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

1. 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

2. 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

3. 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

4. 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.

5. 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

6. 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)

7. 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

8. Liquid crystal spatial light modulator inputpulse out in f Programmable ultrafast pulse shaping

9. 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”

10. 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

11. 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

12. Computational adaptive feedback--results Original pulse Amplitude filter Optimal pulse shape Direction of propagation

13. 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

14. Raman shift during soliton formation in 100 meters in PM fiber

15. 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)

16. GA optimization at low input power - 10 mW

17. GA optimization at medium input power - 15 mW

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

19. 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.

20. 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.

21. 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

22. 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

23. 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.

24. 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).

25. 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.

26. Summary: Observation Manipulation Control out in V f (CH2Cl)2 CH3OH CH2Br2