Ultrafast Dynamics of Polariton Cooling and Renormalization in an

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Ultrafast Dynamics of Polariton Cooling and Renormalization in an Organic Single Crystal Microcavity under Non-Resonant Pumping Kenichi Yamashita, Uyen Huynh, Johannes M. Richter, Lissa Eyre, Felix Deschler, Akshay Rao, Kaname Goto, Takumi Nishimura, Takeshi Yamao, Shu Hotta, Hisao Yanagi, Masaaki Nakayama, and Richard H. Friend ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00041 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Ultrafast Dynamics of Polariton Cooling and Renormalization in an Organic Single Crystal Microcavity under Non-Resonant Pumping

Kenichi Yamashita1,2*, Uyen Huynh1, Johannes Richter1, Lissa Eyre1, Felix Deschler1, Akshay Rao1, Kaname Goto2, Takumi Nishimura2, Takeshi Yamao3, Shu Hotta3, Hisao Yanagi4, Masaaki Nakayama5, and Richard H. Friend1

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Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK

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Faculty of Electrical Engineering and Electronics, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

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Faculty of Materials Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan

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Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan

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Department of Applied Physics, Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan

Abstract Microcavity systems with organic luminescent materials have a hot prospect for room-temperature cavity-polariton devices. The polariton dispersion relation of organic microcavities is significantly different from that of inorganic microcavities due to the strong localization of Frenkel exciton. Also photo-excited particles will undergo a different cooling mechanism until they reach the polariton ground state. In the characterization of efficient polariton condensate, therefore, the polariton cooling dynamics as well as the kinetics of polariton eigenstate should be measured. Here we present experimental studies on ultrafast dynamics of cavity polaritons in an organic single crystal microcavity under non-resonant pumping. In time-resolved photoluminescence measurements we

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observed, for the first time, an ultrafast dynamics of stimulated cooling of the organic cavity polariton. Transient transmission measurement enabled us to investigate the detailed renormalization dynamics of polariton eigenstate. The results clearly demonstrated the prospect of organic microcavity for the room-temperature polaritonic device.

Keywords Polaritons, Light-Matter Interactions, Microcavity, Rabi Splitting, Organic Crystals, Ultrafast Spectroscopies

A strong coupling between an exciton dipole and a cavity photon mode in an optical microcavity results in emergence of a half-light and half-matter quasiparticle known as cavity polariton1-3. High-temperature and low-threshold Bose-Einstein condensate is possible for the cavity polariton particles due to their bosonic nature and extremely small effective mass4-6. In contrast to low-bandgap III-V inorganic semiconductors, wide-bandgap inorganic semiconductors like GaN7,8 and ZnO9, perovskite nanoplatelet10, and organic semiconductors11-15 are attractive for the room-temperature polariton condensate because the excitons formed in those materials have large binding energies and are stable even at the room-temperature. The organic compounds are particularly attractive because (i) the large transition dipole moments can make a strong coupling readily and (ii) a large Rabi splitting energy is considered to inhibit the collapse of strong coupling even at a high-density polariton population. Previous studies demonstrate that the room-temperature polariton condensates are possible with various organic materials such as molecular crystal11, π-conjugated polymer film12, evaporated oligomer molecule13, fluorescent protein film14, and molecular dye dispersed in a matrix polymer15. The polariton cooling dynamics have also gathered interests, especially in the microcavities with the J-aggregate of cyanine dyes16-19, because the rapid thermalisation into the polariton ground state plays an important role for the efficient condensate3. The polariton cooling mechanisms have been well studied in inorganic semiconductor

