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1 Polariton-mediated energy transfer between organic dyes in a strongly coupled optical microcavity David M Coles Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, United Kingdom Niccolo Somaschi School of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom and IESL-FORTH, P.O. Box 1527, Heraklion, Crete, Greece Paolo Michetti Institute of Theoretical Physics, Technische Universität Dresden, Dresden, Germany Caspar Clark Helia Photonics, Rosebank Park, Livingston, West Lothian EH54 7EJ, United Kingdom Pavlos G Lagoudakis School of Physics & Astronomy, University of Southampton, Southampton, SO17 1BJ, United Kingdom Pavlos G Savvidis IESL-FORTH, P.O. Box 1527, Heraklion, Crete, Greece and Department of Materials Science and Technology, University of Crete, Heraklion, Crete, Greece David G Lidzey Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, United Kingdom NATURE MATERIALS 1

2 I. TIME RESOLVED DECAY OF J-AGGREGATE EXCITONS Time-resolved PL decay measurements were performed using a frequency doubled Ti:Sapphire laser (400 nm, 80 MHz) with emission recorded using a Hamamatsu C5680 streak camera with 2 ps resolution. The time-resolved emission of TDBC and NK-2707 in gelatine are shown in Supplementary Fig. S1a and b respectively (black circles). The amplitudes of the decay components are also summarised in Supplementary Table I. In the data presented, the concentration (in gelatine matrix) of the individual aggregates are the same as used in the 3:1 blend film (see Fig. 1 caption in the main text). We find that the TDBC decay is composed of fast and slow decay components having a lifetime of 7 ps and 21 ps. The fast component has been attributed to exciton-exciton annihilation that occurs at high excitation density. 1 5 The NK-2707 is longer lived and also has fast and slow decay components of 12 ps and 59 ps. The decay dynamics of TDBC and NK-2707 in the thin-film blend are shown as red circles in Supplementary Figs. S1a and b respectively. For TDBC, we find that the fast component of the decay is identical (within the resolution of the detector) in both a pure FIG. 1. Fluorescence decay dynamics of J-aggregate films. Time decays of a TDBC (at 587 nm) and b NK-2707 (at 636 nm) in single component (black circles) and blended (red circles) films. 2 NATURE MATERIALS

3 TDBC NK-2707 Fast Slow Fast Slow Single τ 1 =7 ps τ 2 =21 ps τ 1 =12 ps τ 2 =59 ps A 1 =0.92 A 2 =0.28 A 1 =0.26 A 2 =0.72 Blended τ 1=7 ps τ 2 =26 ps τ 1 =11 ps τ 2 =39 ps A 1 =0.93 A 2 =0.11 A 1 =0.47 A 2 =0.53 TABLE I. Decay constants of the J-aggregate emission in a single and blended films film and in the TDBC:NK-2707 blend. The slow component of the decay is slightly longer in the blend film (although the change is of the same order as the streak camera resolution). This indicates that excitons do not undergo energy transfer from the TDBC to the NK-2707 which would manifest itself as a reduction in the TDBC lifetime in the blend film. In the case of the NK-2707, the fast decay component is similar in both the pure NK-2707 film and in the blend. Note however there is a distinct shortening of the slow decay component of the NK-2707 decay in the blend. The reason of this is not understood, however we speculate that the size of the aggregate domains may differ in the blend film compared to the pure film. If the NK-2707 aggregates are smaller in the blend, then there may well be additional quenching processes that occur that are associated with electronic states at the edge of an individual aggregate. NATURE MATERIALS 3

4 II. PHYSICAL STRUCTURE OF FILM CONTAINING TWO J-AGGREGATE DYES We have used atomic force microscopy (AFM) imaging to explore the surface structure of a film containing a blend of TDBC and NK-2707 in gelatine as described in the main body of the paper. A typical scan is shown in Supplementary Fig. S2, together with images of gelatine-tdbc and gelatine-nk-2707 reference films. It can be seen that in all cases, a similar, complex film morphology is observed, that seems to imply the existence of semicrystalline domains having a range of length-scales (typically 500 nm and below). While this is commensurate with previously observed J-aggregate morphology, 6 it is not possible to determine whether these are individual physical J-aggregates (distinct from coherently coupled molecules which define the optical properties of the aggregate, which has been estimated to be between 10 and 50 molecules depending on material system. 7 9 ). We see no obvious fine structure on the scale of the calculated Förster radius (which is at the limit of the AFM resolution). 4 NATURE MATERIALS

