Author Topic: Vibrational coupling in plasmonic molecules  (Read 138 times)

Offline S. M. Enamul Hoque Yousuf

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Vibrational coupling in plasmonic molecules
« on: May 11, 2018, 12:06:19 PM »
Intermolecular and intramolecular energy transfer and redistribution,
associated with many bond-selective reactions, are
usually mediated by internal vibrational or rotational mode coupling
(1–8). A coupled harmonic oscillator picture provides the simplest
approximation to model vibrational interactions in molecules (9).
When two vibrations are coupled, the result is a blueshift of the
higher-frequency mode, while the lower-frequency mode is redshifted
with a magnitude governed by the coupling strength (Fig.
1A). A quantum mechanical treatment becomes necessary to describe
the more complex coupling in larger molecules that support
many vibronic modes (10). As a consequence of quantum mechanical
state mixing, unique spectroscopic features emerge, including
frequency splitting and shifting, that can be exploited to deduce
molecular structure through vibrational spectroscopy (10, 11). One
such phenomenon is known as a Fermi resonance, which occurs
when two energetically similar vibrations mix, resulting in frequency
shifts and intensity redistribution of the uncoupled vibrational resonances
(12). Often, one of the two interacting vibrational modes is
an overtone, and coupling is maximized when ωA = 2ωB.
Many concepts of molecular spectroscopy have been successfully
applied to clusters of proximal nanoparticles supporting plasmon
resonances, known as plasmonic molecules. These include the hybridization
of plasmons to form coupled modes due to strong nearfield
coupling, in analogy with molecular orbital theory (13, 14); the
emergence of Fano interference (14–16); and the application of
group theory to determine selection rules (17). The excitation of
coherent lattice vibrations has also been observed through Raman
spectroscopy (18, 19), ultrafast imaging (20, 21), and transient extinction
spectroscopy (22–24). In this last method, the strong absorption
of an ultrafast laser pulse by the plasmonic nanoparticle
impulsively launches acoustic breathing modes that are detected
optically with a time-delayed probe pulse (22, 25–27). The measured
acoustic modes of isolated nanoparticles are well described
by continuum elastic theory (22, 23, 28). However, vibrational
coupling in plasmonic molecules has been virtually unexplored.
The few ultrafast studies on groupings of plasmonic nanoparticles
have reported widely contradicting results. As the
interparticle gap in gold nanodisk dimers was decreased from
212 to 7 nm, an increase in the oscillation period was measured and
attributed to stronger near-field coupling (29). However, no change
in vibration frequency was observed for gold nanocuboid dimers
with varying separations (30, 31). In closely spaced dimers of gold
nanospheres prepared by chemical methods, a second lowerfrequency
acoustic vibration was detected, corresponding to the
stretching mode of a connected dumbbell (32). A similar stretching
mode was seen in Raman measurements of nanosphere dimers that
were mechanically coupled by surrounding polymer molecules,
consistent with predictions based on continuum elastic theory (33,
34). By using individual nanostructure building blocks with highly
controlled geometries and precise interparticle spacings to create
plasmonic molecules, these observed disparities could be resolved.
This approach has yielded much current fundamental insight into
the near-field interactions of plasmonic modes (13, 16, 25, 35), but
has not been applied to the vibrations of plasmonic molecules.
Here, we report an experimental observation of vibrational coupling
within plasmonic molecules, occurring through the acoustic
modes of their constituent nanoparticles. We use transient extinction
spectroscopy of individual plasmonic molecules with wavelength
tunable optical probe pulses that detected specific acoustic modes in
the metal nanoparticles launched impulsively through ultrafast laser
excitation. The precise control of size, spacing, and arrangement of
the individual nanoparticle constituents of each plasmonic molecule
is accomplished by using electron-beam lithographic fabrication,
allowing us to tune and control the coupling strength. These results
demonstrate the breakdown of classical continuum elastic theory and
suggest that coherent phonons of the substrate play an important
role in the acoustic mode coupling in these systems.
For details please read the attached article.
« Last Edit: May 13, 2018, 10:52:53 PM by S. M. Enamul Hoque Yousuf »

Offline abdussatter

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Re: Vibrational coupling in plasmonic molecules
« Reply #1 on: May 11, 2018, 10:25:59 PM »
 :)
(Md. Dara Abdus Satter)
Assistant Professor, EEE
Mobile: 01716795779,
Phone: 02-9138234 (EXT-285)
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Offline mdashraful.eee

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Re: Vibrational coupling in plasmonic molecules
« Reply #2 on: May 12, 2018, 08:30:14 AM »
good to know
Kind Regards,

Md. Ashraful Haque
Senior Lecturer
Department of (EEE)
Daffodil International University, (DIU)
Permanent Campus

Offline Md. Billal Hossain

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Re: Vibrational coupling in plasmonic molecules
« Reply #3 on: May 14, 2018, 02:31:52 PM »
 :) :) :) :) :)

Offline Nahid_EEE

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Re: Vibrational coupling in plasmonic molecules
« Reply #4 on: May 26, 2018, 06:13:52 PM »
Thank you for your information.
M. Nahid Reza
Lecturer,
Dept. of EEE