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microcavities2,4,20. In most of the low-bandgap semiconductor microcavities, the initial state of excitation is a pair of free carriers or one hot exciton with large k. The excited particles are relaxed energetically along the exciton-like dispersion curve with acoustic and optical phonon interactions and reserved at the inflection point of polariton dispersion curve due to the sudden decrease in the density of state. The ‘stimulated’ cooling via the exciton-exciton (or polariton-polariton) scattering from the reservoir state induces coherent condensate into the polariton ground state. Some recent papers suggest that these cooling pathways result in the tens to hundreds ps delay in the ground state population21,22. Furthermore the ns-order renormalization dynamics of the polariton eigenstates have also been presented23,24. Different polariton cooling mechanism should be considered for the organic microcavity systems as the Frenkel excitons have large effective masses as compared to that of the Wannier exciton and thus their dispersion is almost flat in k space3. The initial state of the excitations under the non-resonant pumping is also different, i.e. the pumping photons are absorbed by a molecular vibrational replica of the singlet exciton (S1) state, generating hot excitons. The hot excitons can be quickly relaxed to the energy minimum of S1 state, which behaves as a non-radiative ‘dark’ state and as reservoir of the excitons to supply the polariton particles25. Some recent papers suggest that the excitons in this reservoir state can be directly cooled to the polariton ground state radiatively or with emission of localized molecular vibration25-27. There are, however, few studies on the direct observation of the cooling dynamics17 even though some groups have discussed them under the ‘resonant’ pumping scheme18,19,28. It remains an open question whether the cooling dynamics includes a ‘stimulated’ process. Furthermore renormalization of the polariton eigenstates caused by high-density polariton population might give a negative impact on the preservation of strong coupling. This also has been unclarified yet with the time-domain measurements. In this article we present ultrafast polariton dynamics in a molecular single crystal microcavity studied by time-resolved photoluminescence (TRPL) and transient transmission (TT) spectroscopic techniques with the non-resonant optical pumping. The results of TRPL clearly showed the emergence of stimulated cooling into the polariton ground state. The TT results exhibited two

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distinguishable polariton renormalization kinetics that we attributed to the stimulated polariton cooling and the long-time accumulation in the exciton reservoir. These results would be very intriguing to achieve fully understanding and control of the room-temperature condensate of organic cavity polaritons.

Results and Discussion Material, Device & Steady State Optical Properties The material used here is an organic single crystal of 2,5-bis(4'-cyanobipheny-4-yl)thiophene referred as BP1T-CN (Fig. 1a)29,30. The large transition dipole moment (12.5 Debye) lies on the long molecular axis31. In a platelet-like single crystal (CCDC registration code 897042), the long axis of each molecule is strongly oriented to one of the in-plane direction of the crystal surface (Ref. 29, Fig. 1b and Fig. S1 in Supporting Information). This in-plane molecular orientation can maximize the coupling strength with the cavity modes polarized to the same direction. More importantly we can readily obtain the top and bottom surfaces with a high optical flatness even in the as-grown crystals. The orientations of the surface planes correspond to the crystallographic {001} planes29. This means that the top and bottom surfaces of the as-grown crystal are parallel each other and thus the crystals are ideal as the active media with a constant cavity length. The spatial uniformities in the photonic and excitonic potentials are key requirements to avoid polaritonic potential disorder inhibiting the formation of spatial coherence among the polariton particles5,32. Therefore we consider that BP1T-CN single crystal is a very promising material for cavity polariton studies. The microcavity studied here is shown schematically in Fig. 1c. The bottom distributed Bragg reflector (DBR) is a dielectric thin-film multilayer of SiO2 and Ta2O5 deposited on a silica substrate. A platelet-like single crystal with the thickness of ~510 nm (Fig. 1d) was simply placed on the top surface of the bottom DBR. Closed contact between the DBR and the crystal is easily obtained due to the electrostatic force. The top DBR is a multilayer of SiO2 and HfO2. Use of a physically robust organic crystal as the cavity material enables a direct sputtering deposition of the top DBR. The reflection bands of the bottom and top DBRs cover the exciton energy (Eex ~ 2.7 eV) and almost the

