CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE Ultrafast spectroscopy of a single metal nanoparticle Fabrice Vallée FemtoNanoOptics group LASIM, CNRS - Université Lyon 1, France
Metal Nanoparticles: from IV...to XXI century Metallic particles in glasses: jewelry, ornament Ag Au Ancient cup (Central Europe) Lycurgus Cup: a Roman Nanotechnology Roman Era (4th Century A.D). It appears green in scattered light...and red in transmitted light.
Optical response of metal nanoparticles Metal nanosphere (ε = ε 1 + iε 2 ) in a matrix (ε m ): Mie theory for sphere R << λ (dipolar): σ abs 18πVε = λ 3/ 2 m ( λ) 2 2 [ ε ( λ) + 2ε ] + ε ( λ) 1 ε 2 m 2 E O ε(ω) E int R + + + + ε m resonance for ε 1 (λ) + 2ε m Surface Plasmon Resonance Ensemble of identical nanoparticles: α = N part σ abs Ag particles in glass Resonance depends on: - environment - shape + light polarization (ellipsoids, rods,...) manipulation of light at subwavelength plasmonics
Optical response of metal nanoparticles Metal dielectric function ε( ω) = ε bound electrons (interband) b ( ω) ω 2 p ( i τ) ω ω + free electrons (intraband) interband transitions E F CB intraband transitions d - bands SPR Frequency: ε ib 1 ΩR ) + 2εm ΩR ωp ε1 + 2 ( ε m 5 4 Ag - D = 13 nm - p = 2x1-4 Surface Plasmon Resonance 1 Au - colloids - D = 1 nm 3 Interband Transitions Threshold Ω R αl 2 1 Ω ib Ω R 2 25 3 35 4 45 5 Wavelength (nm) 4 5 6 7 Wavelength (nm)
Ultrafast spectroscopy of metal nanoparticles
Femtosecond investigation: pump-probe hν Matrix τ p-m e e τ th τ e-ph Lattice τ p-m Matrix Electron excitation (hν) + Femtosecond probing (nonlinear optical response) Coherent response Nonequilibrium electron kinetics Intrinsic electron scattering processes - Internal thermalization ( T e ) electron-electron : τ th ~ 3 fs - External thermalization (T e T L ) electron-lattice : τ e-ph ~ 1 ps Confined vibrational modes Extrinsic processes: nanoparticle - environment coupling Nonlinear optical response
Pump Femtosecond excitation and probing Probe I S T x I S ΔT/T Sample Femtosecond excitation: intraband absorption f T e = T f() f(t) T e > T hω P +hω P Femtosecond probing: E F t < t = t > Athermal electron distribution Thermalization + Cooling Transmission change ΔT/T dielectric function change Δε(t D ) Probe wavelength different electron interactions C.K.Sun, et al., Phys. Rev. B 5, 15337 (1994) T.Tokizaki, et al., Appl. Phys. Lett. 65, 941 (1994) ; J.Y.Bigot, et al., Phys. Rev. Lett. 75, 472 (1995) C.Voisin et al., J. Phys. Chem B 15, 2264 (21). E F E F
Experimental setup: femtosecond pump-probe Verdi Argon Laser Laser Ti:sapphire laser 78 8 - - 13 9nmnm 11W W - 25fs fs BBO ω 2ω 3ω Reference Chopper ((f) ω) Variable Delay prism pair fω Pump: red pulse (25 fs) (or 2ω) Probe: red pulse (ω ; 25 fs) Sample blue (2ω ; 3 fs) or UV (3ω ; 55 fs) Perturbation : ΔT e ~ 1 K - 2 K Sensitivity : depends on mean power on detector maximum: ΔT/T ~ 1-7 (f = 1 khz) Signal Computer Signal Lock-in Amplifier + - Reference
Electron energy losses: electron-phonon coupling Ag - D = 3 nm in Al 2 O 3 1. 1 B.C..8 ΔT/T.1 E F ΔT/T.6.4.1 1 2 Probe Delay (ps) Blue probe UV probe bandes d d-bands.2 1 2 3 Probe Delay (ps) pump: IR (93nm) probe: Blue (465nm) or UV (31 nm) UV probe : Risetime: electron thermalization electron-electron interactions Decay: energy transfer to the lattice electron-lattice coupling Blue probe: Energy in the electron gas electron-lattice coupling Size effect?
