Battling absorptive losses by plasmon–exciton coupling in multimeric nanostructures

Abstract

The strong inherent optical losses present in plasmonic nanostructures significantly limit their technological applications at optical frequencies. Here, we report on the interplay between plasmons and excitons as a potential approach to selectively reduce ohmic losses. Samples were prepared by functionalizing plasmonic core–shell nanostructures with excitonic molecules embedded in silica shells and interlocked by silica spacers to investigate the role played by the plasmon–exciton elements separation. Results obtained for different silica spacer thicknesses are evaluated by comparing dispersions of plasmonic multimers with respect to the corresponding monomers. We have observed fluorophore emission quenching by means of steady-state fluorescence spectroscopy, as well as a significant shortening of the corresponding fluorescence lifetime using TCSPC data. These results are accompanied by the simultaneous enhancement of Rayleigh scattering and transmittance, revealing more effective absorptive loss mitigation for multimeric systems. Moreover, upon decreasing the thickness of the intermediate silica layer between gold cores and the external gain functionalized silica shell, the efficiency of exciton–plasmon resonant energy transfer (EPRET) was significantly enhanced in both multimeric and monomeric samples. Simulation data along with experimental results confirm that the hybridized plasmon fields of multimers lead to more efficient optical loss compensation with respect to the corresponding monomers.

---

1 Introduction

In the past decade, plasmonic nanostructures have attracted great interest because of their extraordinary and promising physical properties, assuming an important role in emerging science and technological applications. In particular, the wide cross-disciplinary features of plasmonics are being recognized in new research fields, leading to interesting fundamental insights and promising applications in materials science, biology and medicine.

It is well known that the particular optical properties of noble metal nanoparticles (NPs) are attributed to the localized surface plasmon resonance (LSPR), which is the collective coherent oscillation of free electrons (with respect to the positive ionic background) on the metal surface in resonance with the electric component of an incident electromagnetic wave. LSPR contributes to the strong confinement of optical radiation and a large enhancement of the electromagnetic field in their proximity. Current research efforts in nanoplasmonics have led to various photonics, opto-electronic and biomedical applications such as single-particle detection and biomolecular recognition,^1,2 optical probes for high resolution imaging,³ photo-thermal therapies,⁴ photovoltaics,⁵ Raman biosensing,^6–8 colorimetric sensing,⁹ hyper-lenses,¹⁰ and optical metamaterials.^11–13 Negative values of the real part and low values of the imaginary part of the dielectric permittivity are key for plasmonic applications. However, plasmonic NPs are not ideal resonators and exhibit considerable ohmic losses at optical wavelengths that are undesirable for most of their potential applications.

In this respect, some theoretical^14–16 and experimental studies^17–19 have demonstrated how non-radiative resonant energy transfer (RET) processes may play a role in modifying the imaginary part of the effective dielectric function of the hybrid systems. Non-radiative energy transfer efficiency in hybrid (plasmon–exciton) systems strongly depends on physical parameters^20–26 such as: (i) the geometry and size of the metal nanostructures, (ii) inter-particle distance, (iii) relative spectral position between the excitonic band and surface plasmon band (SPB) of metal NPs, (iv) relative orientation of the chromophore’s transition dipole moment with respect to the NP’s plasmon modes, (v) strength of the transition dipole moment, (vi) concentration and molar extinction coefficient of the plasmonic and excitonic particles. Recently, Bo Peng et al.¹⁸ have reported that the incorporation of gain material at a controlled distance from gold NPs allows for control and optimization of RET mechanisms. In our former studies, we have investigated the influence of incorporating excitonic material (chromophores) in the high-local-field areas of plasmon nanostructures to induce coherent RET processes from chromophores (donors) to plasmon nano-entities (acceptors), both in gain assisted (NP–dye dispersions) and gain-functionalized (dye encapsulated into the silica shell) systems.^27–29

In the present study, we investigate NP dispersions of multimeric (e.g., monomers, dimers, trimers, quadrimers, etc.) nanostructures consisting of gold-core/silica spacer/silica shell, in which chromophores (Rhodamine B by exciton) have been grafted at the interface between the silica spacer and the protective external shell. By varying the intermediate silica shell thickness from 10 to 30 nm, two series of multimeric nanostructures have been synthesized and dissolved in ethanol solution: passive (without chromophores) and active (functionalized with chromophores). In addition, corresponding passive and active single NPs (monomers) have been synthesized and used as control samples.

Steady-state fluorescence spectroscopy accompanied by ultrafast time resolved spectroscopy shows a stronger coherent coupling between the plasmonic cores and embedded excitonic molecules in multimers as compared to the similar monomers. Moreover, Rayleigh scattering measurements and transient absorption spectroscopy, performed on these two sets of samples, show a stronger plasmon–exciton coupling in multimeric nanostructures with respect to single nanoparticles (monomers), revealing a stronger optical loss mitigation in the case of multimeric nanostructures. Simulation results demonstrate the occurrence of an enhanced local field in multimers with respect to the corresponding monomers which can be understood within the context of plasmon hybridization theory in multimeric nanostructures.^30–34 In addition, we found that a shorter separation distance between NP and dye molecules lead to more efficient non-radiative RET processes, which results in further absorptive losses mitigation. Finally, these results confirm that the EPRET process occur at separation distances longer than those occurring for^35–37 Förster resonant energy transfer (FRET).³⁸

2 Results and discussion

In this experimental study, two categories of multi-silica shell NPs have been investigated: active and passive multimeric and monomeric nanostructures (control samples). Passive NPs consist of ethanol solutions of gold-cores (60 nm in diameter), coated with silica shells 10 and 30 nm thick, labeled as Mo10 and Mo30, respectively. Correspondingly active NPs, namely Mo + 10 and Mo + 30, have been synthesized by starting from the Mo10 and Mo30 samples, then grafting Rhodamine B isothiocyanate (RhB) dye molecules onto the spacer silica shell via click chemistry. Finally, a second 10 nm protective silica shell has been used to cover all the samples and protect the RhB molecules on the Mo + 10 and Mo + 30 samples.