5 FIG. 2. AFM images of gelatine films doped with J-aggregate dyes µm AFM tapping mode topology scans of gelatine doped with a, TDBC b, NK-2707 and c, both TDBC and NK The concentrations in all cases are the same as used in the microcavity presented in the main text. d-e, As a-c with a scan area of 2 2 µm. NATURE MATERIALS 5

6 III. CORRECTION OF THE MEASURED ABSORPTION SPECTRA The absorption spectra we calculate from the reflectivity of the cavities represent the total absorption of the sample (i.e. absorption in the active region and absorption in the mirrors). We correct the measured absorption by using a transfer matrix method to calculate the total absorption of the structure (A total ) and the absorption that occurs in the active layer alone (A active ). The fraction of light absorbed in the active layer is given by A active /A total. We use this factor to correct the measured absorption spectra before calculating the relaxation efficiency. The calculated absorption of the microcavity presented in the main text is shown in Supplementary Fig. S3. Parts a and b show the absorption in the active layer and the total absorption of the structure respectively. The correction factor for each branch is shown in part c. The same transfer matrix approach is used to correct all of the data presented in the supplementary information (with each structure being modelled separately). We use the n and k dispersion of silver given by Palik, 10 and a Lorentzian oscillator model to create the dispersions for the J-aggregate layers using the absorption spectra shown in the main text (Fig. 2a). 6 NATURE MATERIALS

7 FIG. 3. Total and active layer absorption of microcavity. a, Calculated absorption in the active layer of the microcavity structure presented in the main text. b, Calculated total absorption. c, Correction applied to observed cavity absorption when calculating the efficiency presented in the main text. NATURE MATERIALS 7

8 IV. HYBRID ORGANIC MICROCAVITY BASED ON TWO PHYSICALLY SEP- ARATED J-AGGREGATE LAYERS To completely exclude the possibility of energy transfer between the J-aggregates, a microcavity was prepared in which the two J-aggregate layers are spatially separated. The structure consisted of a 200 nm thick silver mirror, a 200 nm thick film of NK-2707 in gelatine, a 205 nm thick film of PMMA (spincast from a toluene solution), a 200 nm thick film of TDBC in gelatine and a 35 nm thick silver mirror. As the J-aggregated films used here were thinner than that used in the blend system, the concentration of cyanine dyes used was three times greater than those used in the blend film to maintain a similar total oscillator strength. This created a 3λ/2 cavity with similar detuning to that of the blend film (photon 75 mev below the NK-2707 exciton and 255 nm below the TDBC exciton at normal incidence). In the cavity, we evidence a Rabi splitting energy of 144 mev between the LPB and MPB, and a splitting of 226 mev between the MPB and UPB. The angular resolved PL is shown in Supplementary Fig. 4a. It can be seen that the emission profile is very similar to that seen in the blend-film cavity (compare with main text, Fig. 3a) with the emission from the LPB being more intense than that of the UPB or MPB. Some differences in the multilayer film cavity are however evident. In particular, emission from the TDBC exciton reservoir emission is relatively stronger than is observed in the blend-film cavity. Such effects are ascribed to disorder within the cavity. The LPB emission is also broader, having a linewidth of 16 nm at normal incidence compared to 10 nm in the cavity containing the blend film. This again likely to result from increased disorder and surface roughness in the organic layers, as is expected in structures composed of three consecutively spin-cast layers. The population distribution (see Supplementary Fig. S4b) is also similar to that of the cavity containing the blend film, with the LPB and MPB both supporting a larger population of states at increasing angle. The UPB population is however relatively suppressed in the cavity with separated layers. The reason for this is not known. The polariton mixing fractions of the UPB, MPB and LPB are shown in Supplementary Fig. S4c, d and e respectively. The PLE signal collected at the bottom of the LPB from the microcavity with spatiallyseparated J-aggregate films is shown in Supplementary Fig. S5a. The PLE signal is again almost identical to that recorded from the cavity containing the blend of cyanine dyes (main 8 NATURE MATERIALS

