Observation of Mode Splitting in Photoluminescence of Individual

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Observation of mode splitting in photoluminescence of individual plasmonic nanoparticles strongly coupled to molecular excitons Martin Wersäll, Jorge Cuadra, Tomasz J. Antosiewicz, Sinan Balci, and Timur Shegai Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04659 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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Observation of mode splitting in photoluminescence of individual plasmonic nanoparticles strongly coupled to molecular excitons Martin Wersäll1, Jorge Cuadra1, Tomasz J. Antosiewicz1,2,*, Sinan Balci3 and Timur Shegai1,* 1

Department of Physics, Chalmers University of Technology, 421 96, Göteborg, Sweden.

2

Centre of New Technologies, University of Warsaw, Banacha 2c, 02-097 Warsaw, Poland.

3

Department of Astronautical Engineering, University of Turkish Aeronautical Association,

06790, Ankara, Turkey. KEYWORDS: strong coupling, Rabi splitting, plasmon-exciton interactions, photoluminescence, plexciton

ACS Paragon Plus Environment

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ABSTRACT

Plasmon-exciton interactions are important for many prominent spectroscopic applications such as surface-enhanced Raman scattering, plasmon mediated fluorescence, nanoscale lasing and strong coupling. The case of strong coupling is analogous to quantum optical effects studied in solid state and atomic systems previously. In plasmonics similar observations have been almost exclusively made in elastic scattering experiments; however, the interpretation of these experiments is often cumbersome. Here, we demonstrate mode splitting not only in scattering but also in photoluminescence of individual hybrid nanosystems, which manifests a direct proof of strong coupling in plasmon-exciton nanoparticles. We achieved these results due to saturation of the mode volume with molecular J-aggregates, resulting in splitting up to 400 meV, i.e. ∼20% of the resonance energy. We analyzed the correlation between scattering and photoluminescence and found that splitting in photoluminescence is considerably less than in scattering. Moreover, we found that splitting in both photoluminescence and scattering signals increased upon cooling to cryogenic temperatures. These findings improve our understanding of strong coupling phenomena in plasmonics.


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Plasmon-exciton interactions and in particular strong coupling phenomena in plasmonics have recently attracted significant research interest, motivated mainly by room temperature performance and deeply subwavelength confinement of electromagnetic modes1-5. Strong coupling is a non-perturbative regime of light-matter interactions in which the coupling strength dominates over any dissipative processes within the system5, 6. To achieve strong coupling, two requirements need to be satisfied: strong mode confinement and strong transitional dipole moment in accordance with = | | ∝ /√ , where is the coupling strength, is the vacuum field and is the mode volume5, 6. The former is achieved via plasmonic nanoparticles, while the latter is provided by a certain class of materials possessing a high transition dipole moment of the electronic excitation, such as J-aggregates7-16, quantum dots17, 18, perovskites19 and most recently 2D transition metal dichalcogenides20,

21

. Demonstration of

strong plasmon-exciton coupling has been shown using surface lattice resonances22-24, propagating surface plasmons9, 12, Fabry-Pérot (FP) cavities7, 8, 11, 15, 25, 26 and localized surface plasmons27-29. Dynamic observations of Rabi oscillations in the strong plasmon-exciton coupling regime have also been demonstrated30-32. Studying quantum optical phenomena with hybrid plasmon-exciton systems naturally requires experiments on individual plasmonic nanoparticles and individual quantum emitters. Observation of individual plasmonic nanoparticles in turn requires an appropriate spectroscopic technique, which in practice almost exclusively reduces to some variant of dark-field (DF) microscopy. However, strong coupling data obtained via DF scattering is sometimes difficult to interpret, as these might be confused with other effects such as enhanced absorption, plasmonic quenching33-35, Fano resonances in the weak coupling regime36,

37

or various polarization and

symmetry artifacts. These subtle effects are different from strong coupling in their physical


