Plasmon-exciton-polariton lasing

Citation for published version (APA):Ramezani, M., Halpin, A., Fernández-Dominguez, A. I., Feist, J., Rodriguez, S. R. K., Gómez-Rivas, J., &Garcia-Vidal, F. J. (2016). Plasmon-exciton-polariton lasing. arXiv, 1-25.http://www.mendeley.com/research/plasmonexcitonpolariton-lasing

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http://www.mendeley.com/research/plasmonexcitonpolariton-lasinghttps://research.tue.nl/en/publications/plasmonexcitonpolariton-lasing(71c96a05-04ed-4004-995d-4e4aad989e33).html

Plasmon-exciton-polariton lasing

Mohammad Ramezani and Alexei HalpinFOM Institute DIFFER, P.O. Box 6336, 5600 HH Eindhoven, The Netherlands

Antonio I. Fernández-Domı́nguez and Johannes FeistDepartmento de F́ısica Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC),

Universidad Autónoma de Madrid, E-28049 Madrid, Spain

Said Rahimzadeh-Kalaleh RodriguezCentre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud,

Universit Paris-Saclay, C2N - Marcoussis, 91460 Marcoussis, France

Francisco J. Garcia-VidalDepartmento de F́ısica Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC),

Universidad Autónoma de Madrid, E-28049 Madrid, Spain andDonostia International Physics Center (DIPC), E-20018 Donostia/San Sebastian, Spain∗

Jaime Gómez RivasFOM Institute DIFFER, P.O. Box 6336, 5600 HH Eindhoven, The Netherlands and

Department of Applied Physics, Eindhoven University of Technology,P.O. Box 513, 5600 MB Eindhoven, The Netherlands†

Metallic nanostructures provide a toolkit for the generation of coherent light below the diffractionlimit. Plasmonicbased lasing relies on the population inversion of emitters (such as organic fluo-rophores) along with feedback provided by plasmonic resonances. In this regime, known as weaklightmatter coupling, the radiative characteristics of the system can be described by the Purcelleffect. Strong lightmatter coupling between the molecular excitons and electromagnetic field gener-ated by the plasmonic structures leads to the formation of hybrid quasi-particles known as plasmon-exciton-polaritons (PEPs). Due to the bosonic character of these quasi-particles, exciton-polaritoncondensation can lead to laser-like emission at much lower threshold powers than in conventionalphoton lasers. Here, we observe PEP lasing through a dark plasmonic mode in an array of metal-lic nanoparticles with a low threshold in an optically pumped organic system. Interestingly, thethreshold power of the lasing is reduced by increasing the degree of lightmatter coupling in spite ofthe degradation of the quantum efficiency of the active material, highlighting the ultrafast dynamicresponsible for the lasing, i.e., stimulated scattering. These results demonstrate a unique roomtem-perature platform for exploring the physics of exciton-polaritons in an open-cavity architecture andpave the road toward the integration of this on-chip lasing device with the current photonics andactive metamaterial planar technologies.

I. INTRODUCTION

Exciton-polaritonshybrid lightmatter quasi-particlesformed by strong exciton-photon couplinghave inspiredmore than two decades of highly interdisciplinary re-search [1]. Polariton physics has largely focused on semi-conductor microcavities, where the strong nonlinearitiesassociated with quantum well excitons [2, 3] combinedwith the high quality factor cavities available throughstate-of-the-art epitaxial techniques have enabled thefirst observations of BoseEinstein condensation (BEC) [4]and superfluidity [5] in optics. A key early vision of thefield focused around the possibility of achieving a coher-ent light source at low threshold powers without the needfor population inversion: a polariton laser [6]. Polariton

∗ fj.garcia@uam.es† j.gomezrivas@differ.nl

lasers have remained in the realm of proof-of-principleexperiments [7, 8] and their widespread usage has notyet been adopted as efforts continue to lower thresholdsand optimize operating parameters. In an effort to over-come some of the material-related limitations hamperingapplications of exciton-polaritons, as well as to explorenovel light-matter states associated with distinct typesof excitons, several researchers have recently turned theirattention to organic materials [9–13]. While organic sys-tems are generally disordered, their optical transitionscan have large transition dipole moments allowing themto couple strongly to light at room temperature. Severalindependent studies have already reported polariton las-ing/BEC [14, 15] and nonlinear interactions with organicexcitons [16] in microcavities.

