Coherent Control of the Raman Fingerprint Spectrum via Single-Pulse CARS

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Coherent Control of the Raman Fingerprint
Spectrum via Single-Pulse CARS

• Toni Taylor

Condensed Matter and Thermal Physics
Group Materials Science and Technology
Division Los Alamos National Laboratory
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Collaborators Richard D. Averitt
(LANL) Jaewook Ahn (LANL) Anatoly Efimov
(LANL) Fiorenzo Omenetto (LANL) Benjamin
P. Luce (LANL) Dave Reitze (U. of
Florida) Mark Moores (Intel)

• Talk Outline
• Principles of coherent control
• Coherent control experiments
• fs pulse propagation in fibers
• - Coherent control and single-pulse CARS

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Principles of adaptive feedback/coherent control
?
Control
?
puzzled theorist
www.science.uva.nl
typical laser experimentalist
Goal Use ultrafast optical pulse shaping
techniques combined with adaptive feedback to
selectively excite materials to prepare unusual
nonequilibrium states
enlightened theorist
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Experimental achievements in adaptive control-
some examples

• Idea Judson, Rabitz (1992)
• AFC of molecular fluorescence Bardeen, et al.
  (1997)
• Adaptive pulse compression Yelin, et al. (1997)
• Adaptive pulse shaping Meshulach, et al. (1998)
• AFC of chemical reactions Assion, et al. (1998)
• Amplified pulse compression Efimov, et al.
  (1998)
• AFC optimization of X-rays Feurer (1999)
• Compression with deformable mirror, Zeek, et al.
  (2000)
• AFC optimization of vibrations Hornung, et al.
  (2000)
• AFC of HHG, Bartel, et al. (2000)
• AFC of semiconductor nonlinearity (Kunde et
  al.)
• AFC of CARS Silberberg (2002)

• Recent results in controlling chemical reactions
  
• Optimization of competing reaction pathways
• Selective excitation of a specific vibrational
  mode.
• Nontrivial control arises from the cooperative
  interaction of the laser pulse shape and phase
  with an evolving wavepacket such that the product
  is sensitive to the pulses structure.

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Coherent control requires observation,
manipulation, and control of ultrafast pulses.
We can observe an ultrafast pulse in great
detail. We can precisely manipulate the
pulse through shaping techniques. We can
control nonlinear processes with adaptive
feedback.
wavelength
phase

• phase sensitive pulse detection techniques

time
time
Input
time
time

• programmable femtosecond pulse shaping

• adaptive feedback control in combination with fs
  pulse shaping

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Phase sensitive measurement techniques--FROG
Experiment
Numerics
Frequency-Resolved Optical Gating
228 pJ
255 pJ
294 pJ
318 pJ
time
time (fs)
time (fs)
Soliton formation in 10 m of SMF-28 fiber
Trebino et al., Rev. Sci. Instr., 68, 1997, 3227
F. Omenetto et al.Optics Letters 24, 1392, (1999)
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Ultrafast pulse shaping - a simple example
Transformation of a square wave in the spectral
domain yields a sinc in the time domain
Calculated spectrogram of the sinc function
wavelength
time
Experimental results - shaping at 1550 nm
wavelength
p phase jumps in temporal phase indicate zero
crossing
time
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Programmable ultrafast pulse shaping
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Implementation of adaptive feedback control
Feedback on the experiment until a desired result
is achieved- observation of the final state
provides information on the physical system
under investigation
EXPERIMENT
ultrashort laser pulse
detector
Feedback signal
fs PULSE SHAPER
Programmable light modulator
GA
feedback loop
Control signal
Searching through a very large space of possible
solutions (pulse shapes) requires efficient
global search algorithms (Genetic algorithms,
Fuzzy Logic, Neural Nets, Simulated Annealing )
Algorithm should be able to tolerate experimental
noise.
1992 Judson and Rabitz, Phys. Rev. Lett. 68 (10)
p. 1500 Teaching Lasers to Control Molecules
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Genetic algorithm- a simple example

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Computational adaptive feedback
GOAL transmit the shortest pulse possible
through a link (100 m) of fiber in
anomalous dispersion regime
AMPLITUDE shaping in the spectral domain binary
filtering
Model Feedback Signal
Pulse Shaper Model
fiber propagation (NLSE)
Initial filter
Evaluation
Fitness/selection
Genetic operations
New Population
Crossover
Mutation
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Computational adaptive feedback--results
Original pulse
Amplitude filter
Optimal pulse shape
Direction of propagation
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Experimental nonlinear optimization in 10 m of
fiber
Initial pulse
Dispersion length LDt02/ b2 50 cm
l 1550 nm, t 200 fs, P 25 mW
Nonlinear length LNL1/ (g P0) 20 cm
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Raman shift during soliton formation in 100
meters in PM fiber
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Adaptive feedback control - Experimental setup
for soliton Raman control
Stimulated Raman scattering
gain spectrum of silica
E2
hnsignal
hnpump
hnphonon
E1
-
100 fs, 330mW,
87MHz, 1550 nm
input from OPO
optical fiber
d300 lines/mm
deformable mirror
OKO technologies
f30cm
membrane deformable mirror
gold coated, 19 actuators
feedback loop (GA)
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GA optimization at low input power - 10 mW
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GA optimization at medium input power - 15 mW
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GA optimization at high input power, 25 mW
Chaos, Cherekov THG
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Coherent Anti-Stokes Raman Scattering
The vibrational frequencies of a molecule depend
on the structure hence vibrational spectroscopy
is a powerful tool for molecular identification
and detection.