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whole of emission band (see Fig. S2 in Supporting Information). In recent studies 31,33, we have already confirmed that the BP1T-CN crystal microcavities are in the strong coupling regime that is owing to the large transition dipole moment and the strong in-plane molecular polarization. Steady-state PL and transmission spectra in Fig. 2a exhibit three lower polariton branches (LPB1, LPB2, and LPB3) at energies below Eex. The emergence of multiple polariton modes is due to the multimode photonic resonance in the cavity with a length of ~510 nm. The angular dependence of the LPBn energies (Fig. 2b) enables us to evaluate the coupling strength in this cavity. While the upper polariton branches (UPBn) are unobservable in both the PL and transmission measurements, probably due to the ultrafast energy relaxation and the strong absorption by the continuous band at E > ~2.7 eV, respectively, the dispersion of LPBn can be accounted for by the coupled oscillator model (see also Fig. S5 in Supporting Information). The Rabi splitting energy ℏ estimated to be ~188 meV (see Fig. 2c) is consistent with the recent results31,33, and would be moderate for room-temperature polariton studies11-15. The polariton lifetimes evaluated from the spectral linewidths of transmission peaks are in a range of 550 – 650 fs (see Fig. S4 in Supporting Information). These polariton lifetime have been verified in detail as shown in Sec. 4 of Supporting Information. The cavity photon lifetimes for the LPB1, LPB2 and LPB3 modes are estimated from the modeled quality factors (Q ~ 413, 5750, and 9960) to be 119, 1520, and 2480 fs, respectively. The exciton decoherence time has been roughly deduced from these results to be in a range of 170 – 470 fs, which is acceptable for organic material34. To estimate cavity photon lifetime, we have accurately evaluated the group refractive index dispersion as described in Sec. 3 of Supporting Information. As a result, the polariton lifetimes mentioned above can be explained well by the average of cavity photon lifetime and exciton decoherence time weighted by the Hopfield coefficients. These consistent experimental results ensure the strong coupling formation in the BP1T-CN microcavity. Actually, when we replaced the BP1T-CN crystal with a passive polymer thin film as the cavity material, the energy of transmission mode became more dispersive than that in the microcavity sample (see Sec. 6 in Supporting Information). This is due to a small effective refractive index (~1.3) in the passive polymer cavity and the weak coupling regime.

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Ultrafast Time-Resolved Photoluminescence To track the polariton cooling process in this organic microcavity, we performed room-temperature ultrafast TRPL measurement using a transient grating method35. The microcavity was pumped non-resonantly at 400 nm (~3.1 eV). The results are summarized in Figs. 3a-3c and shown also by contour maps in Fig. S7 in the Supporting Information. As with previous studies3,25-27, we assume that the hot excitons generated by the non-resonant pumping relax quickly to the energy minimum of the S1 state and that particles are provided from this exciton reservoir state to the LPBn states. The long-lifetime kinetics found after ~1 ps (see Figs. 3b and 3c) can account for this assumption, i.e. in a case of incoherent polariton cooling, the temporal profiles for the LPB emissions follow that for the exciton population in the reservoir state because the lifetime of the exciton reservoir is much longer than that of polariton state. As the decay profile shows no distinct variation with the observation angle θ, the polariton branches around k ~ 0 are populated all at once. For the LPB3 mode, however, the decrease in the relative emission intensity (see Fig. 3a) and the earlier rise and decay in the temporal profiles (see the top of Fig. 3b) were observed at θ = 20 º. These features might show existence of a k-dependent polariton cooling in the LPB band as a minor process. At early times, < ~1 ps, the LPB3 mode clearly shows an ultrafast emission component (see Fig. 3c). This ultrafast emission component is not amplified spontaneous emission (ASE) because, as shown in Sec. 8 in Supporting Information, the ASE component of a bare BP1T-CN crystal has ~50 ps time constant,29,36,37 and is two orders of magnitude longer than the result for microcavity sample. We compared the observed emission dynamics with a modelled dynamics for polariton population in the LPB minimum. Numerical simulation for obtaining the modelled dynamics was based on a Gross-Pitaevskii equation for the condensed polariton wavefunction and a rate equations for the exciton density in reservoir state, i.e. this simulation includes the stimulated polariton cooling process3,13,38 (see Sec. 7 in Supporting Information for more details). The kinetics calculated under the below- and above-threshold conditions can completely account for the observed slow and fast components, respectively. The polariton lifetime is deduced from this simulation to be ~130 fs,

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which is comparable to that expected in the steady state transmission measurement (see Fig. 2a). To our knowledge, this is the first direct demonstration of stimulated polariton cooling in an organic microcavity. The possibility of normal photon lasing is negligible because it should occur preferentially at the energy with highest optical gain (~2.48 eV)30,39. Nevertheless, our present study did not show the polariton condensate directly. In order to manifest the polariton condensate, we have to find a condensation threshold in the emission intensity, thermalisation of the emitted photon, and the spatial and/or temporal coherence. However this is not the core of current study and is to be investigated in near future. At present we can state that the polariton ground state is rapidly populated within ~1 ps. This leads to a phenomenon of drastic renormalization dynamics in the polaritonic eigenstates as described in the next section. Another important point in the TRPL results is ‘simultaneous observation’ of the fast and slow components. Similar results can be found also for the LPB2 and LPB1 modes (see middle and bottom of Fig. 3c), in which the observed dynamics are represented by the superposition of the fast and slow components as is the case with LPB3 emission. It is easily expected that, in the steady state PL measurement or in the transient PL method with a conventional time resolution (> ~ps) , the slow decay component will be dominantly observed and thus the ultrafast signal of the polariton population dynamics will be overlooked. This might be a reason of very few reports on the polariton condensate in organic microcavities. The fs-order ultrafast PL measurement must be a useful technique to find the polariton cooling.

