Experimental evidence of exciton-plasmon coupling in densely packed dye doped coreshell nanoparticles obtained via microfluidic technique A. De Luca, A. Iazzolino, J.-B. Salmon, J. Leng, S. Ravaine, A. N. Grigorenko, and G. Strangi Citation: Journal of Applied Physics 116, 104303 (2014); doi: 10.1063/1.4895061 View online: http://dx.doi.org/10.1063/1.4895061 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Practical protein removal using atmospheric-pressure helium plasma for densely packed gold nanoparticle arrays assembled by ferritin-based encapsulation/transport system Appl. Phys. Lett. 101, 073702 (2012); 10.1063/1.4745508 Self-assembled broadband plasmonic nanoparticle arrays for sensing applications Appl. Phys. Lett. 100, 031102 (2012); 10.1063/1.3674982 Plasmonic core-shell gold nanoparticle enhanced optical absorption in photovoltaic devices Appl. Phys. Lett. 98, 113119 (2011); 10.1063/1.3559225 Plasmonic coupling in nondipolar gold colloidal dimers Appl. Phys. Lett. 98, 083122 (2011); 10.1063/1.3560456 Plasmonics of thin film quasitriangular nanoparticles Appl. Phys. Lett. 96, 133104 (2010); 10.1063/1.3373918
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JOURNAL OF APPLIED PHYSICS 116, 104303 (2014)
Experimental evidence of exciton-plasmon coupling in densely packed dye doped core-shell nanoparticles obtained via microfluidic technique A. De Luca,1,a) A. Iazzolino,2 J.-B. Salmon,2 J. Leng,2 S. Ravaine,3 A. N. Grigorenko,4 and G. Strangi5,b) 1
Department of Physics and Licryl Lab. (CNR-IPCF), UNICAL, Via P. Bucci–87036, Rende, Italy LOF, UMR 5258 Rhodia-CNRS-Bordeaux 1, 178 Avenue du Docteur Schweitzer, F-33608 Pessac, France 3 CRPP, Centre de Recherche Paul Pascal, CNRS and University of Bordeaux, 115 Avenue Schweitzer, 33600 Pessac, France 4 School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom 5 Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, Ohio-44106-7079, USA 2
(Received 18 July 2014; accepted 27 August 2014; published online 9 September 2014) The interplay between plasmons and excitons in bulk metamaterials are investigated by performing spectroscopic studies, including variable angle pump-probe ellipsometry. Gain functionalized gold nanoparticles have been densely packed through a microfluidic chip, representing a scalable process towards bulk metamaterials based on self-assembly approach. Chromophores placed at the hearth of plasmonic subunits ensure exciton-plasmon coupling to convey excitation energy to the quasi-static electric field of the plasmon states. The overall complex polarizability of the system, probed by variable angle spectroscopic ellipsometry, shows a significant modification under optical excitation, as demonstrated by the behavior of the ellipsometric angles W and D as a function of suitable excitation fields. The plasmon resonances observed in densely packed gain functionalized core-shell gold nanoparticles represent a promising step to enable a wide range of electromagnetic properties and fascinating applications of plasmonic bulk systems for advanced optical materials. C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4895061] V
I. INTRODUCTION
Plasmonic systems consisting of metal nanoparticles (NPs) assemblies represent one of the cutting edge research field which recently gained wide scientific interest for the fascinating physics and for promising nano-photonic applications.1–4 Nano-chemistry is playing a leading role in this field, permitting to design new synthesis and functionalization approaches for metal based core-shell NPs as well as selfassembling techniques to fabricate nanoscale metal-dielectric structures for nano-optic and bio-photonic purposes.5,6 This bottom-up approach is based on the spontaneous or directed organization of functionalized nano-objects that, under the effect of complex pair interactions, create 3D structures of various symmetries. Unfortunately, being based on metals, these systems suffer from a strong radiation damping and significant high values of the imaginary part of permittivity, that produce high absorptive losses just in correspondence of the plasmonic resonances. Theoretical studies, as well as experimental works, have extensively shown that bringing donor dye molecules in close vicinity to metal subunits can reduce these losses, facilitating the development of unconventional applications.7–12 The main idea consists in optimizing plasmon-gain dynamics so that coherent and nonradiative energy transfer processes between excitonic and plasmonic states can effectively occur. Programming this interplay by controlling the dominant parameters of plasmon-gain a)
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b)
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interaction (e.g., shape and size of the constituents, concentration, interdistance) provides a powerful tool to trigger relevant physical effects as optical loss compensation, superabsorption, enhanced photoluminescence, surface-enhanced Raman scattering, and laser action. Experimental results obtained in dispersions have demonstrated induced optical transparency accompanied by a considerable enhancement of the Rayleigh scattering intensity as a function of gain.7–9 At the same time, reduction of the lifetime of gain materials in presence of plasmonic NPs evidenced the strong exciton-plasmon coupling occurring via non-radiative energy transfer processes.8,9 More recently, mesoscale systems, as porous silica capsules, embedding gold NPs grown on the inner surface, or gold nanoshells, have been used to demonstrate that, at an intermediate scale between the single plasmonic nanostructure and bulk materials, plasmon-gain interplay is dominated by the spatial distribution of the gain medium with respect to the local field profile. In particular, these structures allow regions of uniform plasmonic field where the energy transfer occurring between chromophoric donors and the surrounding plasmonic acceptors gives rise to a broadband loss attenuation.13,14 Results obtained up to now in this field showed that plasmon-exciton coupling can be extensively studied in the visible range at nano- and meso-scale, but the effective design and fabrication of realistic three-dimensional materials based on nanoparticle size subunits represents still a challenging task. Self-assembly and template assisted methods are considered two of the most promising techniques for design and fabrication of two- and three-dimensional structures towards real metamaterials.
