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