Recently, plasmonic systems have emerged as a promis-ing alternative platform for exploring exciton-polaritonsin an open architecture. The cavity defining the res-onator is no longer a multilayer dielectric stack pos-sessing a complex spectral response, and facilitates the

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integration of exciton-polariton devices with integratedphotonics circuits. In these plasmonic systems, previ-ously shown to be highly suitable for photon lasing [17–21], the excitonic material can be easily integrated bysolution processing. The quality factors of plasmonicresonances are much lower than their counterparts indielectric microcavities. However, the subwavelengthfield enhancements generated by resonant metallic nanos-tructures can significantly boost the lightmatter cou-pling. Indeed, strong plasmon-exciton coupling has al-ready been observed [11, 13, 22–27], but earlier attemptstoward achieving plasmon-exciton-polariton (PEP) las-ing remained unsuccessful due to the inefficient relax-ation mechanism of PEPs and the saturation of strongcoupling at large pumping fluences [23]. Here, wedemonstrate PEP lasing from an optically pumped ar-ray of silver nanoparticles coated by a thin layer of or-ganic molecules at room temperature, occurring at a lowthreshold [8]. Strong coupling between excitons in theorganic molecules and collective plasmonic resonancesof the array forms PEPs. By increasing the PEP den-sity through optical pumping, we observe a pronouncedthreshold in the emission intensity accompanied by spec-tral narrowing. Besides these generic lasing character-istics, our system exhibits two rather distinct features:first, the threshold power for PEP lasing is reduced inparallel with a degradation of the quantum efficiency ofthe material. This counterintuitive behavior, from thestandpoint of conventional laser physics, is intimately re-lated to the onset of strong coupling and the emergenceof new eigenstates, i.e., PEPs. A second distinct featureof our PEP laser stems from the fact that the nanopar-ticle array supports dark as well as bright modes. Themode that first reaches the lasing threshold is in fact darkbelow the threshold. While dark-mode photon lasing hasattracted significant interest in the plasmonics commu-nity for several years, we provide the first report of lasingfrom a dark mode in a strongly coupled plasmon-excitonsystem. Lasing from this dark (below threshold) modealso manifests in an abrupt polarization rotation of theemitted light by 90◦ above threshold.

We first characterize strong coupling between excitonsin an organic dye and the lattice modes supported by anarray of silver nanoparticles through optical extinctionmeasurements. Subsequently, using numerical and semi-analytical techniques, we analyze the modes supportedby the array and the composition of the associated PEPs,respectively. Photoluminescence (PL) measurements onthe sample pumped off-resonance provide the emissionresponse at increasing PEP densities. At high emitterconcentrations, we observe Rabi splitting, the signatureof plasmon-exciton strong coupling, together with the ap-pearance of stimulated scattering and PEP lasing. Bymeasuring the dispersion of the PL at several pump flu-ences for both polarizations, we identify the dark moderesponsible for PEP lasing in this system.

II. RESULTS AND DISCUSSION

Fig. 1 shows the normalized absorption and PL spectraof a layer of PMMA doped with organic dye molecules.We use a rylene dye [N,N′-Bis(2,6-diisopropylphenyl)-1,7-and -1,6-bis(2,6-diisopropylphenoxy)-perylene-3,4:9,10-tetracarboximide] as an emitter [28], due to its highphotostability and low propensity towards aggregationat high concentrations. Two distinct peaks correspond-ing to the main electronic transition at E = 2.24 eVand first vibronic sideband at E = 2.41 eV are evidentin the absorption spectra of the molecules. A layer ofPMMA doped with dye molecules with thickness of 260nm is spin-coated on top of the plasmonic array of silvernanoparticles. A SEM image of the fabricated array isdepicted in the inset of Fig. 1. The array consists of par-ticles with dimensions of 200 nm× 70 nm× 20 nm andthe pitch sizes along the x and y directions are 200 nmand 380 nm, respectively.