Electron interactions: Size dependences.9.8 Ag film 3 Ag film τ e-ph (ps).7.6.5 Ag nanoparticles polymer BaO-P 2 O 5 Al 2 O 3 MgF 2 deposited.4 5 1 15 2 25 3 Nanoparticle diameter (nm) τ th (fs) 2 Ag nanoparticles BaO-P 2 O 5 matrix Al 2 O 3 matrix Deposited on glass 1 5 1 15 2 25 3 Nanoparticle diameter (nm) Electron lattice interaction: τ e-ph (A. Arbouet, PRL 9, 17741 (23)) Tin and Gallium: τ e-ph D (M. Nisoli et al., PRL 78, 3575 (1997)) Electron electron scattering: τ th (C. Voisin, et al., PRL 85, 22 (2)) Screening reduction close to the surface Many studies on large ensembles (1 4 to 1 6 particles) Single nanoparticle Size and shape fluctuations
Femtosecond spectroscopy of a single nanoparticle 1) Optical detection and characterization: linear absorption spectroscopy 2) femtosecond pump - probe: nonlinear response Pump P t Probe I S T x I S ΔT/T Sample Sample
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 perturbation T. Klar et al. Phys. Rev. Lett. 8, 4249 (1998) Far field: focused beam 3-5 nm diluted sample ( < 1 particle / µm 2 ) - Scattering ( V 2 ; size 2 nm): Dark field microscopy C. Sönnichsen et al., Appl. Phys. Lett. 77, 2949 (2) K. Lindfords et al. Phys. Rev. Lett. 93, 3741 (24) - Absorption ( V ; small particle): Focused laser beam: 3-5 nm Gold nanosphere D = 2 nm - 5 nm: absorption of 1-3 -1-5 of the incident light Photothermal technique D. Boyer et al., Science 297, 116 (22) Spatial modulation technique (quantitative) A. Arbouet et al., Phys. Rev. Lett. 93, 12741 (24)
Optical detection of a single metal nanoparticle (A. Arbouet et al., Phys. Rev. Lett. 24) Absorption by a single nanoparticle: - Focused laser beam: 3-5 nm - Gold nanosphere D = 2 nm - 5 nm: absorption of 1-3 -1-5 of the incident light Spatial Modulation Technique: Modulation of the nanoparticule position f Modulation of transmitted light f or 2f microscope objective 1x sample f f, 2f lock-in amplifier Light piezo f Transmitted power P XY scanner
Gold nanoparticles - <D> = 1 nm Transmitted power: t i ext (I: intensity profile at the focal spot) ( ) P = P σ I x, y δ y Modulation of the particle position at f : y P P σ t i ext I( x, y ) σ ext I y y δ y σ sin(2 πft) 2 y + δ y sin(2πft) ext 2 y I 2 y δ 2 y sin 2 (2πft) y I(x,y ) Sample image: X/Y scan - λ = 532 nm Diluted sample (< 1 particle/μm 2 ): 1 nm gold nanoparticle spin-coated on a substrate y ΔP/P ΔP/P X (µm) X (µm) f Y (µm) 2f Y (µm)
Absorption spectroscopy of a single nanoparticle Ti- sapphire femtosecond oscillator 1 mw - 78 nm - 2fs grating Non-linear photonic crystal fiber Supercontinuum λ > 45 nm SMS microscope Optical absorption signature
Single nanoparticles: optical characterization λ = 532 nm; modulation along Y at f = 1 khz ΔT/T 4 19.5 nm Tunable source 3 Spectroscopy σ abs (nm 2 ) 2 X (µm) Y (µm) 1 18 nm 45 5 55 6 Wavelength (nm) Counts 15 1 5 Nanoscope <D> = 16.6 nm Counts 12 8 4 <η > =.9 Nanoscope Absolute value of the absorption cross-section + polarization dependence Counts 12 13 14 TEM15 16 <D> 17 18= 16.2 19 2 nm21 6 Diameter (nm) 5 4 3 2 1 12 13 14 15 16 17 18 19 2 21 Diameter (nm) Counts.5.6.7.8.9 1. 6 TEM Aspect ratio c/a <η > =.9 4 2.5.6.7.8.9 1. Aspect ratio c/a Optical identification of a nanoobject: size and shape and orientation O.L.Muskens et al., Appl. Phys. Lett. 88, 6319 (26)
Femtosecond optical nonlinearity of a single nanoparticle femtosecond pump - probe study : Pump Probe I S T x I S ΔT/T Sample
Femtosecond spectroscopy of a single nanoparticle t D CF tunable Ti:Al 2 3 fs oscillator Chameleon BBO Ch x y f PC DVM Lock-in PD CF
