Ultrafast%20techniques

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Ultrafast techniques

• Laser systems
• TiSaph oscillator/regen, modelocking
• NOPAs
• Pump-probe absorption difference spectroscopy
• Two-color
• Dispersed detection
• Fluorescence spectrosopy
• Photon counting
• Streak Camera imaging
• Upconversion
• Nanosecond time scale, FTIR

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Elementary Reactions in Biology
Free Energy
Reactant
Product
Configuration
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Lasers

• Light Amplification by Stimulated Emission
  Radiation
• Population inversion
• Cavity
• Gain medium -gt Titaniumsapphire
• Single mode, CW laser
• Many modes with phase relation leads to a pulse
  in the cavity

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Cavity
Pump laser
Leaky mirror
For Tisapphire oscillators? 800 nm, Rep. rate
80 MHz Low power 10 nJ Pulses can be as short
as 10 fs
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Amplify from nanoJ to milliJoules -gt peak power
20 fs pulse if focussed to 100 micrometer
1012W/cm-2 1000 times damage treshold most
materials!
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Regenerative amplifier
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White light generationby Self Phase Modulation
n(I) n0 n2I .
The electrical laser field is E(x,t)
E(t)cos(?t-kx) f ?t-kx ?t ?nx/c
?(t-n0z/c) n2 ?z/cI(t) ? df/dt ? A dI/dt
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Parametric generation or amplification
The splitting of one photon in two ?pump
?signal ?idler Conservation of momentum kpump
ksignal kidler This can be done in nonlinear,
birefringent crystals were the index of
refraction depends on the polarization
?1
?1?2
?2
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Noncollinear optical parametric amplification

• When using a non-collinear phase matching angle
  in BBO pumped at 400 nm, the phase matching angle
  becomes independent of wavelength over a large
  part of the spectrum, for an angle of a 3.7o
  between pump and signal (Gale,Hache 1994) ?
  large bandwidth
• The spatial walk-off (from the extraordinary pump
  beam) is 4.0o, with Pp farther from optical axis
  than kp. This is coincidently close to the
  noncollinear angle a! ? high gain
• Sub-10 fs with µJ energies can be obtained
  (efficiency 10-30)

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optic axis
idler angle
signal angle
ks
ki
a
kp
Optimize bandwidth by matching the signal and
idler group velocities (degeneracy for collinear
beams) VS VI cosO Expressed in terms of a and
? and solved for large bandwidths, one finds a
3.7o and ? 32o
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400 nm pump
6.4o
white light seed

• Shorter pulses by
• minimizing dispersion of white light (no
  dispersive optics)
• or even lengthening pump pulse
• optimal compression (small apex angle prisms or
  gratings)

• Tune by
• changing delay since white light is dispersed
• phase matching angle and noncollinear angle

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NOPA
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Amplified TiSapphire Laser
0.5mJ 50 fs 1khz
Oscillator-stretcher-amplifier-compressor
NOPA

Sapphire
1 mm 3 fs
Optical Delay Line
Moving cell

Grating
Diode Array
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Amplified TiSapphire Laser
NOPA
OPA
1 mm 3 fs
Optical Delay Line
Moving cell

photodiode
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The instrument response function
The cross- or auto correlationis given by
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ES 2
Excited stateaborption
ES 1
Stimulated emission
Ground state
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A
or
DA
l
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Protochlorophyllide Oxido Reductase
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Ultrafast Spectral Evolution in POR
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Important experimental aspects

• Repetition rate of laser must be slower than
  photocycle, or samplemust be refreshed for every
  shot
• Excitation density must be low, only when less
  than 10 ofcomplexes are excited you are in a
  linear regime -gt annihilation,saturation due to
  stimulated emission, orientational saturation
• Population dynamics are measured under the
  magic angle54.7o, at other angles
  orientational dynamics are measuredanisotropy
  r (?DOD// -?DOD?) / (?OD// 2?OD?)

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Saturation
The probability to excitea complex is
(E.µ)2Since E2 I n, n(T) n cos2 T
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Time-Resolved Polarized Absorption
Probe
Pump
Dt
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Anisotropy
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Pump-probe spectroscopy is a self-heterodyned
third order spectroscopy
A third order polarization is induced P(3)(w,t)
?3EprEpuEpu This nonlinear polarization is the
source of a new generated field (Maxwell equation
slowly varying envelop
give)
Pu
Pu
Pr
t1
Stimulated emission
Time to absorb a photon, either determined by
pulse length of pump, or by the dephasing time of
the optical coherence i.e. ?/absorption bandwidth
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Heterodyne detection, observation of
superposition of local oscillator field (
probe field) and signal field I(t) n(?s)c/4p
Elo(t) Es(t)2 ILO(t) IS(t) 2 n(?s)c/4p
ReELO(t).ES(t)
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Fluorescence techniquesI. Photon Counting
Laser
Spontaneous emission
Instrument response30-50 ps High sensitivity,
thoughmostly used with highrep rate systems,
gt100 KHz
start
stop
Time to amplitude converter
Monochromator or filter
photomultiplier
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II. Streak Camera Fluorescence
Time resolution 3 ps Whole spectrum at
once Moderate sensitivity
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III Fluorescence Upconversion
Laser
Spontaneous emission
?laser?signal
Monochromator
1 mm 3 fs
Very thin BBO crystal 50 mm
detector
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Slow Absorption difference spectroscopy

• Fast detector
• Relatively more probe light than in a fs-ps
  experiment,
• actinic??

Ns laserpulse
?OD
Monochromator
Lamp
sample
Photomultiplieror photodiode
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Step-scan FTIR
Lamp
IR detector MCT
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