At the same time, multimeric samples have been produced by selecting monomers, dimers, trimers, quadrimers, heptamers, etc., of both passive and active monomers with 10 and 30 nm plasmon–chromophore separation distances. Active multimers (main samples) and the corresponding passive ones (control samples) have been labeled as Mu + 10, Mu + 30 and Mu10, Mu30, respectively. A sketch of the multimeric system is presented in Fig. 1(a).

Fig. 1 (a) Sketch of the active multimers, (b) statistical analysis of core diameters performed on the Mu + 10 system. TEM images of the (c) Mo + 10 monomers and (d) Mu + 10 multimers with core diameter of 60 nm, silica spacer of 10 nm and silica protective shell of 10 nm.

The monodispersity of the monomeric samples and the uniformity in size and shape of both systems have been investigated by means of transmission electron microscopy (TEM, Fig. 1(c) and (d)). Average size uniformity is obtained by statistical analysis of the core diameter, as shown in Fig. 1(b). Further information regarding the core size and silica shell thickness is provided in the Methods section.

The extinction cross-section spectra of both systems have been measured. Fig. 2(a) and (b) show the normalized extinction cross-section profiles of both passive and active monomers and multimers, respectively. It is well known that the plasmon response of the nanostructures can be influenced by the thickness of the silica shell, refractive index of the surrounding environment and configuration of the nanostructures.^39–41 In Fig. 2(a) and (b), the SPBs show a slight red-shift for multimeric systems with respect to monomeric ones. The increase of the local field intensity due to coherent plasmon–plasmon interactions leading to hybridization processes^30–32 can result in a more efficient scattering cross-section of multimers accompanied by a significant red-shift with respect to the absorption efficiency spectrum. Of note is the increase of the red-shift of the extinction curves for multimers with respect to monomers for a shorter gold core–fluorophores separation (10 nm) as compared to thicker separation layer samples.

Fig. 2 Normalized extinction spectra of (a) passive and (b) active monomers and multimers, including absorption (λ_abs,max = 543 nm) and emission (λ_em,max = 570 nm) spectra of RhB dye molecules all dispersed in ethanol. The insets of (a) and (b) precisely show the red-shift of the multimers’ plasmon bands as compared to those of the corresponding monomers.

2.1 Numerical calculation results

In order to qualitatively investigate the effect of the local field enhancement on the plasmon–exciton coupling efficiency, the local field distribution of both active monomers and multimers are simulated by full wave electromagnetic simulation software based on the finite element method (FEM) (see Methods for details). The electromagnetic properties of the gain material have been modeled by a generic four-level system described by a standard semi-classical equivalent model.^42,43 This approach allows us to simulate the plasmon–exciton coupling in the same experimental conditions in which the dye molecules are excited. The approximate active constitutive relation for a gain medium in the linear regime is expressed by the following Lorentzian dispersion formula⁴⁴Δω_a is the bandwidth of the dye transition at the emitting angular frequency ω_a, σ_a is the coupling strength of polarization density of gain to the external electric field, τ₂₁ is the decay rate of fluorophores from the metastable energy level, Γ_pump03 is the pumping rate from level 0 to energy level 3 and ε_h stands for the permittivity of the environment, surrounding gain elements.

Simulations of the normalized local field intensity (ψ) of both plasmonic systems are shown in Fig. 4, in which ψ = |E/E_i|² is the absolute ratio of the local field intensity (E²) surrounding a nanostructure calculated at maximum fluorophore emission intensity wavelength, with respect to the intensity of incident plane wave field (E_i²). The obtained results demonstrate that gain functionalized plasmonic systems amplify of local fields with respect to passive ones, as evident upon comparing the ψ values for active monomers in Fig. 3(c) and (d) with the corresponding passive nanostructures in Fig. 3(a) and (b). Such enhancement is due to the near field coupling between the dipoles of plasmons and excitons, resulting from satisfying the aforementioned EPRET conditions.

Fig. 3 Simulation results for the local field intensity of passive (a) monomers; active (c) monomers, (e) dimers, (g) trimers (orientation 1), (i) aligned trimers (k) trimers (orientation 2) with 10 nm silica shell thickness, and passive (b) monomers; active (d) monomers, (f) dimers, (h) trimers (orientation 1), (j) aligned trimers, (l) trimers (orientation 2) with 30 nm silica shell thickness. The normalized field profiles (ψ) are calculated at the maximum emission wavelength of RhB dye molecules in ethanol and plotted in the YZ cross section plane (for details see the Methods section).