• Single-pulse CARS
• When the pulsewidth is less than the vibrational
  period of the molecule, the excitation can be
  induced within a single pulse via intrapulse
  4-wave mixing.
• However, using a transform limited pulse, the
  spectral resolution is limited by the pulse
  bandwidth and the nonresonant background is
  enhanced
• Coherent control techniques can be used to
  selectively excite a particular vibrational level
  in the pulse bandwidth, significantly enhancing
  resolution
• Suppression of the nonresonant background follows
  from the longer pulsewidth and harmonic
  excitation.

This time frequency approach enables CARS to
be performed with a single beam! This is not just
a technique to measure a CARS spectrum - a new
signature for a particular molecule is determined.
CARS is a powerful nonlinear optical
technique that detects these vibrational modes
using two or more beams.
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Single-Pulse CARS
Coherent control in CARS

• 10 fs pulses enough spectral bandwidth to
• extend S-CARS to the fingerprint region.
• (b) Adaptive feedback to maximize molecular
• coherence for complex molecules.
• (c) Two SLM for phase and amplitude control of
• the pulses (640 pixels X 2 1280 knobs)

By controlling the spectral amplitude and phase
of the short pulses we can use single pulse for
high resolution (10 cm-1), broad coverage (400
1800 cm-1), with a suppressed nonresonant signal.
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Single-pulse CARS
Suppression of nonresonant background by more
than 1 order of magnitude by adding higher
harmonic orders to the phase mask this is a
very general approach to reducing the
peak intensity and associated nonresonant signal
Broad bandwidth of an ultra-short laser pulse was
coherently altered to perform the Coherent
Anti-Stokes Raman Scattering, revealing the
Raman bands in spectral resolution of 30 cm-1.
CH3OH
(CH2Cl)2
CH2Br2
Single beam CARS imageCH2Br2 in glass
Using a single 128 pixel SLM phase mask with a
sinusoidally modulated phase
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Single-pulse CARS
Ba(NO3)2
Phase modulation of the form F(w) 1.25 cos
tm (w- wo) Leading to a train of pulses
separated by tm Vary tm from 400 fs to 1 ps CARS
signal peaks when tm is commensurate with a
vibrational period
Diamond
Toluene
Dudovich, Oron, Silberberg, J. Chem. Phys. 118,
9208 (2003).
Lexan
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Proposed single-pulse CARS instrument

• Ultra-short pulse laser (lt10 fs pulse width)
• High-resolution spatial light modulator (2640
  optical masks for amp.phase control)
• Fast data acquisition (Megahertz Lock-in)
• Computer controlled feedback loop

• Proposed Goal
• Spectral Raman resolution of 10 cm-1
• Access Raman fingerprint region (1000-1500cm-1)
• Coherent control of molecular identification
• Use adaptive feedback to develop catalog of phase
  masks identifying different molecules.

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Raman fingerprint spectrum

• S-CARS access the fingerprint spectra in the
  region of 1000-1700cm-1 closely packed with
  coupled modes of C-C stretching and C-C-H bending
  motions show distinctive spectral differences
  among these PAH molecules.
• Tailored pulse shapes selectively access Raman
  vibrational bands.

Raman spectra of simple polycyclic aromatic
hydrocarbons (PAH) Benzaanthracene(A),
Naphthacene(B), Chrysene(C), and Tiphenylene(D).
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Summary/advantages of single-pulse CARS

• Compact, simple, and smart spectroscopy.
• Single-pulse CARS (S-CARS) utilizes shaped single
  pulses whose filtered output provides the signal.
  Its a compact, simple, but smart spectroscopy.
• Coherently controlled spectroscopy
• Uses techniques developed for selective
  photo-dissociation of molecules.
• Address a simpler problem -- control vibrations
  to simply probe them, (not to break bonds).
• Fast and selective molecular classification
• The quantum coherence, even in large molecules,
  is created and probed by phase-controlled combs
  of a single laser pulse.
• By determining the molecular signatures
  singlepulse CARS should provide a practical
  method of molecular identification in complex
  environments.

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Summary Observation Manipulation
Control
(CH2Cl)2
CH2Br2
CH3OH