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nature but may still result in similar experimental observations, namely asymmetry or split-like lineshape of spectroscopic signal15, 34, 35, 37. The absence of alternative experimental techniques combined with simplicity and robustness of DF explains the popularity of the method; however, at the same time makes it non trivial to draw conclusions on the nature of plasmon-exciton interactions. In part this is because plasmonic cavities, as opposed to their photonic counterparts, are multi-mode and lossy38-41. An additional complication is a lack of tunability in plasmonics, which makes it difficult to experimentally verify anti-crossing behavior using one and the same hybrid nanostructure. Previously, several works have demonstrated strong coupling on a single nanoparticle level27-29 and claims have been made even at the single nanoparticle - single quantum

dot/molecule

level42-44.

These

works

however,

utilized

DF

rather

than

photoluminescence (PL) for strong coupling investigations, which is in sharp contrast to past established works in solid state and atomic physics where strong coupling in PL has been demonstrated in the quantum regime6, 45. Although in our previous studies on a single particle level significant broadening of PL was observed, which could be interpreted as signs of strong interactions, no obvious splitting was observed28, 29. In bigger systems, such as thin metallic films and FP cavities, fluorescence originating only from the lower polariton (LP) branch was reported, as the upper polariton (UP) is inactive at room temperature due to the fast nonradiative energy transfer to uncoupled incoherent states9,

11, 46, 47

. Although some recent data suggest

observation of complex spectra in PL of nanoparticle solutions on ensemble level18,

48

, PL

splitting on a single particle level has not been reported. Here, we demonstrate splitting in PL of strongly coupled plasmon-exciton hybrids at an individual nanoparticle level. We achieve this by fully saturating the mode volume, resulting in DF scattering splitting of up to 400 meV, which approaches the ultrastrong coupling regime16, 49.


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The mode splitting observed in PL is substantially smaller (nearly 2-fold) than the obtained DF values. We study hybrid plexciton systems in both DF and PL as a function of temperature in vacuum. In addition, we perform a polarization-resolved study, which shows a nearly isotropic response, suggesting random orientation of J-aggregates with respect to the local field. The data are highly consistent and reproducible, which is in line with the near complete mode volume saturation with J-aggregates observed in SEM. This result is the first demonstration of the strong coupling regime observed in the PL of individual nanoparticles, which is analogous to earlier works in solid state and atomic systems45,

50, 51

. One thus could envision the emergence of

quantum optical applications such as non-linearities at the single photon level and quantum optics using plasmonics at room temperature in similar plasmonic systems4, 22, 52, 53.

Figure 1. Dark-field scattering spectra of an uncoupled plasmonic Ag nanoprism (cyan) and a coupled single nanoprism plexciton structure (blue). Insets display SEM images of the nanoparticles imaged on top of Au surface to visualize J-aggregate shell (right) as well as graphical sketches of the nanoprisms with J-aggregates shown in red. The splitting reaches nearly 400 meV, a value significantly above the full width at half maximum of an uncoupled nanoprism.


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The spectral scattering data originating from single bare Ag nanoprisms and a single plexcitonic particle are displayed in Fig. 1. SEM shows that in both cases the spectra originate from individual triangular nanoprisms. The scattering spectrum reveals a massive splitting (~400 meV) indicating that the structure is deep in the strong coupling regime. This type of spectral change is not possible to attribute to other DF artifacts, as morphological analysis performed by SEM shows that Ag prisms are isolated nanostructures and are completely embedded in the J-aggregate matrix (grey regions around the Ag nanoparticle, see inset in Fig. 1). For comparison, bare Ag nanoprisms are not surrounded by a corresponding grey region. Hyperspectral imaging signals in this study were obtained using a liquid crystal filter and an EM-CCD detector (see Methods). The liquid crystal filter plays a role of a linear polarizer, which allows studying polarization dependence of the optical signal. By performing the measurements using two different orthogonal orientations of the filter, we recorded polarization resolved data. Most of the structures displayed a nearly isotropic response, which is consistent with a homogenous distribution of TDBC molecules around the Ag nanoprisms in a core-shell geometry. This was confirmed by SEM image visualizing the distribution of dye molecules around the Ag prism in Fig. 1. However, several symmetric triangular nanostructures exhibited a non-isotropic response, which we attribute to a non-homogenous distribution of molecules around the plasmonic structures. Indeed, a polarization dependent coupling constant may arise from