In addition, the retrieved local field intensity of gain functionalized samples with different silica spacer thicknesses are presented in Fig. 3(c), (e), (g), (i) and (k) (10 nm silica spacer) and Fig. 3(d), (f), (h), (j) and (l) (30 nm silica spacers). In fact, the plasmonic local field of thinner silica spacer nanostructures (10 nm) is much more intense, providing better conditions for plasmon–exciton coupling. Moreover, these numerical calculations predict that the multimeric configurations are promising plexcitonic systems for enhanced near fields as a consequence of the plasmon hybridization effect. Plasmon hybridized modes originate from linear interaction of the fundamental plasmon modes of the individual NPs, in analogy to molecular orbital theory.^45,46 Such combination leads to the splitting of the plasmon resonances into two new resonances, the lower energy symmetric or “bonding” plasmon and the higher energy anti-symmetric or “anti-bonding” plasmon.^31,32,46 As the gap between NPs decreases the coherent (bonding) mode is more pronounced with respect to the anti-bonding one, as a result of the increasing energy difference between the two modes. The presence of such a strong coherent mode results in a stronger field intensity in multimeric systems.

2.2 Experimental results

Steady-state fluorescence measurements have been performed on the investigated systems. As evidenced in Fig. 4, the emission spectral position of encapsulated RhB in our NPs shows a significant change with respect to the same molecules dispersed in solution (green dashed dotted line in Fig. 2(b)). In fact, the surrounding environment⁴⁷ and probable formation of dye aggregates during the grafting process can modify the absorption and emission curves.⁴⁸

Fig. 4 Fluorescence quenching observed in multimers as compared to monomers under the same pump energy value. (a) Fluorescence emission maxima of the Mo + 10 sample (black squares) with respect to the Mu + 10 sample (red circles). (b) Fluorescence emission maxima of the Mo + 30 sample (black squares) with respect to the Mu + 30 sample (red circles).

Moreover, excited dye molecules exposed to intense plasmonic fields may undergo radiative decay from energy levels triggered by plasmon modes. Steady-state photoluminescence (PL) studies show that the fluorescence peak corresponding to multimers is red-shifted with respect to the monomers (Fig. 4(a) and (b)). This effect appears more pronounced in the Mu + 10 sample (thinner silica spacer thickness) as compared to Mu + 30, corroborating our hypothesis that strong plasmon fields can trigger radiative resonance with the plasmon modes, that are expected to be at longer wavelength for Mu + 10 (see Fig. 2(b)).

In addition, fluorescence quenching effects can be observed because of resonant non-radiative energy transfer processes between excited fluorophores and plasmon nanostructures. We have observed fluorescence quenching of RhB dye molecules in both multimeric systems. The emission spectra of gain materials grafted in the Mu + 10 sample show a 2.8 fold intensity reduction compared with Mo + 10 (Fig. 4(a)), whereas only 1.5 fold lowering was observed in Mu + 30 (Fig. 4(b)). The obtained results demonstrate that the fluorescence quenching process is more efficient in multimers with a thinner spacer, because of the more effective EPRET process in cases when the excitonic molecules are placed in close proximity to plasmonic NPs.

To perform a systematic study of gain-induced optical loss modifications, time-resolved fluorescence spectroscopy,^49–51 transmission and Rayleigh scattering spectroscopies have been performed on both monomeric and multimeric systems.

The fluorescence lifetime of all samples has been measured and compared with pure dye solution. Fig. 5(a) shows the scheme of the ultrafast time resolved spectroscopy⁵⁰ setup used to acquire time-correlated single-photon counting (TCSPC) data from active samples (see the Methods section for details of the setup). The fluorescence time decay of dissolved RhB molecules was fitted by a single exponential function, while for both monomeric and multimeric systems the time decay curves were fitted by using tri-exponential decay functions (green dots and blue line fit in Fig. 6). The fitting procedure of the emission intensity decays I(t), uses a tri-exponential model according to the following expression:

where τ_i are the decay times and α_i represents the amplitudes of each component at t = 0.⁴⁷ The presence of a very fast decay time (τ₁) is attributed to a significant enhancement of the non-radiative decay rate due to a strong EPRET process as represented in Fig. 5(b). The intermediate decay time (τ₂) is related to FRET interactions, which may occur via two main coupling channels: direct coupling (i.e. the excitonic coupling) and indirect coupling (i.e. the excitonic coupling assisted by the plasmon).^52–54 Finally, the long-lived emission decay kinetics (τ₃) is attributed to a small percentage of unbound dye molecules that may have persisted throughout purification of samples or those dye molecules which have not participated in energy transfer processes.

Fig. 5 (a) Scheme of the spectrofluorometer set up to perform ultrafast time-resolved fluorescence spectroscopy. (b) Sketch of different coupling configurations between grafted dye molecules and gold cores.

Fig. 6 Time-resolved fluorescence decays and relative fitting plots of (a) monomer (Mo + 10) and multimer (Mu + 10) systems, (b) monomer (Mo + 30) and multimer (Mu + 30) systems. Panels (c–f) show residuals decay fits of Mo + 10 and Mu + 10 systems and of Mo + 30 and Mu + 30 systems, respectively.