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C 2014 AIP Publishing LLC V
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Here, we report about the microfluidic technique based on self-assembly approach and used to fabricate highly organized 3D structures of core-shell NPs. A microfluidic device works by extraction of the solvent from an initially dilute mixture containing NPs, which eventually leads to concentrated, dense states of the mixture. Microfluidic technique makes possible the manipulation of very small quantities of NP solution (ll), in order to concentrate nanocolloids by controlled extraction of the solvent. More specifically, microchips are made of an elastomer (polydimethylsiloxane–PDMS) which is permeable to water (and other solvent such as ethanol, acetone, etc.) but not to NPs; we can therefore selectively extract the solvent. While doing so, a net flow is induced in the chip which compensates the solvent evaporation and in turn drains the solution from the reservoir. An increasing number of NPs get accumulated in the micro-channel and their concentration increases with time, in a manner which is easily predictable for ideal solutions and calculable for interacting species. The main objective of the device is its ability to concentrate different species–from ions to colloids–at a well controlled rate, starting from a very dilute solution up to a dense state. The latter may be obtained once a high enough concentration has been reached to overcome the solubility limit of the solute. Several tests of the colloidal stability of many NP solutions have been performed, in order to define a proper model system for our experiments, prevent colloidal aggregation, and obtain a crystal-like dense state of core-shell NPs. The goal is to concentrate mixtures of NPs with gain material properly encapsulated into the silica shell of each NP (gain functionalized NPs), using a proper designed microfluidic channel. The formation of these densely packed cells can be used to obtain a high concentration of active material at the right interdistance from each gold core that, coupled with plasmonic resonances, can induce the localization of electromagnetic modes in the visible range. The formation of a three dimensional structure of active plasmonic resonators represents the closest step towards a real active metamaterial. II. EXPERIMENTAL RESULTS AND DISCUSSIONS
Here, we report experimental results that demonstrate the strong coupling occurring in functionalized core-shell NPs made of exciton molecules and metal cores, densely packed in a microfluidic flexible channel. The investigations have been performed on Au@SiO2 NPs composed of a gold core (diameter 12 6 2 nm) covered by a thin silica shell (about 12 nm) with gain molecules (DCM–Dicyanomethilene) functionalized into the shell. Gold NPs were synthesized by following the procedure published by Grabar et al.,15 and coated with a 12 6 5 nm fluorescent silica shell according to the method reported by Graf et al.16 In order to obtain a densely packing of these gain functionalized core-shell NPs, a microfluidic device has been designed and fabricated using standard soft lithography.17 It involves the replica of a defined structure on a soft elastomer mold (PDMS), consisting essentially of three steps. After printing the desired design on a transparent mask at high resolution (20 000 dpi), photolithography is used to realize a mold with a fingerprint of a mask.
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FIG. 1. (a) Sketch of the designed chip used as mold into PDMS. Purple area R represents the reservoir of NP solutions, whereas final part L of the long micro-channels (L0 ¼ 11 cm) is used for ellipsometric investigations. (b) SEM image of a micro-channel, showing the obtained crystal-like 3D assembly. White bar is 10 lm. (c) Zoom of image (b), scale bar is now 1 lm. (d) Three dimensional sketch of the densely packed gain-functionalized Au NPs. (e) Variable Angle Pump-probe Ellipsometric set-up (VAPE). External CW excitation source at k ¼ 532 nm.
