Ultrafast spectroscopy of a

single metal nanoparticle

Fabrice Vallée

FemtoNanoOptics groupLASIM, CNRS - Université Lyon 1, France

CENTRE NATIONALDE LA RECHERCHESCIENTIFIQUE

Metallic particles in glasses:jewelry, ornament

Lycurgus Cup: a Roman NanotechnologyRoman Era (4th Century A.D). It appears green in scattered light...and red in transmitted light.

Metal Nanoparticles: from IV...to XXI century

Ag Au

Ancient cup (Central Europe)

Optical response of metal nanoparticles

• Metal nanosphere (ε = ε1 + iε2) in a matrix (εm):Mie theory for sphere R << λ (dipolar):

→ Resonance depends on: - environment- shape + light polarization (ellipsoids, rods, ...)

→ manipulation of light at subwavelength→ plasmonics

[ ] )(2)()(18

22

21

22/3

λε+ε+λε

λελεπ

=σm

mabs

VR

EO

Eint

ε(ω)

εm+

+

+

+

resonance for ε1(λ) + 2εm ≈ 0→ Surface Plasmon Resonance

Ag particles in glassEnsemble of identical nanoparticles: α = Npartσabs

Optical response of metal nanoparticles

EFintrabandtransitionsinterband

transitions

CB

d - bands

( )τ+ωωω−ωε=ωε ipb 2)()(

bound electrons (interband)

free electrons (intraband)

Metal dielectric function

mib1pRmR1 202)( ε+εω≈Ω⇔≈ε+ΩεSPR Frequency:

400 500 600 7000

1

αL

Wavelength (nm)

Au - colloids - D = 10 nm

ΩR

200 250 300 350 400 450 5000

1

2

3

4

5

ΩRInterband Transitions

Threshold

Ωib

Surface Plasmon Resonance

Wavelength (nm)

Ag - D = 13 nm - p = 2x10-4

Ultrafast spectroscopy of metal nanoparticles

Femtosecond investigation: pump-probe

Electron excitation (hν) + Femtosecond probing (nonlinear optical response)→ Coherent response→ Nonequilibrium electron kinetics→ Intrinsic electron scattering processes

- Internal thermalization ( → Te) → electron-electron : τth ~ 300 fs- External thermalization (Te → TL) → electron-lattice : τe-ph ~ 1 ps

→ Confined vibrational modes→ Extrinsic processes: nanoparticle - environment coupling→ Nonlinear optical response

MatrixLatticeτe-ph

e ↔ eτth τp-mτp-m

Matrix

hν

Femtosecond excitation and probing

f

E

f(0)

F E F

0 f(t)

E F

Te = T0 Te > T0

+hωP

hωP

t = 0 t > 0t < 0

Femtosecond excitation: intraband absorption

→ Athermal electron distribution → Thermalization + Cooling

Femtosecond probing: • Transmission change ΔT/T ⇔ dielectric function change Δε(tD)

• Probe wavelength → different electron interactions

IS T x ISΔT/T

Sample

Probe

Pump

C.K.Sun, et al., Phys. Rev. B 50, 15337 (1994)T.Tokizaki, et al., Appl. Phys. Lett. 65, 941 (1994) ; J.Y.Bigot, et al., Phys. Rev. Lett. 75, 4702 (1995)C.Voisin et al., J. Phys. Chem B 105, 2264 (2001).

Experimental setup: femtosecond pump-probe

Chopper ( ω)

Computer

Variable Delay

Lock-inAmplifier

ω

+ -

Signal Reference

Argon Laser

Reference

BBO

Sample

Signal

Ti:sapphire laser

800 - 900nm1W - 25fs

prismpair

Pump: red pulse (25 fs) (or 2ω)Probe: red pulse (ω ; 25 fs)

blue (2ω ; 30 fs)or UV (3ω ; 55 fs)

Perturbation : ΔTe ~ 10 K - 2000 K

Sensitivity : depends on mean power on detector maximum: ΔT/T ~ 10-7 (f = 100 kHz)

ω

2ω

3ω(f)

f

Verdi Laser

Ti:sapphirelaser

780 - 1030 nm1 W - 25 fs

Electron energy losses: electron-phonon couplingAg - D = 3 nm in Al2O3

UV probe : Risetime: electron thermalization → electron-electron interactionsDecay: energy transfer to the lattice → electron-lattice coupling

Blue probe: Energy in the electron gas → electron-lattice coupling

→ Size effect ?

bandes d

B.C.