First, we measure the angle-resolved extinction of thenanoparticle array when covered by an undoped layer ofPMMA (Figs. 2(a,c)). The polarization of the incidentlight is fixed, and set to be either perpendicular (top row)or parallel (bottom row) to the long axis of the nanopar-ticles as indicated by the arrow in the inset. Here we takeadvantage of a particular type of plasmonic modes thatare supported by periodic arrays of metal nanoparticles,the so-called surface lattice resonances (SLRs). Thesemodes are the result of the radiative coupling betweenlocalized surface plasmon (LSP) resonances in the indi-vidual nanoparticles enhanced by the in-plane diffractedorders of the array, i.e., the Rayleigh Anomalies (RAs).Energy dispersions and quality factors of these SLRscan be tailored by varying the geometrical parametersand energy detuning between RAs and LSP resonances[29, 30]. In addition, the enhanced in-plane radiative cou-pling reduces the radiative losses associated to localizedresonances [31] and the redistribution of the electromag-netic field around the particles also reduces Ohmic losses[32], creating narrow resonances with high quality factors[31, 33, 34].

By probing the sample under different polarizationsand angles of incidence, we couple to different resonanceswith distinct electric field distributions and symmetriesdepending on which RAs are probed. The resonance withparabolic dispersion in Fig. 2(a) demonstrates the com-bination of both strong extinction and narrow linewidthin SLRs, which results in this case from the enhancedradiative coupling of (0,±1) RAs to a dipolar LSP res-onance supported by the individual nanoparticles (seebelow). Similarly, by rotating the polarizer and align-ing the incident electric field along the long axis of thenanoparticles, we probe the coupling of (0,±1) RAs to aquadrupolar LSP resonance supported by the rods (seealso below) [35]. Differences between the extinction mea-surements of Figs. 2(a,c) can be observed, where the mostimportant is the vanishing SLR at kx = 0 mrad/nm forincident electric field parallel to the long axis of the rods.

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For this case the electric field distribution is not dipole-active along the polarization direction. As a result, cou-pling of the free space radiation into this mode is inef-ficient and the mode is dark at normal incidence. Thisbehaviour implies a strong reduction of radiation lossesof this mode which, as we will show later, is a key ingredi-ent to achieve PEP lasing [36, 37]. Due to the multipolarcharacter of the dark mode, the net dipole moment is notzero (Fig. 2). This residual dipole moment is responsiblefor the out-coupling of the PL above threshold that is re-ported ahead. Extinction measurements of the same ar-ray in the presence of a 260 nm thick PMMA layer dopedwith dye molecules are shown in Figs. 2(b,d). The dyeconcentration (C) in the polymer is 35 wt% and the mea-surements are referenced to the extinction of the dopedlayer without the nanoparticles. Increasing the molec-ular concentration modifies the extinction dispersion asa result of the hybridization between the SLR and themolecular transition. For both polarizations we observean anticrossing in the extinction dispersion and an associ-ated Rabi splitting, ~Ω = 200 meV, at kx ≈ 7 mrad/nm,which reveals the emergence of strong coupling betweenthe SLR and molecular excitations. The coupling leadsto the creation of upper and lower PEPs where hybridiza-tion occurs for both bright and dark modes. In Fig. 2(d),another resonance slightly blue shifted with respect tothe lower PEPs is visible. This resonance corresponds tothe guided-mode supported by the polymer layer. Theappearance of this guided mode is due to the increase ofthe refractive index of the doped polymer layer when themolecular concentration is increased.

In order to analyze the hybrid light-matter nature ofthe PEPs formed by strong coupling of the SLRs to thedye molecules, we use a few-level Hamiltonian that re-produces the measured extinction dispersions and allowsthe determination of the Hopfield coefficients defining theexciton and photon components of the strongly coupledsystem. We treat each field polarization separately, withHamiltonian (for each in-plane momentum)

H =

ESLR g1 g2g1 Edye,1 0g2 0 Edye,2

where ESLR is the energy of the SLR, Edye,1 (Edye,2)

is the dispersionless energy of the first (second) absorp-tion peak of the dye, and g1 (g2) describes the couplingbetween the SLR and the molecular modes. In Fig. 2,both the input energies of the “bare” states and the en-ergy of the lowest PEP modes are depicted. As the ac-tual system contains additional states at higher energiesthat we do not treat, we only show the lowest coupledstate in the figure. From this analysis we can extract theHopfield coefficients of the lower PEP, indicating that atkx = 0 mrad/nm it is composed of 75% SLR and 25%molecular excitations, with similar values obtained forboth polarizations. We note here that while the SLR forthe polarization along the long axis of the particles is

dark under far-field excitation at kx = 0 mrad/nm, thecoupling to the molecules occurs in the near-field andthus attains similar strengths for both polarizations.

