Kansas State University

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Single Attosecond Pulses and XUV Supercontinuum
Shambhu Ghimire, Bing Shan, and Zenghu Chang
J. R. Macdonald Laboratory
Kansas State University
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Applications of Attosecond Pulses
as
ps
zs
fs
10-21 s
10-18 s
10-15 s
10-12 s
Time
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Generation of attosecond pulsesHigh harmonic
generation
Step1 Ionization Step2 Acceleration Step3
Recombination
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Previous workAttosecond pulse at cutoff
85 eV
135 ev
Attosecond pulse train
Single as pulse
A. BALTU KA et.al, Nature 421, 611( 2003)
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Attosecond pulse train
Elaser
Electron trajectory
t (fs)
IHHG
t (fs)
0
1
-1
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Discrete harmonic orders in the plateau-Spatial
analogy of pulse train interference
Discrete pattern at plateau analogy to multi-slit
diffraction
Single slit
Double slit
Multi slit
Diffraction patterns (spatial frequency)
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Our goal

• The drawbacks of previous work
• Attosecond pulse train in the plateau
• Single attosecond pulse only at the cutoff
• Harmonic intensity is low at the cutoff
• Covers a narrow spectrum range

• We intend to generate single attosecond pulses in
  the plateau range

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Generation of single attosecondpulse in the
plateau

• Approach 1- Half cycle laser pulse
• Reduce laser pulse to half of a cycle.
• Very hard to do.
• Approach 2- Polarization gating
• The laser pulse can have a few cycles.

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Polarization gating

• Laser is linearly polarized

HHG emission

• Laser is circularly polarized

No HHG emission

• Polarization gating linear portion is less than
  half of a cycle

t (fs)
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Generation of ellipticity- depended pulse -with
birefringence optics
Optics axis
Time delay
Time delay
Laser field
e-ray
o-ray
L
R
e-ray
o-ray
Optic axis
Quartz plate
¼ waveplate
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Generation of ellipticity-depended pulse
Time (fs)
Time (fs)
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Pulse duration measurement Frequency-resolved
optical gating
wavelength
Compensating plate
I(t)
time
BBO Crystal
I(t)
BS
Time Delay Stage
Computer
lens
Spectrometer and cold CCD
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Pulse duration measurement Hollow-core fiber
output
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Attosecond pulse generation experimental setup
KLS 4 mJ, 25fs 0.8 mm
Hollow-core fiber
0.5 mJ, 8fs
Gas nozzle
CCD
Grating
MCP Phosphor
Filter
¼ waveplate
HHG Spectrum
Quartz plate
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Spectra broadening for shorter pulse
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21
23
25
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With a linear, 25 fs pulses (10 laser cycle) (
the interference of 20 as pulses)
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21
23
25
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With two circular 12 fs pulses, linear portion
3 fs (1 cycle)
( the interference of 2 as pulses)
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The effect of polarization gating
With two pulses gt9.2fs, 1.7fs linear polarization
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Single Attosecond Pulsesand XUV Super continuum
45 nm
20 nm
With two pulses 8 fs
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Simulated spectrum of HHG Without polarization
gating
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Simulated spectrum of HHG With polarization gating
Continuum at plateau
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Summary

• Single attosecond pulse at plateau for the first
  time.
• Birefringence optics was used to produce the
  ellipticity-dependent laser pulse.
• The single attosecond over a broader spectrum
  range.
• The attosecond at the plateau is more intense
  than that at the cutoff.