plexcitonic optical response of core-shell hybrid ...a plethora of plasmonics from the laboratory...
TRANSCRIPT
“Hybrid devices consisting of nanoscale optical, electronic as well as molecular-scale
materials hold great promise for enhancing and achieving new functionalities based
on light-matter interactions [1].”
[1] Taken from the flyer of the European Materials Conference, Spring Meeting 2012, May 14-18,
Strasbourg, France (www.european-mrs.com).
There is considerable current interest in coupling excitonic and plasmonic states
as a means of modifying the OR of the noble metal nanostructures, and thereby to
develop new photonic devices that combine their nanoscale optical responses.
Such systems hold potential for tunable nanophotonic devices for imaging,
chemical sensing, and resonance energy transfer (sensors, switches, lasers, light-
harvesters etc.).
Plexcitonic optical response of core-shell hybrid nanoparticles
Demet Gülen
Part of the work was conducted at the Physics Dept., METU
El-Sayed, Some interesting properties of metals confined in time and nanometer space of different
shapes. Acc. Chem. Res. 34, 2001.
Noble metal nanoparticles and localized surface plasmon resonance (LSPR)
Nanostructures of noble metals exhibit collective oscillations of their conduction
electrons with respect to the positive ion background in the applied electric field.
Halas et al., A plethora of plasmonics from the laboratory for nanophotonics at Rice University.
Advanced Materials, 24, 2012.
JA’s are self-assembled, ordered aggregates of organic molecules (e.g., cyanines, Chls).
Nanoscale aggregates with interesting “collective” optical properties.
Interactions among the transition dipole moments (TDM) of the constituent molecules
delocalized excited states/excitonic states optical response
J-aggregates (JA) and their excitonic states
350 400 450 500 550 600wavelength (nm)
0.0
2.0
4.0
6.0
0
0.2
0.4
0.6
monomer
J-band
ab
so
rba
nc
e
A sharp, red-shifted absorption band (J-band)
A superstate with a large TDM/oscillator strength
Absorption peaking around typical Au/Ag LSPR
Spectral tuning: different monomeric molecules, different aggregation/coherence lengths
monomer J-aggregate
Gülen et al., Chem. Phys. Lett., 2004; Gülen, Photosyn Res, 2006; Birkan, Gülen, Özçelik, J Phys
Chem B, 2006; Gülen, Özçelik, J Lum, 2008; Özçelik, Gülen, J Lum, 2008; Gülen, J Comp and Theo
Nanoscience, 2009; Gülen, Atasoylu , Özçelik , Chem Phys, 2009.
Wurthner et al., J-Aggregates: From serendipitous discovery to supramolecular engineering of
functional dye materials, Angewandte Chemie, 2011.
Restriction of the absorbing medium in a nanoscale volume offers the ability to
tune the resonance against the dielectric constant (s) of the surrounding (s).
“ confinement”
Nanoshells: gifts in a gold wrapperMark L. Brongersma, Nature Materials 2, 296 (2003)
Increased control over the positions, the splitting and the strength of the two LSPRs
Size is not an effective control for particles <<the excitation wavelength
The systems of nanoshells surpass this scale invariance.
Variable “shell thickness” + Layers of different core, spacer and shell materials
Plexcitonic nanoparticles
non-absorbing
plasmonic
excitonic
A plasmonic noble metal nanoshell or a core covered by an excitonic JA shell
Plexcitonic NPs exhibit strong exciton-plasmon coupling.
(considerable experimental evidence in the last 5-10 years)
Mixed exciton-plasmon states
Plasmonic states can absorb very well.
Excitonic states can fluoresce well.
Tunable nanophotonic devices for imaging, chemical sensing, light-
harvesting and lasing.
Core-shell gold J-aggregate nanoparticles for highly efficient strong coupling applications.Lekeufack et al., App. Phys. Lett., 2010.
Extinction spectra of Au NPs with different
sizes coated with J-aggregates.
Extinction spectra of bare gold NPs with
different sizes (from 10 to 45 nm).
The J-aggregate band (around 586 nm).
TEM images
……… As the size of gold particle is tunable,
the precise optimization of the strong coupling
between the electronic transitions of organic
components TDBC and the plasmon modes of
the gold NPs is achieved corresponding to
a Rabi energy of 220 meV, a value not yet
obtained in such a system.......
Typical plexcitonic absorption data
The OR/plasmonic response of noble metal NPs was grasped well through analytical
as well numerical methods (extensively studied in the last two decades).