Electromagnetic calculations were also performed tosimulate the extinction properties of the experimentalsamples. Both the finite-difference time-domain (FDTD)(Lumerical) and finite element method (FEM) (ComsolMultiphysics) methods were employed, finding an excel-lent agreement with the extinction maps in Fig. 2 (seeSupplementary Information for details). Geometric andmaterial parameters were extracted from SEM imagesof the samples and previous literature [38], respectively.The right insets in Fig. 2 render electric field amplitudeand induced surface charge density maps evaluated at kx= 0 mrad/nm. The top insets (Fig. 2(e,g)) show that,for a polarization along the short axis, the near-field isgoverned by the bright dipolar-like LSP supported by therods, i.e., the so called (λ/2) LSP, as anticipated above.On the contrary, the bottom insets (Fig. 2(f,h)) indicatethat a dark quadrupolar LSP, i.e., the (3λ/2) LSP, res-onates for incoming light polarized along the long axisof the rod. Importantly for lasing purposes, both polari-tonic modes spectrally overlap within an energy windowof a few meV. The vacuum Rabi frequency Ω defining thecoupling strength is proportional to

√N , where N is the

number of excitons within the mode volume of the reso-nance. Therefore, achieving strong coupling with organicmolecules requires increasing their concentration withinthe polymer matrix such that the exciton density is max-imized [19, 24, 25]. However, one immediate drawbackexpected from increasing C for the emission is the emer-gence of considerable inter-molecular interactions, whichcan lead to spontaneous aggregation (modifying the spec-tral properties of the sample) and also to a reductionof fluorescence lifetime (τ) due to the enhancement ofnon-radiative decay channels (concentration quenching ofthe emission) [39]. Importantly, the dye molecules usedhere do not suffer from aggregation at high C as the nor-malized absorption spectrum for different C remains un-changed (See Fig. S4 at the Supplementary Informationfor details). To quantify the concentration quenching,we have measured the lifetime and quantum efficiencyof the polymer layers at different C (see SupplementaryInformation for details). One can see that at low C thequantum efficiency of the molecules is close to the unityand the lifetime of the excited molecules is 5.8 ns. How-ever, by increasing C to 35 wt% the quantum efficiencyis reduced to 3 % with a corresponding τ = 400 ps. Theemissive properties of high C samples are thus unsuit-able for stimulated emission and photon lasing, wherea high quantum yield is desired for achieving gain. Infact, the prediction that lasing without inversion could beachieved by exploiting many-body coherence in stronglycoupled exciton-photon systems has been a major mo-tivation for the development of exciton-polariton lasers[6].

In order to investigate PEP lasing, we optically pumpthe sample nonresonantly and measure the PL spectra as

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a function of the incident excitation power for differentconcentration of the molecules. The wavelength of thepump laser is centered at λexc = 500 nm (Eexc = 2.48 eV)and the pump polarization is fixed along the short axisof the nanoparticles for all experiments. The spectra ofthe PL for the sample with C = 35 wt% in the for-ward direction for different absorbed pump fluences isshown in Fig. 3(a), where we observe the appearance ofa very sharp peak in the emission spectrum at a fluenceof 18 µJ/cm2. In Fig. 3(b), we plot the maximum ofthe PL intensity as a function of absorbed pump fluencefor three different concentrations of dye. For the lowconcentration sample, C = 15 wt%, the PL intensity in-creases linearly with the incident power. This sample isat the onset of the appearance of exciton-SLR hybridiza-tion in extinction (see Supplementary Information). AtC = 25 wt%, we observe the emergence of a threshold inthe PL, followed by a superlinear increase of the emis-sion. The nonlinear response of the system is furtherincreased at C = 35 wt% where the threshold fluence islowered by a factor of 2. This threshold fluence is oneof the lowest values observed in optically pumped or-ganic polariton lasers [14–16] and also is lower than thereported threshold for plasmonic based photon lasers inthe weak coupling regime [20, 40]. Note that in our exper-iments, photo-degeradation occurs before the transitionto the weak coupling regime [23], so the photon lasing isprecluded by the damage threshold. Moreover, it is in-teresting to note that below the threshold, the emissiondecreases as C increases. This decrease is mostly due tothe reduction of the photoluminescence quantum yield(PLQE) (See Supplementary Information).

