Coherent Control of the Coherent Control of the Raman Fingerprint Raman Fingerprint

Spectrum via Single-Spectrum via Single-Pulse CARSPulse CARS

Toni Taylor

Condensed Matter and Thermal Physics Group

Materials Science and Technology Division

Los Alamos National Laboratory

Talk Outline

-Principles of coherent control

-Coherent control experiments: -fs pulse propagation in fibers

- Coherent control and single-pulse CARS

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)

Control

puzzledtheorist

www.science.uva.nl

typical laserexperimentalist

enlightenedtheorist

smart computer

satisfied experimentalist

sensitivedetector

Adaptive Control

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Principles of adaptive feedback/coherent Principles of adaptive feedback/coherent controlcontrol

Goal: Use ultrafast optical pulse shaping techniques combined with adaptive feedback to selectively excite materials to prepare unusual nonequilibrium states

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.

• 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)

• …

Experimental achievements in adaptive control- Experimental achievements in adaptive control- some examplessome examples

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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.

• phase sensitive pulse detection techniques

• programmable femtosecond pulse shaping

• adaptive feedback control in combination with fs pulse shaping

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Coherent control requires observation, Coherent control requires observation, manipulation, and control of ultrafast pulses.manipulation, and control of ultrafast pulses.

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Frequency-Resolved Optical Gating

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ωω (

AC(AC())Spectrometer

CCD

Trebino et al., Rev. Sci. Instr., 68, 1997, 3227

Soliton formation in 10 m of SMF-28 fiberF. Omenetto et al.Optics Letters 24, 1392, (1999)

318 pJ318 pJ

294 pJ294 pJ

255 pJ255 pJ

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Phase sensitive measurement techniques--FROGPhase sensitive measurement techniques--FROGExperiment Numerics

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Ultrafast pulse shaping - a simple exampleUltrafast pulse shaping - a simple example

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Calculated spectrogram of thesinc function

Experimental results - shaping at 1550 nm

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~phase jumps in temporalphase indicate zero crossing

Transformation of a square wave in the spectral domain yields a sinc in the time domain

input pulse

Liquid crystalLiquid crystalspatial light modulatorspatial light modulator

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Programmable ultrafast pulse shapingProgrammable ultrafast pulse shaping

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ultrashort laser pulse

Searching through a very large space of possible Searching through a very large space of possible solutions (pulse shapes) requires efficient global solutions (pulse shapes) requires efficient global search algorithms (Genetic algorithms, Fuzzy Logic, search algorithms (Genetic algorithms, Fuzzy Logic, Neural Nets, Simulated Annealing …) Algorithm should Neural Nets, Simulated Annealing …) Algorithm should be able to tolerate experimental noise.be able to tolerate experimental noise.

detector

Programmablelight

modulator

fs PULSE SHAPER

EXPERIMENT

feedback loopGA

Control signal

Feedbacksignal

Implementation of adaptive feedback controlImplementation 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

1992 Judson and Rabitz, Phys. Rev. Lett. 68 (10) p. 1500“Teaching Lasers to Control Molecules”

Genetic algorithm-Genetic algorithm- a simple examplea simple example

Fitness Function :

11 11 11 11

f = i=1-8xi

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

01 11 01 10 f=5

f=201 00 00 01

Crossover : fittest individuals produce new offspring:

01 10 01 10

01 01 00 01 f=3

f=4

Selection : Calculate f for each individual (chromosome):

01 01 00 01 f=3

01 01 01 1001 01 00 01

01 10 01 10

11 01 00 01

….

