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

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 CH2 groups (=2850 cm-1) along all-trans alkyl chains are antiparallel to each other, therefore their contributions to SFG are small.


2750 2850 2950 3050

Infrared Frequency (cm-1)

0.0

0.4

0.8

1.2

1.6

SF

G I

nte

nsi

ty (

arb

. u

nit

s)

OTS 1080C

OTS 22 0C

Typical properties of SFG vibrational spectroscopy from symmetry breaking

2750 2850 2950 3050


0.0

0.4

0.8

1.2

1.6

SF

G In

ten

sity

(ar

b. u

nit

s)

OTS 1080C

OTS 108 0C

2750 2850 2950 3050


0.0

0.4

0.8

1.2

1.6

SF

G In

ten

sity

(ar

b. u

nit

s)OTS 1080C

OTS 67 0C


Each cis-trans defect causes unpaired CH2 groups, which then contribute to SFG activity at =2850 cm-1.


Chain-chain interaction between LC molecules and surfactant monolayer could be the first event in aligning LC molecules on surface.

2750 2850 2950 3050


0.00

0.04

0.08

0.12

0.16

SF

G In

ten

sit

y (

arb

. un

its

)

OTS 1080C

DMOAP8CB/DMOAP

The uniaxial alignment of the first LC layer on an alignment surface is revealed by an azimuthal dependence of the CN 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


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 tech

nique known as two-dimensional vibrationa

l spectroscopy, which can be used to

determine static structure of peptides an

d proteins ;

examine fast processes such as protein fol

ding and peptide conformational dynamics;

map the relationship between individu

al 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.

),(),(

);(~

);(~

),(

2121

2121

tt

t

iAS

IIC

2D Infrared Correlation Spectroscopy

Synchronous Asynchronous

)(sin 02

0 UAAp

Insight into the Synchronous 2D IR Correlation Plot

Isotropic component A0 can not affect the auto peaks of the synchronous correlation plot.


)(sin 02

0 UAAp

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.


)(sin 02

0 UAAp

Insight into the Synchronous and Asynchronous 2D IR Correlation Plot


)(sin 02

0 UAAp

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


)(sin 02

0 UAAp

Surface interactions can unwind the spontaneous helix, , which then yields a uniform FLC alignment with sec Response

BistabilityWide 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 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.


70 sec after applying -10V

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.

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.


Third harmonicgeneration

Three photonfluorescence

Two photonfluorescence

tt

NLO Processes for Multi-Photon Microscopy


(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.


Coherent-control optical contrast enhancement in nonlinear optical microscopy

SLM

Grating 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.


Conclusions

SFG can probe into the interfacial molecular structure, SFG can probe into the interfacial molecular structure, which controls the bulk alignment and therefore the which controls the bulk alignment and therefore the material properties.material properties.

Time-resolved 2D IR correlation spectroscopy had been Time-resolved 2D IR correlation spectroscopy had been

used to reveal intra- and intermolecular motions in an used to reveal intra- and intermolecular motions in an electro-optical switching FLC.electro-optical switching FLC.

Control molecular response by laser pulses beyond simple Control molecular response by laser pulses beyond simple pump-probe scheme is possible via coherent control pump-probe scheme is possible via coherent control technique.technique.

Documents

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