probing into the molecular world with light jung y. huang department of photonics and institute of...
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![Page 1: Probing into the Molecular World with Light Jung Y. Huang Department of Photonics and Institute of Electro-Optical Engineering, NCTU](https://reader035.vdocuments.site/reader035/viewer/2022062721/56649f1b5503460f94c3174e/html5/thumbnails/1.jpg)
Probing into the Molecular World with Light
Jung Y. Huang
Department of Photonics and Institute of Electro-Optical Engineering, NCTU
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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.
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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.
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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.
Sum-frequency vibrational spectroscopy of surfaces and interfaces
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
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2750 2850 2950 3050
Infrared Frequency (cm-1)
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
Infrared Frequency (cm-1)
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
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.
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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.
2750 2850 2950 3050
Infrared Frequency (cm-1)
0.00
0.04
0.08
0.12
0.16
SF
G In
ten
sit
y (
arb
. un
its
)
OTS 1080C
DMOAP8CB/DMOAP
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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
Sum-frequency vibrational spectroscopy of surfaces and interfaces
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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.
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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.
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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
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Insight into the Synchronous 2D IR Correlation Plot
Isotropic component A0 can not affect the auto peaks of the synchronous correlation plot.
2D Infrared Correlation Spectroscopy
)(sin 02
0 UAAp
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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.
2D Infrared Correlation Spectroscopy
)(sin 02
0 UAAp
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Insight into the Synchronous and Asynchronous 2D IR Correlation Plot
2D Infrared Correlation Spectroscopy
)(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.
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Summary of the Synchronous and Asynchronous 2D IR Correlation Plot
2D Infrared Correlation Spectroscopy
)(sin 02
0 UAAp
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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
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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
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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
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Tracking correlated motion of submolecular fragments of a complex FLC mixture
U and 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.
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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.
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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.
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(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
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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
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Third harmonicgeneration
Three photonfluorescence
Two photonfluorescence
tt
NLO Processes for Multi-Photon Microscopy
Control of Molecules with Ultrafast Light
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(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
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Coherent-control optical contrast enhancement in nonlinear optical microscopy
SLM
Grating Grating
spectrometer
Objective lens
sample
Beam splitter
XY scanning stage
Input pulses
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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
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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.