For an ethanolic solution of RhB molecules, the time-resolved fluorescence decay curve (black line in Fig. 6(a) and (b)) can be fitted with a single-exponential function resulting in a lifetime of about τ_D ∼ 3 ns (χ² = 1.192). The observed results are consistent with previous studies which have been performed in RhB doped silica beads.⁵⁵

Fig. 6 compares the TCSPC data obtained at 572 nm, for dye molecules functionalized in monomeric (green lines) and multimeric (red lines) systems when irradiated with a 375 nm ultrafast pulsed laser. The results of the TCSPC data fit of these samples (Fig. 6, blue dots for monomers and black dots for multimers) are presented in Table 1.

Table 1 Time resolved fluorescence decay results for monomeric and multimeric systems with 10 nm and 30 nm fluorophore–NP separation distances. λ_ex and λ_em stand for excitation and emission wavelengths, respectively. τ₁, τ₂ and τ₃ are the components of the tri-exponential function used to fit the TCSPC data correlated to each one of active samples. χ² is the chi-square function which shows the goodness of the applied fit

Sample	λex (nm)	λem (nm)	τ1 (ps)	τ2 (ps)	τ3 (ns)	χ^2
Mu + 10	375	572	70 ± 1	332 ± 3	3.00 ± 0.01	1.27
Mo + 10	375	572	88 ± 2	971 ± 56	2.90 ± 0.01	1.14
Mu + 30	375	572	325 ± 7	1582 ± 40	5.32 ± 0.01	1.18
Mo + 30	375	572	452 ± 10	1860 ± 20	3.2 ± 0.1	1.07

---

The experimental data show a significant reduction of the lifetime of exciton states in multimeric systems with respect to a single NP (monomer), reflecting a stronger plexcitonic coupling in multimers. The obtained results are in agreement with the steady-state fluorescence spectroscopy data, where multimers show a higher fluorescence quenching efficiency with respect to the corresponding monomers (Fig. 4), and are also corroborated by the numerical simulations of local field intensities presented in Fig. 3, as the plasmon–exciton coupling strength is mediated by the enhanced local field of multimers. Besides the direct coupling between fluorophores embedded in a single NP, the nanospheres constituting a multimeric structure enhance the indirect coupling between dye molecules via the plasmonic gold core, which results in a shortening of τ₂ for the case of Mu + 10 as compared to Mo + 10. The long-living emission decay times τ₃ of both samples are almost the same, being associated with the fluorescence decay time of non-interacting supernatant dye molecules.

A comparative set of lifetime measurements has been performed on Mo + 30 and Mu + 30 nanostructures, where the chromophores are located 30 nm apart from the gold cores (see Table 1). The acquired lifetimes of Mo + 30 and Mu + 30 systems reveal that Mu + 30 samples show a reduction in both short- and intermediate lifetimes with respect to Mo + 30 monomers, indicating the occurrence of more effective dye–NP and dye–dye couplings in the Mu + 30 system.

Hence, these data indicate that: (i) non-radiative RET processes occur between chromophores and metal cores leading to very short lifetimes (τ₁); (ii) plasmon–exciton RET is drastically affected by their separation distance; (iii) as a result of the plasmon hybridization process, multimeric systems generally create more intense plasmonic hot-spots (high local fields), providing stronger plasmon–exciton coupling with respect to monomeric systems; (iv) the intermediate decay times (τ₂) infer that the decrease of silica shell thickness, as well as enhanced local fields, further promotes indirect couplings, irrespective of monomeric or multimeric systems; (v) these results show that energy transfer processes between exciton and plasmon states occur over 30 nm separation distances. The presence of RET for such inter-distances proves that in contrast to FRET (max inter-distance 10 nm), plexcitonic RET can occur even for longer interparticle distances, up to 30–40 nm.

In order to gain further understanding of the effects related to the interplay between excitons and plasmons in the investigated systems, pump–probe experiments have been performed. According to the Beer–Lambert law, simultaneously measuring Rayleigh scattering and transient absorption, both in the absence and presence of gain, allow us to determine if the extinction curve of the plasmonic structure is affected by resonant energy transfer processes.

Thus, modifications of the Rayleigh scattering and transmitted intensity of probe beams have been monitored as a function of the pump energy for all systems (excitation wavelength = 355 nm). A pin-hole and a notch filter were used just before the spectrometer; in order to avoid stray light affecting the experiment. The pumping wavelength was selected to avoid direct excitation of the NP plasmon states and to excite only the RhB dye molecules. We also made sure that the optical pumping was not inducing significant changes in the sample by measuring transmitted and scattering intensity before and after pumping events.

As first proposed by Lawandy,⁵⁶ the localized SP resonance in metallic nanospheres is expected to show a singularity when the surrounding dielectric medium provides a critical value of optical gain. This can be evidenced by an increase in the Rayleigh scattering signal because of the enhancement of the local field around the metal nano-antenna. Pump–probe Rayleigh scattering experiments have been performed by collinearly launching a probe beam in a small portion of the volume excited by a pump beam at λ = 355 nm. The modification of the scattered probe beam is detected at an angle of 70° relative to the beam propagation direction. The scattered probe light, acquired by means of the optical fiber of a high-resolution spectrometer (HORIBA Jobin-Yvon MicroHR Symphony), was observed in the spectrum as a relatively narrow line centered at 532 nm.

