Slide sem t

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X-ray generation with ultrashort light pulses
Lino Misoguti Instituto de Física de São Carlos
- USP JILA- University of Colorado
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• Motivation
• Frequency generation using nonlinear crystal
• Frequency generation using gas and hollow-core
  fiber
• Non-linear processes to generate UV, VUV, EUV and
  SXR
• Experiments results
• Summary and perspectives

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• To develop tunable, efficient, ultrashort pulse
  sources from UV to SXR
• nonlinear wavemixing and high harmonic generation
  with ultrashort pulses in gases
• Generation of sub-femtosecond pulse
• Several application for ultrashort UV-SXR pulses
• Chemical dynamic/control
• High-density plasma interaction
• X-ray nonlinear optics
• Material machining, lithography and microscopy
• Biophysics DNA, water-window holography
• A unique light source - not possible using
  crystal NLO

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The Soft X-Ray (SXR) and Extreme Ultraviolet
(EUV) are characterized by the presence of the
primary atomic ressonances and absorption edges
of most of low and intermediate Z elements
(Zatomic number) The most common source are
synchroton radiation machine
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Second Harmonic Generation (SHG)
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-For high-efficiency conversion -High
nonlinear coefficient (?(2)) -Phase-matching
-Anisotropic materials (birefringence)
-Uniaxial (KDP, LiNbO3, BBO, LBO)
-Biaxial (KTP, DAST)
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Two different index of refraction for a uniaxial
crystal
Index of Refraction
Wavelength

• Angular phase-matching
• Temperature tuning

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• Nonlinear crystal is very efficient for
  conversion in the Visible and Infrared region
• Optical transmission range (typically gt200nm)
• Ultrafast pulses have very broad spectrum
• Limited phase-matching bandwidth
• walk-off
• Very thin crystal should be used (low efficiency)

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• Gases as material for UV and SRX region
• Transparency
• Low-density, low walk-off, low color dirpersion
• Isotropic
• ?(3) is the first effective nonlinear coefficient
  
• How to achieve phase-matching in isotropic
  material?
• No birefringence to compensate the color
  dispersion
• Phase-matching is necessary to high-efficiency
  frequency conversion

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Hollow-core Fiber
Pump
UV, VUV
EUV, SXR
Idler
Noble Gas Inside (Ar, Kr, Xe)

• Use hollow-core fiber geometry
• frequently used to broaden the spectrum and
  generate ultrashort pulse by SPM (Self-Phase
  Modulation)
• extends the interaction length
• pressure tuning coupled with fiber mode
  propagation allows phase-matching
• increasing efficiency

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• The gas-filled hollow-core fiber acts as a
  waveguide
• By adjusting the pressure, fiber diameter, or
  spatial mode of beams, the k-vector of the light
  can be adjusted

u11 2.405 ? EH11 ? TEM00 u21 3.862 ? EH21 u12
5.520 ? EH12 u31 5.136 ? EH31 ...

• The negative contribution of waveguide helps to
  achieve phase-matching condition

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Can these processes be phase-matched?
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?k2kpump- kidler- ksignal
DF 4-Wave Mixing
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Broad Band Oscillator
Diode Pumped Sources
Ring Amplifiers with Pellicle
Deformable Mirror Pulse Shaper
5mJ 1kHz 25fs
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l0.25 m a125 ?m E(400nm)25 ?J E(800nm)25
?J E(267nm)EH112 ?J Eff.8 Pulsewidth lt 15fs
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EH11
l0.8 m a125 ?m E(400nm)25 ?J E(800nm)25
?J E(267nm)EH117.5?J Eff.30 Pulsewidth lt
8fs
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Capillary Diameter
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Depletion of 400nm
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Many pathways/processes are possible

• Which path are most important?
• phase-matching
• effective order nonlinearity

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D.M..
?, 2?
3?, 4?, 5?
?
2?
Cascaded
1mJ, 1 KHz 25 fs laser
B.S.
BBO
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Efficient Cascaded Four-Wave Mixing
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Phase-matching conditions use non-depleted
growth of third harmonic as source for cascading
Example
Where
Nonlinear polarization is modulated by
intermediate field E4 grows under several
different phase matching conditions... Quasi
phase-matching process
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?k267nm2k400nm- k800nm- k267nm
?k200nm2k267nm- k400nm- k200nm
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Single section fiber
Experiment (200nm)
Signal Intensity (mW)
Pressure (torr)
and
cascade
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Single section fiber
cascade
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D.M..
?, 2?
3?, 4?, 5?
?
2?
Cascaded
1mJ, 1 KHz 25 fs laser
B.S.
BBO
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Two section fiber
and
cascade
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Krypton
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Index of Refraction of Krypton
n(fundamental)-n(harmonic)0
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Six-wave mixing and THG in Krypton in VUV
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• High harmonics are generated by the coherent
  interaction of an intense laser field and an atom