orientation

of

the

transition

dipoles

with

respect

to

vacuum

field

-

= | | ( ) cos , where denotes the angle between the ith J-aggregate transition dipole moment and vacuum electric field vectors and ( ) – the vacuum field at the position of the ith J-aggregate (see Supporting Information Figs. S2-3). The collective coupling constant is given by = + + ⋯ + = √!| | 〈cos 〉 and thus depends on


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the local molecular orientation with respect to the vacuum field. Hence, polarization-resolved data allows us to conclude that this orientation is random and homogeneously averaged out over many possible orientations for a majority of nanoparticles. We observe several interesting properties in the investigated hybrid systems. The splitting overall is significantly greater than previously reported for similar structures29, supporting the hypothesis of mode volume saturation with J-aggregates. Furthermore, the size of the nanoparticles here is smaller than in our previous studies29, meaning that the mode volume is further compressed, which in turn explains a higher coupling strength in this case in accordance with ∝ !/ , since for small single particles approaches the (decreasing) geometrical volume39. Having studied the elastic optical response of the coupled structures, we now turn to PL. The PL signal originating from an individual plexcitonic nanoprism at room- (293 K) and liquid He temperatures (4.5 K) is shown in Fig. 2a. Corresponding DF and SEM data are also displayed. Note that room temperature PL was recorded both prior and after the cooling, to assure photodegradation issues play no role in the temperature dependent analysis (see Fig. 2a – blue and red solid lines). The data shows observations of splitting both in DF scattering and PL at cryogenic temperatures (Fig. 2a,b), which we attribute to plasmon-exciton hybridization observed in PL of individual plexcitonic nanostructures. However, the splitting in PL at both room and low temperatures is significantly smaller than splitting in scattering for the same nanostructure. Moreover, we observe that the PL signal depends on the temperature. At room temperature we observe significant PL broadening (in comparison to bare J-aggregate’s PL), but only small splitting, while at low temperature PL splitting is increased. Note also that the higher


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energy peak in the PL signal is close to the uncoupled molecules, which indicates that it may be not due to emission from the upper polariton, but rather due to uncoupled molecules. We return to this point in the Discussion section.

Figure 2. a) Photoluminescence of a single hybrid nanoprism at room and liquid helium temperatures. Inset shows an SEM image of the nanoparticle (scale bar is 50 nm), b) dark-field scattering recorded at room temperature for the same nanoprism as in a). The green vertical dotted line shows the position of the excitation laser. c) PL signal recorded at room and liquid helium temperatures (inset: SEM scale bar is 50 nm), d) dark-field scattering recorded at room and liquid helium temperatures for the same nanoprism as in c). Black vertical dotted lines in all panels show the position of bare J-aggregate emission ($ = 588 nm). In the shown examples, nanoparticles’ plasmon resonances


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were approximately degenerated with the J-aggregate resonance at room temperature, i.e. %&' ≈ %) . Smooth Lorentzian lines below each PL spectrum are obtained through fitting of the data to the coupled oscillator model.

There can be several reasons behind these temperature-dependent observations. One possibility is increased exciton delocalization in J-aggregates at low temperatures. Indeed, at room temperature the exciton in TDBC J-aggregates is delocalized over ∼15 monomer molecules54, while at low temperature the delocalization is increased to ∼30-45 molecules55. Another possibility is that the quantum yield * of the upper polariton branch is highly temperature dependent, which was demonstrated in various microcavity systems8, 9, 46, 47, 56. To understand the reasons for stronger PL splitting at low temperatures, we performed additional experiments in which both DF and PL were measured as a function of temperature (Fig. 2c,d). Importantly, here we observe that splitting in DF and position of the plasmon resonance are also temperature-dependent (Fig. 2d). This implies that splitting in PL is not solely due to temperature-dependent *, but also due to stronger coupling at low temperature. It is known from the literature that plasmon resonance is weakly temperature-dependent due to insignificant electron-phonon contribution to the total Ohmic loss at optical frequencies57,

58

.

