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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. Collaborators: Richard D. Averitt (LANL) Jaewook Ahn (LANL)

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coherent control of the raman fingerprint spectrum via single pulse cars

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

slide2

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
slide3

Principles of adaptive feedback/coherent control

Adaptive Control

smart

computer

sensitive

detector

satisfied

experimentalist

?

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

slide4

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 pulse’s structure.
slide5

Unoptimized

out

wavelength

Optimized

out

time

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
slide6

Spectrometer

AC(t)

CCD

t

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)

slide7

l

time

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

slide8

Liquid crystal

spatial light modulator

inputpulse

out

in

f

Programmable ultrafast pulse shaping

slide9

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”

slide10

Genetic algorithm-

a simple example

Selection : Calculate f for each individual (chromosome):

TASK: find the array of 8 bits containing all 1\'s:

f=3

Crossover : fittest individuals produce new offspring:

Fitness Function :

f=3

f= Si=1-8xi

1

1

1

1

1

1

1

1

f=4

Initial population

f=5

1

1

1

1

1

1

0

1

0

0

0

0

0

1

1

1

1

1

0

0

1

0

0

0

0

1

0

0

1

0

0

0

0

0

0

0

1

1

1

0

0

1

1

1

0

0

0

0

1

0

0

1

1

0

1

1

f=2

Mutation : randomly flip the value of one bit (allele):

f=2

0

0

0

0

0

0

1

1

1

0

0

0

1

1

1

0

0

0

0

1

0

0

0

0

1

0

0

1

1

0

0

1

….

f=3

NEW POPULATION

slide11

Computational adaptive feedback

0

0

1

1

0

1

0

0

GOAL:transmit the shortest pulse possible through a

link (100 m) of fiber in anomalous dispersion regime

AMPLITUDEshaping in the spectral domain: binary filtering

Model

Feedback

Signal

Pulse

Shaper

Model

fiber propagation (NLSE)

Initial filter

Evaluation

Fitness/selection

New Population

Genetic operations:

Crossover

Mutation

slide12

Computational adaptive feedback--results

Original pulse

Amplitude filter

Optimal pulse shape

Direction of propagation

slide13

Unoptimized out

Optimized out

Experimental nonlinear optimization in 10 m of fiber

Initial pulse

l = 1550 nm, t = 200 fs, P= 25 mW

Dispersion length LD=t02/| b2| ~50 cm

Nonlinear length LNL=1/ (g P0) ~20 cm

slide15

SHG

E

E

1

2

PD

Adaptive feedback control - Experimental setup for soliton Raman control

gain spectrum of silica

Stimulated Raman scattering

E2

hnsignal

hnpump

hnphonon

E1

-

100 fs, 330mW,

87MHz, 1550 nm

input from

OPO

optical fiber

d=300 lines/mm

deformable

mirror

OKO technologies

f=30cm

membrane deformable mirror

gold coated, 19 actuators

feedback loop (GA)

c oherent a nti stokes r aman s cattering
Coherent Anti-StokesRamanScattering

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.

slide20

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.

single pulse cars
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 image—CH2Br2 in glass

Using a single 128 pixel SLM phase mask

with a sinusoidally modulated phase

slide22

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

proposed single pulse cars instrument
Proposed single-pulse CARS instrument
  • Ultra-short pulse laser (<10 fs pulse width)
  • High-resolution spatial light modulator (2*640 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.
raman fingerprint spectrum
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): Benz[a]anthracene(A), Naphthacene(B), Chrysene(C), and Tiphenylene(D).

summary advantages of single pulse cars
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. It’s 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 single–pulse CARS should provide a practical method of molecular identification in complex environments.
slide26

Summary:

Observation Manipulation Control

out

in

V

f

(CH2Cl)2

CH3OH

CH2Br2

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