Fano Interference in the Optical Absorption of an Individual Gold

Sep 20, 2016 - Fano resonances are central features in the responses of many systems including atoms, molecules, and nanomaterials. They arise as a co...
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Fano interference in the optical absorption of an individual gold-silver nanodimer Anna Lombardi, Marcin P. Grzelczak, Etienne Pertreux, Aurélien Crut, Paolo Maioli, Isabel Pastoriza-Santos, Luis M. Liz-Marzán, Fabrice Vallée, and Natalia Del Fatti Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02680 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016

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Fano interference in the optical absorption of an individual gold-silver nanodimer Anna Lombardi1, Marcin P. Grzelczak2, Etienne Pertreux1, Aurélien Crut1, Paolo Maioli1, Isabel Pastoriza-Santos2, Luis M. Liz-Marzán2,3,4, Fabrice Vallée1 and Natalia Del Fatti1* 1

FemtoNanoOptics group, Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F69622 Villeurbanne, France 2 Departamento de Química Física, Universidade de Vigo, 36310 Vigo, Spain 3 Bionanoplasmonics Laboratory, CIC biomaGUNE, 20009 Donostia-San Sebastián, Spain 4 Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain * email: [email protected]

Abstract

Fano resonances are central features in the responses of many systems including atoms, molecules, and nanomaterials. They are consequences of interferences between two channels, most frequently associated to two system modes. In plasmonic materials, Fano interferences between optical modes have been shown, experimentally and theoretically, to induce narrow features in their scattering spectra. By investigating individual silver-gold heterodimers, we first experimentally demonstrate that Fano interference is also a key effect in the optical absorption of plasmonic nano-objects, in agreement with theoretical predictions. Conversely to previously investigated systems, the two interacting modes at the origin of absorptive Fano effect are mostly localized on either one or the other dimer component. Experimental results were obtained by selectively monitoring the optical absorption of one dimer component using a two-color nonlinear time-resolved technique. This also opens the way to full optical far-field noncontact investigations of charge or energy exchanges between nano-objects with a spatial resolution much smaller than the optical wavelength. Keywords Fano resonance, plasmonics, gold-silver dimers, absorption, interferences

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Fano effect is a consequence of interference between two channels ruling the response of a system, e.g., an atom, a molecule, or a nano-object or nanostructure. The corresponding Fano resonance results from interference between the material responses due to a spectrally narrow mode with that due to a much broader one, or a continuum, an effect leading to a characteristic asymmetric spectral line shape. Discovered by Fano in the autoionizing states of helium atoms1, it is present in many systems.2–5 It has received a large attention in nanophysics, and particularly plasmonics, as Fano interference between two plasmonic modes can induce narrow spectral features exploited for designing ultrasensitive detectors.6,7 Up to now, the optical Fano resonances investigated in plasmonic materials show-up in their scattering spectra. As in atomic or molecular systems, they are due to interference in the radiative response between two spectrally overlapping modes of the full system. The most frequent example is interaction of a highly radiative (“superradiant”) plasmon mode with a “subradiant” one associated to a much narrower dark resonance.8–11 However, such effects can only be observed in highly scattering systems, i.e., relatively large nano-objects (typically > 50nm). Recently, investigations in semiconductor devices showed a Fano effect in both their far-field scattering response and optical absorption indirectly probed by photocurrent measurements.12 Fano resonances in the optical absorption of small nano-systems were theoretically predicted few years ago in bimetallic heterodimers.13 However, no experimental evidence was reported up to now, due to the difficulty in their controlled synthesis and in designing appropriate experiments to detect them. In this context, nano-objects formed by a gold and a silver nanoparticle are model systems where Fano resonance can take place between optical excitations of different nature, namely the localized surface plasmon resonance (SPR) of the silver component and the continuum of interband transitions of the gold one.13,14 Furthermore, conversely to previously investigated atomic-, molecular-, or nano-systems, these two involved modes mostly correspond to excitation of either one or the other nanodimer component. This introduces a large versatility for tailoring their Fano resonance and controlling optical excitation of each nano-component by adjusting the dimer morphology, light wavelength and polarization. In gold-silver nanodimers, Fano effect is theoretically predicted to predominantly modify light absorption by one of the two materials, namely the gold one.13 This effect cannot be unambiguously evidenced by linear far-field optical spectroscopy, whose limited spatial resolution does not allow for specific interrogation of a dimer component (specific excitation being achievable with electron microscopies15). Linear approaches only provides access to the 2 ACS Paragon Plus Environment