Thus, stronger DF splitting at low temperature is likely due to excitons. In particular, in Jaggregates is temperature-dependent54,

55

. It is important to note that in addition to stronger

coupling we also observe a red shift in plasmon resonance frequency upon cooling (Fig. 2d). This is in line with earlier works on individual gold nanostructures57. This red shift correlates with red shift in the lower energy PL peak and thus results in observation of stronger splitting in PL at low temperature (high energy PL peak is relatively temperature-independent).


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We further report statistics on all the data measured in this study. Mode splittings shown in Fig. 3a were extracted using the coupled oscillator model that takes losses into account. The splitting between the polariton branches is given by %+ − %- = 4 + / − (0&' − 0) ), where is the total coupling strength, / is the detuning between plasmon and exciton resonance frequencies, i.e. / = %&' − %), and 0&',) are the plasmon and exciton linewidths correspondingly. At first we extracted resonance positions of upper and lower polaritons, %+ and %- . Further, by assuming that %+ + %- = %&' + %) and noting that the exciton position is always fixed at %) ≈2.11 eV, we calculated the detuning and then estimated splitting at zero detuning as Ω3 = 4 − (0&' − 0) ) = 2(%+ − %) )(%) − %- ). In this manner we extracted Ω3 for both DF and PL signals. We observed that splitting is almost independent of the particle size, thus supporting the hypothesis of full saturation of the mode volume with molecules (Fig. 3a). This contrasts with our previous observations, where saturation of Rabi splitting was not reached29. Mode splitting in DF and PL as a function of detuning is shown in Fig. 3b. The data exhibits an anti-crossing behavior in both cases; however, splitting in DF is always greater than in PL. The lower energy peak in PL displays dependence on the detuning based on which we attribute it to the lower polariton branch. The average linewidth of the lower energy peak is slightly reduced upon cooling – from 104±16 meV at 293 K to 88±20 meV at 4 K. Note that the lower energy PL peaks are blue shifted with respect to the corresponding scattering peaks. This may be due to emission from not fully thermally relaxed lower polariton states, as well as due to differences in mode splittings of different experimental observables that we discuss below. In Fig. 3c we plot this blue shift in terms of ∆= %-67 − %-89 . We observe a clear correlation


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between ∆ and splitting in scattering, suggesting that the blue shift may totally disappear at lower coupling strengths. Moreover, ∆ depends on temperature such that the blue shift is higher at room temperature. This may be related to the red shift of %&' upon cooling57, as ∆ at both room and liquid helium temperatures was calculated using the room temperature value of %-89 (T = 293K) (due to complexity of DF measurements of small particles inside the cryostat, see Methods). We return to these observations in the Discussion section.

Figure 3. a) Distribution of mode splitting in DF and PL (at T=4.5 and 293 K) as a function nanoprisms’ side length L. Note that a typical value for 0&' ≈150 meV, while 0) ≈100 meV. b) Anti-crossing in DF and PL (at T=4.5 and 293 K) as a function of detuning. Red and blue dashed lines show plasmon and exciton frequency correspondingly. Green dotted line shows 532 nm laser excitation. c) Blue shift between PL and DF signal of the lower polariton branch, ∆= %-67 − %-89 , shown as a function of splitting in scattering. Blue and red dashed lines are linear fits of the data and serve as guides for an eye.