Fig. 7(a) shows the enhancement of Rayleigh scattering signal as a function of excitation energy in Mu + 10 multimers. The increase of the scattered signal above a given threshold value of the pump energy demonstrates the enhancement of the Rayleigh scattering cross section mediated by EPRET processes.^27–29 Finally, we have measured the transmittance of the probe beam traversing the excited volume upon varying the pump rate and compared this with the transmittance in the absence of excitation. Fig. 7(b) shows the increase of transmission peaks of the probe signal at 532 nm as a function of the excitation energy for the Mu + 10 system. Accordingly, as mentioned earlier, the simultaneous enhancement of Rayleigh scattering and optical transmittance imply the mitigation of absorptive losses, specifically in the Mu + 10 hybrid system.

Fig. 7 Normalized Rayleigh scattering (a and c) and transmission (b and d) signals of the probe beam (532 nm) as a function of the pump energy (355 nm) for multimeric systems: (a and b) Mu + 10 and (c and d) Mu + 30. Lower threshold values of transmittance and scattering are observed for the Mu + 10 system compared to Mu + 30.

Fig. 7(c) and (d) show the enhancement of the Rayleigh scattering intensity and optical transmission at 532 nm, as a function of the pump rate, for the Mu + 30 sample. This phenomenon confirms the inter-distance dependence of the non-radiative energy transfer rate among NP–chromophore according to EPRET theory, dipolar interactions are expected to scale as r⁻⁴, r being the NP–chromophore separation distance.^57,58 Furthermore, critical behavior of the transmission was observed above a given threshold value of about 0.45 mJ per pulse for Mu + 10, whereas the threshold value for Mu + 30 was found to be about 0.63 mJ per pulse, showing a change in the RET efficiency as a function of the gain.⁵⁶

A comparative set of experiments, including both Rayleigh scattering and optical transmission have been performed on the Mo + 10 and Mo + 30 monomers. The results reported in Fig. 8 indicate a striking enhancement in the scattered and transmitted light above a certain gain threshold value. Evidently, the Mo + 10 system demonstrates lower threshold values (∼0.55 mJ per pulse) compared to the Mo + 30 system (∼0.7 mJ per pulse). In addition, the threshold values of monomers are higher than the corresponding values for multimers. Therefore, appropriate exciton–plasmon distances in combination with stronger local fields result in a more efficient absorptive loss mitigation in hybrid plasmonic systems.

Fig. 8 Normalized Rayleigh scattering (a and c) and transmission (b and d) signals of the probe beam (532 nm) as a function of the pump energy (355 nm) for monomeric systems (a and b) Mo + 10 and (c and d) Mo + 30. Lower threshold values of transmittance and scattering signals are observed for the Mo + 10 system relative to Mo + 30.

3 Conclusions

To summarize, the exciton–plasmon interplay in monomers and multimers of gain functionalized multi-shell nanostructures with different silica shell thickness (10 and 30 nm) has been investigated. These nano-composite systems permitted a systematic investigation of the RET process between excitonic molecules and localized plasmons as a function of NP configurations and spacer thicknesses. Time-resolved fluorescence spectroscopy allowed us to investigate a remarkable resonant coupling difference between monomers and multimers. The obtained results, in combination with steady-state measurements, demonstrate that multimers experience a more effective exciton–plasmon RET because of the enhancement of the local field related to plasmon hybridization effects. Moreover, our results show that reducing the silica shell thickness results in a better coupling strength not only between exciton and plasmon, but also between NPs in a multimeric configuration, as evidenced by significant changes of fluorescence decay times. In fact, the observed enhancement of Rayleigh scattering and transmission above a critical value of gain, in correlation with inter-particle distance, have demonstrated the magnification of the local plasmon field of the gain functionalized nanostructures and the increasing of their optical transparency in the presence of fluorescent molecules. The calculated plasmon field distribution and intensity of both monomeric and multimeric samples support the experimental observations.

Hence, the obtained results suggest that by properly bringing gain to strongly absorptive meta-subunits we can remarkably modify the imaginary part of the polarizability of these media, resulting in a reduced extinction coefficient for frequencies corresponding to the exciton–plasmon bands. In particular, engineering multimeric systems with proper values of metal–gain inter-distance could be very promising to tailor effective gain assisted plasmon hybridization processes. In the framework of a bottom-up systems approach based on plasmonic materials, these results may guide towards real applications of plasmonic metamaterials at optical frequencies.

4 Methods

4.1 Numerical simulation

Simulations were conducted using COMSOL software, which is full wave electromagnetic simulation software based on FEM. In simulations, the incident field is a Transverse Electromagnetic (TEM) plane wave with electric and magnetic fields in the Z and Y directions respectively, while the wave propagates in the X direction. Along with this excitation, electric dipole resonators are formed in the Z direction. The normalized field profiles were calculated at the maximum emission wavelength of RhB dye molecules in ethanol (570 nm) and plotted in the YZ cross section plane.

The gold cores of nanostructures were modeled as 60 nm diameter spheres with a frequency dependent dielectric function taken from Johnson and Christy,⁵⁹ while spacer (10 nm or 30 nm) and coating shell (10 nm) were simulated using the dielectric properties of silica according to the Sellmeier dispersion equation.⁶⁰ The surrounding environment of the nanostructures was considered as ethanol with a refractive index of 1.36. The thickness of the dye shell was assumed as 5 nm, while in the simulation of the corresponding passive monomers this layer was considered as silica.