EF

UV probe

d-bands

Blue probe

0 1 2 3

0.2

0.4

0.6

0.8

1.0

ΔT/

T

Probe Delay (ps)

0 1 2

0.01

0.1

1

ΔT/

T

Probe Delay (ps)

pump: IR (930nm)probe: Blue (465nm) or UV (310 nm)

Electron interactions: Size dependences

0 5 10 15 20 25 30100

200

300

Ag nanoparticles

Ag film

BaO-P2O5 matrix Al2O3 matrix Deposited on glass

Nanoparticle diameter (nm)

τ th

(fs)

0 5 10 15 20 25 300.4

0.5

0.6

0.7

0.8

0.9

Ag nanoparticles

Ag film

polymer BaO-P2O5

Al2O3

MgF2 deposited

Nanoparticle diameter (nm)

τ e-ph

(ps)

Electron – lattice interaction: τe-ph

(A. Arbouet, PRL 90, 177401 (2003))Tin and Gallium: τe-ph ∝ D (M. Nisoli et al., PRL 78, 3575 (1997))

Electron – electron scattering: τth

(C. Voisin, et al. , PRL 85 , 2200 (2000))Screening reduction close to the surface

Many studies on large ensembles (104 to 106 particles) ⇒ Size and shape fluctuations→ Single nanoparticle

Femtosecond spectroscopy of a single nanoparticle

1) Optical detection and characterization:linear absorption spectroscopy

Pt

Sample

IS T x ISΔT/T

Sample

Probe

Pump

2) femtosecond pump - probe:nonlinear response

Optical detection and spectroscopy of a single metal

nanoparticle

Optical study of a single metal nanoparticle

Non luminescent object: → Detection of light scattering or absorption

♦ Near field: local environment perturbationT. Klar et al. Phys. Rev. Lett. 80, 4249 (1998)

♦ Far field: focused beam 300 - 500 nm → diluted sample ( < 1 particle / µm2 )- Scattering (∝ V2 ; size ≥ 20 nm): → Dark field microscopy

C. Sönnichsen et al., Appl. Phys. Lett. 77, 2949 (2000)K. Lindfords et al. Phys. Rev. Lett. 93, 37401 (2004)

- Absorption (∝ V ; small particle): Focused laser beam: 300 - 500 nmGold nanosphere D = 20 nm - 5 nm: absorption of 10-3 - 10-5 of the incident light

→ Photothermal techniqueD. Boyer et al., Science 297, 1160 (2002)

→ Spatial modulation technique (quantitative)A. Arbouet et al., Phys. Rev. Lett. 93, 127401 (2004)

Optical detection of a single metal nanoparticle(A. Arbouet et al., Phys. Rev. Lett. 2004)

Absorption by a single nanoparticle:- Focused laser beam: 300 - 500 nm- Gold nanosphere D = 20 nm - 5 nm: absorption of 10-3 - 10-5 of the incident light

lock-in amplifier

Transmitted power P

f

piezof

microscopeobjective

100xf , 2f

XY scanner

Light

sample

Spatial Modulation Technique:Modulation of the nanoparticule position f ⇔ Modulation of transmitted light f or 2f

Gold nanoparticles - <D> = 10 nm

yy0

I(x0,y0)• Transmitted power:

(I: intensity profile at the focal spot)( )00, yxIPP extit σ−= δy

• Modulation of the particle position at f : y0 → y0 + δysin(2πft)

)2(sin2

)2sin(),( 222

2

00

00

ftyIft

yIyxIPP y

y

exty

yextextit πδ

∂∂σ

−πδ∂∂

σ−σ−≈

2f

X (µm)

Y (µm)

ΔP/P

f

X (µm)

Y (µm)

ΔP/P

• Sample image: X/Y scan - λ = 532 nmDiluted sample (< 1 particle/μm2): 10 nm gold nanoparticle spin-coated on a substrate