To fabricate the mold, we used a negative photoresist (SU-8 by MicroChem) which can reticulate under UV irradiation. The result is a thick mold (thickness h) containing twodimensional networks of channels in relief on the silicon wafer. The third step consists in molding the network of channels on PDMS. In our case, the designed PDMS microevaporator (see Fig. 1(a)) is deposited over a membrane and the system is left at 65 C for 2 h, allowing the binding of the two layers. The chip and the membrane are then peeled off from the wafer, and the final chip is placed on two glass slides to be easily managed. The assembly of NPs is then obtained using microfluidic evaporation. As the solvent evaporates through the semi-permeable membrane, the solutes are continuously concentrated in the tip of the channel and, upon sufficient accumulation time, a dense state of nanoparticles nucleates. The rate at which the device extracts solvent has been quantified in details18–20 and depends on the membrane efficiency, the channel length (L0), and the concentration of NPs in the reservoir. By following all the necessary steps, we
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were able to obtain a densely packed crystal-like of active gold NPs (see SEM images in Figs. 1(b) and 1(c)). The above mentioned experimental approaches used to investigate loss mitigation in NPs solutions can be considered necessary but not sufficient to analyze the collective resonance properties of three dimensional assemblies of metal NPs. The strong coupling effects present in bulk systems have to be studied by means of suitable techniques specifically used to extract macroscopic physical quantities. In this sense, variable angle spectroscopic ellipsometry represents an extremely powerful technique to investigate optical properties of thin films. In these specific experiments, ellipsometry (usually considered as a probing technique) has been implemented with a secondary pump source, used to excite the guest chromophores functionalized into the plasmonic nanostructures, in order to promote a resonant energy transfer (RET) process (see sketch in Fig. 1(e)). The measurement of the ellipsometric angles were found not to be affected by the pump light, since the detecting system is based on a rotating analyzer. Therefore, the detected signal is locked in phase with respect to the input signal ruling out from the analysis any stray light, including the pump light. Interestingly, the expected RET process during the excitation time produces significant modifications of the optoplasmonic properties of the system. In particular, ellipsometry measures the complex reflectance ratio, q, of a system, which may be parameterized by the amplitude component W and the phase difference D.21–24 The polarization state of the light incident upon the sample may be decomposed into an s and a p component. The amplitudes of the s and p components, after reflection and normalized to their initial value, are denoted by rs and rp, respectively. The complex reflectance ratio q is defined as the ratio between rs and rp, thus being equal to tan (W)exp(iD). Here, tanðWÞ ¼ jrp j=jrs j is the amplitude ratio upon reflection, and D ¼ drp drs is the phase shift. Figure 2(a) represents the transmission of p-polarized light through a cuvette of 1 mm path length, measured by means of the spectroscopic ellipsometer (V-VASE by Woollam). A plasmon resonance peaked at 532 nm has been clearly observed. On the other hand, when we deposit the solution on a glass substrate and wait until it is completely dried, what we observe in terms of plasmon resonance is slightly different. The angle dependence of the ellipsometric angle W is reported in Fig. 2(b), showing a plasmon band shift with respect to the solution around 530 nm, due to aggregations present of the glass substrate. This measurement represents a precursor of the behavior we aspect from an assembly of gain functionalized Au NPs. By following the procedure described above, we have obtained a 3D crystal-like structure of densely packed NPs functionalized with excitonic molecules (see 3D sketch in Fig. 1(d)). In this configuration, the plasmon resonance band can be considered as the result of the hybridization process of strongly interacting plasmon nanoparticles.25,26 The overall opto-plasmonic behavior is represented in Fig. 3(a) (black solid curve), in terms of the ellipsometric angle W. A well defined collective resonance is observed around 561 nm, where D assumes its maximum value (see Fig. 3(b), red solid curve). The presence of excitonic molecules inside the silica shell of NPs
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FIG. 2. (a) Transmission of p-polarized light measured by means of ellipsometer into a 1 mm cuvette solution. (b) Spectroscopic behavior of W parameter as function of the incidence angle for the same solution dispersed on a glass substrate and dried, showing a collective resonance shift around 530 nm, due to aggregation of metal NPs.
promotes a resonant energy transfer and as result induces a significant modification of the bulk optical properties, as revealed by the dispersion curves of the ellipsometric parameters, under the action of an external excitation source. A diode laser at 532 nm has been used to excite the same surface area probed by the ellipsometer, in a pump-probe configuration. It is worth to notice how, in the spectral region where plasmon band and emission band overlap (see green dash-dot curve in Fig. 3(a)), we find a strong modification of W and D curves (black and red dashed curves in Fig. 3(a) and b, respectively), while pumping the material with respect to the absence of excitation. It is worth to notice that we do not observe any modification of the ellipsometric parameters at the excitation wavelength (532 nm), but only in spectral region where gain and plasmon bands overlap. This different optical response is the result of a change in the physical properties of bulk materials induced by a resonant energy transfer process. The exciton-plasmon coupling ensured by the designed topological and spectral configuration dominates the mechanism of conveying excitation energy nonradiatively into the plasmon states. The complex permittivity of the system, modified by optical excitation, is directly measured through the ellipsometric probe. However, by modeling W and D behavior for different angles, it is possible to determine the overall refractive index and extinction coefficient of the the entire bulk system, illuminated by the ellipsometer fiber spot. This unique setup offers the vantage to capture directly information about phase and amplitude
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FIG. 4. Average refractive index hni and extinction coefficient hki (incident angle 65 ) of a densely packed 3D structure of Au@PEG NPs with R6G dispersed in solution obtained via microfluidic technique. Black solid and dashed curves represent the hni behavior when external optical excitation at 532 nm is switched OFF and ON, respectively. Red solid and dashed curves represent the hki behavior when external excitation at 532 nm is switched OFF and ON, respectively. A reduction of the extinction coefficient hki in the 525–750 nm band represents a key factor for the energy transfer process occurring from gain material to gold NPs.