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extinction of the whole dimer, which is dominated by the silver response around its SPR13,14,16,17 (as also observed on ensembles of Au-Pd stacked dimers18,19). We demonstrate here that the optical absorption in the gold part of a gold-silver nanodimer can be selectively monitored using two-color femtosecond pump-probe nonlinear spectroscopy. In these experiments, the absorption of a pump pulse heats-up differently each component, depending on its absorption cross-section within the dimer. Heating of the gold component is subsequently selectively monitored by a second time-delayed probe pulse, with a properly adjusted wavelength. Experiments were performed in single dimers, which permits light polarization-dependent measurements and individual transmission electron microscopy (TEM) characterization of the dimer morphology, key points for quantitative comparison with theory. This nonlinear experiment demonstrates the presence of a polarization-dependent Fano interference in the spectral profile of gold absorption within the dimer, in agreement with theoretical predictions. Model heterodimers are prepared by electrostatic assembly of gold nanospheres (about 60 nm diameter) and silica-coated silver nanospheres (Ag@SiO2 with about 40 nm diameter and 15 nm thick shell) synthesized by wet chemistry.16 The silica shell prevents silver from oxidation and acts as the spacer setting the Au-Ag distance (Figure 1). The optical extinction spectrum of individual dimers deposited by spin-coating on a silica membrane substrate is measured in the visible range using far-field spatial modulation spectroscopy (SMS), which gives access to the amplitude of the nano-object extinction cross-section, σext (Figure 2a).20,21 Extinction spectra of individual heterodimers were measured using a frequency doubled tunable Ti:Sapphire oscillator and a visible optical parametric oscillator (OPO). The measured spectra (Figure 1a) exhibit two resonances at wavelengths close to that of the SPR of Ag Au ≈ 420 nm) and Au ( λSPR ≈ 530 nm) nanoparticle forming the dimer. As Ag@SiO2 ( λSPR

reported before, their amplitudes and, to a lesser extent, their spectral positions, depend on light polarization direction (Figure 1a), with an enhancement of σext for light polarized parallel to the dimer axis, as a consequence of electromagnetic coupling of the Au and Ag components.16 The experimental σext spectra and amplitudes are consistent with those numerically computed by finite-element simulations21,22 using the morphology of the same dimer determined by TEM (Figure 1b and Figure S2 of the Supporting Information). For the investigated sizes, the absorption cross-section, σabs, is larger than the scattering one, σsca,

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Ag except close to λSPR . Absorption being computed via a spatial integration of resistive

heating,16 this modeling also permits to selectively compute the individual contributions from Au Ag the gold, σ abs , and the silver, σ abs , component to the total dimer optical absorption. These are

shown Figure 1c,d, together with the computed absorption cross-sections of the assumed isolated Au and Ag@SiO2 particle components. Apart from enhancing gold absorption around Au λSPR , Au-Ag electromagnetic interaction within the dimer is theoretically predicted to induce Ag a pronounced asymmetric Fano profile around λSPR in its interband absorption spectrum. For

light polarized along the dimer axis, this shows up as a strong reduction and enhancement of the absorption on the blue and red side of the silver SPR (Figure 1c, solid orange line), as compared to the almost undispersed interband absorption of an isolated gold nanosphere (Figure 1c, dotted orange line). A reverse spectral behavior, of smaller amplitude, is obtained for orthogonal polarization (Figure 1d, solid orange line). These Fano profiles result from interference between the quasi-continuum of absorbing states, the undispersed gold interband transitions, and a spectrally narrow mode, the silver SPR.13,16 They can be understood in the framework of a simple dipole-dipole interaction model16,23 (see Note 1 and Figure S1 of the Supporting Information), which yields the polarizability of the gold component of the dimer, 0 eff 0 , as a function of those of isolated gold and silver nanoparticles, α Au and α Ag , and the α Au

(

)

