Enhancement in photoemission from He + by simultaneous irradiation of laser and soft x-ray pulses

1. JMA3 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)

2. 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 @ l = 13.5 nm (Ti:Sapphire H59) • 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) • University of Tokyo, ISSP (S. Watanabe) • 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) Appl. Phys. Lett. 84, 4 (2004) Hergott et al. Phys. Rev. A 66, 021801 (2002) Yoshitomi et al. Opt, Lett. 27, 2170 (2002)

3. High-order harmonic generation Soft x-ray XUV High-field physics in the soft x-ray ranges may be within experimental reach ! 0.33 mJ @ l = 29.6 nm (Ti:Sapphire H27) 4.7 mJ @ l = 62.3 nm (Ti:Sapphire H13) focused to an area of 10mm2 by a mirror Assuming the pulse duration < 30 fs 1015 W/cm2 1014 W/cm2

4. 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) Photoemission Ionization Laser (Ti:S)

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

7. Fundamental + H27 He+ ion Soft x-ray (Ti:S 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] HHG of even higher orders Laser (Ti:S) 17 orders of magnitude ! Pulsewidth = 10 fs

8. Field ionization Dependence of ionization on fundamental wavelength E = 0 H27-fund. 2p H27: 1013 W/cm2 H27-2xfund. 40.8 eV H27 H27 Field ionization 1s He+ 2s excitation (8%) E = 0 fundamental 2s 40.8 eV H27 1s He+

9. Harmonic generation from a coherent superposition of states Fundamental wavelength = 800 nm • 92% 1s, 8% 2s + laser (800nm) 3x1014 W/cm2 • 100% 1s + laser (800nm) 3x1014 W/cm2 + H27 1013 W/cm2 The two spectra are strikingly similar to each other both in peak heights and positions !

10. Dependence of ionization on fundamental wavelength H27-fund. • The harmonic intensity does not depend much on the fundamental wavelength. H27: 1013 W/cm2 H27-2xfund. Photoemission spectra H27 Laser (3x1014W/cm2)+H27 (1012W/cm2) Population after the pulse 2s : 1.0 x 10-4 2p : 7.7 x 10-6

11. Two-color frequency mixing Fundamental wavelength = 785 nm Dominant contribution: direct process from the ground state(two-color frequency mixing) Laser Superposition of 1s, 2s, and 2p 1s Laser + H27 Large discrepancy between the two spectra From the superposition of states

12. E = 0 Virtual state 2p E = 0 40.8 eV H27 fundamental 2s 1s He+ 40.8 eV H27 1s He+ Field ionization Field ionization Mechanism of the enhancement 800 nm 785 nm Harmonic generation from a coherent superposition of states Two-color frequency mixing Optical field ionization (OFI) from a virtual state Watson et al., Phys. Rev. A53, R1962 (1996)

13. 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. Figure 3 in the Technical Digest … rmax = 125 a.u. rmax = 250 a.u. Artifact due to the reflection from the calculation boundary ! Sorry for this …

15. Conclusions Combined soft x-ray (Ti:S H27) and fundamental laser pulse • 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 Dramatic enhancement of harmonic photoemission compared with the case of the fundamental pulse alone

16. The yield by [fundamental+H27] >>>>> [fundamental alone] or [H27 alone] • H27plays 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 ! He2+ yield (ionization) Decrease of the yield !!

17. H27-fund. E = 0 fundamental H27: 1013 W/cm2 2s H27-2xfund. H27 H27 40.8 eV 1s He+ Field ionization The fundamental pulse plays three roles 800nm • 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

18. Dependence on fundamental intensity The He2+ yield is NOT a monotonically increasing function of fundamental intensity!