Transient Transmission In contrast to the PL measurements, which enable us to evaluate the polariton population, the transmission measurements give us information on the polariton eigenstates. As well as polariton energies, the photon and exciton fractions in a polariton eigenstate, i.e. the Hopfield coefficients40, can also be qualitatively evaluated from the transmission measurements because the intensity of transmitted light is proportional to the square of electric field strength in the cavity. Here we show the time evolution of transmission spectrum under optical pumping, i.e. TT spectroscopy, for the

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BP1T-CN microcavity. The kinetics of the polariton eigenstate obtained in the TT spectroscopy is complementary information to the polariton population dynamics obtained in TRPL. The TT spectroscopy was performed with the same setup with the conventional transient absorption measurement. Figure 4a shows the results of non-resonantly pumped TT measurement for the LPB2 at room-temperature. The signal amplitude ∆/ corresponds to − / , where and are time-resolved transmittance spectra under the pumping and no-pumping conditions, respectively. The sensitive detection of photo-induced change in the transmission is possible by measuring the ∆/ kinetics. Note that a derivative-like spectral feature observed in Figs. 4a and 4e reveals the blue-shift of transmission peak energy by the pumping. This fact can be confirmed also in the raw experimental data (see Fig. S8 in Supporting Information). From the ∆/ signal we reproduced the normalized spectrum varied with the delay time as shown in Figs. 4b and 4f (see Sec. 9 and Fig. S9 in Supporting Information for more details). The LPB1 mode exhibited the similar results as shown in Fig. S10 in Supporting Information, but the TT measurement was difficult for the LPB3 mode because of its small transmittance even at the steady state (see Fig. 2a). The relative changes of the peak energy and peak intensity, ∆ and / , for the spectra are shown in Figs. 4c and 4d, respectively. Two kinetics showing the fast (< ~1 ps) and slow (< ~100 ps) components coexist in both ∆ and /. We have confirmed that, for a passive cavity, in which the organic crystal was replaced by a passive matrix, no ∆/ signal observed in these time regions (see Fig. S11 in Supporting Information). This fact means that the ultrafast signal observed in the organic microcavity was not an experimental background effects like the coherent artifacts. First we focus on the slow component of > ~1 ps (see Fig. 4c). This component exhibits the dynamics of a long-timescale renormalization in the polariton eigenstate with a blue-shift of ~0.7 meV. From the analogy with the TRPL results we can attribute the dynamics in this timescale to the population in the exciton reservoir state. Actually, a simple BP1T-CN crystal also showed a ∆/ signal corresponding to the slow component (see Sec. 11 in Supporting Information). A recent study on a III-nitride microcavity has suggested two mechanisms for the blue-shifted polariton 8


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renormalization induced by population in the exciton reservoir41; one is blue-shift of the exciton energy due to repulsive interactions between excitons, and the other is saturation effects in the oscillator strength42. The former is unlikely for our case because of the spatial localization and weak inter-particle interaction of the Frenkel excitons. For the latter case the reduction in the exciton oscillator strength directly weakens the coupling strength with the cavity photon43, and thus it results in the reduction in ℏ leading to the blue-shift in the LPB energies. This mechanism can consistently account for our result because, for the LPB2 mode with the negative detuning Δ (see Fig. 2c), decrease in the coupling strength causes increase in the photonic nature (i.e. Hopfield coefficient giving the photon fraction, ||). This can be found as the increase in the transmission intensity as found in the bottom of Fig. 4c. The photo-induced reduction in the refractive index might well be the origin of blue-shift44. However it would not be dominant because, in that case, the Hopfield coefficient giving the exciton fraction, || , should be increased. Accordingly, the transparency of the polariton mode have to be decreased ( / should be lower than the unity). This expectation conflicts with the experimental observation. On the other hand, it is apparent from the analogy with the TRPL results that the fast component observed at < ~1 ps is attributed to the population in the polariton ground state. The blue-shift found in this kinetics (see middle of Fig. 4d) shows the polaritons interaction energy. Importantly, we have succeeded for the first time in separately evaluating two kinds of polariton renormalization kinetics induced by the exciton and polariton state populations. The difference in the natures between these two renormalization effects can be clearly found in Fig. 4g. As compared to the slow component originated from the exciton-reservoir population, the fast component shows a more sensitive / behavior to ∆. This result means that the photonic nature in the polariton ground state is rapidly increased up under the high pumping fluence. But actually the increase in ∆ is small enough as compared to ℏ and spectral linewidth of polariton mode, and thus the strong coupling formation can be ensured during the dynamics.