The linewidth of the PL as a function of the absorbedpump fluence is shown in Fig. 3(c), where a strong reduc-tion of the linewidth above threshold and hence, substan-tial increase of the temporal coherence, can be observed.In Fig. 3(d) we display the polarization of the emissionbelow and above the threshold for the sample with C = 35wt%. Below the threshold, the emission of the sample ismainly polarized along the short axis of the particles, i.e.,90◦. This emission is associated with PEPs originatingfrom the hybridization of the bright SLR with excitons(see Fig. 2(b)). Interestingly, above the threshold, thepolarization of the emission rotates by 90◦ and is pri-marily oriented along the long axis of the particles. Thispolarization rotation strongly indicates that the emissionabove threshold is dominated by the PEPs created fromthe dark modes whose polarization point to the long axisof the particles (see Fig. 2(d)).

After excitation the molecules relax to the PEP band.In this incoherent relaxation, the in-plane momentum isnot conserved, leading to a broad distribution of excita-tion across the whole band. Most of the excitation thusends up in high-kx reservoir modes that are almost un-coupled from the SLR [4]. In order to achieve polaritonlasing, the population of a single state has to becomelarge enough to obtain significant bosonic stimulation.In semiconductor microcavities, the necessary relaxation

from the reservoir typically relies on exciton-polaritonand polariton-polariton scattering. In contrast, micro-scopic models for organic polariton lasing have suggestedthat vibronic coupling can play a more important role fordissipating excess momentum than exciton-polariton andpolariton-polariton scattering [41–43]. In this picture,high frequency intramolecular vibrational modes allowexciton-polaritons to scatter directly from the reservoirtowards the lower energy levels [44]. This implies that or-ganic exciton-polariton lasing is most efficient when therelaxation from the reservoir to the lasing state is reso-nant with a strong optical phonon line [14]. As shownin the top inset of Fig. 3(a), this condition is exactlyfulfilled in the current sample: the energy difference,∆ = ∆1 = ∆2, between the vibronic subpeaks of themolecule (the phonon energy) corresponds exactly to theenergy difference between the lowest peak and the lasingstate. Moreover, one needs to take into account that inaddition to the role of the lattice parameters in modifyingthe detuning between the SLR and the molecular exciton,the change of the molecular concentration can also alterthe detuning through the change in the refractive indexof the layer. The specific energy at which the polaritonlasing occurs also poses an interesting question: while en-ergy shifts are seen as the smoking-gun for interactionsbetween polaritons in semiconductors microcavities, thereported spectral behaviour of polariton lasers in organicsystems has been variable [14–16]. In our system as thePL below and above threshold is dominated by differentmodes, we must distinguish the values of the energy shiftfor these two regimes. In Fig. 3(c), one can see that abovethe threshold, the dominant photoluminescence originat-ing from the dark mode shifts by 1.3 meV and locks at2.038 eV. Locking of the energy shift above thresholdhas been predicted by the model in Ref. [16]. This isin agreement with our measurements considering organicpolariton interaction within the condensate. Similar tothe extinction measurements shown earlier, we can useangular-resolved measurements to study PEP emission atincreasing pump fluences as we approach the critical den-sity of PEPs [23]. In Fig. 4, we display the measured dis-persion of the emission both below and above thresholdfor two orthogonal detection polarizations correspondingto the bright and dark modes seen in Figs. 2(b,d). InFigs. 4(a,d), where the pump fluence is below threshold,we recover the same dispersions as those shown previ-ously in the extinction measurements, with a bright modefor horizontal polarization and a dark mode for vertical.We observe emission over the whole range of kx indicat-ing that, as mentioned above, the molecular relaxationprocess populating the PEPs after high-frequency exci-tation does not conserve kx = 0 mrad/nm. Moreover, thePL from the uncoupled molecules lead to the green andcyan background in Figs. 4(a,d). Upon increasing thepump fluence, we observe a collapse in the emission pat-tern towards kx = 0 mrad/nm over the narrow spectralrange seen in the spectra of Fig. 3(a). As we mentionedearlier, while the system exhibited no vertically polar-

