cleo2004 k. l. ishikawa no. 0 enhancement in photoemission from he + by simultaneous irradiation of...

CLEO2004 　 K. L. Ishikawa No. 1

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: [email protected]

K. Ishikawa, Phys. Rev. Lett. 91 , 043002 (2003)

JMA3


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)


4.7　4.7　J　@　J　@　　=　62.3　nm　　=　62.3　nm　(Ti:Sapphire　H13)(Ti:Sapphire　H13)

4.7　4.7　J　@　J　@　　=　62.3　nm　　=　62.3　nm　(Ti:Sapphire　H13)(Ti:Sapphire　H13)

0.33　0.33　J　@　J　@　　=　29.6　　=　29.6　nm　(Ti:Sapphire　H27)nm　(Ti:Sapphire　H27)0.33　0.33　J　@　J　@　　=　29.6　　=　29.6　

nm　(Ti:Sapphire　H27)nm　(Ti:Sapphire　H27)

1014　W/cm21015　W/cm2

Soft　x-rayXUV

High-order harmonic generation

focused　to　an　area　of　10m2　by　a　mirrorAssuming　the　pulse　duration　<　30　fs　

High-field　physics　in　the　soft　x-ray　ranges　may　be　High-field　physics　in　the　soft　x-ray　ranges　may　be　within　experimental　reach　!within　experimental　reach　!


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+　ionSoft　x-ray　(Ti:S　H27)

Laser　(Ti:S)Photoemission

Ionization


Simulation model

■ 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

Time-dependent　Schrodinger　equationTime-dependent　Schrodinger　equation

€

i∂Φ r,t( )

∂t= −

12∇2 −

2r

− zE t( ) ⎡ ⎣ ⎢

⎤ ⎦ ⎥Φ r,t( )

Field　of　the　combined　harmonic　and　fundamental　pulse

Field　of　the　combined　harmonic　and　fundamental　pulse


HeHe++

Ti:S　H27　〜 40　eV

Cut-off　energy　of　HHG

€

Ec = IP + 3.17UP

Ionization　potentialPonderomotive　energy

€

UP =e2E2

4mω2

40.8　eV40.8　eV

11ss

2s,　22s,　2pp

EE　=　0　=　0

54.4　eV54.4　eV

Field　ionization

Classical　motion

Recombinationphotoemission

P.　Corkum　(1993)

■ In the case of He+...Higher energy cut-offBut… Extremely low efficiency

Pulsewidth　=　10　fs

High-order harmonic generation(Fundamental pulse alone)

Three-step　model


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+　ionSoft　x-ray　(Ti:S　H27)

Laser　(Ti:S)HHG　of　even　higher　orders


Dependence of ionization on fundamental wavelength

H27

H27-fund.

H27-2xfund.

H27: 1013 W/cm2

H27

40.8　eV

11ss

22pp

EE　=　0　=　0

HeHe++Field　ionization

40.8　eV

11ss

22ss

EE　=　0　=　0

HeHe++

H27

Field　ionization

fundamental

2s　excitation　(8%)


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　!


Dependence of ionization on fundamental wavelength

H27

H27-fund.

H27-2xfund.

H27: 1013 W/cm2

Laser　(3x1014W/cm2)+H27　(1012W/cm2)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


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 statesLarge　discrepancy　between　the　two　spectra

Fundamental wavelength = 785 nm


Mechanism of the enhancement

40.8　eV

11ss

22ss

EE　=　0　=　0

HeHe++

H27

Field　ionization

fundamental

40.8　eV

11ss

22pp

EE　=　0　=　0

HeHe++

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　n

m

785　n

m


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

70

65

60

55

50

45

40

35

Order of the harmonic emittedat the nuclear position

350300250200150100500

Phase of the field at the moment of ionization

Laser + H53 Laser + H27


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　…


Conclusions

Dramatic　enhancement　ofharmonic　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


He2+ yield (ionization)

Fundamental　(800nm)

H27 He2+　yield

3×1014　W/cm2 2.29×10-15

1014　W/cm2 4.14　×10-4

3×1014　W/cm2 1014　W/cm2 0.815

3×1014　W/cm2 1013　W/cm2 0.170

3×1014　W/cm2 1012　W/cm2 1.85×10-2

3×1014　W/cm2 1011　W/cm2 1.86×10-3

1015　W/cm2 1013　W/cm2 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　!!


The fundamental pulse plays three roles

H27

H27-fund.

H27-2xfund.

H27:　1013　W/cm2

40.8　eV

11ss

22ss

EE　=　0　=　0

HeHe++

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


Dependence on fundamental intensity

The　He2+　yield　is　NOT　a　monotonically　increasing　function　of　fundamental　intensity!

cleo2004 k. l. ishikawa no. 0 enhancement in photoemission from he + by simultaneous irradiation of...

Documents