Au 0 0 0 ∝ Im α Au + Cα Au α Ag / 2 , showing strength C of their coupling. For parallel polarization, σ abs

the impact of electromagnetic interactions between the gold and silver components. Substituting the actual polarizabilities of silver and gold with simplified expressions (a DrudeAu like polarizability and a constant, purely imaginary one) leads to a Fano profile for σ abs ,

allowing determination of its Fano parameter q (see Supporting Information and Figure S1). This simplified model leads to q ≈ 0.3 for our experimental conditions, this value constituting however only a crude estimation affected by the strong simplifications made. The estimation of q can be refined by directly fitting the finite-element computed absorption spectra using ideal Fano profiles (corresponding to Eqs. 3 and 4 of the Supporting Information), leading to accurate reproduction of the spectra for q ≈ 0.6 (Figure S1b). Though electromagnetic interaction also alters absorption of the silver component and of the whole dimer ( Au Ag σ abs = σ abs + σ abs ), it mainly leads to modifications of the amplitude and weak shift of the Ag-

like SPR (Figure 1). Direct experimental demonstration of the presence of a Fano interference in the optical absorption of a dimer thus requires selective investigation of the individual 4 ACS Paragon Plus Environment

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Au absorption in its gold component, i.e., σ abs , which we have done using a two-color

femtosecond pump-probe spectroscopy (Figure 2a). Under illumination by a pump light pulse of wavelength λpp and fluence Fpp, the gold and silver parts of the dimer are differently heated, proportionally to the energy they absorb: m m Eabs (λ pp ) = σ abs (λ pp )Fpp , where m stands for the Au or Ag component. Dimer thermalization

by interparticle energy exchanges through the silica shell being relatively slow (typically few tens of picoseconds24), the dimer components can be driven out of equilibrium when excited by a femtosecond pulse and their energy exchanges neglected on a few ps time scale. In this regime, light absorption heats-up the electrons in each component, their electronic temperature further independently decaying by electron-lattice energy exchange on a typically 1 ps time-scale.25,26 For sufficiently weak excitation (electron temperature rise of few tens of m degrees), the maximum electron temperature increase, ∆Texc , induced in each component is

m (λ pp ) 27,28: proportional to σ abs

m (λ pp ) = T02 + 2 Eabsm (λ pp ) /(Vm c0m ) − T0 ≈ Eabsm (λ pp ) /(Vm c0mT0 ) ∆Texc

(1)

where T0 is the room temperature, c0m the electronic heat capacity coefficient of gold and silver 29 ( c0Au , Ag ≈ 65 J m-3 K-2), and Vm the volume of each particle component. These rises of the electronic temperatures in the Ag and Au parts of the dimer induce changes of their complex dielectric function, ε m = ε 1m + iε 2m 27,28,30,31 which translate into modification, ∆σext, of the whole dimer extinction cross-section. This can be detected, for a given pump-probe time delay tD, by monitoring the relative change of transmission ∆Tr/Tr of a second femtosecond probe pulse focused onto the dimer, with ∆Tr/Tr = −∆σext(λpr, tD)/Spr (where λpr and Spr are the probe wavelength and focal spot size).31 Though the detected ∆σext(λpr, tD) a priori depends on the pump-induced dielectric function changes in both Au and Ag (i.e. on the individual electronic temperature in both components at time tD), the contribution of the latter can be made negligible by probing the dimer extinction change around the gold-like SPR. More precisely, for weak excitations, ∆σext(λpr) can be written as:

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∆σ ext (λ pr , tD ) =

 ∂σ  ext ∑  ∂ε1m m = Au , Ag 

∆ε 1m (λ pr , tD ) + λ pr

∂σ ext ∂ε 2m

 ∆ε 2m (λ pr , t D )  λ pr 

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(2)

where the four coefficients ∂σ ext ∂ε 1m, 2 are set by the linear optical properties of the dimer. Using the same model that yields a good description of σext (Figure 1), they can be obtained by numerical derivation of σext (Figures 3a and S3 for light polarization parallel or orthogonal , Ag to the dimer axis). Plasmonic effects lead to large enhancement of ∂σ ext ∂ε 1Au around the ,2

gold and silver-like SPR. In particular, this is making ∂σ ext ∂ε 1Au much larger than ,2 Au around λSPR (Figure 3a), enhancing the gold contribution to ∆σext in this ∂σ ext ∂ε 1Ag ,2

wavelength range. Furthermore, as on a few ps time scale the induced changes ∆εm of the dielectric functions of noble metals mostly reflect alteration of their interband transitions,27,28 they are larger for gold than for silver around the gold-like SPR which lies close to the onset of gold interband transitions, λibAu ≈ 520 nm, and far from the silver one λAg ib ≈ 320 nm. This is confirmed by computing the transient changes of ε1,2 in gold and silver for the same electron ref temperature rise, ∆Texc , using numerical resolution of Boltzmann equation and deducing the