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ized emission at normal incidence below threshold, abovethreshold the lasing peak is mainly polarized along thisdirection. This behaviour highlights one of the majordifferences between open systems defined by plasmoniclattices and traditional microcavities: the bright modecorresponding to the SLR of Fig. 2(a) represents a lossymode due to the radiation losses as it can efficiently cou-ple out into free-space. On the other hand, the darkmode is significantly less lossy at kx = 0 mrad/nm dueto suppression of radiation losses, which favors PEP las-ing at this mode. The PEPs created via this mode canbe accumulated with a much lower probability of decay.The energy dispersion of the bare modes associated withuncoupled SLRs for the dark and bright modes are shownin Fig. 4 indicating that the system remains in the strongcoupling regime. We also observe in Fig. 4(c),a residualPL emission with a flat dispersion at the energy in whichthe lasing occurs. This emission is due to the scatteringand polarization conversion of the PEP lasing emissionfrom local imperfections in the sample.

CONCLUSION

In conclusion, we demonstrate the first polariton laserbased on a plasmonic open cavity using an organic emit-ter. By exploiting diffractive coupling in a metallicnanoparticle array, we obtain spectrally sharp surface lat-tice resonance modes leading to the formation of PEPsin a dye-doped layer above the surface of the array. Alasing threshold in this system appears alongside the on-set of strong-coupling at high dye concentrations, whena sufficiently large number of molecules interact with theSLR mode, leading to an avoided crossing as seen in ex-tinction measurements. At subsequently higher concen-trations, the threshold pump energy diminishes and weobserve a very low lasing threshold in organic polaritonlaser systems. Though the observation of strong-couplingappears in far-field measurements through coupling ofa bright mode to molecular resonances, the radiativelydark mode supported by the array plays a critical role inthe near-field providing a low-loss channel in which po-laritons can accumulate. This ultimately results in PEPlasing at an orthogonal polarization to that of the brightmode. While the optimal conditions for polariton lasingin plasmonic arrays remain to be determined, subsequenttailoring of both the mode structure of the array and theemitter could conceivably lower the lasing threshold re-ported here further (for instance by making use of pump-enhancement). Note that the energy difference betweenthe RAs and the LSP resonance in arrays of nanoparticlesdefines the confinement of SLRs to the surface, i.e., themode volume and also their quality factor which affectthe threshold. Therefore, arrays of metallic nanoparticlesconstitute a rich system that requires a through investi-gation. In addition, the ease in fabrication of large areaarrays by nanoimprint lithography and the open natureof the cavity, opens a window through other interesting

phenomena occurring in polariton systems with an archi-tecture that provides potentially straightforward imple-mentation into devices.

METHODS

Fabrication

The array of silver nanoparticles was fabricated by sub-strate conformal nanoimprint lithography following theprocedure described in Ref.[45] on glass (Eagle 2000) andencapsulated by 8 nm of SiO2 and 20 nm of Si3N4 to pre-vent oxidation of silver.

Characterization

The extinction measurements have been performed us-ing a collimated polarized white light beam generatedfrom a halogen lamp. The zeroth-order transmissionof the white light from the sample under different in-cident angles (T0(θ)) is collected. For the reference mea-surement, the transmitted light through the same sub-strate and the polymer layer in the absence of the ar-

ray (T ref0 (θ)) is measured. The extinction is defined as

1− T0(θ)/T ref0 (θ).All photoluminescence measurements except for the

polarization measurement below threshold in Fig. 3(b)have been done with amplified pulses generated from anoptical parametric amplifier with an approximate pulseduration of 100 fs and a repetition rate of 1 kHz. Exci-tation using a low repetition rate but high pulse energiesallows to generate large densities of PEPs without thethermal damage associated to high duty-cycle excitation.The excitation laser is focused on 100 µm diameter spoton the sample. The polarization measurements belowthreshold were done with a continuous-wave diode laseremitting at λ = 532 nm.