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The nature of the higher energy resonances in PL is not entirely clear based on this data. The high energy peaks may be due to uncoupled molecules46, 59 or dark polaritons60 and not due to UP emission as was suggested previously48. The fact that its dispersion is essentially flat speaks in favor of the former, despite being slightly blue shifted with respect to uncoupled molecules. In such a case the mode splitting in PL calculated earlier does not fully account for the energy difference between the upper and lower polariton states, thus explaining a nearly 2fold difference between splitting in scattering and PL (see Fig. 3a). The linewidth of the higher energy resonance is also slightly reduced upon cooling – from 107±11 meV at 293 K to 99±10 meV at 4 K, which is in agreement with free J-aggregates temperature induced narrowing. Note that the fluorescence filter cube used in our measurements limits spectral range of observations to wavelength longer than ∼545 nm, which in turn limits the detuning range in PL measurements (see Fig. 3b). Note also that because of the unclear nature of the high energy PL peak, estimation of the %&' and correspondingly / = %&' − %) = %+ + %- − 2%) in case of PL may be inaccurate. To elucidate on this issue, we plot /67 vs /89 for the same particles in Fig. S4 (see SI). We find that the offset typically does not exceed ∼50 meV, implying that the interpretation provided in Fig. 3b is valid. To summarize, we observe temperature dependent mode splitting in PL. In all measured cases splitting in PL at low temperature was always stronger than at room temperature. Moreover, splitting in PL was always smaller than in DF. These observations are consistent and reproducible. We further discuss these observations, as well as several related issues, such as relaxation within the excited states, polariton lifetime and quantum yield.


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Discussion: Splitting in scattering vs photoluminescence: We first turn to the question of why splitting in PL is smaller than splitting in DF. To shed more light on this problem, we performed numerical calculations of plasmon-exciton coupling (see Methods). Indeed, Fig. 4 shows splitting both in absorption and scattering, indicative of the system being in the strong coupling regime. Furthermore, we are able to separate the absorption in the coupled system into absorption in the dye and in the silver nanoparticle contributions. Both of these contributions exhibit splitting, although the former shows a smaller one. Additionally, a peak/shoulder at 588 nm is observed, which results from absorption in uncoupled dye. The similarity between the relatively weak splitting of absorption in the dye and the PL signal (when compared to elastic scattering) is striking. In order to observe the PL signal, the energy first needs to be absorbed by the dye. Assuming that in the coupled system the Stokes shift is also close to zero as it is for the free J-aggregate61, one can envision how the PL spectrum should look alike. Hence, one could expect that the splitting in PL would be smaller than that of the elastic scattering as shown in Fig. 4.


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Figure 4. FDTD calculations of the optical response of the coupled silver nanoprism J-aggregate system. Silver core is homogenously covered by a finite molecular (TDBC) shell (thickness 3 nm). The nanoprism side length and the thickness were taken to be 40 nm while 11 nm correspondingly, in agreement with HR-TEM data. The corners were rounded (r = 6 nm). Vertical dashed lines show the position of upper and lower resonances for total absorption (blue), scattering (purple) and J-aggregate absorption (orange); J-aggregate resonance at 588 nm (black). Evidently, splitting in scattering is greater than splitting in total absorption, which in turn is greater than splitting in absorption of the J-aggregate layer.

To elucidate on the issue of the uncoupled molecules, we have performed additional FDTD calculations, in which molecular aggregate shell thickness was systematically varied (see Fig. S5). These calculations show that contribution of uncoupled molecules to both absorption and scattering of strongly coupled systems become significant in thick molecular shells. However, for thinner shells molecules mostly contribute through the collective strong coupling. In addition to FDTD data, previous works on strong coupling in quantum well semiconductor microcavity systems suggested that different quantities, such as absorption, transmission, reflection and PL all exhibit different values of observable mode splittings62. Moreover, all these splittings are actually different from the Rabi splitting itself - Ω3 = 4 − (0&' − 0) ). This raises an important practical question about the means of measuring the actual structure of plasmon-exciton polariton energy levels. In our previous works splitting in scattering, extinction and absorption have been shown to differ from one another, such that splitting in scattering was typically stronger than that in absorption28, 34, 35, 37. Observations in Fig. 4 are consistent with these earlier results. Hence, we conclude that mode splittings for different experimental observables may be different. A simple explanation for this can be