9 FIG. 4. Hybrid polariton emission, population and mixing co-efficients. a, Angular dependent microcavity (with separated J-aggregate layers) emission following excitation at 473 nm. White dotted lines are polariton branch energies, while exciton and photon energies are shown with black and red dotted lines respectively as calculated from equation (1) in the main text. b, Polariton population distributions. The shaded regions represent the experimental uncertainty. Polariton branch mixing fractions for c UPB, d MPB and e LPB. text, Fig. 4a) with all features being reproduced (see main text for details). Absorption and efficiency plots (Supplementary Fig. S5b and c respectively) closely resemble those determined in the cavity containing the blend film. We therefore conclude that none of the spectral properties determined in the cavity containing the blend of cyanine dyes result from direct Förster energy transfer. We note that the efficiency (and all other efficiencies shown) is calculated with the cavity absorption corrected for the absorption in the metal mirrors (see section Correction of the measured absorption spectra for discussion). NATURE MATERIALS 9

10 FIG. 5. Angular and wavelength dependent photoluminescence excitation of the k =0 lower polariton branch state, polariton absorption and relative relaxation efficiency into the LPB ground state. a, PLE signal as a function of angle and wavelength recorded at k = 0 on the LPB for a microcavity having spatially separated J-aggregate layers. b, Angular dependent absorption. c, Angular dependent relative relaxation efficiency to k = 0 on the LPB. The magnitude of the PLE, absorption and efficiency for each branch are shown in parts d, e and f respectively. The shaded regions represent the experimental uncertainty. 10 NATURE MATERIALS

11 V. EMISSION AND PLE MEASUREMENTS ON MICROCAVITIES HAVING DIFFERENT EXCITON-PHOTON DETUNINGS We have measured PL emission and PLE data for cavities having different exciton-photon detunings. Cavities were created by varying the thickness of the organic layer. In all cases the organic layer composition and mirror thicknesses are the same as described in the main text. Supplementary Fig. S6 shows angular dependent photoluminescence maps, population distributions and mixing fractions of the polariton branches for four cavities having a detuning (i) = ( 60, 207) mev, (ii) = ( 6, 166) mev, (iii) = (127, 32) mev and (iv) = (83, 240) mev where = (E γ (0) E ex1,e γ (0) E ex2 ) as determined through a coupled oscillator fit. It can be seen in parts (i)a and (ii)a in which the photon mode energy is negatively detuned from both exciton energies that the cavity emission occurs most strongly from the LPB, with emission from the MPB being far less intense, with very little emission observed from the UPB. The polariton population is shown in parts (i)b and (ii)b. It can be seen that the polariton population is greatest along the lower polariton branch. In cavity (iii), the cavity mode energy lies between the exciton reservoirs at normal incidence. This greatly reduces the mixing between the two excitons in the branches as can be seen in (iii)c-(iii)e. With the increasingly positive detuning, the emission intensity from the MPB becomes greater with respect to the LPB, as can be seen in part (iii)a, with the MPB population exceeding that of the LPB at large angles as can be seen in (iii)b. When the photon mode is positively detuned above both exciton states, there is very little mixing in the MPB, with almost all of the emission originating from the MPB as seen in (iv)a. In part (iv)b it can be clearly seen that the polariton population in the MPB exceeds that of the LPB for all angles. We find therefore that as the exciton mixing fraction in the MPB decreases with increasing positive detuning (i.e. from cavity (i) to (iv)), the relative population of polaritons found in the MPB increases, indicating that scattering of polaritons from the MPB to the LPB (occurring via the ex1 reservoir) is suppressed. In Supplementary Fig. S7 we show the lower branch PLE, cavity absorption and relaxation efficiency to LPB for the same cavities shown in Supplementary Fig. S6. Cavity (i) is similar in detuning to the cavity presented in the main text, with the efficiency of scattering to the LPB (part (i)f) is strongest from the MPB close to the ex1 reservoir (the absorption of the MPB at small angles goes below the detection limit hence the efficiency NATURE MATERIALS 11

