Probing into the Molecular World with Light

1. Probing into the Molecular World with Light Jung Y. Huang Department of Photonics and Institute of Electro-Optical Engineering, NCTU

2. In many cases, material properties are strongly affected by the structure and type of species on surface or at interface. • Sum-frequency vibrational spectroscopy can be employed to reveal interfacial molecular structure.

3. Sum-frequency vibrational spectroscopy of surfaces and interfaces • Unique finger printing features of vibrational modes: • highly localized; • can be well characterized by theory. • SFG: (2)eff =(2)eff(bulk) +(2)s(surface) • In a medium with an inversion symmetry: (2)eff(bulk)= 0, (2)s (surface)0 • In a medium with polar structure : (2)eff(bulk) 0 and dominates. Resonance can be employed to yield sensitivity to molecular species.

4. Sum-frequency vibrational spectroscopy of surfaces and interfaces Typical properties of SFG vibrational spectroscopy from symmetry breaking • The IR transition moments of the CH2 groups (=2850 cm-1) along all-trans alkyl chains are antiparallel to each other, therefore their contributions to SFG are small.

5. Sum-frequency vibrational spectroscopy of surfaces and interfaces Each cis-trans defect causes unpaired CH2 groups, which then contribute to SFG activity at =2850 cm-1.

6. Sum-frequency vibrational spectroscopy of surfaces and interfaces Chain-chain interaction between LC molecules and surfactant monolayer could be the first event in aligning LC molecules on surface.

7. Sum-frequency vibrational spectroscopy of surfaces and interfaces 8CB on Rubbing surfaces The uniaxial alignment of the first LC layer on an alignment surface is revealed by an azimuthal dependence of the CN stretching mode of LC molecules. Although for a molecular system the bulk structure can be strongly affected by the interface structure, the material properties are determined by the bulk.

8. Two-dimensional Vibrational Spectroscopy Ultrashort laser can be employed to probe the internal workings of molecular materials. A major development in this area may be a technique known as two-dimensional vibrational spectroscopy,which can be used to • determine static structureof peptides and proteins ; • examine fast processes such as protein folding and peptide conformational dynamics; • map the relationship between individual bondswithin or among molecular species.

9. Two-dimensional Infrared Correlation Spectroscopy • 2D IR methodology emulates techniques currently used in NMR. • Typical vibrational relaxation rates (picoseconds) are orders of magnitude faster than typical spin relaxation rates (microseconds). Therefore 2D IR with sub picosecond IR pulses can monitor molecular structures on a picosecond timescale. • We used a much slower process (such as time, or polarization angle of the incident IR) to perturb the molecular system of interest. • To generate 2D IR correlation spectra, IR spectra were collected sequentially as a function of the perturbing parameter.

10. 2D Infrared Correlation Spectroscopy By spreading peaks along the second dimension, one can often sort out complex or overlapped spectral features that cannot be detected along the first dimension. Synchronous Asynchronous

11. 2D Infrared Correlation Spectroscopy Insight into the Synchronous 2D IR Correlation Plot • Isotropic component A0 can not affect the auto peaks of the synchronous correlation plot.

12. 2D Infrared Correlation Spectroscopy Insight into the Synchronous 2D IR Correlation Plot The cross peaks of the synchronous correlation plot vary linearly with the uniaxial parameter U: A=0.1318 U1U2.

13. 2D Infrared Correlation Spectroscopy Insight into the Synchronous and Asynchronous 2D IR Correlation Plot 0-dependence can also be found in the asynchronous plot. The cross peaks of the synchronous correlation plot can vary with 0.

14. 2D Infrared Correlation Spectroscopy Summary of the Synchronous and Asynchronous 2D IR Correlation Plot

15. Time-resolved 2D Infrared Correlation Spectroscopy Time-resolved 2D IR Correlation SpectroscopyTracking correlated motion of sub molecular fragments in an electro-optical switching FLC mixture • Surface interactions can unwind the spontaneous helix, which then yields a uniform FLC alignment with • sec Response • Bistability • Wide Viewing Angle

16. Time-resolved 2D Infrared Correlation Spectroscopy Time-resolved 2D IR Correlation SpectroscopyTracking correlated motion of sub molecular fragments in an electro-optical switching FLC mixture Time-resolved azimuthal patterns of IR absorption peaks at 1608 (black) and2924 (red) cm-1

17. Time-resolved 2D Infrared Correlation Spectroscopy Time-resolved 2D IR Correlation Plot • Cross peaks of the synchronous plot are sensitive to both U and 0. • Cross peaks of the asynchronous plot are sensitive to 0 only. • The thermal fluctuations in the azimuthal angle of SmC*-FLC (Goldstone mode) are suppressed by the electric field. 70 sec after applying +10V 70 sec after applying -10V

18. Time-resolved 2D Infrared Correlation Spectroscopy Tracking correlated motion of submolecular fragments of a complex FLC mixture U and 0 0 By using ultrashort IR pulses, this technique can be employed to reveal correlated intra- or inter-molecular motions at fast time scales.

19. Control of Molecules with Ultrashort Light • We can go beyond the simple pump-probe spectroscopic techniques and actually use the laser pulses to influence the course of the molecular dynamics directly. • This work is often carried out in a feedback loop with some form of pulse shaping element being controlled by a computer.

20. Control of Molecules with Ultrashort Light • An issue with coherent control is the inverse problem, from knowingwhat the optimal pulse is togain information about the system under interrogation. • Techniques that can be employed to(1) characterize ultrafast pulsesand then(2) modify them appropriate to the experiments being carried outhave recently been developed.

21. Control of Molecules with Ultrafast Light (1) Complete-field characterization of coherent optical pulses • New spectral-phase freezing algorithm had been developed to directly and rapidly provide complete-field information.

22. Control of Molecules with Ultrafast Light Complete-field characterization of coherent optical pulses • Magnitude and phase distortion in a femtosecond pulse reflected from an semi-conductor InAs QD saturable absorber can be rapidly and reliably determined.

23. Third harmonic generation t t Two photon fluorescence Three photon fluorescence Control of Molecules with Ultrafast Light NLO Processes for Multi-Photon Microscopy

24. Control of Molecules with Ultrafast Light (2) Coherent-control optical contrast enhancement with multiphoton optical microscopy • By making use of the pulse shaping techniques, it is possible to selectively excite individual probes leaving the others in their ground states.This can be used to increase image contrast when exciting probe molecules exposed to differing chemical environments. M. Dantus, et al., Opt. Express 11, 1695 (2003) PH-sensitive dye in area with different PH value.

25. Coherent-control optical contrast enhancement in nonlinear optical microscopy spectrometer Input pulses Beam splitter Objective lens XY scanning stage sample SLM Grating Grating

26. Control of Molecules with Ultrafast Light Coherent-control enhanced optical contrast in nonlinear optical microscopy • Coherent control contrast enhancement as large as a factor of three can be achieved at regions where the spectral peak wavelengths differ only 18 nm. • Coherent control study offers an additional degree of freedom for distinguishing coherent and incoherent nonlinear optical processes.

27. Conclusions • SFG can probe into the interfacial molecular structure, which controls the bulk alignment and therefore the material properties. • Time-resolved 2D IR correlation spectroscopy had been used to reveal intra- and intermolecular motions in an electro-optical switching FLC. • Control molecular response by laser pulses beyond simple pump-probe scheme is possible via coherent control technique.