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inferred based on the coupled oscillator model36. However, in addition to these difficulties, there may be other reasons, related to excited state dynamics, that further complicate the picture in case of PL. We consider these below. Photoluminescence mechanism: When ! J-aggregates strongly couple to a plasmon field, two bright - |+> and |−>, and (! − 1) dark polariton states arise. Although these dark states are weakly absorbing, they may contribute to the PL, as was suggested recently60. In addition, an ensemble of incoherent uncoupled molecules may be present in our structures due to random molecular orientations and thick dye overlayers spanning beyond the plasmon near-field (see SEM image in Fig. 1 inset). We also note that strong coupling to dark plasmonic states, i.e. quenching, can play an important role here, especially for the molecules in the nearest vicinity of the metal surface63. Additionally, exciton-exciton interactions in high density molecular layers may lead to self-quenching61. With this complex structure in mind, we start discussing the mechanism of PL in the strongly coupled plexcitons. In order for PL to occur, absorption of a laser photon must be followed by some inelastic nonradiative energy loss mechanism(s). These processes have been studied by Agranovich and co-workers for the case of strongly coupled organic microcavities filled with J-aggregates46, 59. Following these works, the dynamics in the present case can be thoughts to proceed as follows. At first the hybrid system is excited to the vibronic polariton state by absorbing a 532 nm laser photon |, 0 >AB) → |+>AD) (here E denotes vibrational quantum number along some vibrational normal mode). Excitation is then followed by a rapid relaxation (∼50 fs) to uncoupled molecules46, where the population can reside for a relatively long time (∼1-10 ps)59. An alternative route of direct relaxation to the lower polariton branch


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|+>AD) → |−>AD) was estimated to occur on pico second time scale and is thus inefficient64. The subsequent dynamics strongly depends on the relation between the Rabi splitting and the vibrational modes of the molecules. In particular, if Rabi splitting exceeds the phonon energy, in addition to standard photoluminescence of uncoupled molecules, there appears a relaxation pathway consisting of nonradiative transition to the lower polariton branch accompanied by G-HH

emission of a high-energy intramolecular phonon59. In the present case %AF 300 meV, so Ω3 ≈ 2%AF , implying that this pathway is indeed viable. Both processes, namely direct PL from the uncoupled molecules and population of lower polariton branch gives rise to PL. Based on the arguments above, as well as on data shown in Figs. 2-3, we assign the highenergy PL peak to emission from uncoupled molecules - (Process 1), while the low-energy PL peak to emission from the lower polariton branch |−> - (Process 2) correspondingly. We note that the PL of the uncoupled molecules is modified with respect to the free space emission due to interaction with the plasmon (Purcell effect), however, keeping in mind that relaxation of Jaggregates is typically dictated by fast nonradiative processes55, this modification is expected to be small. Both Processes (1-2) occur on a pico second time scale, i.e. much slower than the polariton lifetime (∼10 fs in accordance with 0± ≈ (0&' + 0) )/2), and thus represent the bottleneck of the PL in strongly coupled systems. Such picture agrees with the lifetime measurements in strongly coupled microcavities at room temperature reporting J=1-3 ps66, 67. An alternative view involves coherent dark polariton states in addition to incoherent uncoupled molecules60. This process also gives rise to PL at around J-aggregate resonance of uncoupled excitons, which thus can be responsible for the higher energy PL peak.


