Exciton-Plasmon Interactions and
Fano Resonances
in Nanostructures
Alexander Govorov
Department of Physics and Astronomy,
Ohio University, Athens, Ohio, USA
Supported by:
NSF, Air Force Research Office, A. v. Humboldt Foundation,
BNNT Institute at Ohio U.
Collaborators:
Pedro Hernandez, Ohio U.
Fan Zhiyuan, Ohio U.
Wei Zhang, Ohio U. (now in China)
Nanoparticles as building blocks
Interaction between nanocrystals
Exciton + plasmon + photons
Linker
(polymer)
liquid
Quantum emitter
Amplification or
suppression of PL
++
+
+
-
--
-
exciton
plasmon
Au
CdTe
++
+
+
-
--
-
Au
CdTe
plaser
emissionexcp E
Incoherent exciton-plasmon interaction
2
, int
2 2ˆ( ) | 0; ;0 | ( ) Im[ ( )]non rad metal exc exc f exc
f T
f V exc
Field enhancement factor:
2
2
2
)( photon
tmetalno
t
actual
E
E
E
P
1exc
0exc
0
Metal NPSemiconductor
NP
++
+
+
-
- -
-
Au
CdTe
Book: Joseph R. Lakowicz
Principles of Fluorescence Spectroscopy
Energy transfer (FRET) rate:
0 0 0( ) ( )exc rad exc non rad transfer exc ldn P n P Idt
Slocik, J.M.; Govorov, A.O.;
and Naik, R.R.,
Supramolecular Chemistry,
2006.
N Au-NP -- CdSe-NP
Number of Au NPs
NAu=1 NAu=2 NAu=4 NAu=6
400 500 600 700 800 900 1000
0.8
0.9
1.0
1.1
1.2
NAu=4,2
NAu=1NAu=6
E
na
hn
ce
m
en
t f
ac
to
r
Wavelength, (nm) 0 1 2 3 4 5 6 7 8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
theory
exper
Number of Au NPs, NAu
emiss=610 nm
laser=400 nm
E
m
is
si
on
ra
tio
A
(N
A
u)
0
0
( )
(0)
emiss Au NPs
emiss transfer
I NA
I N
Emission ratio:
Govorov, et al., Nano Lett., 2006
Coherent interactions.
Fano effect
1
2 k
0V v
w
2 2 2
0
2 2 2
2
0
( )
2
E v qAbs
w
w
V wq
v
-40 -20 20 40
0.05
0.1
0.15
0.2
0.25
0.3
12
Absorption
Detuning,
X-ray absorption spectrum of He
-40 -20 20 40
0.05
0.1
0.15
0.2
0.25
0.3
Fano effect in He
2 1s1 k
0V v
w
X-ray absorption spectrum of He
Fano lineshapes
1
2 k
0V v
w
2
2 2
12
2
0
( )qAbs
w
V wq
v
-10 -5 0 5 10
0.0
0.5
1.0
1.5
-10 -5 0 5 10
0.0
0.5
1.0
1.5
A
bs
or
pt
io
n
-10 -5 0 5 10
0.0
0.5
1.0
1.5
q=0.01
q=2
Detuning
q=100
12,
k
+
+
+
+
-
--
-
exciton plasmons
MNPSQD
Coulomb interaction
kCoulk
k
Coulkk
k
k accUaccUaaccEccEH ˆˆˆˆˆˆˆˆˆˆˆˆˆ 21
*
122221110
1
2
exciton plasmon
states
ground
state
Colloidal nanocrystals
Two paths for excitation of plasmon
interference effect (Fano effect)
Optical absorption
k
1
2
exciton plasmon
states
ground
state
+
+
+
+
-
--
-
exciton plasmons
MNPSQD
Coulomb interaction
Fano effect
1
2 k
0V v
w
-40 -20 20 40
0.05
0.1
0.15
0.2
0.25
0.3
12
absorption
2 2 2
0
2 2 2
2
0
( )( )
2
E v qAbs
w
w
V wq
v
Light
ttV
tVHH
HHi
t
accUaccUaaccEccEH
opt
opt
kCoulk
k
Coulkk
k
k
cos)(ˆ
)(ˆˆˆ
ˆˆ)ˆˆˆˆ(
ˆ
ˆˆˆˆˆˆˆˆˆˆˆˆˆ
0
0
21
*
122221110
Er
Strong de-coherence in the metal NP
(fast relaxation of plasmon)