The applied dielectric function of Rhodamine B dye molecules (eqn (1)) in the simulation of the local fields were calculated with parameter values of ε_h = 2.25, λ_a = 570 nm (ω_a/2π = 526 THz), Δλ_a = 24 nm, and with respect to Δv_a = c₀ × Δλ_a/λ_a² = 22.2 THz (c₀ = 3 × 10⁸ m s⁻¹), consequently Δω_a = 44.2π (rad s⁻¹). Also, by considering τ₂₁ = 3 ns and δ_rad = 3 × 10⁸ s⁻¹, we have σ_a = 8.25 × 10⁻⁸ C² kg⁻¹, Γ_pump03 = 2.7 × 10⁹ s⁻¹ and N₀ = 8.75 × 10¹⁹ cm⁻³.

4.2 Nanoparticles synthesis

Gold NPs with a diameter of (60 ± 5) nm were synthesized through the reduction of hydrogen tetrachloroaurate(III) (HAuCl₄) in the presence of sodium citrate (Na₃C₆H₅O₇·2H₂O) and sodium borohydride (NaBH₄) according to the procedure published by Brown et al.⁶¹ Firstly, passive gold nanoparticles (without dye) were coated with a silica shell (monomer) according to the procedure published by Graf et al.⁶² The thickness of the shell was controlled by changing the amount of silica precursor (tetraethyl orthosilicate, TEOS). The active nanoparticles (with dye) were obtained from the passive ones by firstly grafting a Rhodamine B derivative with ethoxy-silano group, which was previously prepared by reacting Rhodamine B isothiocyanate (RhB) with aminopropyltriethoxysilane in absolute ethanol,⁶³ on the silica shell. To do so, an amount corresponding to 15 RhB molecules per nm² of silica surface was added into an ammonia (6% v/v)/ethanol suspension of passive particles. The reactive medium was heated at 70 °C during one hour. The monomers were collected by centrifugation and washed three times with absolute ethanol. A second protective silica shell of 10 nm thickness was then grown by drop wise addition of an ethanolic solution of TEOS.⁶⁴ The increase of the concentration of passive and active monomers during the addition of the TEOS solution leads to the formation of multimers which are a mixture of dimers, trimers, quadrimers, heptamers, etc. These multimers are formed as a result of collisions between monomer particles and are permanently fixed via the hydrolysis and condensation of TEOS molecules on their surface.⁶⁵ Synthesized multimers were filtered via ultrafiltration membranes to separate monomers from multimers. However, filtrating all monomers is not experimentally achievable. Statistical analyses on several multimeric nanostructures have been performed in order to determine the core average size and the shell average thickness. The average core diameter is about 63.8 nm. For Mu + 10 and Mo + 10 samples (with 10 nm exciton–plasmon silica gap and 10 nm protective layer) the average shell thickness is approximately 18 nm, while for Mu + 30 and Mo + 30 samples (with 30 nm exciton–plasmon silica gap and 10 nm protective layer) it is about 44 nm.

4.3 Characterization and measurements

Extinction bands were measured by means of a spectrophotometer (Cary5E by Varian), whereas the steady-state emission spectra were recorded on a HORIBA Jobin-Yvon Fluorolog-3 FL3-211 spectrometer equipped with a 450 W xenon arc lamp, double grating excitation and single-grating emission monochromators (2.1 nm mm⁻¹ dispersion; 1200 grooves per mm), and a Hamamatsu R928 photomultiplier tube. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. TEM observations were performed with a Hitachi H-600 microscope operating at 75 kV, where particles were cast on a glass substrate by leaving a drop of a diluted suspension evaporating.

The fluorescence lifetime of the two systems was acquired with respect to that of pure dye solution, by means of an advanced fluorescence lifetime spectrometer (Edinburgh, FLS980 Series) using the TCSPC data option. The used system, an ultrafast time-resolved fluorescence spectroscopic instrument, consists of a Ti:sapphire pulsed laser as a tunable excitation source in the 680–1080 nm range (repetition rate = 80 MHz, pulse width = 140 fs, by Coherent Inc.), and represents the core of a customized set-up that presents a pulse picker used to decrease the repetition rate in the range between 1 to 5 MHz, in order to be synchronized with a multi pronged spectrofluorometer able to perform both steady state measurements and TCSPC investigations. The excitation laser source has been adjusted at 750 nm, in order to perform time-resolved measurements on samples. A Second Harmonic Generation (SHG) module is used to decrease the excitation wavelength in the range of 340–540 nm (blue beam). All the above mentioned investigations are performed inside the spectrofluorometer chamber (sample position S1). A multi channel plate, synchronized with incoming pulses is used to acquire the fluorescence decay lifetime in the proper temporal range. A broadband half-wave plate (λ/2) and high damage threshold polarizer (P) are used to control the power of excitation pulses. The mounted periscope (PE) has been used to have an in-plane beam at a different height. The photons collected at the detector are correlated by a time-to-amplitude converter (TAC) to the excitation pulse. Signals were collected by using a photon counting module, and data analysis was performed using the commercially available F900 software. The fit of data has been acquired by instrument software and quality of fit was assessed by minimizing the reduced χ² function and visual inspection of the weighted residuals.

