Room temperature organic exciton-polariton flow exploiting high-speed, high-Q

propagating modes

Giovanni Lerario1,2,3

, Dario Ballarini1*

, Alessandro Cannavale2,3

, Federica Mangione1,

Salvatore Gambino1,2

, Lorenzo Dominici1,2

, Milena De Giorgi1, Giuseppe Gigli

1,3, Daniele

Sanvitto1

1 NNL, Istituto Nanoscienze - CNR, Via Arnesano, 73100, Lecce, Italy

2 CBN-IIT, Istituto Italiano di Tecnologia, Via Barsanti, 73100, Lecce, Italy

3 Università del Salento, Via Arnesano, 73100 Lecce, Italy

* email: dario.ballarini@nano.cnr.it

Exciton-polaritons, bosonic quasi-particles formed by the interaction of light and matter, have

shown a plethora of exciting phenomena that have been chiefly restricted to inorganic

semiconductors and low temperature operation. Only recently, polariton condensation and

non-linear effects have been demonstrated with polymers and organic molecules, making

these systems suited for a realistic new generation of all-optical devices. However, polariton

propagation in the plane of the device, essential for on-chip integration, is still limited by the

very strong dissipation inherent to present organic microcavities. Here, we demonstrate

strong-coupling of organic excitons with a Bloch surface wave (Q ≈ 3000) which sustains

polariton propagation for distances longer than 300 m and polariton lifetimes of about 1 ps,

a record value in organic devices. The group velocity of the polariton mode is found to be

≈ 50% the speed of light, about two order of magnitude higher than in any planar

microcavity.

Introduction

Exciton polaritons are hybrid quasi-particles, arising from the strong coupling between excitons and

photons, which retain both the photonic and excitonic features of their bare components.1,2,3

Photons, which are massless and non-interacting particles, lighten the polariton mass up to 3 orders

of magnitude smaller than the one of bare excitons,4,5,6

while excitons carry their nonlinear

properties, which are 4 orders of magnitude higher than in standard nonlinear optical media.7,8

These assets led, in the last 10 years, to the observation of fascinating new physics in solid state

systems, such as polariton condensation,9,10,11

superfluidity,12,13

quantized vortices14,15

and their

non-linear dynamics.16

Recently, exciton polaritons have also attracted a strong technological interest as high-speed,

dissipative-less, optical devices.17,18,19,20,21

On the one hand electronics is facing the intrinsic limits

set by power heating in case of high rates of information manipulation and transmission, while on

the other hand, state of the art photonic devices require extremely high switching and operating

powers. Moreover, realistic optical gates need to fulfill the properties of gain, fan-out and

cascadability, something still out of reach in many optical architectures.22

In this context, exciton

polaritons are perfect candidates for all-optical devices and logic circuits: the excitonic component

makes them extremely sensitive to small power changes, while the photonic part allows for

information transmission and manipulation with high speed, efficiency and no heat dissipation.23

Conventional polaritonic devices are planar microcavities, where the optical active layer, typically

semiconductor quantum wells, is embedded between two distributed Bragg reflectors (DBR). In

inorganic microcavities, polariton propagation has been used to demonstrate all-optical logic gates

(AND and OR)24

with fast switching times,25

yet maintaining all the condition for a cascadable

circuit. The key point is to make polariton flowing in the plane of the cavity, in particular the need

for fast group velocities (higher data transmission speeds) and long lifetimes (low dissipation rate).

A missing yet very important improvement is room temperature operation, which is essential for the

use of polariton devices in present technology. For their ease of fabrication, low costs and high

binding energies,26,27

organic polaritons are an emerging field with outstanding potentialities, as

demonstrated by the recent observation of nonlinear effects and room temperature condensation.28,29

However, the difficulties in the growth of DBR structures on top of organic materials and their

inhomogeneities have rendered very difficult the realization of high Q cavities. Moreover, also in

case of long-living polaritons, the data transfer rate—or, in other words, the maximum theoretical

throughput—is limited by the "low" group velocity (typically 1-2 m/ps) of standard planar

microcavities. Indeed, fast propagation of high-Q polariton fluxes in organic systems would be a

fundamental milestone towards the realization of optical devices operating at room temperature and

with minimal dissipation.

