An Ultrafast UV Pulse Shaper for Photochemistry

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An Ultrafast UV Pulse Shaper for Photochemistry
Coherent control using the pump probe method
Abigail Nunn Department of Chemistry,
UCL CoCoChem Graduate Student Meeting University
of Birmingham 24th April 2008
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Dual array liquid crystal spatial light modulator

• SLMs pulse shaping in the visible and near IR
  gives higher resolution than AOPDF

• Direct pulse shaping in the UV (AOPDF and MEMS)
  is still in its infancy

• Shaping in the UV is more difficult, generally
  requires a nonlinearlity after shaping indirect
  shaping

• After the pulse shaper, the light can be
  converted to the UV by SFG (Nuernberger et al.)
• OR SHG (Our choice). The pulse is frequency
  doubled in a 100 mm BBO crystal

New phase and amplitude high resolution pulse
shaper, Monmayrant et al., Rev. Sci. Instrum. 75,
2668 (2004).
Generation of shaped ultraviolet pulses at the
third harmonic of titanium-sapphire femtosecond
laser radiation, Nuernberger et al., Appl. Phys.
B. 88, 519 (2007).
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Pulse shaper set-up
Folded near Littrow Geometry
Traditional 4f Zero Dispersion geometry
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UV shaped pulse characterisation

• 640 pixel dual array pulse shaper therefore, we
  can make pulses up to 7 ps long before replicas
  appear, from an input pulse of 35 fs at the
  transform limit

• Our pulses may be quite weak, 1 mJ, hence
  sensitivity important

• We want to be able to characterise all possible
  phase and amplitude pulse shapes

• We need a characterisation method with fast
  feedback to help us understand the doubling
  process

• SPIDER phase retrieval will fail if the phase
  varies too quickly, and spectral interferometry,
  using a reference pulse, is difficult if we shape
  to very large (ps) durations

• We cannot make a FROG measurement of a 254 nm UV
  pulse with a frequency doubling crystal

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Characterisation

• Our solution characterise an unshaped reference
  pulse using FROG method, and then use this to
  characterise the UV light in an XFROG

E(t)
795 nm
Esignal(w,t)
Eg(tt)
t
398 nm
795 nm
Nonlinear medium
SHG crystal
Commercial instrument (GRENOUILLE)
Strong 795 nm will amplifies weak shaped UV pulse
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FROG trace inversion
The FROG error is measured as the difference
between this and the experimental trace, and is
iteratively reduced
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Our shaping and characterisation experiment design
OPA
Amplifier
Chamber
Delay line
Doubling crystal
SLM
XFROG
Grenouille
We use a thin crystal for the XFROG (10 mm), and
we use the chamber to find time zero.
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Unshaped Pulses
1604085
Time / fs
Wavelength / nm
Temporal FWHM 90 fs Spectral FWHM 1 nm the
minimum FROG error is 0.008
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Shaped Pulses pulse pairs
?
Frequency doubling
FT
??
?t ??????
t
??
Frequency doubling of phase-modulated, ultrashort
laser pulses, Hacker et al., Appl. Phys. B 73,
273 (2001).
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Shaped Pulses pulse pairs
Dt 600 fs
Dt 800 fs
Dt 2600 fs
Dt 1800 fs
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Shaped Pulses sine functions
Phase that is a sine function will double to give
an intensity sine function
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Further work

• Characterise the 508 nm shaped light directly
  using an 508 nm 795 nm SFG XFROG

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Acknowledgements
Dorian Parker
Dr Russell Minns
Prof Helen Fielding
All the rest of the Fielding group
Thank you for listening!