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Exciton-Plasmon Interactions and Fano Resonances in · PDF file 2010-09-14 · Exciton-Plasmon Interactions and Fano Resonances in Nanostructures Alexander Govorov Department of Physics

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  • 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

    1Fanoq

    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

    1Fanoq

  • 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

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