Femtosecond spectroscopy of a single Ag nanosphere Optical characterisation of a single nanoparticle (linear absorption spectrum) Microscope Objective 1x & femtosecond pump - probe study : Transmitted Power ΔT T 1nanoparticle Δσ = σ ext ext pump σ S ext probe IR excitation / SPR probing (425 nm) σ ext (x 1-15 m 2 ) 1 5 Ag - D = 3 nm D = 21 nm ΔT/T (x 1-4 ) 1.2.8.4 ΔT/T max (x 1-4 ) 1..5. 2 4 P P (µw) 35 4 45 5 55 Longueur d'onde (nm). 1 2 3 Probe delay (ps)
Electron-phonon energy exchange in single Ag nanospheres Mechanism ΔT/T electron excess energy Decay: electron-lattice energy exchange τ e-ph 1.5 τ e-ph (ps) 1.2.9 τ e ph Strong excitation regime excitation dependent decay: τ e (T e ) Weak excitation regime ΔT decay with τ e-ph = c T / G.6..2.4 Pump power (mw) Thermal distributions: Two temperature model T e ; T L ; G = e-lattice coupling constant C e (T e ) = c T e ; C L : heat capacities C C e L dte ( Te ) = G( Te T dt dtl = G( Te TL ) dt L )
Electron-phonon coupling in single Ag nanospheres 1.5 max Known nanoparticle known excitation T e -T τ e-ph (ps) 1.2.9.6..2.4 Pump power (mw) Comparison wih two temperature model: pump power T e max Same electron-phonon coupling as in ensemble measurements (in glass) comparison with the two-temperature model τ e-ph / τ e-ph No environment dependence (large excitation).1 1 No e-ph coupling dependence (T max -T e ) / T on excitation regime O.Muskens, N. Del Fatti and F. Vallee, Nano Letters 26 2. 1.5 1. ΔT e max : 11-43 K (3nm) ΔT e max : 22-38 K (21nm)
Acoustic vibration of a single nano-object: pair of nanoprisms Vibrational acoustic modes: Frequency: size and shape dependent Damping: environment / size and shape distribution Single nanoparticle
Gold nanoprisms: detection TEM image M. El-Sayed Georgia Inst. Tech., Atlanta Nanosphere lithography: Organized nanoprisms: size 12 nm thickness 3 nm (W. Huang et al., Nano Lett. 4, 1741 (24)) Optical image (at 41 nm) AFM image Optical observation of prism pairs 5 x 5 µm 2
Gold nanoprism pair: acoustic vibrations J. Burgin et al., J. Phys. Chem. C 112, 11231 (28) - Breathing mode: single T 3 = 12 ps ; ensemble: T 3 = 14 ps. Prism pair Ensemble ΔT/T (x1-3 ) -.2 5 1 15 Probe delay (ps) ΔT/T (a.u.) -5 5 1 15 Probe delay ( ps ) -Twomodes: pair: T 1 = 64 ps, T 2 = 49 ps ; ensemble: T 1 = 67 ps, T 2 = 4 ps - Pair: period fluctuations + slower relaxation (reduced inhomogeneous damping) First isotropic modes of a triangle:
Gold nanoprisms: acoustic vibrations Gold nanoprisms: acoustic vibrations.2 6 ΔT/T (x1-3 ).1. 5 1 15 Probe delay (ps) Main mode periods: T 1 and T 2 T 2 (ps) 55 5 45 τ 1 (ps) 6 4 2 6 65 7 75 T 1 (ps) 6 65 7 75 T 1 (ps) Period fluctuations: Correlated fluctuations shape/size effect Damping: Energy tranfer to the substrate - 1 ps τ 1 6 ps <τ 1 > 26 ps - ensemble measurement: τ 1 7 ps (inhomogeneous damping) - No τ 1 - T 1 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 Si 3 N 4 substrate Optical TEM Silica sphere markers Pair of interacting Au nanospheres: 9 1 nm σ ext (λ) (1 4 nm 2 ) 5 4 3 2 1 1 μm 9 D = 12 nm 4 5 6 7 8 Agreement with Mie theory 6 Light polarization 9 Wavelength (nm) 4 5 6 7 8 9 σ ext (λ) (1 4 nm 2 ) σ ext (λ) (1 4 nm 2 ) 2 15 1 5 4 2 Light polarization Wavelength (nm)
Conclusion 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 μm 2 ) 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
Acknowledgements Université Bordeaux 1 D. Christofilos A. Arbouet O. Muskens J. Burgin P. Langot Université Lyon 1 FemtoNanoOptics Group V. Juvé (Ph.D) H. Baida (Ph.D) P. Maioli A. Crut N. Del Fatti Université Lyon 1 - France J.R. Huntzinger P. Billaud E. Cottancin M. Pellarin J. Lermé M. Broyer G. Bachelier A. Brioude Universidad Vigo - Spain L. Liz-Marzan Université Paris VI - France M.P. Pileni Georgia Institute of Technology - USA M. El-Sayed