Conclusions

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Ultrafast dynamics of organic cavity polariton under the non-resonant pumping has been clarified. The TRPL and TT results enabled us to clearly understand the dynamics of polariton population and polariton renormalization, respectively. The results manifest that ultrafast stimulated cooling of polariton particles is possible in the BP1T-CN single crystal microcavity. Also two kinds of renormalization effects due to the exciton and polariton populations were identified for the first time. These findings will be useful for fully understanding and controlling of polariton dynamics and demonstrate the prospect of organic materials for the room-temperature cavity polariton devices.

Methods Crystal Growth Powdery BP1T-CN was purchased from Sumitomo Seika Chemicals and used as received. The BP1T-CN crystal was grown by the sublimation-and-recrystallization method in a glass cylinder filled and purged with the dried nitrogen gas. The temperature inside cylinder was spatially graded using a couple of band heaters; the temperatures for sublimation and recrystallization sections were set to be 270 – 290 ºC and 230 – 240 ºC, respectively. With this procedure we can obtain platelet-like BP1T-CN crystals with a size of ~100 × 100 µm2.

Microcavity Fabrication The bottom DBR was dielectric thin film multilayer of SiO2 and Ta2O5, which was deposited by the rf-magnetron sputtering method on a silica substrate with a thickness of 500 µm. The refractive indices of SiO2 and Ta2O5 are 1.46 and 2.16, respectively. Deposition of twelve pairs of 80-nm SiO2 and 60-nm Ta2O5 films results in a high reflectivity larger than 99 % at the 440 – 550 nm band. A vapor-grown BP1T-CN crystal was picked up with a thin metallic wire and transferred onto the bottom DBR mirror. The thickness of crystal was measured to be ~510 nm using a contact-type thickness meter (Bruker, Dektak XT-S). The rf-magnetron sputtering method was employed again to deposit dielectric thin film multilayer of SiO2 and HfO2 as the top DBR. The refractive index of

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HfO2 is 2.13. Deposition of ten pairs of 80-nm SiO2 and 58-nm HfO2 films results in a reflectivity larger than 98 % at the 450 – 530 nm band.

Steady State Spectroscopies For the steady state PL measurements a continuous wave laser diode of 405 nm was used for the sample excitation. The excitation density was ~120 mW/cm2. The emission was collected using a quartz optical fiber with a core diameter of 1 mm. The emission spectra were measured with a CCD spectrometer with a resolution as high as ~0.6 nm. The angular dispersion measurement was performed by moving the optical fiber tip concentrically to the sample. The angular resolution was ~1.1 °. For the steady state transmission measurement, collimated white light from a halogen lamp was focused onto the sample surface using an optical lens with a 200-mm focal length. The transmitted white light was collected by the optical fiber and detected by the CCD spectrometer. The sample placed on the optical axis was rotated to obtain the angular dependent transmission property.

