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Laser-assisted photoionization for attosecond pulse measurements

Laser-assisted photoionization for attosecond pulse measurements. Z. X. Zhao. KSU AMO seminar 9-29-2004. Outline. Motivation Review on ultrashort pulse measurements Theory of laser assisted photoionization Spectra of circularly polarized laser assisted XUV photoionization of argon

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Laser-assisted photoionization for attosecond pulse measurements

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  1. Laser-assisted photoionization for attosecond pulse measurements Z. X. Zhao KSU AMO seminar 9-29-2004

  2. Outline • Motivation • Review on ultrashort pulse measurements • Theory of laser assisted photoionization • Spectra of circularly polarized laser assisted XUV photoionization of argon • Pulse retrieving • Summary

  3. Left circularly polarized IR Right circularly polarized IR Polarization gating Gas target ? as pulse IR Gas Spectra Motivation • Attosecond pulse generated by Zenghu’s group using polarization gating • Measure it? • In this work: • Using circularly polarized laser pulses • laser-assisted photoionization of Argon • Study the procedures of measuring attosecond pulses as pulses?

  4. Review on ultrashort pulse measurement • Autocorrelation • The pulse is split into two parts and then overlapped temporally in a nonlinear medium. • Limitation on wavelength. • X-ray pulses generated too weak. • Cross-correlation • Laser-modified photoionization spectrum provides the nonlinearity linking the x-ray to the laser pulse • The atomic gas serves as the nonlinear medium. • For long XUV pulses (>T0): • For sub-laser-cycle pulses (this talk)

  5. X-ray Linear or circular  Initiate atomic process Laser Attosecond streak camera: cross-correlation • Cross-correlation • Probe atomic dynamics Time-resolved spectra

  6. Theory of laser-assisted photoionizaton

  7. Quantum mechanical model Strong field approximation: neglect Coulomb field Assuming no depletion of ground state, no structure Assume : XUV: ionization Laser: modify energy Stationary phase equation: ts: Saddle point

  8. y   x  Linear polarized laser assisted photoionization classical model: Linear polarization: Electron energy at observation angle :

  9. t1 Linear polarized laser assisted photoionization XUV pulse Laser-free momentum distribution t0 A(t) (drift velocity)

  10. y  x   Circularly polarized laser assisted photoionization Circular polarization: (Replace  by ’ in that of linear case and noted that the definition of  is different from PRL88,173903)

  11. t1 t-1 Circularly polarized laser assisted photoionization Laser-free t0 XUV pulse A(t) (drift velocity)

  12. HOW to characterize attosecond pulses from Spectra of circularly polarized laser assisted XUV photoionization of argon?

  13. Laser-free photoionization of Argon Starting from 3P ground state, reduced dipole moment to s and d cont.: Total cross section proportional to Angular distribution: Asymmetry parameter  can be calculated from R- and R+ Single active electron model of Ar:

  14. Ix() Laser-free photoionization:Cross section and asymmetry parameter XUV:1012W/cm2,0.1-2fs, 35 ev (21HG)

  15. Transform-limited vs chirped pulses Transform-limited: Chirped: Do laser assisted photoionization to get pulse information Laser:5x1013W/cm2,5fs, 1.65 eV (750 nm,2.5fs) XUV:1012W/cm2,0.1-2fs, 35 ev (21HG)

  16. No chirp– dependence on the phase angle of circularly polarized laser no laser xuv along x axis 0.1 fs for xuv

  17. Dependence on the Chirp

  18. Pulse retrieving

  19. Procedures of pulse retrieving 1) Laser-free PI spectra as input: 2) Free guess of the phases: 3) Construct XUV pulse: 4) Calculate laser-assisted spectra: 5) Compared with measured one: 6) Find best fit of the phases: 1. genetic algorithm 2. 5 parameter fitting

  20. Straightforward Genetic Algorithm Discretize the phases: Genetic algorithm: 15 bits, 200 parameters, 200 population, 200 generation 1fs, chirp 10 as an example

  21. 5-parameter GA Taylor expansion of the phase:

  22. Transform limited (no chirp) XUV pulses 0.2 fs • Energy width decreases as pulse duration increases • The angular distribution of final momentum • For given energy • broader as XUV pulse duration increases • For XUV duration approaching laser cycle: • image expands in all direction • Sidebands begin to emerge 0.5 fs 2 fs no laser

  23. Double-pulse XUV light (a) no laser (b),(c),(d) laser phase with 0, /4 and /2

  24. mapping

  25. Chirp-dependence Stationary phase equation (no chirp): ts: Saddle point Linearly chirped XUV pulse (, chirp parameter): Energy center of gravity at given angles: spiral curve

  26. Summary • Calculated spectra • Retrieved electric field of attosecond pulse • Retrieving method can be further improved

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