Initial population

NEW POPULATION

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

01 01 00 01

f=201 00 00 01

f=3

fiber propagation (NLSE)

Genetic operations:

PulseShaper Model

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

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R ModelFeedback

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Computational adaptive feedback Computational adaptive feedback

Direction of propagation

Direction of propagation

Amplitude filterOptimal pulse shape

Original pulse

Computational adaptive feedback--results Computational adaptive feedback--results

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Dispersion length LD=t02/| β2| ~50 cm

Nonlinear length LNL=1/ (γ P0) ~20 cm

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

Experimental nonlinear optimization in 10 m of Experimental nonlinear optimization in 10 m of fiberfiber

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Raman shift during soliton formation in 100 meters Raman shift during soliton formation in 100 meters in PM fiberin PM fiber

PD

SHGE 1 E 2optical fiber

input from OPO

d=300 lines/mm

f=30cmdeformable

mirror

feedback loop feedback loop (GA)(GA)

100 fs, 330mW,87MHz, 1550 nm

OKO technologiesmembrane deformable mirrorgold coated, 19 actuators

Adaptive feedback control - Experimental setup for soliton Raman controlAdaptive feedback control - Experimental setup for soliton Raman control

hphononE1

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hsignalhpump

Stimulated Raman scattering gain spectrum of silica1.00.80.60.40.20.0norm. gain coef.

3020100frequency (THz)

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GA optimization at low input power - 10 mWGA optimization at low input power - 10 mW

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GA optimization at medium input power - 15 mWGA optimization at medium input power - 15 mW

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GA optimization at high input power, 25 mW: GA optimization at high input power, 25 mW: Chaos, Cherekov THGChaos, Cherekov THG

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Coherent Anti-Stokes Raman Scattering

The vibrational frequencies of amolecule depend on the structure –hence vibrational spectroscopy is apowerful tool for molecular identification and detection.

CARS is a powerful nonlinear optical techniquethat detects these vibrational modes usingtwo or more beams.

This time – frequency approach enables CARS to be performed with a single beam!This is not just a technique to measure a CARSspectrum - a new signature for a particularmolecule is determined.

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.

Single-Pulse CARSSingle-Pulse CARSCoherent control in CARS:Coherent control in CARS:

(a) 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 amplitudeand phase of the short pulses we canuse single pulse for high resolution (10 cm-1),broad coverage (400 –1800 cm-1), witha suppressed nonresonant signal.

CH3OH CH2Br2 (CH2Cl)2

Using a single 128 pixel SLM phase mask with a sinusoidally modulated phase

Single beam CARS image—CH2Br2 in glass

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.

Single-pulse CARSSingle-pulse CARS

Suppression of nonresonantbackground by more than 1 orderof magnitude by adding higher harmonicorders to the phase mask – this is a verygeneral approach to reducing the peakintensity and associated nonresonant signal

Single-pulse CARSSingle-pulse CARS

Ba(NO3)2

Diamond

Toluene

Lexan

Phase modulation of the form:

F(ω)=cos [m(ωω)

Leading to a train of pulses separated by m

Vary m from 400 fs to 1 ps

CARS signal peaks when m is commensurate with a vibrational period

Dudovich, Oron, Silberberg, J. Chem. Phys. 118, 9208 (2003).

Proposed single-pulse CARS Proposed single-pulse CARS instrumentinstrument

1. Ultra-short pulse laser (<10 fs pulse width)

2. High-resolution spatial light modulator (2*640 optical masks for amp.+phase control)

3. Fast data acquisition (Megahertz Lock-in)4. Computer controlled feedback loop

Proposed Goal1. Spectral Raman resolution of 10 cm-1

2. Access Raman fingerprint region (1000-1500cm-1)

3. Coherent control of molecular identification

4. Use adaptive feedback to develop catalog of phase masks identifying different molecules.

Raman fingerprint spectrumRaman fingerprint spectrum

Raman spectra of simple polycyclic aromatic hydrocarbons (PAH): Benz[a]anthracene(A), Naphthacene(B), Chrysene(C), and Tiphenylene(D).

• 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.

Summary/advantages of single-pulse Summary/advantages of single-pulse CARSCARS

• 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.

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Summary:Summary:Observation Manipulation ControlObservation Manipulation Control

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(CH2Cl)2CH2Br2

CH3OH