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

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

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. Sum-frequency vibrational spectroscopy of surfaces and interfaces Resonance can be employed to yield sensitivity to molecular species. Unique finger printing features of vibrational modes: highly localized; can be well characterized by theory.

The IR transition moments of the CH 2 groups ( =2850 cm -1 ) along all-trans alkyl chains are antiparallel to each other, therefore their contributions to SFG are small. Sum-frequency vibrational spectroscopy of surfaces and interfaces Typical properties of SFG vibrational spectroscopy from symmetry breaking

Sum-frequency vibrational spectroscopy of surfaces and interfaces Each cis-trans defect causes unpaired CH 2 groups, which then contribute to SFG activity at =2850 cm -1.

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.

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. 8CB on Rubbing surfaces Sum-frequency vibrational spectroscopy of surfaces and interfaces

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 structure of peptides and proteins ; examine fast processes such as protein folding and peptide conformational dynamics; map the relationship between individual bonds within or among molecular species.

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.

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. 2D Infrared Correlation Spectroscopy SynchronousAsynchronous

Insight into the Synchronous 2D IR Correlation Plot Isotropic component A 0 can not affect the auto peaks of the synchronous correlation plot. 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= U 1 U 2. 2D Infrared Correlation Spectroscopy

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

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

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

Time-resolved 2D IR Correlation Spectroscopy Tracking correlated motion of sub molecular fragments in an electro- optical switching FLC mixture Time-resolved azimuthal patterns of IR absorption peaks at 1608 (black) and 2924 (red) cm-1 Time-resolved 2D Infrared Correlation Spectroscopy

Time-resolved 2D IR Correlation Plot 70  sec after applying +10V 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. Time-resolved 2D Infrared Correlation Spectroscopy 70  sec after applying -10V

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

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.

Control of Molecules with Ultrashort Light An issue with coherent control is the inverse problem, from knowing what the optimal pulse is to gain information about the system under interrogation. Techniques that can be employed to (1) characterize ultrafast pulses and then (2) modify them appropriate to the experiments being carried out have recently been developed.

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

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

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

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. Control of Molecules with Ultrafast Light

Conclusions SFG can probe into the interfacial molecular structure, which controls the bulk alignment and therefore the material properties. 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. 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. Control molecular response by laser pulses beyond simple pump-probe scheme is possible via coherent control technique.