FUNDING INFORMATION

This research was financially supported by the Ned-erlandse Organisatie voor Wetenschappelijk Onderzoek(NWO) through the project LEDMAP of the Technol-ogy Foundation STW and through the Industrial Part-nership Program Nanophotonics for Solid State Light-ing between Philips and the Foundation for Funda-mental Research on Matter FOM. This work has beenalso funded by the European Research Council (ERC-2011-AdG proposal No. 290981), by the EuropeanUnion Seventh Framework Programme under GrantAgreements FP7-PEOPLE-2013-CIG-630996 and FP7-PEOPLE-2013-CIG-618229, and the Spanish MINECOunder Contract No. MAT2014-53432-C5-5-R.

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ACKNOWLEDGMENTS

We sincerely thank Martin Könemann for providing usthe organic dye. We are grateful of Marc A. Verschuuren

for the fabrication of the samples.

SUPPLEMENTAL DOCUMENTS

See Supplementary document for supporting content.

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FIG. 1. (a) Normalized absorption (blue) and photoluminescence (red) spectra of the layer of PMMA doped with dye moleculesin the absence of the plasmonic array. The inset shows an SEM image of the array of silver nanoparticles. (b) Schematicillustration of the array covered with a thin layer of PMMA doped with dye molecules.

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FIG. 2. The extinction measurements of the sample with incident light polarized along the short axis of the nanoparticles inthe absence (a) and the presence (b) of the dye molecules. The extinction of the same sample illuminated by the light polarizedalong the long axis of the nanoparticles without (c) and with (d) the dye. The white dashed lines indicate the energies ofthe two strongest vibronic transitions in the molecules (horizontal lines) and the SLR dispersion whereas the dashed blacklines correspond to PEP modes. Maps of the induced charge density (e,f) and normalized electric field amplitude (g,h) for thecorresponding lowest PEP modes at kx = 0 mrad/nm. (e,g) correspond to the bright (dipolar-like) resonance, while (f,g) tothe dark (quadrupolar) resonance at kx = 0 mrad/nm.

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FIG. 3. (a) Emission spectra along the forward direction for the array of nanoparticles covered with PMMA with the dyeconcentration of 35 wt % at increasing absorbed pump fluences. (Inset) Close view of the lasing peak. Upper panel: Absorptionspectrum of the dye (solid curve). The energies of the lasing emission, the main electronic transition and the first vibronicside band of the molecule are indicated by the gray shaded areas. Note that the energy differences ∆1 and ∆2 are equal. (b)Photoluminescence peak intensity as a function of absorbed pump fluence for three samples at different dye concentrations. (c)Linewidth and energy shift of the photoluminescence peak as a function of absorbed pump fluence for the sample with C = 35wt% dye concentration. The linewidths and peak energies are extracted by fitting a Gaussian function to the spectrum. Theerrorbars in peak energy plot are set by the resolution of the spectrometer (≈ 1 meV) (d) Polarization of the emission fromthe sample with C = 35 wt% below and above threshold (P = 1.5Pth). The long axis of the nanoparticles is oriented along thevertical direction (θ = 0◦).

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FIG. 4. Normalized angular-resolved emission measurements for two detection polarizations, parallel and orthogonal to thelong axis of the nanoparticles, for the array of nanoparticles covered with PMMA with the dye concentration of 35 wt %. Thecartoon in the inset of each panel depicts the orientation of the nanoparticles with an arrow indicating the direction of thetransmission axis of the polarization analyzer. The emission intensity in unit of counts/s at E = 2.04 eV and kx = 0 mrad/nmfor each detection condition is indicated at the bottom of each panel. (a-c) Angle-resolved PL for different pump fluences withthe analyzer along the short axis of the nanoparticle visualizing to the bright mode along (0,±1) RAs supported by the array.(d-f) Angle-resolved PL for different pump fluences with analyzer along the long axis of the nanoparticle. In this configuration,the dark mode excited by (0,±1) RAs is probed. The bare modes associated with the uncoupled SLRs are shown by whitedashed line in each panel.

Plasmon-exciton-polariton lasingAbstractI INTRODUCTIONII Results and Discussion Conclusion Methods Fabrication Characterization

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