Although propagation has been reported in organic nanofibers and waveguides,30,31

the flow of

organic polaritons in the plane of the device—essential for the architecture of polariton transistors

and gates—has never been observed so far.

In this paper, we demonstrate propagating organic polaritons with typical lengths over 120 m (1/e)

exploiting a mode with Q ≈ 3000 and having a group velocity exceeding 50% the speed of light in

vacuum. To obtain such high speeds and long lifetimes, we couple an organic exciton (Lumogen

Red F305) with a Bloch surface wave (BSW). The BSW is an optical mode propagating on the

surface of a Distributed Bragg Reflector (DBR) beyond the critical angle:32,33

in the total internal

reflection regime, the field is confined at the interface between the stack of the photonic crystal and

air. The BSW is a lossless optical mode that differs from other surface modes like Tamm states or

surface plasmons since, although possessing a very high group velocity,34,35

it does not undergo

dissipation caused by metal losses.

Strong coupling is achieved by deposition of a thin layer of an organic dye on the surface of the

DBR structure, where the confined electromagnetic field is maximum.32

In this configuration, the

top DBR, necessary for the formation of polaritons in planar microcavities, is no longer needed.

This leads to an important advantage since such DBR in standard microvavities has to be deposited

on top of the organic active layer with strong limitation on the mirror quality and consequent cavity

finesses. Indeed, in the Bloch surface wave polariton (BSWP) the organic dye couples instead with

a very high Q mode, giving polariton lifetimes of 1 ps, much longer than those (about 150 fs) found

in the best planar organic microcavities28,29

and with a propagation group velocity 100 times higher.

Finally, we highlight the key importance of the deposition quality for the propagation properties of

polaritons by comparing two organic layers with high and low deposition homogeneity.

Results

In a previous work36

we have demonstrated the strong coupling between a j-aggregating organic

TDBC-based spin coated film and a BSW mode. Strong coupling was observed both in reflection

and emission measurements.36,37

However the deposition method used to obtain a uniform organic

layer (spin coating) does not allow for a very high level of homogeneity, which manifests in a

relatively broad polariton emission linewidth. We will discuss at the end of this section the

importance of the layer homogeneity on the propagation properties of BSWP, starting here with a

smooth deposition obtained by thermal evaporation of a different organic molecule.

A 35 nm thick layer of Lumogen Red F305, a small molecule with the absorption and emission

spectra reported in figure 1a, is thermally evaporated on top of a DBR made of 7 couples of

TiO2/SiO2 (85 nm/120 nm) deposited on glass. In order to study BSWP propagation properties, we

use a leakage radiation microscope (Fig. 1b), coupled to a monochromator and CCD camera, which

allows energy-resolved spatial- and momentum-mapping. The excitation beam is focused on the

sample from the organic deposition side, avoiding the coupling of the excitation beam with the

propagating optical mode. Emission is detected from the other side of the sample by a high

numerical aperture (N.A.=1.49) microscope objective, that allows detection beyond the critical

angle.

a

b

Figure 1 | Material and optical setup. a, Chemical structure, absorption and emission spectra of an evaporated layer of

Lumogen Red F305 on a glass substrate. b. Illustration of the leakage radiation microscope setup.

Figure 2 shows the emission spectrum beyond the critical angle. The strong coupling between the

Lumogen and the BSW mode is clear from the comparison of the bare optical mode with the BSWP

dispersion. The experimental data are backed up by transfer matrix calculations (parameters given

in the Method Section) superimposed on the same plot. The polaritonic mode (orange line) diverges

from the bare optical BSW (blue line) showing the anticrossing behavior typical of polaritons. The

short-propagating evanescent modes associated to the limit of the DBR optical band-gap, also

visible in Fig. 2 at low energies and high momenta, confirm the excellent agreement of the

experimental data with the transfer matrix calculations of the structure.