This knowledge was integrated into the interperation of the OR of the plexcitonic NPs.
The nanoscale OR of the excitonic shell was ignored at large .
(i.e.,the nanoshell OR was assumed to be the same as the bulk OR)
Such an understanding was absent in the literature.
Bottom-up control of the optical response (OR)?
Core in the effective dielectric
medium of “water”+shell
Shell in the effective dielectric
medium of “water”+core
A hybridization model for the plasmon response of complex nanostructures
Prodan et al., Science, 2003
Dispersion should not matter.
Othogonal resonance doublet should also pertain for the organic molecular/
the J-aggregate nanoshells with an adequate dispersion relationship.
Plasmon hybridization in metal nanoshells:
the interaction between the sphere and cavity plasmons.
The two nanoshell plasmons are antisymmetrically
coupled (antibonding) w+ plasmon mode and a
symmetrically coupled (bonding) w- plasmon mode.
Drude nanoshells have two orthogonal resonances.
The excitonic J-band is a huge TDM that simply oscillates in the applied electric field.
Lorentzian dispersion is a reasonable model to start with.
It is widely acknowledged that classical electrodynamics provides a
reasonable quantitative description of the optical response of noble metal NPs.
If the size of the system is much smaller than the wavelength of the incident
light it is sufficient to consider only the dipolar response to light and
the quasi-static approximation is appropriate. (MaxwellLaplace)
Rationale
Bohren & Huffman, Absorption and Scattering of Light by Small Particles, Wiley, 2004.
σ(ω)=constant. ωIm[α(ω)]
α(ω)-polarizability: induced dipole moment/electric field strength
Absorption cross-section:
dielectric function/constants of the core, shell, and medium +
structural parameters; core radius, shell thickness
Polarizability acquires its w-dependence through w-dependence
of the dielectric functions of the core/Drude and the shell/Lorentzian.
D. Gülen, Optical Response of Lorentzian Nanoshells in the Quasistatic Limit.
J. Phys. Chem. B 117 (2013) 11220-11228.
D. Gülen, Optical Response of Lorentzian Nanospheres in the Quasistatic Limit.
J. Lum. 132 (2012) 795-800.
A new methodology is developed in which the functionalities of the dispersive
properties of the spherical shell and the nanoenvironment in tuning the OR are
clearly separated.
This separation allows a convenient control for a bottom-up manipulation of
the OR of the spherically symmetric nanoshells with different dispersions.
NPs with Lorentzian dispersion/excitonic NPs are blue shifters of the optical
resonance(s) contrary to Drude /plasmonic NPs (well-known red shifters).
Optical signature of the Lorentzian nanoshells is an orthogonal pair of
resonances. Both are blue-shifted w.r.t. the “bulk resonance.”
Convenient resonance rulers for the typical Drude (plasmonic) and
Lorentzian (excitonic) nanoshells are provided.
Results
D. Gülen, A Numerical Spectroscopic Investigation on the Functionality of Molecular
Excitons in Tuning the Plasmonic Splitting Observed in Core/Shell Hybrid Nanostructures.
J. Phys. Chem. C 114 (2010) 13825-13831.
This 3-level scheme can explain the key features of the experimental absorption data.
The results can be instrumental for the bottom-up engineering of the plexcitonic OR.
The classical electrodynamics based response of
the core-shell hybrid can be understood as a 3-level
system of two distinct subsystems:
a simple 2-state system in which the plasmon and
the blue-most excitonic states are coupled+
an uncoupled excitonic state.
en
erg
y
Plasmonic response of the intact core
=
embedding medium: water600 640 680 720 760
wavelength (nm)
0
10
20
30
(
a. u.)
LSPR peak ≈ 680 nmAu core parameters
spherical radius=20 nm
aspect ratio=3
ε∞=9.84
bulk plasmon frequency: hωp=9 eV (138 nm)
plasmon relaxation rate:
hγp=70 meVplasmonic core
non-absorbing
medium
non-absorbing
shell
Drude dispersion
Excitonic response of the intact shell system
650 675 700 725
wavelength (nm)
0.00
0.05
0.10
(a
. u
.)
in solution OR (680 nm)
660 670 680 690wavelength (nm)
0.0
1.0
2.0
(a
. u.)
-for
narr
ow
0.0
0.5
1.0
d/a=0.12
f
0.03
0.05
0.07
0.09
non-absorbing
medium
excitonic shell
non-absorbing
core
Resolution of the doublet
at a narrow band width
Optical signature of the Lorentzian nanoshells is
a blue-shifted excitonic doublet.