• Broad comb of frequencies generated
  simultaneously from 4.5eV (UV) up to 600eV (SXR)

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• Harmonics are generated when ionized electrons
  recombine with an ion
• Phase accumulated by the electron trajectory
  determines the harmonic phase - many such
  trajectories contribute to a given harmonic
• The total harmonic intensity is determined in
  part by interferences between different
  trajectories from different 1/2-cycles of the
  laser pulse

laser field
electron
(Kulander
et al
,
Corkum
et al)
Ion
electron
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• Increase the interation length and phase matching
  can be achieved in the waveguide

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Cascaded process in the EUV
Two-color frequency-mixing w and 2w (wltlt 2w)
Blue (40mW) IR(5mW)
Blue (40mW) IR(3mW)
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Blue Only (40mW)
13
12
14
16
11
15
14
11
17
13
10
10
10
15
17
18
12
16
18
18
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Mixing harmonics can be generated by -
14?7(2?)
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Two-color frequency-mixing w and 2w (wltlt 2w)
P(Blue)40mW

• Blue harmonics (10th, 14th, 18th) DEPLETE,
  indication of cascaded four-wave mixing (e.g.
  15? (14?)2?-? )
• Harmonics such 11th, 15th and 13th, 17th GROW as
  (?)2, and harmonic such 16th, 12th GROW as (?)4 .
  This indicates that more than one cascaded
  process contributes to the same harmonic

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Two-color frequency-mixing w and 2w (wgtgt2w)
IR (90mW) Blue(15mW)
IR Only (90mW)
Harmonics can be generated by low-order or
high-order processes -
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P(IR)90mW
P(Blue)5mW

• High odd harmonics (27th, 25th, 23th) DEPLETE
  with 2? power
• Low odd harmonics (13th, 15th, 17th) GROW
  linearly with 2? power
• Pairs of harmonics are CORRELATED (e.g. 16th and
  17th, 18th and 19th, 20th and 21th) as function
  of ? power

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• DEPLETION of high odd harmonics (27th, 25th,
  23th) is expected for cascaded four-order
  wavemixing processes (26? (27?)?-2?)
• Linear GROWTH of low odd harmonics (13th, 15th,
  17th) as function of 2? power, means 1 blue
  photon process rather than 2 blue photons as
  high-order processes predicted (17? 13(?)2(2?)
  or 17? 21(?)-2(2?) )
• If 16? is driven by 17? (16? (17?)?-2?) we
  expected that both harmonics are going to be
  CORRELATED as function of ? power

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• We have demonstrated frequency conversion from UV
  to SXR using nonlinear processes in a hollow core
  fiber filled with gases
• Cascaded frequency conversion into the VUV and
  EUV has been observed
• Spectral bandwidth of harmonics are ultra-broad,
  sufficient to support ultrafast femtosecond
  pulses
• New type of compact light source for VUV up to
  SXR

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• Improve the conversion efficiency for
• X-ray pulse duration measurement
• X-ray nonlinear optic application
• Seed for X-ray pulsed laser
• New approach to compress ultrashort pulse in the
  deep UV, X-ray region
• Chirped mirror
• Group velocity and phase pre-compensation
• Development of few-cicle laser pulse with high
  energy stabilized phase
• Pulseshaper technique to control nonlinear
  processes, enhance harmonic conversion, etc

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Evolutionary Strategy
Deformable
Mirror
Grating
X-ray cell
X-ray CCD
Focusing
Mirror
Perfect Pulse i.e. transform-limited
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1st demonstration of feedback control on a very
high-order quantum nonlinear system Controls
phase of electron wavefunction using light can
optimize a SINGLE harmonic order Total x-ray
energy increases. Increase brightness up to
x11 Works on all gases in all wavelength ranges