12 FIG. 6. Hybrid polariton emission, population and mixing co-efficients for a series of cavities with different exciton-photon detunings. The detuning of each cavity is described by the parameter = (X, Y ) where X = E γ (0) E ex1 and Y = E γ (0) E ex2. The detunings shown are i =( 60, 207) mev, ii =( 6, 166) mev, iii = (127, 32) mev and iv = (83, 240) mev. a, Angular dependent microcavity emission following excitation at 473 nm. White dotted lines indicate the position of the polariton branches, while exciton and photon energies are shown with black and red dotted lines respectively, with all features determined using a fit to the PL spectra using equation (1) in the main text. The color palette is on a linear scale. b, Polariton population distributions. Polariton branch mixing for the c UPB, d MPB and e LPB. PL characterization of the cavity was performed at room-temperature following the method described in Ref. 11. becomes un-physically large). Cavity (ii) is less negatively detuned with the cavity mode 12 NATURE MATERIALS

13 almost resonant with the ex1 reservoir at normal incidence. From the relaxation efficiency shown in part (ii)f we see that the MPB and LPB are now similar in magnitude at all angles, indicating a relative suppression of the MPB relaxation efficiency due to the reduced mixing. The relaxation efficiency of the UPB remains weak. In cavity (iii), we do not observe the LPB as it has an almost flat dispersion that results in the excitation and collection energies becoming almost degenerate. Here, the MPB relaxation efficiency (see part (iii)f) is greater than that of the UPB except at large angles. In cavity (iv) (the most positively detuned structure examined) we now find that the relaxation efficiencies of MPB polariton states are smaller in magnitude than those of the UPB states (see part (iv)f). Our results show therefore that as the exciton mixing in the MPB is reduced (going from cavity (i) to (iv)), there is a general reduction in the relaxation efficiency of MPB states towards the LPB. In the most positively detuned cavity, the relaxation efficiency of UPB states towards the LPB are are greater than that of MPB states. This should be compared to the cavity in which there is effective mixing between exciton states (cavity (i)), in which the relaxation efficiency of the MPB is very much larger than that of both the UPB and LPB. In Supplementary Fig. S8 we plot the ratio of MPB to LPB populations integrated over all angles studies as a function of exciton mixing in the MPB. We quantify the mixing by calculating the product of the two exciton components in the MPB ( αex1 MPB 2 αex2 MPB 2 ) for each angle and taking the maximum value. We see that as the maximum mixing increases, the population of the LPB relative to the MPB increases linearly. NATURE MATERIALS 13

14 FIG. 7. Angular and wavelength dependent photoluminescence excitation of the k =0 lower polariton branch state, polariton absorption and relative relaxation efficiency into the LPB ground state for a series of cavities with different exciton-photon detunings. The detunings shown are i =( 60, 207) mev, ii =( 6, 166) mev, iii = (127, 32) mev and iv = (83, 240) mev. a, Angular dependent PLE signal recorded at k = 0 on the LPB. b, Angular dependent absorption measured simultaneously with PLE. c, Angular dependent relative relaxation efficiency to k = 0 on the LPB. The magnitude of the PLE, absorption and efficiency of each branch are shown in parts d, e and f respectively. The shaded regions represent the experimental uncertainty. PLE characterization of the cavity was performed at room-temperature following the method described in Ref NATURE MATERIALS

15 FIG. 8. Ratio of LPB to MPB integrated population as a function of exciton mixing in the MPB. The total excitonic component of the MPB is calculated, and the maximum value taken for a figure of merit of the exciton mixing. This is plotted against the ratio of polariton populations in the MPB and LPB integrated over all angles studied. NATURE MATERIALS 15