FIG. 3. (a) W parameter of the densely packed 3D structure obtained via microfluidic technique. Black solid and dashed curves represent the W behavior when external CW excitation at 532 nm is switched OFF or ON, respectively. (b) Red solid and dashed curves represent the D behavior when external excitation at 532 nm is switched OFF or ON, respectively. A reduction of D values in the 606–706 nm band is accompanied by an enhancement in close bands, verifying the causal nature of the response of materials via KK dispersion relations.
relations of the probing electric field. The coupling of electric field of the incident electromagnetic waves with the collective plasmon, during transfer of excitation energy, is responsible for significant changes of W and D. Experimental curves are acquired by taking into account the possible permanent modification of the optical properties of the bulk sample. W and D spectra have been acquired each time before and after the optical excitation, verifying the complete restoring of the initial behavior for amplitude and phase. Fig. 3(a) shows a significant decrease of W in the RET band, indicating a change of the extinction term of the material that is undergoing a resonant energy transfer process. It is also important to notice that, according to the KramersKronig (KK) relations, D turns out to be selectively modified within the RET band. A decreasing of D in the band of 606–706 nm, related to permittivity modifications induced in a limited spectral region, corresponds to an enhancement of the response in neighbor spectral bands, 537–606 nm and 706–839 nm. As reported in Ref. 9, a simple model that takes into account the principle of causality through KK relations reveals the possibility to predict this general behavior in a simple way, by considering a single gold core/silica shell nanoparticle with gain encapsulated into the shell (negative value for e002 ðx0 Þ), resulting in good agreement with the experimental data.
Similar results have been obtained in a system in which different dye molecules (Rhodamine 6G, R6G) were added to the solution during the microfluidic evaporation process of Au@PEG nanoparticles, used to obtain dense states of gainassisted core-shell subunits. Figure 4 represents the behavior of the average refractive index and extinction coefficient (hni and hki, real and imaginary part of the complex index of refraction) obtained by fitting W and D at an incident angle of 65 . The used ellipsometric model assumes a bulk gold layer of 0.1 mm and an upper gold layer of 13 nm (Au-Palik) on a glass substrate. Black solid and dashed curves represent the hni behavior when the optical excitation at 532 nm is switched OFF and ON, respectively, whereas red solid and dashed curves are referred to hki behavior in absence and in presence of the external stimulus, respectively. Even in this case, it is well evident the modification of the two physical quantities hni and hki in presence of an excited gain medium, due to the strong coupling between the two species. As well as the gain functionalized sample, the modification of hni respects the KK relations, in a way that it decreases in the range 450–525 nm, but it increases in the adjacent band (525–750 nm). More important is the behavior obtained for the extinction coefficient hki, which resulted reduced in the case of laser ON in the 570–700 nm band. Normalized emission curve of R6G has been overlapped to the hni and hki curves in order to show the overlapping region (green dash-dot line). III. CONCLUSIONS
Thus, the presented experimental observations represent a clear demonstration of the strong coupling between collective plasmon resonance and excitons, in densely packed 3D structures obtained via microfluidic technique. Selective modification of ellipsometer parameters W and D, as well as fitted hni and hki, under an external excitation source represents a significant evidence of the effective energy transfer
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processes occurring between the two species. This strong coupling resulted in a frequency dependent behavior of the dielectric functions of the gain-plasmon system. Thus, it implies that fascinating optical properties and potential novel applications are expected since optical loss can be reduced and controlled in adjacent spectral regions. Placing gain media right at the heart of metal NPs may enable multiple strong plasmon-exciton couplings that are at the basis of extraordinary optical properties of designed artificial materials. ACKNOWLEDGMENTS
The research leading to these results has received support and funding from the Ohio Third Frontier Project Research Cluster on Surfaces in Advanced Materials (RCSAM), the European Union’s Seventh Framework Programme (FP7/2008) METACHEM Project under Grant Agreement No. 228762 and from the Italian Project PRIN 2012, Protocol No. 2012JHFYMC. 1
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