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Spectral lineshape and its dependence on temperature: Previous works on free Jaggregates showed that their optical properties can be tuned by temperature. In particular, an increase in the *, narrowing of the PL and shortening of J was observed at low temperatures55, 56, 68

. It is thus possible that similar effects can be important for temperature-dependent strong

plasmon-exciton interactions studied here. When the lower polariton branch is pumped via emission of a phonon from a reservoir of uncoupled molecules, the emitting state will be determined by the rate of the competitive processes involved. A good measure characterizing this competition could be the blue shift between the energy of lower polariton in PL and in DF ∆= %-67 − %-89 . This quantity is shown in Fig. 3c. We clearly observe that ∆ grows with the splitting. By extrapolating this result to smaller splittings, we estimate values at which the blue shift should totally disappear. This happens for splitting of about 150 meV – i.e. approaching the high-energy phonons of the system. In the same figure we also observe that the blue shift is offset by about 40 meV at T = 4.5 K, which could be explained by the ∼10 nm temperature-induced red shift in %&' 56.. Previous literature has reported observations of similar blue shifts in some cases47,

66

,

while in others these were not observed8, 9, 67, 69. As we show in Fig. 3c, the blue shift is reduced for smaller coupling strength, which may be determined by the relation between the Rabi splitting and the phonon energy. In the abovementioned examples, cavities with the largest Rabi splittings – 330÷500 meV47, 66 indeed showed blue shifts in PL. In cases of weaker couplings8, 9, 67, 69

, blue shifts were not observed, although splitting could have been as large as 230 meV – i.e.

above the phonon energy. In our previous works, splitting was much weaker than here, yet the


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blue shift was still observed28, 29. In those cases, however, partial or complete photodegradation of the dye rapidly occurred and thus could be responsible for this blue shift.

Conclusions In conclusion we demonstrated vacuum Rabi splitting in PL of individual plexciton nanoparticles. This was achieved due to full saturation of the plasmonic mode volume with Jaggregates, which resulted in a giant Rabi splitting of up to ∼400 meV in DF measurements. Moreover, we showed that splitting in PL is temperature-dependent and that it is smaller than splitting in scattering. Based on the anti-crossing data, we assigned the lower energy resonance in PL to the lower polariton emission, while the higher energy resonance to emission of uncoupled molecules. We analyzed the blue shift between the lower polariton emission in PL and DF as arising due to competition between relaxation and emission rates and due to differences in splitting measured in absorption and scattering of nanoparticles. At lower temperature %&' is slightly red shifted57, which explains a stronger PL splitting observed at low temperature. Even though previous works performed on ensemble level have suggested visualization of both UP and LP branches in PL18, 48, the current study gives a more thorough understanding of the dynamics in single strongly coupled plasmon-exciton structures. Our measurements ensure that the spectral modes indeed arise from plasmon-exciton interactions rather than from any ensemble related issues such as complexity in particle clusters and/or structures with odd morphology. Our experimental approach allows for a careful correlation between scattering and photoluminescence spectra with morphological details of each individual nanoparticle. The system parameters in terms of particle size, J-aggregate shell characteristics,


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temperature and vacuum were reproducible, controllable and stable, which greatly simplified the analysis. We expect that future studies could focus on systematic variation of the splitting with respect to phonon energy, excitation spectra, temperature effects, quenching, lifetime and quantum yield measurements. Also proper theoretical understanding of strongly coupled plasmon-exciton systems is required. This work is the first important step in confirming strong coupling behavior on the single plexciton particle level in photoluminescence, which opens up prospects for future realizations of quantum plasmonics systems provided the mode volume is further compressed.

Methods: Synthesis of plasmon-exciton hybrids: We prepared the samples following Ref.

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Briefly, the Ag nanoprisms with precise control over the prism edge length were wet-chemically synthesized by using a seed-mediated protocol. High resolution TEM shows that Ag nanoprisms are high quality single crystalline nanostructures (dimensions: ca. 50 nm side length and 10 nm height), which is essential for minimizing Ohmic losses (see Supporting Information Fig. S1). At first small seed Ag nanoparticles were synthesized, which was then followed by a slower growth of nanoprism. Plasmon-exciton hybrid nanoparticles were synthesized by self-assembly of a Jaggregate dye (TDBC, 5,5’,6,6’-tetrachloro-di-(4-sulfobutyl) benzimidazolocarbocyanine, FEW Chemicals) on Ag nanoprism surfaces. The color of the solution changed immediately indicating plexcitonic nanoparticle formation. The excess TDBC molecules were removed by centrifugation. This allows saturation of the mode volume of the nanoparticles with the dye and