Ag Au Ag ref = ∆Texc = ∆Texc induced ∆ε1Au, from Rosei band structure models (Figure 3b, for ∆Texc = 10 ,2

K, which will be used as the reference temperature increase in the following). Consequently, Au = 530 nm ∆σext(λpr) is expected to be largely dominated by the gold contribution at λ pr ≈ λSPR

(about 300 times larger than the silver one), yielding selective access to its dielectric function change ∆εAu. To selectively measure the gold component response of an individual Au-Ag dimer, two-color femtosecond pump-probe experiments were performed using a high sensitivity setup (Figure 2). The dimer is excited by a first pump pulse with wavelength λpp tunable from 400 to 420 nm around the Ag-like SPR (i.e., in the spectral region of the expected Fano resonance, Figure 1). Its extinction change, ∆σext(λpr), is monitored using a second timedelayed probe pulse at a fixed wavelength close to the Au-like SPR (λpr = 530 nm). The measured transmission change of the probe pulse, ∆Tr/Tr, is illustrated in Figure 2 for three different pump wavelengths and the same pump fluence, Fpp= 1.2 µJ/cm2. The ∆Tr/Tr temporal shape is typical of the response of noble metals28. Its rise reflects electron heating due to pump-pulse absorption, and its subsequent decay cooling of the hot electrons by energy 6 ACS Paragon Plus Environment

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transfer to the lattice. For the pump fluences used here, the maximum electron temperature Au , Ag rises, ∆Texc , in the gold and silver components are always less than 100K. In this low

perturbation regime, the overall amplitude of ∆Tr/Tr = −∆σext / Spr scales linearly with Fpp and its kinetics is independent of it and of λpp. This transient optical response directly reflects the energy losses of the electrons in gold to their environment. The temporal evolution of the experimental extinction changes deduced from ∆Tr/Tr, ∆σext(tD) at λpr = 530 nm, is shown Figure 3d for different pump wavelengths and light polarization parallel to the dimer axis (and Figure S3d for orthogonal polarization). After substraction of the small amplitude long delay thermal background, all experimental signals show a mono-exponential decay with the same relaxation time, τe-ph = 1ps (inset of Figure S3d). This is consistent with the electron-phonon energy transfer time measured in bulk gold and large gold nanoparticles,26 further confirming that the experimental signals are sensitive to the evolution of the electronic temperature in the gold part of the dimer, with absolute amplitudes providing the information about initial gold excitation. The computed temporal dynamics (from Eq. 2) of the transient extinction changes for same excitation and probing conditions than in experiments are also shown Figure 3c. In this low perturbation regime, both the real and imaginary parts of ∆εAu scale linearly with the electronic temperature increase in gold.27,28 The full amplitude of ∆σext is thus proportional to Au the maximum electron temperature increase, ∆Texc (Eq. 1), and one can simply write Eq. 2 as:

∆σ ext (λ pr , t D ) =

Au ∆Texc (λ pp )

∆T

ref exc

ref Au ∆σ ext (λ pr , t D ) ≈ σ abs (λ pp )

Fpp E

ref abs

ref ∆σ ext (λ pr , t D )

(3)

ref where ∆σ ext is the induced dimer extinction change after rising the gold electron temperature ref ref by ∆Texc , due to absorption of the corresponding energy Eabs (Eq. 1). Measurement of ∆σext

then permits a direct reading of the absorbed energy at the tunable pump wavelength in the Au (λ pp ) and comparison of its gold part of the dimer, i.e., quantitative investigation of σ abs

spectral dependence with the predicted Fano-like one (Figure 1c,d) induced by electromagnetic interactions between the gold and silver components of the dimer. The ∆Tr/Tr amplitude (and thus the |∆σext| one, Figure 3 and S3) is found to increase with λpp over the 400 to 420 nm range (by a factor of about 2.5) for polarization along the 7 ACS Paragon Plus Environment