The BSWP is highly sensitive to optical indexes perturbation at the surface, therefore any material

inhomogeneity in space would result in a broader BSWP dispersion. The sharpness of the

polaritonic mode, well clear in the not saturated emission image reported in the inset of Fig. 2, is a

first hint of the high local homogeneity of the organic deposition. Although the Q factor of the BSW

mode cannot be extracted from these measurements given the linewidth is below the spectral

resolution of the setup, it can however be obtained from the polariton propagation lengths and group

velocities, as explained later in this section and in the Supporting Information, confirming the high

quality of the structure.

The flow of polaritons is shown in Fig. 3 by filtering in momentum space only the region that

includes the BSWP, cutting out most of the leaky modes of the DBR at higher k-wavevectors. The

energy-resolved polariton propagation along the x-direction, upon non-resonant laser excitation at

position x0 = 0, can be observed in Fig. 3a. The overall polariton propagation length d, defined as

, evaluated fitting the energy-integrated decaying signal, is measured to be 120 m, more

than one order of magnitude higher than inorganic planar microcavities of similar Q factors.24

Figure 2 | Dispersion of the BSWP. Saturated experimental emission (black background color) superimposed to

calculated dispersions (grey background color) beyond the critical angle. The light cone is shown with a red line, while

the blue line indicates the bare optical BSW and the yellow line the exciton energy (2.13 eV). The theoretical BSWP

(orange line) fits perfectly the experimental results. In the inset the not saturated experimental emission is reported with

the theoretical fitting of the BSWP dispersion.

In addition, a close inspection of the spectra in Fig. 3a reveals that BSWPs relax towards lower

energies during propagation. Indeed, differently from light in bare optical BSWs,34,38

polaritons, as

interacting particles, undergo scattering and relaxation. This is the first time that relaxation along

the polariton branch is directly observed in organic materials, setting the time span for such effect.

This feature is traced with white dashed lines in Fig. 3a.

Three different cross sections of the energy-resolved polariton propagation, corresponding to

different exciton fractions of the polariton mode39

, are shown in Fig. 3b. At high energies (1.98 eV),

propagation distances are relaxation limited: here the excitonic component is relevant (>22%) and

polaritons relax towards lower energy states during their time of flight, limiting the propagation

length to 20 microns. In principle the presence of non-radiative channels could also contribute to

reduce the propagation distance, yet two factors exclude this possibility: the high emission

efficiency of the dye under consideration40

and the bending of the BSWP emission towards lower

energies at longer times. This is also confirmed by the presence of an initial rise of the population,

due to feeding from higher-energy polariton states, as seen in Fig. 3a and in the cross section at

1.88 eV (red line in Fig. 3b), showing propagation distances of 200 m. In the energy range of

maximum emission intensity, around 1.92 eV (shifted with respect to the peak intensity emission of

the bare molecule by 80 meV), the BSWP, with an exciton fraction of 12.7%, propagates for lengths

of 120 micron, as shown by the black line in Fig. 3b. In fact, polariton emission can be observed

even for distances of more than 400 m with 10% of signal intensity.

a

b

Figure 3| Polariton propagation. a, Propagation of the BSWP after filtering in momentum-space. The red dashed line

indicates the position of the excitation spot while the white dashed lines are guide to the eyes to highlight the polariton

energy relaxation during the time of flight. b, Cross sections along the propagation directions at 1.98 eV (blue arrow),

1.92 eV (black arrow) and 1.88 eV (red arrow) showing 20 m, 120 m and 200 m of propagation length respectively.

In this last case it is also possible to observe a rise of the signal, due to polariton relaxation, appearing after the shape of

the laser spot at x0 = 0.

