Femtosecond Coherent Control for Precision Nonlinear Spectroscopy

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Femtosecond Coherent Control for Precision
Nonlinear Spectroscopy
D. Oron, N. Dudovich and Y. Silberberg, Physics
of Complex Systems Weizmann Institute of
Science Rehovot, Israel
CREOL, April 2004
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Narrow transitions induced by broad band pulses
Loss of spectral resolution
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Quantum Coherent Control
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Simplest nonlinear interactionTwo photon
absorption
Nonresonant TPA
I
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Nonresonant TPA
All the paths are in-phase
Transform-limited pulses maximize transition
rates Antisymmetric phase functions maintain
efficiency
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Energy level structure of Cesium
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Nonresonant TPA Control Experimental results
Step location/bandwidth
Antisymmetric phase has no effect on transition
probability Specific spectral phase mask can
annihilate the absorption rate ?Selective
excitation
Meshulach Silberberg, Nature, 396, 239
(1998),Phys. Rev. A 60, 1287 (1999)
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Two photon absorption
II
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Angular momentum control
Px
Py
ExEx transitions
Brixner and Gerber, Opt. Lett. 26, 557 (2002)
Two degenerate orthogonal states can be
separately controlled
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Separate control of non-interfering paths
?
Px
Py
Ey
Ex
Phase and polarization
Phase mask
Ex
Ex
N. Dudovich, D. Oron and Y. Silberberg, accepted
for publication in Phys. Rev. Lett. (2004).
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Nonlinear laser scanning microscopy

• Multiphoton microscopy enables inherent optical
  sectioning capabilities due to nonlinear nature
  of the process. Signal originates only from the
  focal spot.
• NIR illumination enables imaging through
  scattering samples
• Typically applied as laser scanning microscopy in
  which the sample is scanned point by point.
• First demonstrations
• CARS microscopy Duncan et al., Opt. Lett. 7, 350
  (1982)
• two-photon fluorescence microscopy Denk et al.,
  Science 248, 73 (1990).
• Extended to coherent processes
• SHG Peleg et al., Bioimaging 4, 215 (1996).
• THG Barad et al., Appl. Phys. Lett. 70, 922
  (1997).
• CARS (revisited) Zumbucsh et al., Phys. Rev.
  Lett. 82, 4142 (1999)

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Laser scanning microscopy - setup
Photomultiplier tube
lock-in amplifier
computer
filter
condenser
z
sample
y
x
microscope objective
Optical scanners
femtosecond laser source
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THG images of biological specimen
fossil
Yeast cell
Mouse bone
Xenopus embryo
Drosophila ovary
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Optical sectioning capabilities
Optical sections of a neuron
Sections separated by 1mm
Yelin et al., Appl. Phys. B 74, S97 (2002)
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Coherent Anti-Stokes Raman Scattering (CARS)

• In a CARS process a pump and a Stokes photon
  coherently excite a vibrational level. A probe
  photon interacts with the excited level to emit a
  signal photon.
• Large, directional and coherent signal (compare
  to Raman scattering).
• Attractive for microscopy applications
• -provides a vibrational imaging with 3D
• sectioning capability.

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Single Pulse CARS
When the pulse duration is shorter than the
vibrational period of the molecule, the CARS
process can be induced within a single pulse.
The spectral resolution of this process is
limited by the pulse bandwidth High nonresonant
background
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Resonant vs. Nonresonant CARS
Resonant CARS is always accompanied by a
nonresonant background
Nonresonant background is maximal for transform
limited pulses (highest peak intensity)
Usually dealt with by using longer pulses and by
polarization techniques
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CARS Selective Excitation
Transform-limited pulses maximize transition
rates Periodic phase functions maintain
efficiency
Weiner et al., Science 273, 1317 (1990) Oron et
al., Phys. Rev. A 65, 043408 (2002) Gershgoren et
al., Opt. Lett. 28, 361 (2003)
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Single-pulse CARS with periodic phase
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Single-pulse CARS with periodic
phaseSpectroscopy by selective excitation
Spectral phase
Temporal profile
Population amplitude (monitor 577cm-1 level)
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Single-pulse CARS experimental setup
N. Dudovich, D. Oron and Y. Silberberg, Nature
418, 512 (2002).
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CARS spectroscopy
Spectrocopy in the fingerprint region
Modulated spectral phase function
t