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dimer axis, a reversed and less marked behavior (decrease by a factor of about 1.6) being observed for orthogonal polarization (Figure 2b,c). This is consistent with the expected Fano spectral profile in the absorption of a gold nanoparticle involved in a dimer, the computed Au σ abs (λ pp ) increasing (or decreasing) with increasing λpp for the parallel (or orthogonal)

polarization in this frequency range (Figure 1c,d, solid orange lines). The large variations of the dimer ∆Tr/Tr over a relatively limited spectral range strongly contrasts with the absence of any variation, computed and measured, for isolated individual gold nanospheres (in agreement with the undispersed absorption of gold over this spectral range, Figure 1c). This large dispersion yields experimental evidence for the presence of a Fano resonance in gold component absorption, due to interaction with the silver component. It can be precisely characterized by plotting the experimentally measured maximum extinction change, ∆σ ext

max

, as a function of pump wavelength (Figure 4a). For both light polarizations, its measured Au (λ pp ) for the relative variations are consistent with the computed Fano dispersions of σ abs

investigated dimer around the silver-like SPR wavelength, showing similar spectral behaviors (Figure 4a,b, and Figure 4d,e for another dimer). Au (λ pp ) can be estimated These comparisons can be made even more quantitative, as σ abs

ref max using Eq. 3 from the experimental ∆σ ext and the computed ∆σ ext :

Au (λ pp ) = σ abs

ref ∆σ ext Eabs ref Fpp ∆σ ext

max

(4)

max

ref Here Fpp is experimentally known, and ∆σ ext

max

can be numerically calculated using Eq. 2,

ref with the ∆ ε 1Au , 2 values computed for absorption of the reference energy E abs in the gold Au component of the dimer (Figure 3). The thus experimentally deduced σ abs is in excellent

agreement with the predicted one, showing the same relative dispersion and amplitude for Au both light polarizations (Figure 4b,c, Figure 4e,f). The electron temperature rise ∆Texc due to

absorption of the pump pulse can then also be experimentally determined (using Eq. 1), showing a large variation in the 20 - 70 K range with pump wavelength and light polarization (Figure 4-c,f, right scale bar), quantitative consequence of Fano effect. In conclusion, using femtosecond pump-probe nonlinear spectroscopy, we have experimentally demonstrated the existence of a strong Fano interference in the absorption of multi-material nano-objects, gold-silver heterodimers as model systems. Due to interaction 8 ACS Paragon Plus Environment

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between two material excitations of very different nature, i.e., silver plasmonic mode and gold interband transitions localized in each dimer component, the way light is absorbed in each component is strongly modified at nanoscale. This polarization-dependent Fano effect is masked in the linear optical spectrum of the whole dimer, but can be evidenced using our nonlinear two-color femtosecond approach that permits to selectively detect the energy absorbed by one of the components. Measurements performed on individual nanoparticles are found to be in excellent quantitative agreement with theoretical modeling. The possibility to selectively optically excite and monitor a part of a multicomponent nano-object by wavelength selection also offers many opportunities for optical investigations of charge and energy exchange at nanoscale with a spatial resolution much smaller than the optical wavelengths. Fano effect demonstrated here also opens the possibility of using heterodimers as versatile tunable absorbing devices. For instance, using dimers with nanoparticles linked by molecules having thermally or optically induced conformational changes, the absorption in one component could be externally modified by adjusting the separation to the other one. This externally adjusts the interaction between the two components, and therefore actively controls the amplitude of absorption and its dependence on light polarization direction, acting as an external gate in a field-effect transistor.

Associated content Supporting Information. Dipolar modeling of nanodimers and numerical simulations with light polarization orthogonal to their axis. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements. The authors thank G. Bachelier and M. Broyer for fruitful discussions and acknowledge financial support by ANR program TRI-CO under contract ANR-11NANO-025. L.M.L.-M. acknowledges funding from the European Research Council (ERC Advanced Grant PLASMAQUO (267867)).

Author information Corresponding author * Natalia Del Fatti, email: [email protected] Present addresses Marcin P. Grzelczak, Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, United Kingdom. 9 ACS Paragon Plus Environment

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Anna Lombardi, NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK. Notes The authors declare no competing financial interest.

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Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Saunders, Ed.; Saunders College Publishing: Philadelphia, 1976.

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Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370–4379.

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Baida, H.; Mongin, D.; Christofilos, D.; Bachelier, G.; Crut, A.; Maioli, P.; Del Fatti, N.; Vallée, F. Phys. Rev. Lett. 2011, 107, 057402.