Furthermore, Förster non-radiative energy transfer processes necessary to mitigate the high metal losses can be observed through indirect experiments. All of them are based on a pump–probe setup. The sample is optically pumped with 4 ns pulses of the third harmonic of a Nd:YAG laser (Brio by Quantel), λ = 355 nm. A probe beam (CW diode-pumped solid-state laser at 532 nm by Laser Quantum), with a fixed low power, was focused within the excitation region of the sample. The probe wavelength was chosen in the proximity of the overlapping region between the dye fluorescence and plasmon band maxima, where localized surface plasmon modes are expected. The probe light emitted, scattered, or transmitted by Au NPs (λ @ 532 nm) was collected by an optical fiber together with the emission of dye, in a position depending on the particular experiment. The fiber was positioned at an angle of 70° relative to the beam propagation direction for scattering (90° for fluorescence) experiments, within several millimeters from the cuvette; transmission signals were acquired far from the sample, on the same pump beam direction and with a high neutral filter in front of the fiber head, in order to prevent any possible signal different from the probe one (high stable power).

Acknowledgements

The authors thank C. Versace, M. Albani, A. Vallecchi, N. Scaramuzza and M. La Deda for the fruitful scientific discussions. We acknowledge support of the Ohio Third Frontier Project, “Research Cluster on Surfaces in Advanced Materials (RC-SAM) at Case Western Reserve University”. The research leading to these results has received funding from the EU-FP7 (FP7/2008) METACHEM Project under Grant Agreement no. 228762.