Electromagnetic enhancement
due to the plasmon
+
+
+
+
-
--
-
exciton plasmons
MNPSQD
W. Zhang, A. Govorov, et al., PRB 2008.
Dipole limit:
tot MNP SQDQ Q Q
12
10
1
metal NP semiconductor NP
metal NPR nm
meV
100nm
2.0 2.2 2.4 2.6
0
2
4
6
8
10
12
14
Photon energy (eV)
E
xt
in
ct
io
n,
(1
08
c
m
-1
M
-1
)
W. Zhang, A. Govorov, et al., PRB 2008.
In the regime of strong de-coherence:
Strong anti-resonances in
absorption and light scattering
Weak lines become visible as
anti-resonances
Multipoles are important
100nm
NPtorsemiconducNPmetal
CdTe-Au complex
12nm
tot MNP SQDQ Q Q
12 1
10metal NP
meV
R nm
Nonlinear Fano effect
1
2 k
0V
v
w
Strong light field
Transition 1 2 is partially saturated
1 2 22 11Absorption
Experiments in Munich
A dot with weak tunnel coupling to a continuum
Fano factor
Narrow exciton resonances
Self-assembled quantum dots at low temperatures
GaAsInAs
Quantum well
1Fanoq
AlGaAs
-0.5
0.0
0.5
1.0
-50 0 50 -50 0 50 -50 0 50 -50 0 50 -50 0 50-50 0 50
-0.5
0.0
0.5
1.0
0.41µeV 2.1µeV 6.9 µeV 16 µeV9.5 µeV
0.33 nW 168 nW90 nW8.3 nW
22 µeV
481 nW 935 nW
L - 01 (µeV)
N
or
m
al
iz
ed
e
xt
in
ct
io
n
Martin Kroner, Khaled Karrai, at al., experiments
Theory
M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A.
Badolato, P. M. Petroff, W. Zhang, R. Barbour, B. D. Gerardot, R. J.
Warburton & K. Karrai, Nature, 2008.
N
or
m
al
iz
ed
d
iff
er
en
tia
l a
bs
or
pt
io
n
power
Nonlinear Fano effect
Low power High power
At low power, the natural broadening prohibits
observation of weak processes
At high power, we can observe weak coherent processes
The discrete resonance is saturated
Transitions to the continuum become more important
powerab
so
rp
tio
n
Discrete
continuum
laser
1Fanoq
metal NP molecule
Light
0
0
metal NP
molecule
CD
CD
Optical chirality (circular dichroism)
Circular dichroism spectroscopy
0
10log
lcp
lcp lcp
lcp
lcp rcp
CD lcp rcp
I
A L c
I
A A A
0
( )lcp rcpI ( )lcp rcpI
k
Chiral objects:
No mirror symmetry planes
Example: Helix
1
2
molecule density of
states of NP
photons
Coulomb
NP
dye
m
R
NPa
k
150 200 250 300 350 400 450 500
-40
-20
0
20
40
60
80
C
D
,
(c
m
-1
M
-1
)
Wavelength (nm)
plasmon-induced
*,
*, ||
*, ||n
150 200 250 300 350
0
2000
4000
6000
8000
Wavelength (nm)
E
xt
in
ct
io
n
(c
m
-1
M
-1
)
Ag
*,
*, ||n
*, ||
-helix
Ag NP
W. Moffitt, J