Lorentzian Nanoshell
hγ0=50 meV
(not identical to the in solution (”bulk”) optical response as assumed in the literature at large)
Lorentzian dispersion
600 650 700 750wavelength (nm)
0
5
10
15
20
(a.
u.)
0
0.5
1
E+/+
E-/-
non-absorbing
medium
excitonic shell
plasmonic core
Plasmon band
In solution J-band
Fixed aspect ratio: 3
Different oscillator strengths :
0.03, 0.05, 0.07, 0.09
Fixed shell thickness:
(thickness/core radius=0.12)
uncoupled excitonic band
Optical response of the plexcitonic system(Drude core-Lorentzian shell)
The splitting (exciton-plasmon coupling?) observed in the experiments is
produced faithfully.
Nature of the hybrid bands
650 675 700wavelength (nm)
0.1
1
10
100
1000
log
(a.
u.)
fixed exciton relaxation rate (2 meV)
different plasmon relaxation rates
0.7 meV , 7 meV, 25 meV , 70 meV
650 675 700wavelength (nm)
0.1
1
10
100
1000
log
(a.
u.)
fixed plasmon relaxation rate (0.7 meV)
different exciton relaxation rates
0.7 meV , 2.5 meV, 5 meV, 7.5 meV
The coupling is between the LSPR of the core and the blue-most excitonic
transition of the shell. The excitonic state of lower energy is uncoupled.
Exciton-plasmon coupling leads to a pair of hybridized bands around
“the originally degenerate” exciton and plasmon resonances.
(Nanoshells offer a pair of orthogonal resonances.)
600 650 700 7500
10
20
600 650 700 7500
10
20
600 650 700 7500
10
20
600 650 700 7500
10
20
Wavelength (nm)
Ab
so
rba
nce (
a.
u.)
f=0.03
f=0.05
f=0.07
f=0.09
d/a=0.12
0.06 0.12 0.18d/a
V (
me
V)
20
40
60
80
0.05 0.10 0.15 0.20d/a
0.3
0.4
0.5
0.6
0.7
sin2ξ
cos2ξ
The plexcitonic spectra can be quantified in terms of a QM 2-state model.
coupling strength mixing
en
erg
y
A 3-level system of two distinct subsystems:
a 2-state system in which the plasmon and
the blue-most excitonic states are coupled+
an uncoupled excitonic state.
Optical signature of the Lorentzian/excitonic
nanoshells is a pair of orthogonal resonances.
Both resonances are blue-shifted w.r.t. the
“bulk resonance”.
Nano Lett. 2010, 10, 77-84
A 3-level system of two distinct subsystems:
a 2-state system in which the plasmon and
the blue-most excitonic states are coupled+
an uncoupled excitonic state.
Optical signature of the Lorentzian/ excitonic
nanoshells is a pair of orthogonal resonances.
Both resonances are blue-shifted w.r.t. the
“bulk resonance”.
Core-shell gold J-aggregate nanoparticles for highly efficient strong
coupling applications.Lekeufack et al., App. Phys. Lett., 2010.
Extinction spectra of Au NPs with different
sizes coated with J-aggregates. Extinction spectra of bare gold NPs with
different sizes (from 10 to 45 nm).
The J-aggregate band (around 586 nm).
For designing the plexcitonic nanoparticles with maximum splitting one should prefer
a J-band on the red of the plasmon resonance/ tune the plasmonic resonance on the blue
of the in-solution excitonic band.
In addition to contributing to an increased control over tuning the OR of
core-shell plexcitonic NPs for their potential applications, these results can
stimulate interest for analytical approaches aiming at a more fundamentally
sound understanding of the exciton-plasmon coupling.
Overall
A more fundamental understanding of the mechanism of coupling?
Zhang, Govorov, Garnett, PRL 97 (2006)
Semiconductor-metal nanoparticle molecules: Hybrid excitons and the nonlinear Fano effect
R
SQD in E
SQD in
Einduced by the MNP MNP in E
MNP in
Einduced by the SQD
Explore the dynamics of excitations in a typical two-state problem,
e. g., Stochastic Liouville Equation-add some statistical mechanics.
How coherent is the exciton-plasmon coupling?
Possible connection to the real-world: Dynamics of the Rabi oscillations.
This will possibly follow experimentally-ultrafast spectroscopy.
R. S. Knox, D. Gülen, “Theory of polarized fluorescence from molecular pairs: Förster
transfer at large electronic coupling.”, Photochem. and Photobiol., 57, 40-43 (1993).