Exciton-Plasmon Interactions and Fano Resonances in Nanostructures · 2010-09-14 ·...
Transcript of Exciton-Plasmon Interactions and Fano Resonances in Nanostructures · 2010-09-14 ·...
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
+++
+---
-
excitonplasmon
AuCdTe
+++
+---
-Au
CdTe
plaser
emissionexcp E
Incoherent exciton-plasmon interaction
2, int
2 2ˆ( ) | 0; ;0 | ( ) Im[ ( )]non rad metal exc exc f excf T
f V exc
Field enhancement factor:
2
2
2
)( photon
tmetalno
tactual
EE
EP
1exc
0exc
0
Metal NPSemiconductor NP
+++
+-- -
-Au
CdTe
Book: Joseph R. LakowiczPrinciples of Fluorescence Spectroscopy
Energy transfer (FRET) rate:
00 0( ) ( )exc
rad exc non rad transfer exc ldn P n P I
dt
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 10000.8
0.9
1.0
1.1
1.2
NAu=4,2
NAu=1NAu=6
Ena
hnce
men
t fac
tor
Wavelength, (nm) 0 1 2 3 4 5 6 7 80.0
0.2
0.4
0.6
0.8
1.0
1.2
theory
exper
Number of Au NPs, NAu
emiss=610 nmlaser=400 nm
Em
issi
on ra
tio A
(NA
u)
0
0
( )(0)
emiss Au NPs
emiss transfer
I NAI N
Emission ratio:
Govorov, et al., Nano Lett., 2006
Coherent interactions. Fano effect
1
2 k
0V v
w
2 2 20
2 2 2
2
0
( )2
E v qAbsw
wV wqv
-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
122
0
( )qAbs
wV wqv
-10 -5 0 5 100.0
0.5
1.0
1.5
-10 -5 0 5 100.0
0.5
1.0
1.5
Abs
orpt
ion
-10 -5 0 5 100.0
0.5
1.0
1.5
q=0.01
q=2
Detuning
q=100
12,
k
++
++
---
-
exciton plasmons
MNPSQD
Coulomb interaction
kCoulkk
Coulkkk
k accUaccUaaccEccEH ˆˆˆˆˆˆˆˆˆˆˆˆˆ21
*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 20
2 2 2
2
0
( )( )2
E v qAbsw
wV wqv
Light
ttV
tVHH
HHit
accUaccUaaccEccEH
opt
opt
kCoulkk
Coulkkk
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 2.602468
101214
Photon energy (eV)
Ext
inct
ion,
(1
08 cm
-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 110metal NP
meVR nm
Nonlinear Fano effect
1
2 k
0Vv
w
Strong light fieldTransition 1 2 is partially saturated
1 2 22 11Absorption
Experiments in MunichA 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)
Nor
mal
ized
ext
inct
ion
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.
Nor
mal
ized
diff
eren
tial a
bsor
ptio
n
powerNonlinear 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 saturatedTransitions to the continuum become more important
powerabso
rptio
n
Discrete
continuum
laser
1Fanoq
metal NP molecule
Light
0
0metal NP
molecule
CD
CD
Optical chirality (circular dichroism)
Circular dichroism spectroscopy
0
10log lcplcp lcp
lcp
lcp rcp
CD lcp rcp
IA L c
I
A A A
0( )lcp rcpI ( )lcp rcpI
k
Chiral objects: No mirror symmetry planesExample: 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
CD
,
(cm
-1M
-1)
Wavelength (nm)
plasmon-induced
*,
*, ||
*, ||n
150 200 250 300 3500
2000
4000
6000
8000
Wavelength (nm)
Ext
inct
ion
(cm
-1M
-1)
Ag
*,
*, ||n
*, ||
-helixAg NP
W. Moffitt, J. Phys. Chem. 25, 467 (1956).
Chromophore excitons are delocalized in a helix structure
z
x
y
Govorov, A.O.; Fan, Z.; Hernandez, P.; Slocik, J.M; Naik, R.R., Nano Letters, 2010.
Purely plasmonic CD
Z. Fan, A. O. Govorov, Nano Letters 2010.
The exciton-plasmon interactions
Linear and nonlinear interference effects
Plasmon-induced CD
Conclusions
Low power High power
Ag
150 200 250 300 350 400 450 500-40
-20
0
20
40
60
80
CD
,
(cm
-1M
-1)
Wavelength (nm)
plasmon-induced