatic enhancement of harmonic photoemission Combined soft x-ray (Ti:S H27) and fundamental laser pulse compared with the case of the fundamental pulse alone ■Mechanism Harmonic generation from a coherent superposition of states Two-color frequency mixing ■The higher the photon energy (> ionization threshold), the weaker the photoemission the lower the cutoff energy

16. CLEO2004 K. L. Ishikawa No. 15 He 2+ yield (ionization) Fundamental (800nm) H27He 2+ yield 3×10 14 W/cm 2 2.29×10 -15 10 14 W/cm 2 4.14 ×10 -4 3×10 14 W/cm 2 10 14 W/cm 2 0.815 3×10 14 W/cm 2 10 13 W/cm 2 0.170 3×10 14 W/cm 2 10 12 W/cm 2 1.85×10 -2 3×10 14 W/cm 2 10 11 W/cm 2 1.86×10 -3 10 15 W/cm 2 10 13 W/cm 2 0.083 ■The yield by [fundamental+H27] >>>>> [fundamental alone] or [H27 alone] H27 plays an essential role in 1s → 1s, 2p Fundamental plays a major role in 1s, 2p → continuum ■The yield by [fundamental+H27] is proportional to the H27 intensity. Saturation at the higher intensity Both are necessary for efficient ionization ! Decrease of the yield !!

17. CLEO2004 K. L. Ishikawa No. 16 The fundamental pulse plays three roles H27 H27-fund. H27-2xfund. H27: 10 13 W/cm 2 40.8 eV 1s1s1s1s 2s2s2s2s E = 0 He + H27 Field ionization fundamental ■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. Complicated intensity dependence 800nm

18. CLEO2004 K. L. Ishikawa No. 17 Dependence on fundamental intensity The He 2+ yield is NOT a monotonically increasing function of fundamental intensity!