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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
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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
∗ [email protected]† [email protected]
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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3
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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4
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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5
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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9
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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10
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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11
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
Funding Information Acknowledgments Supplemental Documents
References