Some "residual" emission, which does not propagate, is also detectable close to the excitation beam

position (red dotted line), which is attributed to the short-propagating leaky modes of the DBR

mirror, visible at high k-wavevectors in Fig. 2.

a

b

Figure 4 | Propagation lengths and Lifetimes. a, propagation lengths (black dots) versus energy and exciton fraction;

the blue dashed line indicates the propagation lengths evaluated by TMM calculations which does not consider

relaxation processes and therefore fails to fit the data at high energies, indicating that propagation is relaxation limited

at exciton fractions higher than 12.5% b, Experimental lifetimes (black dots), extracted from the propagation lengths

d(E) and the corresponding BSWP group velocity vg(E), together with the TMM calculated lifetimes (blue dashed line)

showing again a divergence at the same exciton fractions. Relaxation times are shown in red dots.

In Fig. 4a we report the propagation lengths (black dots) as a function of energy and exciton

fraction. In the same plot, the corresponding theoretical lengths, evaluated with transfer matrix

method (TMM), are shown as blue dashed line. Because TMM does not consider any energy

relaxation process between polariton states, it diverges from the experimental results for exciton

fractions above 12.5%. Indeed, as shown by the experimental data in Fig. 3a and Fig. 3b, polariton

relaxation rate increases with exciton fraction, as expected for scattering-assisted processes, and

appears as the dominant decay channel for energies higher than 1.92 eV. In Fig. 4b we display the

polariton lifetimes (pol) at different energies (black dots), calculated from the experimental

propagation length and group velocity (vg), together with the theoretical lifetimes (tmm), in blue

dashed line, which exclude relaxation processes. The flow speeds are calculated directly from the

BSWP dispersion, giving values ranging from 120 m/ps to 250 m/ps (see Supporting Material).

The BSWP lifetimes are ≈ 1 ps, which is long enough to allow the estimation of the polariton

intrabranch relaxation time rel as 1/rel = 1/tmm -1/pol. As shown in Fig. 4b, at exciton fractions

higher than 20%, relaxation times are faster than 300 fs, while this value increases considerably

when lowering the excitonic fraction. At energies below 1.91 eV, with an exciton fraction below

13%, the propagation length is no more relaxation limited and varies in a range from 120 microns

(1.92 eV) to 300 microns (1.86 eV). From the measured propagation distances and group velocities,

it is possible also to extract the quality factor Q of the optical mode at different energies (see

Supporting Materials). At the energy with the maximum emission intensity, the estimated Q-factor

is about 3000, confirming the low-dissipation character of the BSWP compared to planar organic

microcavities (Q≈600).41

On the other hand, this value is not as high as some reported for inorganic

planar microcavities, which have quality factors of some tens of thousands, however they show

propagation lengths of similar extensions,42,43

showing that in our case, the long range propagation

is not only ascribed to a long polariton lifetime, but in particular to a very high speed of

propagation.

Finally, to assess the importance of the organic layer inhomogeneities, we compare these results

with the ones obtained using TDBC molecules as the active layer. Using a DBR with identical

characteristics as those described in the case of Lumogen Red F305, we have deposited a thin film

(52 nm) of PVA:TDBC on top of the structure. The organic layer was deposited by spin-coating

from a solution of methanol:water (1:1 in volume).

a

b

c

Figure 5 | Dispersion and propagation of TDBC-based BSWP. a, Experimental emission superimposed to calculated

dispersions of the TDBC sample. The red line defines the critical angle and the blue line indicates the simulated bare

optical BSW. The orange line is the theoretical fitting of the BSWP mode. b, Space-energy propagation of the

polaritonic mode; the red dashed line indicates the excitation laser position. c, Space-space propagation evidence the

surface inhomogeneities, which strongly limit polariton propagation to values of about 50 m.