SMS microscope

grating

Non-linear photonic crystal fiber

Supercontinuumλ > 450 nm

Ti- sapphire femtosecond oscillator

100 mW - 780 nm - 20fs

Optical absorption signature

Absorption spectroscopy of a single nanoparticle

Single nanoparticles: optical characterization

ΔT/T

X (µm)

Y (µm)

λ = 532 nm; modulation along Y at f = 1 kHz

Absolute value of the absorption cross-section

+ polarization dependence

450 500 550 6000

100

200

300

400

σ abs

(nm

2 )

Wavelength (nm)

Tunable source

Spectroscopy

19.5 nm

18 nm

12 13 14 15 16 17 18 19 20 210

5

10

15

<D > = 16.6 nm

Coun

ts

D iam eter (nm )

N anoscope

12 13 14 15 16 17 18 19 20 210

10

20

30

40

50

60<D > = 16.2 nm

Coun

ts

D iam eter (nm )

TEM 0.5 0.6 0.7 0.8 0.9 1.00

4

8

12 <η > = 0.9

Coun

ts

Aspect ratio c/a

Nanoscope

0.5 0.6 0.7 0.8 0.9 1.00

20

40

60

<η > = 0.9

Coun

ts

Aspect ratio c/a

TEM ⇓Optical identification of a nanoobject: size

and shape and orientationO.L.Muskens et al., Appl. Phys. Lett. 88, 063109 (2006)

Femtosecond optical nonlinearity of a single nanoparticle

IS T x ISΔT/T

Sample

Probe

Pump

femtosecond pump - probe study :

Femtosecond spectroscopy of a single nanoparticle

DVM

PCf

x

y

BBOtunableTi:Al203

fsoscillator

Chameleon

Lock-in

tD

PD

CF

CF

Ch

350 400 450 500 5500

5

10

σ ext

(x 1

0-15 m

2 )

Longueur d'onde (nm)

Ag - D = 30 nm

D = 21 nm

IR excitation / SPR probing (425 nm)

0 1 2 3

0.0

0.4

0.8

1.2

ΔT/T

(x

10-4)

Probe delay (ps)

0 200 4000.0

0.5

1.0

ΔT/T

max

(x

10 -4

)

PP (µW)

probe

ext

pumpext

ext

lenanopartic STT σ

σσ

⋅⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ−=

Δ

1

Transmitted Power

Microscope Objective

100x

Femtosecond spectroscopy of a single Ag nanosphere

Optical characterisation of a single nanoparticle (linear absorption spectrum) & femtosecond pump - probe study :

0.0 0.2 0.40.6

0.9

1.2

1.5

τ e-ph

(p

s)

Pump power (mW)

• Mechanism ⇒ ΔT/T ∝ electron excess energy⇒ Decay: electron-lattice energy exchange → τe-ph

0phe−τ

Strong excitation regime

excitation dependent decay: τe(Te)

Weak excitation regime

ΔT decay with τ0e-ph = c0 T0 / G

⎪⎪⎩

⎪⎪⎨

⎧

−=

−−=

)(

)()(

LeL

L

Lee

ee

TTGdt

dTC

TTGdt

dTTCThermal distributions: Two temperature model

Te ; TL ; G = e-lattice coupling constant

Ce(Te) = c0 Te ; CL : heat capacities

Electron-phonon energy exchange in single Ag nanospheres

0.0 0.2 0.40.6

0.9

1.2

1.5

τ e-ph

(p

s)

Pump power (mW)

Comparison wih two temperature model:→ Same electron-phonon coupling

as in ensemble measurements (in glass)→ No environment dependence

(large excitation)→ No e-ph coupling dependence

on excitation regime

0.1 1

1.0

1.5

2.0

τ e-ph

/ τ 0 e-

ph

(T max e -T0) / T0

pump power ⇒ T

emax

Known nanoparticle ⇒ known excitation Te - T0

→ comparison with the two-temperature model

max

Electron-phonon coupling in single Ag nanospheres

O.Muskens, N. Del Fatti and F. Vallee, Nano Letters 2006

ΔTemax : 110 - 430 K (30nm)

