Laser-assisted photoionization for attosecond pulse measurements

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Laser-assisted photoionization for attosecond
pulse measurements

• Z. X. Zhao

KSU AMO seminar 9-29-2004
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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

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Motivation

• Attosecond pulse generated by Zenghus 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?
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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 (gtT0)
• For sub-laser-cycle pulses (this talk)

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Attosecond streak camera cross-correlation

• Cross-correlation
• Probe atomic dynamics

Time-resolved spectra
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Theory of laser-assisted photoionizaton
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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
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Linear polarized laser assisted photoionization
classical model
Linear polarization
Electron energy at observation angle ?
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Linear polarized laser assisted photoionization
XUV pulse
Laser-free momentum distribution
t0
A(t) (drift velocity)
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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)
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Circularly polarized laser assisted
photoionization
Laser-free
t0
XUV pulse
A(t) (drift velocity)
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HOW to characterize attosecond pulses from
Spectra of circularly polarized laser assisted
XUV photoionization of argon?
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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
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Laser-free photoionizationCross section and
asymmetry parameter
XUV1012W/cm2,0.1-2fs, 35 ev (21HG)
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Transform-limited vs chirped pulses
Transform-limited
Chirped

Do laser assisted photoionization to get pulse
information
Laser5x1013W/cm2,5fs, 1.65 eV (750
nm,2.5fs) XUV1012W/cm2,0.1-2fs, 35 ev (21HG)
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No chirp dependence on the phase angle of
circularly polarized laser
no laser
xuv along x axis
0.1 fs for xuv
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Dependence on the Chirp
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Pulse retrieving
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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
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Straightforward Genetic Algorithm
Discretize the phases
Genetic algorithm 15 bits, 200 parameters, 200
population, 200 generation
1fs, chirp 10 as an example
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5-parameter GA
Taylor expansion of the phase
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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
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Double-pulse XUV light
(a) no laser (b),(c),(d) laser phase with 0, ?/4
and ?/2
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mapping
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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
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Summary

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