In Fig. 5a the emission of the experimental dispersion of the polariton mode is shown superimposed

to the theoretical calculation (see Method Section for more details). Strong exciton-photon

coupling is clearly visible by the deviation of the BSWP (orange line) from the bare optical BSW

(blue line). Exciton at about 2.08 eV is shown with an horizontal yellow line. Fig. 5b shows the

propagation path of the BSWP in the space-energy domain started from the excitation beam

position indicated with the red dashed line. As we have seen in the sample with Lumogen, the

excitonic component reveals itself in the polariton energy relaxation throughout the propagation

distance, resulting in a visible decreasing of energy during the whole polariton emission for

increasing distances from the laser spot. However, differently from the previous case, in which a

single well-defined polariton mode is observed, the BSWP emission of TDBC comes as an

ensemble of polariton branches with slightly different energies, resulting in the inhomogeneously

broadened dispersion spectrum shown in Fig. 5a. The poorer quality of the PVA:TDBC film

compared to the previous one, , clearly visible also in the space-space map of Fig. 5c, inhibits the

polariton propagation to ≈50 microns. Indeed, the bright spots in Fig. 5c are associated to the

presence of strong local inhomogeneities in the organic layer: defects and large aggregates "break"

the BSWP propagation and in addition increase the relaxation rate towards lower energy states (see

Fig. 5b).

Conclusions

In conclusion, we demonstrate long-distance organic polariton propagation on top of a single DBR

structure, coupling the Bloch surface wave of the photonic crystal with the excitons of a thin, highly

homogeneous organic layer. Propagation lengths well beyond one hundred microns are observed,

thanks to the extremely high group velocity and quality factor of the optical mode. The polariton

relaxation along the BSWP manifests as a red-shift of the emission energy, strongly dependent on

the excitonic fraction, during propagation. The BSWP speed exceeds 150 m/ps, which is two

orders of magnitude higher than typical polariton velocities in standard planar microcavity, making

this system well suitable for ultrafast polaritonic flux manipulation and transmission combined with

an easy fabrication process. Indeed, in the present approach, the deposition of the top DBR, strongly

detrimental for the organic layer underneath, is no longer needed. Moreover, this system is

particularly appropriate for surface patterning and lithography of guiding structures directly on the

organic layer. These results can therefore pave the way for a new class of room temperature

polaritonic circuits with low dissipation and fast data-transmission rates.

Methods

Sample fabrication. Titanium dioxide / Silicon dioxide (TiO2 / SiO2) Distributed Bragg Reflectors were deposited on

130 μm thick glass substrates using an e-beam evaporator (Temescal SuperSource2).

The substrate temperature was set at 220 °C and the chamber pressure during the process was 10-4

mbar. The DBR

mirror fabrication process consisted in the deposition of seven TiO2 / SiO2 pairs (dTiO2= 85 nm and dSiO2= 120 nm), with

deposition rates of 3 and 7.2 nm/min, respectively.

The 35 nm thick layer of a perylene derivative (Lumogen Red F305) was deposited onto the SiO2 top layer of the DBR,

by thermal evaporation in a Kurt J. Lesker UHV cluster tool with a base pressure of around 10 -7

mbar, and at a

deposition rate of about 1.0 Å/s.

The polyvinyl alcohol (PVA) (SigmaAldrich) mixed 5,6-Dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulfobutyl)-

benzimidazol-2-ylidene]-propenyl]-1-ethyl-3-(4-sulfobutyl)-benzimidazolium hydroxide, inner salt, sodium salt

(TDBC) (FEW Chemicals) layer was obtained starting from water:methanol (1:1) solution. Set the TDBC concentration

at 0.5 mg/ml, we added PVA in the solution in order to get the optimal deposition thickness (about 52 nm) according to

spin coating parameters.

Characterization. Lumogen Red F305 thin film absorption and emission spectra were measured using commercial

spectrometer and spectrophotometer.TiO2 and SiO2 refractive indexes were evaluated by ellipsometric measurement.

The BSWP dispersions and emissions were revealed using a customized leakage radiation microscope; the excitation

beam is a continuous wave diode laser focalized on the organic layer with a 2 m spot radius; The laser intensity on the

sample surface is limited to 10 W/cm2, preventing organic layer damages. The detection was obtained using an oil

immersion microscope objective (60x, NA=1.49). Two optical configurations in the detection line were adopted in

order to selectively reveal the real- or momentum-space domains; a spectrometer and a high sensitivity CCD camera

were used for collecting energy resolved images.