16 VI. TIME RESOLVED HYBRID-POLARITON EMISSION To determine angular-dependent cavity decay dynamics, the PL emission was collected using a microscope objective with the Fourier plane imaged onto the entrance slit of a monochromator. Here a horizontal slit was used to allow light from collected from a small range of k-space to pass to the streak camera (2 ps resolution) where this emission was spectrally and temporally resolved. In Supplementary Fig. S9 we show the time resolved emission decay of the LPB (black triangles) and MPB (red circles) at emission angles of (a) 0 o, (b) 18 o and (c) 35 o. We find that the decay of both polariton branches can be fit at all angles using a biexponential decay. The amplitudes of the polariton branch decay curves are also summarised in Supplementary Table II. It is not possible to make a comprehensive comparison between PL decay rates recorded in a control film with that of a cavity, as the excitation density in both structures is likely to be different, with previous work showing that the decay lifetime of J-aggregates is highly dependent on density-dependent processes such as exciton-exciton annihilation. 1 5 Nevertheless using the data shown in Supplementary Tables I and II, it is possible to make some general observations. In particular, we find that long component of the decay lifetime of the ex1 (NK-2707) reservoir (39 ps) is commensurate with that of the long component of LPB polariton decay which takes values between 36 and 42 ps. Similarly, the long component of the decay lifetime of the ex2 (TDBC) reservoir (26 ps) is close to that of the MPB, which takes values of 26 ps at 18 o and 19 ps at 35 o. This behaviour is consistent with previous work in which it has been shown that the dynamics of polariton decay are largely governed by the dynamics of the exciton reservoir. 12 This is because the rate at which polaritons are populated by excitons is relatively slow, and thus the relative fraction of polariton states at any one time is small (order 10 4 in J-aggregate systems). Because of this, the strong coupling regime does not (at least at low excitation density) modify the overall recombination dynamics of the exciton reservoir. Our results thus indicate that the polariton population in the LPB and MPB states mostly originate by direct exciton scattering from the ex1 and ex2 reservoirs respectively. Notably however, we find that MPB states close in energy to the ex1 reservoir have a progressively longer lifetime (around 31 ps at 0 o ). As the decay lifetime of the ex1 reservoir (NK-2707) is significantly longer than that of the ex2 (TDBC) 16 NATURE MATERIALS

17 FIG. 9. Fluorescence decay dynamics of hybrid polaritons. Polariton emission decays of the LPB (black triangles) and MPB (red circles) at a 0 o, b 18 o and c 35 o. (39 ps compared to 26 ps), it suggests that such states are partly populated from the ex1 reservoir by thermally assisted scattering and hence their dynamics follow the decay of the ex1 (NK-2707) excitons. This population process is made possible as the energy separation between the MPB and the ex1 at 0 o is < 10 mev; a value less than kt. NATURE MATERIALS 17

18 MPB LPB Fast Slow Fast Slow 0 o τ 1 =6 ps τ 2 =31 ps τ 1 =11 ps τ 2 =36 ps A 1 =1.11 A 2 =0.07 A 1 =0.72 A 2 = o τ 1=6 ps τ 2 =26 ps τ 1 =12 ps τ 2 =42 ps A 1 =1.07 A 2 =0.1 A 1 =0.65 A 2 = o τ 1=5 ps τ 2 =19 ps τ 1 =15 ps τ 2 =41 ps A 1 =0.98 A 2 =0.12 A 1 =0.74 A 2 =0.21 TABLE II. Decay constants of the polariton branch emission in a hybrid cavity 18 NATURE MATERIALS

19 VII. CALCULATING CAVITY EMISSION USING A SIMPLE MODEL FOR PO- LARITON TO EXCITON SCATTERING In the main text of the paper we have described the use the PLE data to determine an effective polariton to exciton relaxation rate R along the three polariton branches. This data was then used in the fit shown in Fig. 5. An alternative approach to using measured data to determine the relaxation rate is to instead assume that the polariton to exciton relaxation rate is simply dependent on the relative exciton fraction of each branch. Our assumption is based on the fact that relaxation of a polariton is accompanied through the release of energy, and that this process is facilitated by interaction with the local vibrational environment. This process is enhanced when a particular polariton state is more exciton-like. We therefore assume the relaxation of UPB, MPB and LPB polaritons is proportional to αex2 UPB 2, αex1 MPB 2 and αex1 LPB 2 respectively. Note, that we only account for the ex1 fraction mixed into the MPB, as it is to this exciton-reservoir to which MPB polaritons undergo relaxation. FIG. 10. Modelling of the polariton emission using the excitonic component. Observed polariton emission intensity (open symbols) following non-resonant cw excitation at 473 nm, and modelled intensity (solid lines) for the a UPB, b MPB and c LPB. In Supplementary Fig. S10, we plot αex2 UPB 2, αex1 MPB 2 and αex1 LPB 2. We use the polariton exciton fractions to re-model the PL emission from the UPB, MPB and LPB as shown in Supplementary Fig. S11. Again, it can be seen that the model is able provide a good fit to the experimental data. The fitting parameters are listed in Table III, and show good NATURE MATERIALS 19