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Figure 1. Linear optical spectroscopy of a single Ag@SiO2-Au nanodimer. (a) SMS measured absolute extinction cross-section spectra, σ ext , of an individual Ag@SiO2-Au nanodimer deposited on silica substrate for light polarized parallel (red squares) and orthogonal (black circles) to the dimer axis. Inset: TEM image of the same dimer (Au and Ag particle mean diameters DAu = 58 nm and DAg = 40 nm, and silica shell thickness t = 16 nm). (b) Computed extinction (dashed red), absorption (solid red) and scattering (dotted red) dimer cross-section spectra using its TEM-determined morphology for light polarized parallel to its Au Ag axis. (c) Individual contributions of the Au, σ abs (solid orange), and Ag@SiO2, σ abs (solid

grey), components to the full dimer absorption cross-section σ abs (solid red), computed for light polarized parallel to the dimer axis. For comparison, absorption cross-section of isolated Au (dotted orange) and Ag@SiO2 (dotted grey) nanoparticles on silica substrate are also shown. (d) Same as (c) for light polarized orthogonal to the dimer axis.

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Figure 2. Ultrafast optical spectroscopy of a single Ag@SiO2-Au nanodimer. (a) Schematics of the experimental setup 20. (b) Time-resolved transmission changes, ∆Tr/Tr, measured on the dimer of Figure 1 for pump and probe polarized parallel to its axis, λpr = 530 nm and λpp = 400 nm (solid violet), 410 nm (dashed blue) and 420 nm (dotted grey), with same incident pump fluence Fpp= 1.2 µJ/cm2 and pulse duration 500 fs. (c) Same as (b), for polarizations perpendicular to the dimer axis.

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Figure 3. Modeling the ultrafast optical response of a single Ag@SiO2-Au nanodimer. (a) Partial derivatives ∂σ ext ∂ε im of the extinction cross-section (Eq. 2) of the dimer of Figure 1 numerically computed for light polarization parallel to its axis, for i = 1 (blue line) and 2 (green lines), and m = Au (full line) and Ag (dashed lines) cases. The arrow shows the experimental probe wavelength, λpr = 530 nm. (b) Computed time evolution of the real (solid blue line) and imaginary (solid green line) part of the interband gold dielectric function Au ref change ∆ε 1Au , 2 at λpr = 530 nm for an electron temperature increase ∆Texc = ∆Texc = 10 K.

Dashed lines: same for ∆ε 1Ag , 2 . (c) Time-dependent extinction changes of the dimer of Figure 1, -∆σext, computed at λpr = 530 nm for parallel polarization (Eq. 2 with excitations corresponding to incident pump fluence Fpp= 1.2 µJ/cm2 and parallel absorption cross-sections Ag from Fig. 1-c), at different pump wavelengths around Fano resonance: λ pp − λSPR ≈ -25 nm (solid violet), -15 nm (dashed blue) and -5 nm (dotted grey). (d) Time-resolved extinction changes of the dimer of Figure 1, -∆σext, with same excitation and probing conditions as (c), deduced from experimental ∆Tr/Tr (Figure 2-b). To account for slightly different spectral positions of silver-like SPR in experimental and computed linear extinction spectra, Ag = 425 nm (experiments) and 420 nm (theory). comparisons are made with respect to λSPR

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Figure 4. Fano interference in the absorption of an individual Ag@SiO2-Au nanodimer. Red (black) symbols and lines correspond to light polarization parallel (orthogonal) to the dimer axis. (a) Maximal amplitudes of the extinction cross-section change measured at λpr = 530 nm for the dimer of Figs. 1-2 as a function of pump wavelength relative to the Ag-like Ag Au SPR wavelength λSPR . (b) Fano dispersion of the computed σ abs in the dimer (see Figure Au 1c,d, solid orange line). (c) Fano dispersion of the experimentally extracted σ abs (left scale), Au and corresponding electron temperature rise ∆Texc (right scale). (d), (e) and (f) same as (a),

(b) and (c) for a different Ag@SiO2-Au nanodimer (DAu = 58 nm, DAg = 38 nm, silica shell thickness t = 14 nm, λpr = 550 nm). Insets: TEM images of the two investigated nanodimers (scale bar: 50 nm).

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