Theoretical model. The theoretical simulations are obtained by transfer matrix model; thicknesses and refractive

indexes of the layers constituting the DBR were evaluated by ellipsometric measurements. For both active materials, the

organic layers is modeled by a single lorentzian oscillator with a FWHM of 0.5 meV and with an integrated strength

equal to the one experimentally measured (thin film deposited on glass substrate).44,45,46

In TDBC:PVA mixture, the

optical indexes of the final layer consider also the volume fractions of the two components.

The exciton and photon fractions of the polariton field at different energies are obtained by solving the equation of two

coupled oscillators and calculating the mixing angle, defined as:

2

2

1

22

2 2

De

ArcCosDe De

V

where De is the energy detuning between the bare BSW and the exciton, 22 / 2V De and V is the Rabi energy of

the system estimated from the Rabi splitting at zero detuning.

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Acknowledgements

G. L. is grateful to Gianluca Latini for the encouragement at the initial stage of his research path.

This work has been funded by the MIUR project Beyond Nano and the ERC project POLAFLOW

(Grant 308136).

Author contributions

G.L., A.C., F.M and S.G. fabricated the samples. G.L. built the experimental set up, performed the

measurements and fitted the data. G.L., D.B., D.S., L.D. and M.D.G. conceived the project and

contribute to the data analysis and interpretation. G.L., D.B. and D.S. co-wrote the manuscript with

inputs from all other authors. D.S. and G.G. supervised and coordinated the research.

Supporting information

Room temperature organic exciton-polariton flow exploiting high-speed, high-Q

propagating modes

Giovanni Lerario1,2,3

, Dario Ballarini1*

, Alessandro Cannavale2,3

, Federica Mangione1,

Salvatore Gambino1,2

, Lorenzo Dominici1,2

, Milena De Giorgi1, Giuseppe Gigli

1,3, Daniele

Sanvitto1

1 NNL, Istituto Nanoscienze - CNR, Via Arnesano, 73100, Lecce, Italy

2 CBN-IIT, Istituto Italiano di Tecnologia, Via Barsanti, 73100, Lecce, Italy

3 Università del Salento, Via Arnesano, 73100 Lecce, Italy

* email: dario.ballarini@nano.cnr.it

Correction for polariton expansion

The measurements reported in Fig. 3b are taken with the monochromator slits partially closed to

allow energy resolution. The expansion in the direction orthogonal to the flow introduces therefore

a small underestimation of the signal intensity depending on the distance from the excitation area.

To take into account for the lateral expansion, a correction factor F(λ) (1 < F(λ) < 1.4), obtained by

dividing the total counts in the space-space domain by those detected in the energy-space domain, is

applied in the evaluation of the propagation distances.

Quality Factor

The quality factor Q of the optical mode coupled with the molecular excitons is defined by the ratio

Q=E/ΔE, where E is the energy (ΔE is the linewidth) of the BSW. Given that after the deposition

we have directly access only to the coupled mode, whose energy linewidth is given by

, where α and β (γBSW and γex) are the photon and exciton fraction (linewidth),

respectively, the Q factor of the optical mode can be retrieved from .

The dashed blue line in Fig. S1 reports the values Q of the bare BSW versus energy,

showing a quality factor Q≈3000 for all the energy range of interest.

Figure S1 | Quality factor. Ratio E/E of the bare BSW as a function of energy evaluated from the polariton linewidth

and exciton fraction.

Group Velocity

In Fig. S2 the group velocities of the BSWP vs energy are reported. In our range of analysis the

group velocity exceeds 120 um/ps and, when close to the light-line, it reaches values beyond the

light velocity in the organic medium.

Figure S2 | Group velocity. Group velocity obtained directly from the first derivative of the dispersion.