ΔTemax : 220 - 380 K (21nm)

Acoustic vibration of a single nano-object:

pair of nanoprisms

Vibrational acoustic modes: Frequency: size and shape dependentDamping: environment / size and shape distribution

→ Single nanoparticle

Gold nanoprisms: detection

M. El-SayedGeorgia Inst. Tech., Atlanta

Nanosphere lithography:Organized nanoprisms: size 120 nm

thickness 30 nm(W. Huang et al., Nano Lett. 4, 1741 (2004))

TEM image

AFM image

5 x 5 µm2

Optical image (at 410 nm)

⇒ Optical observationof prism pairs

Gold nanoprism pair: acoustic vibrationsJ. Burgin et al., J. Phys. Chem. C 112, 11231 (2008)

- Breathing mode: single T3 = 12 ps ; ensemble: T3 = 14 ps

0 50 100 150-0.2

0.0

ΔT/T

(

x10-3

)

Probe delay (ps)0 50 100 150

-5

0

ΔT/T

(a

.u.)

Probe delay ( ps )

Prism pair Ensemble

First isotropic modes of a triangle:

- Two modes: pair: T1 = 64 ps , T2 = 49 ps ; ensemble: T1 = 67 ps , T2 = 40 ps - Pair: period fluctuations + slower relaxation (reduced inhomogeneous damping)

0 50 100 150

0.0

0.1

0.2

ΔT/T

(

x10-3

)

Probe delay (ps)

• Period fluctuations:→ Correlated fluctuations → shape/size effect

Gold nanoprisms: acoustic vibrationsGold nanoprisms: acoustic vibrations

Main mode periods:T1 and T2

60 65 70 75

45

50

55

60

T1 (ps)

T2

(ps)

60 65 70 750

200

400

600

τ 1 (p

s)

T1 (ps)

• Damping: Energy tranfer to the substrate- 100 ps ≤ τ1 ≤ 600 ps → <τ1> ≈ 260 ps- ensemble measurement: τ1 ≈ 70 ps

(inhomogeneous damping) - No τ1 - T1 correlation

→ fluctuation of the prism-substrate contact

Optical investigation and electron microscopy of a single

nanoparticle

Single nanoparticle spectroscopy: Correlation with electron microscopy - Au particles on Si3N4 substrate

Optical TEM

1 μm

0° 90°

D = 102 nm

400 500 600 700 8000

1

2

3

4

5

σ ext(λ

) (1

04 nm

2 )

Wavelength (nm)

400 500 600 700 800 9000

2

4

6

Light polarization 90°

Light polarization 0°

Wavelength (nm)

σ ext

(λ)

(104 n

m2

)

0

5

10

15

20

σ ext

(λ)

(104 n

m2

)

0°90°

100 nm

Pair of interacting Au nanospheres:

Silica spheremarkers

Agreement with Mie theory

• Single nanoparticle optical absorption detection → spatial modulation technique: direct absorption measurement→ absorption cross section down to 5 nm (gold) - 3nm (silver)→ far-field technique ⇒ dilute sample (< 1 particle per μm2) → spectroscopy: optical identification of a single nanoobject

• Femtosecond time-resolved spectroscopy→ electron-phonon coupling in a single metal nano-object→ acoustic vibration: acoustic properties at a nanoscale→ nonlinear optics with a single nanoobject→ combination with electron microscopy→ Extension to hybrid nanoparticles: semiconductor-metal

Conclusion

Université Bordeaux 1

D. ChristofilosA. ArbouetO. Muskens

J. BurginP. Langot

Université Lyon 1FemtoNanoOptics Group

V. Juvé (Ph.D)H. Baida (Ph.D)

P. MaioliA. Crut

N. Del Fatti

Acknowledgements

Université Lyon 1 - FranceJ.R. Huntzinger

P. BillaudE. CottancinM. PellarinJ. Lermé

M. BroyerG. BachelierA. Brioude

Universidad Vigo - SpainL. Liz-Marzan

Université Paris VI - FranceM.P. Pileni

Georgia Institute of Technology - USAM. El-Sayed