20 qualitative agreement with the parameters given in the methods section of the main text. FIG. 11. Modelling of the polariton emission using the excitonic component. Observed polariton emission intensity (open symbols) following non-resonant cw excitation at 473 nm, and modelled intensity (solid lines) for the a UPB, b MPB and c LPB. C 1 = (218 ps) 1 C5 LPB = (236 fs) 1 p 1 =1.93 C 2 = (908 ps) 1 C5 MPB = (3.2 fs) 1 p 2 =0 C 3 = (42 ps) 1 C 4 = (35 fs) 1 C UPB 5 = (39 fs) 1 TABLE III. Fitting parameters used to produce Supplementary Fig. S NATURE MATERIALS

21 Current address: Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom 1 Brumbaugh, D. V., Muenter, A. A., Knox, W., Mourou, G. & Wittmershaus, B. Singlet exciton annihilation in the picosecond fluorescence decay of 1, 1 -diethyl-2,2 -cyanine chloride dye j-aggregate. Journal of Luminescence 3132, Part 2, (1984). 2 Sundström, V., Gillbro, T., Gadonas, R. A. & Piskarskas, A. Annihilation of singlet excitons in j aggregates of pseudoisocyanine (pic) studied by pico- and subpicosecond spectroscopy. The Journal of Chemical Physics 89, (1988). 3 Stiel, H., Daehne, S. & Teuchner, K. J-aggregates of pseudoisocyanine in solution: New data from nonlinear spectroscopy. Journal of Luminescence 39, (1988). 4 Moll, J., Harrison, W. J., Brumbaugh, D. V. & Muenter, A. A. Exciton annihilation in j- aggregates probed by femtosecond fluorescence upconversion. The Journal of Physical Chemistry A 104, (2000). 5 Akselrod, G. M., Tischler, Y. R., Young, E. R., Nocera, D. G. & Bulović, V. Exciton-exciton annihilation in organic polariton microcavities. Phys. Rev. B 82, (2010). 6 von Berlepsch, H. et al. Supramolecular structures of j-aggregates of carbocyanine dyes in solution. The Journal of Physical Chemistry B 104, (2000). 7 Müller, M., Paulheim, A., Eisfeld, A. & Sokolowski, M. Finite size line broadening and superradiance of optical transitions in two dimensional long-range ordered molecular aggregates. The Journal of Chemical Physics 139, (2013). 8 van Burgel, M., Wiersma, D. A. & Duppen, K. The dynamics of one dimensional excitons in liquids. The Journal of Chemical Physics 102, (1995). 9 Kamalov, V. F., Struganova, I. A. & Yoshihara, K. Temperature dependent radiative lifetime of j-aggregates. The Journal of Physical Chemistry 100, (1996). 10 Palik, E. (ed.) Handbook of Optical Constants of Solids, Volumes I, II, and III (Academic Press - Science & Technology, 1985). 11 Coles, D. M., Grant, R. T., Lidzey, D. G., Clark, C. & Lagoudakis, P. G. Imaging the polariton relaxation bottleneck in strongly coupled organic semiconductor microcavities. Phys. Rev. B NATURE MATERIALS 21

22 88, (2013). 12 Lidzey, D. G. et al. Experimental study of light emission from strongly coupled organic semiconductor microcavities following nonresonant laser excitation. Phys. Rev. B 65, (2002). 22 NATURE MATERIALS