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to stabilize J-aggregates against degradation, resulting in formation of core shell Ag J-aggregate nanostructures. Optical Measurements: Hybrid nanoparticles were immobilized on the oxidized Si substrate and analyzed using an upright microscope. To achieve an appropriate density of nanostructures, a drop of nanoparticle solution was applied on a substrate precoated with polylysine (0.25 mg/mL). The droplet was subsequently removed by an intense nitrogen flow after remaining on the substrate for about 2 min. The substrate contains a lithographically defined square patterns (100×100 µm2), which enables spectroscopic and morphological correlation. DF scattering measurements were performed using a hyper-spectral imaging technique based on a liquid crystal tunable filter29. This allows for a high throughput parallel recording of DF spectra for all structures within the field of view. All room temperature measurements were performed using a reflective DF objective (Nikon, 100× NA=0.8) in an upright microscope. All low temperature measurements were performed in the optical cryostat and a long working distance objective (Nikon, 20× NA=0.45) in an upright microscope. We note that these low temperature measurements allowed the signal only from the biggest nanoprisms to be recorded with sufficient signal to noise ratio. Photoluminescence: It is well-documented that J-aggregates are unstable under humid, oxygen and elevated temperatures conditions61. Thus, to increase the photostability of the molecular aggregates, the hybrid systems were investigated in an optical cryostat chamber. All measurements were conducted using a 532 nm laser in epi-illumination mode with an irradiance of ∼200 W/cm2 and a long working distance objective (Nikon, 20× NA=0.45) in an upright


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microscope. Note that under these conditions, no obvious photodegradation was observed after several hours of continuous irradiation. Numerical Calculations: we performed numerical calculations of plasmon-exciton coupling using the finite-difference time-domain method (FDTD Solutions, Lumerical). On a semi-infinite glass substrate (n = 1.45) we place a J-aggregate coated silver nanoprism (Fig. 4). The side length a = 40 nm and the thickness is h = 11 nm. The corners are rounded (r = 6 nm) and the permittivity of silver is taken from literature70. Such a nanoprism, when placed on the substrate, has a resonance wavelength of ∼530 nm, a value which does not change significantly with polarization. The coupled plasmon-exciton system is properly modeled by assuming a Jaggregate shell that conformably surrounds the nanoprism. We use linearly polarized light (both polarizations, total-field/scattered-field formulation) to excite the structure and measure the scattered signal to match the calculated spectrum with the measured one in terms of peak splitting, peak width and relative amplitude (see Fig. 4). This matching is obtained by varying the shell thickness as well as the optical properties of the dye, which are described by a Lorentzian function - KG-HH (%) = KL − M%) /(%) − % − N0) %). Matching of the experimental and calculated scattering spectra is obtained for a 3 nm conformal J-aggregate layer with the following optical properties. The background refractive index is 1.45 (KL = 2.10) with an oscillator strength of M=0.1 and peak width 0) of 75 meV. The absorption line %) is at 588 nm. We then calculate absorption within the structure.


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Supporting Information. Additional data consisting of high resolution TEM image of Ag nanoprism, polarization resolved data and additional statistical data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected] and [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

ACKNOWLEDGMENT We would like to thank Dr. Ruggero Verre and Dr. Andrew Yankovich for helping with sample preparation and high resolution TEM measurements correspondingly. We acknowledge financial support from Swedish Research Council (VR grant: 2012-4014) and Knut and Alice Wallenberg foundation. T.J.A. thanks the Polish National Science Center for support via the project 2012/07/D/ST3/02152 and the Polish Ministry of Science and Higher Education via the Iuventus


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Plus Project IP2014 000473. S.B. acknowledges The Scientific and Technological Research Council of Turkey (TUBITAK) (112T091).


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TOC FIGURE:


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Observation of Mode Splitting in Photoluminescence of Individual

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