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. 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) 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)

3. Control puzzled theorist www.science.uva.nl typical laser experimentalist enlightened theorist smart computer satisfied experimentalist sensitive detector Adaptive Control ? ? Principles of adaptive feedback/coherent control Goal: Use ultrafast optical pulse shaping techniques combined with adaptive feedback to selectively excite materials to prepare unusual nonequilibrium states

4.  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- some examples

5. 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 time wavelength time Input time Unoptimized out Optimized out wavelength Coherent control requires observation, manipulation, and control of ultrafast pulses. phase

6. Frequency-Resolved Optical Gating  AC(  ) Spectrometer CCD Trebino et al., Rev. Sci. Instr., 68, 1997, 3227 Soliton formation in 10 m of SMF-28 fiber F. Omenetto et al.Optics Letters 24, 1392, (1999) 318 pJ 294 pJ 255 pJ 228 pJ time (fs) time Phase sensitive measurement techniques--FROG ExperimentNumerics

7.  time Ultrafast pulse shaping - a simple example Calculated spectrogram of the sinc function Experimental results - shaping at 1550 nm time wavelength time wavelength ~  phase jumps in temporal phase indicate zero crossing Transformation of a square wave in the spectral domain yields a sinc in the time domain

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

9. ultrashort laser pulse 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. detector Programmable light modulator fs PULSE SHAPER EXPERIMENT feedback loop GA Control signal Feedback signal 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 1992 Judson and Rabitz, Phys. Rev. Lett. 68 (10) p. 1500 “Teaching Lasers to Control Molecules”

10. Genetic algorithm- a simple example a simple example Fitness Function : 11111111 f =  i=1-8  x i TASK: find the array of 8 bits containing all 1's: 0111 0110 f=5 f=2 0100 0001 Crossover : fittest individuals produce new offspring: 01100110 01010001 f=3 f=4 Selection : Calculate f for each individual (chromosome): 01010001 f=3 01010110 01010001 01100110 11010001 …. Initial population NEW POPULATION Mutation : randomly flip the value of one bit (allele): 0101 0001 f=2 0100 0001 f=3

11. fiber propagation (NLSE) Genetic operations: Pulse Shaper 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 00001110 Model Feedback Signal Initial filter Evaluation Fitness/selection New Population Crossover Mutation Computational adaptive feedback

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

13. Initial pulse Unoptimized out Optimized out Dispersion length L D =t 0 2 /|  2 | ~50 cm Nonlinear length L NL =1/ (  P 0 ) ~20 cm  = 1550 nm,  = 200 fs, P= 25 mW Experimental nonlinear optimization in 10 m of fiber

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

15. PD SHG E 1 E 2 optical fiber input from OPO d=300 lines/mm f=30cm deformable mirror feedback loop (GA) 100 fs, 330mW, 87MHz, 1550 nm OKO technologies membrane deformable mirror gold coated, 19 actuators Adaptive feedback control - Experimental setup for soliton Raman control h phonon E1E1 E2E2 h signal h pump Stimulated Raman scattering gain spectrum of silica -

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. C oherent A nti-Stokes R aman S cattering The vibrational frequencies of a molecule depend on the structure – hence vibrational spectroscopy is a powerful tool for molecular identification and detection. CARS is a powerful nonlinear optical technique that detects these vibrational modes using two 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 CARS spectrum - a new signature for a particular molecule 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.

20. Single-Pulse 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 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. CH 3 OH CH 2 Br 2 (CH 2 Cl) 2 Using a single 128 pixel SLM phase mask with a sinusoidally modulated phase Single beam CARS image—CH 2 Br 2 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 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

22. Single-pulse CARS Ba(NO 3 ) 2 Diamond Toluene Lexan Phase modulation of the form:  cos [   m     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).

23. Proposed single-pulse CARS instrument 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 Goal 1.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.

24. Raman 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.

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. f inin outoutSummary: Observation Manipulation Control Summary: V (CH 2 Cl) 2 CH 2 Br 2 CH 3 OH