• Reduced nonresonant background
• Spectral resolution 30 cm-1, 70 times the pulse
  band width

N. Dudovich, D. Oron and Y. Silberberg, J. Chem.
Phys. 118, 9208 (2003).
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Multiplexed CARS
Measured signal is heterodyned with the
nonresonant background
From Muller et al., JPC B 106, 3715 (2002)
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Single-pulse Multiplexed CARS
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Experimental set-up phase and polarization shaping
Brixner and Gerber, Opt. Lett. 26, 557 (2002)
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Narrow probing by an orthogonal polarization
D. Oron, N. Dudovich and Y. Silberberg, Phys.
Rev. Lett. 90, 213902 (2003).
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Narrow probing by phase and polarization shaping
Contribution only by
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Extracted Raman spectrum
D. Oron, N. Dudovich and Y. Silberberg, Phys.
Rev. Lett. 90, 213902 (2003).
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All-optical analysis of Raman spectra
Use broadband probing to induce interferences
between contributions from several Raman levels
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All-optical analysis of Raman spectra
application to spectroscopy of 1,2 dichloroethane
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Summary

• Spectral resolution orders of magnitude higher
  than the pulse bandwidth is achieved in nonlinear
  spectroscopy by applying spectral phase and
  polarization manipulations.
• Nonresonant background can be suppressed,
  directed to an orthogonal channel or used as a
  local oscillator, to enhanced the
  signal/background ratio.
• Coherent control schemes developed for simple
  quantum systems can be utilized as the "building
  blocks" to control various multiphoton
  transitions.

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Nonresonant background reduction
glass
Ba(NO3)2
n1
n2
Relative intensity
n4
Pulse train duration fs
Nonresonant reduction by a factor of 250
Dudovich et al., J. Chem. Phys. 118, 9208 (2003)
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Angular distribution control
Experimental results
Simulation
N. Dudovich, D. Oron and Y. Silberberg, accepted
for publication in Phys. Rev. Lett. (2004).
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Femtosecond pulses motivations and main drawbacks

• Short duration ? Measuring fast evolving
  processes.
• High peak intensity ? efficient for multiphoton
  transitions.
• Large spectral bandwidth ? provide wide band
  spectroscopic information.

However

• In resonant nonlinear transitions two time scales
  are involved
• ? and the life time of the resonant level (ps in
  molecules or ns in atoms)
• Lost of spectral resolution
• Large nonresonant signal

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Separate control of non-interfering paths
Px
Py
Ey
Ex
Phase and polarization
Phase mask
Ex
Ex
N. Dudovich, D. Oron and Y. Silberberg, accepted
for publication in Phys. Rev. Lett. (2004).
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Multiplex CARS Phase control

• Multiplex scheme narrow spectral phase
  manipulation.
• Sign inversion around the resonance ? narrow
  enhancement of the signal can be achieved by
  applying ? phase window

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CARS spectrum Narrow probing
Nonresonant Background from overlap of the pump
and probe
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Narrow probing by an orthogonal polarization
D. Oron, N. Dudovich and Y. Silberberg, Phys.
Rev. Lett. 90, 213902 (2003).
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Weiner et al., Science 273, 1317 (1990) Oron et
al., Phys. Rev. A 65, 043408 (2002) Gershgoren et
al., Opt. Lett. 28, 361 (2003)
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Impulsive excitation
Weiner et al., Science 273, 1317 (1990)
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Impulsive excitation
Weiner et al., Science 273, 1317 (1990)