References

1. G. Raschke, S. Kowarik, T. Franzl, C. Sönnichsen, T. A. Klar, J. Feldmann, A. Nichtl and K. Kürzinger, Nano Lett., 2003, 3, 935.
2. I. H. El-Sayed, X. Huang and M. A. El-Sayed, Nano Lett., 2005, 5, 829.
3. A. F. Scarpettini, N. Pellegri and A. V. Bragas, Opt. Commun., 2009, 282, 1032.
4. V. Zharov, V. Galitovsky and M. Viegas, Appl. Phys. Lett., 2003, 83, 4897.
5. K. R. Catchpole and A. Polman, Opt. Express, 2008, 16, 21793.
6. K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari and M. S. Feld, Phys. Rev. Lett., 1997, 78, 1667.
7. A. M. Michaels, M. Nirmal and L. E. Brus, J. Am. Chem. Soc., 1999, 121, 9932.
8. P. M. Tessier, O. D. Velev, A. T. Kalambur, J. F. Rabolt, A. M. Lenhoff and E. W. Kaler, J. Am. Chem. Soc., 2000, 122, 9554.
9. R. A. Reynolds, C. A. Mirkin and R. L. Letsinger, J. Am. Chem. Soc., 2000, 122, 3795.
10. J. B. Pendry, Phys. Rev. Lett., 2000, 85, 3966.
11. W. Cai, U. K. Chettiar, A. V. Kildishev and V. M. Shalaev, Nat. Photonics, 2007, 1, 224.
12. M. A. Vincenti, S. Campione, D. de Ceglia, F. Capolino and M. Scalora, New J. Phys., 2012, 14, 103016.
13. J. Y. Ou, E. Plum, J. Zhang and N. I. Zheludev, Nat. Nanotechnol., 2013, 8, 252.
14. S. Campione and F. Capolino, Nanotechnology, 2012, 23, 235703.
15. S. Campione, S. Steshenko, M. Albani and F. Capolino, Opt. Express, 2011, 19, 26027.
16. M. A. Noginov, V. A. Podolskiy, G. Zhu, M. Mayy, M. Bahoura, J. A. Adegoke, B. A. Ritzo and K. Reynolds, Opt. Express, 2008, 16, 1385.
17. C. Garcia, V. Coello, Z. Han, I. P. Radko and S. I. Bozhevolnyi, Opt. Express, 2012, 20, 7771.
18. B. Peng, Q. Zhang, X. Liu, Y. Ji, H. V. Demir, C. H. A. Huan, T. C. Sum and Q. Xiong, ACS Nano, 2012, 6, 6250.
19. A. De Luca, R. Dhama, A. R. Rashed, C. Coutant, S. Ravaine, P. Barois, M. Infusino and G. Strangi, Appl. Phys. Lett., 2014, 104, 103103.
20. F. Cannone, G. Chirico, A. R. Bizzarri and S. Cannistraro, J. Phys. Chem. B, 2006, 110, 16491.
21. C. Xue, Y. Xue, L. Dai, A. Urbas and Q. Li, Adv. Opt. Mater., 2013, 1, 581.
22. M. P. Singh and G. F. Strouse, J. Am. Chem. Soc., 2010, 132, 9383.
23. S. Halivni, A. Sitt, I. Hadar and U. Banin, ACS Nano, 2012, 6, 2758.
24. T. Soller, M. Ringler, M. Wunderlich, T. A. Klar and J. Feldmann, Nano Lett., 2007, 7, 1941.
25. P. Viste, J. Plain, R. Jaffiol, A. Vial, P. M. Adam, P. Royer, H. P. Josel, Y. Markert, A. Nichtl and K. Kürzinger, ACS Nano, 2010, 4, 759.
26. T. Ozel, P. L. Hernandez-Martinez, E. Mutlugun, O. Akin, S. Nizamoglu, I. Ozge Ozel, Q. Zhang, Q. Xiong and H. Volkan Demir, ACS Nano, 2013, 13, 3065.
27. A. De Luca, M. P. Grzelczak, I. Pastoriza-Santos, L. M. Liz-Marzán, M. La Deda, M. Striccoli and G. Strangi, ACS Nano, 2011, 5, 5823.
28. G. Strangi, A. De Luca, S. Ravaine, M. Ferrie and R. Bartolino, Appl. Phys. Lett., 2011, 98, 251912.
29. A. De Luca, M. Ferrie, S. Ravaine, M. La Deda, M. Infusino, A. R. Rashed, A. Veltri, A. Aradian, N. Scaramuzzaa and G. Strangi, J. Mater. Chem., 2012, 22, 8846.
30. D. W. Brandl, N. A. Mirin and P. Nordlander, J. Phys. Chem. B, 2006, 110, 12302.
31. H. Wang, D. W. Brandl, P. Nordlander and N. J. Halas, Acc. Chem. Res., 2007, 40, 53.
32. P. Nordlander, C. Oubre, E. Prodan, K. Li and M. I. Stockman, Nano Lett., 2004, 4, 899.
33. S. C. Yang, H. Kobori, C. L. He, M. H. Lin, H. Y. Chen, C. Li, M. Kanehara, T. Teranishi and S. Gwo, Nano Lett., 2010, 10, 632.
34. S. J. Barrow, X. Wei, J. S. Baldauf, A. M. Funston and P. Mulvaney, Nat. Commun., 2012, 3, 1275.
35. C. S. Yun, A. Javier, T. Jennings, M. Fisher, S. Hira, S. Peterson, B. Hopkins, N. O. Reich and G. F. Strouse, J. Am. Chem. Soc., 2005, 127, 3115.
36. T. Pons, I. L. Medintz, K. E. Sapsford, S. Higashiya, A. F. Grimes, D. S. English and H. Mattoussi, Nano Lett., 2007, 7, 3157.
37. J. Griffin, A. K. Singh, D. Senapati, P. Rhodes, K. Mitchell, B. Robinson, E. Yu and P. C. Ray, Chem.–Eur. J., 2009, 15, 342.
38. T. Förster, Ann. Phys., 1948, 437, 55.
39. L. M. Liz-Marzan, M. Giersig and P. Mulvaney, Langmuir, 1996, 12, 4329.
40. A. E. Schlather, N. Large, A. S. Urban, P. Nordlander and N. J. Halas, Nano Lett., 2013, 13, 3281.
41. J. Rodrıguez-Fernandez, I. Pastoriza-Santos, J. Perez-Juste, F. J. Garcıa de Abajo and L. M. Liz-Marzan, J. Phys. Chem. C, 2007, 111, 13361.
42. P. W. Milonni and J. H. Eberly, Laser Physics, Wiley, New York, 2010.
43. A. Fang, T. Koschny, M. Wegener and C. M. Soukoulis, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 79, 241104.
44. S. Campione, M. Albani and F. Capolino, Opt. Mater. Express, 2011, 1, 1077.
45. J. I. Gersten and A. Nitzan, Surf. Sci., 1985, 158, 165.
46. E. Prodan, C. Radloff, N. J. Halas and P. Nordlander, Science, 2003, 302, 419.
47. A. P. Demchenko, Advanced Fluorescence Reporters in Chemistry and Biology, Springer, Berlin, 2011.
48. S. Özçelik and D. L. Akins, J. Phys. Chem. B, 1999, 103, 8926.
49. J. R. Lakowicz, Anal. Biochem., 2001, 298, 1.
50. J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, New York, 2006.
51. Y. Li, Q. Li, Z. Zhang, H. Liu, X. Lu and Y. Fang, Plasmonics, 2015, 10, 271.
52. V. Pustovit, F. Capolino and A. Aradian, J. Opt. Soc. Am. B, 2015, 32, 188.
53. V. N. Pustovit and T. V. Shahbazyan, Phys. Rev. Lett., 2009, 102, 077401.
54. J. R. Lakowicz, Anal. Biochem., 2005, 337, 171.
55. M. Ferriè, N. Pinna, S. Ravaine and R. A. L. Vallèe, Opt. Express, 2011, 19, 17697.
56. N. M. Lawandy, Appl. Phys. Lett., 2004, 85, 5040.
57. S. Bhowmick, S. Saini, V. B. Shenoy and B. Bagchi, J. Chem. Phys., 2006, 125, 181102.
58. J. Seelig, K. Leslie, A. Renn, S. Kluhn, V. Jacobsen, M. van de Corput, C. Wyman and V. Sandoghdar, Nano Lett., 2007, 7, 685.
59. P. B. Johnson and R. W. Christy, Phys. Rev. B: Solid State, 1972, 6, 4370.
60. I. H. Malitson, J. Opt. Soc. Am., 1965, 55, 1205.
61. K. R. Brown, D. G. Walter and M. J. Natan, Chem. Mater., 2000, 12, 306.
62. C. Graf and A. van Blaaderen, Langmuir, 2002, 18, 524.
63. X. Gao, J. He, L. Deng and H. Cao, Opt. Mater., 2009, 31, 1715.
64. S. Kang, S. I. Hong, C. R. Choe, M. Park, S. Rim and J. Kim, Polymer, 2001, 42, 879.
65. M. Ibisate, Z. Zou and Y. Xia, Adv. Funct. Mater., 2006, 16, 1627.

---