Ultrafast Photoluminescence Measurements 80-fs pulses from a Ti:Sapphire amplifier system (Solstice, Spectra-Physics) operating at 1 kHz was split into two lines. One of them was used to generate UV pump beam, and the other was used to build up the transient grating set-up. The pump beam was the second harmonic of the 800-nm pulse generated in a BBO crystal. The fluence for sample excitation was ~200 µJ/cm2. PL from the sample was focused onto a gate medium (fused silica) using a silver parabolic mirror and an optical iris for limiting the angle of emission azimuth. The angular resolution was ~7.5 º. In the transient grating set-up, the 800-nm beam was split into two using a 50/50 beam splitter. The beams were focused onto the gate medium with an angle of ~5 º and temporally overlapped each other. Interference of the two beams resulted in emergence of the transient grating in the gate medium due to the optical Karr effect. PL signal from the sample was diffracted by the transient grating, which could be spatially separated from the undiffracted background signal. The temporally gated PL signal was introduced to a spectrometer (SP 2150, Princeton). The time-resolved PL spectrum was measured using a CCD

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detector (PIMAX4, Princeton). A motorized optical delay line equipped in the UV pump line sequentially changed time delay between the pumping and gating. All measurements were performed for the sample at room temperature.

Transient Transmission The transient transmission measurement was performed with a conventional transient absorption setup described as follows. The output of a Ti:Sapphire amplifier system (Solstice, Spectra-Physics) operating at 1 kHz and generating 80-fs pulses was split into two for the pump and probe lines. The pump beam was the second harmonic of the 800-nm pulse generated in a BBO crystal. The fluence for sample excitation was ~200 µJ/cm2. The probe beam with a spectral width of ~20 nm was generated with a home-built noncollinear optical parametric optical amplifier. The center wavelength of the probe beam was tuned to that of transmission mode. The incident angle of pump beam was within ~10 º respect to the microcavity plane. The optical axis of probe beam was perpendicular to the microcavity plane. The transmitted light was introduced to a spectrometer (SP 2150, Princeton), and measured with an InGaAs dual-line array detector (Hamamatsu G11608-512) driven and read out by a custom-built board from Stresing Entwicklungsbüro. All measurements were performed for the sample at room temperature.

References (1) Weisbuch, C.; Nishioka, M.; Ishikawa, A.; Arakawa, Y. Observation of the coupled exciton-photon mode splitting in a semiconductor quantum microcavity. Phys. Rev. Lett. 1992, 69, 3314-3317. (2) Deng, H.; Haug, H.; Yamamoto, Y. Exciton-polariton Bose-Einstein condensates. Rev. Mod. Phys. 2010, 82, 1489-1537. (3) Sanvitto, D.; Kéna-Cohen, S. The road towards polaritonic devices. Nature Materials 2016, 15, 1061-1073.

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(4) Kasprzak, J.; Richard, M.; Kundermann, S.; Baas, A.; Jeambrun, P.; Keeling, J. M. J.; Marchetti, F. M.; Szymańska, M. H.; André, R.; Staehli, J. L.; Savona, V.; Littlewood, P. B.; Deveaud, B.; Dang, L. S. Bose-Einstein condensation of exciton polaritons. Nature 2006, 443, 409-414. (5) Balili, R.; Hartwell, V.; Snoke, D.; Pfeiffer, L.; West, K. Bose-Einstein condensation of microcavity polaritons in a trap. Science 2007, 316, 1007-1010. (6) Deng, H.; Weihs, G.; Santori, C.; Bloch, J.; Yamamoto, Y. Condensation of semiconductor microcavity exciton polaritons. Science 2002, 298, 199-202.. (7) Christopoulos, S.; Baldassarri Höger von Högersthal, G.; Grundy, A. J. D.; Lagoudakis, P. G.; Kavokin, A. V.; Baumberg, J.J. Room-temperature polariton lasing in semiconductor microcavities. Phys. Rev. Lett. 2007, 98, 126405. (8) Christmann, G.; Butté, R.; Feltin, E.; Carlin, J. -F.; Grandjean, N. Room temperature polariton lasing in a GaN/AlGaN multiple quantum well microcavity. Appl. Phys. Lett. 2008, 93, 051102. (9) Lu, T.-C.; Lai, Y.-Y.; Lan, Y.-P.; Huang, S.-W.; Chen, J.-R.; Wu, Y.-C.; Hsieh, W.-F.; Deng, H. Room temperature polariton lasing vs. photon lasing in a ZnO-based hybrid microcavity. Opt. Express 2012, 20, 5530-5537. (10) Su, R.; Diederichs, C.; Wang, J.; Liew, T. C. H.; Zhao, J.; Liu, S.; Xu, W.; Chen, Z.; Xiong, Q. Room-temperature polariton lasing in all-inorganic perovskite nanoplatelets. Nano Letters 2017, 17, 3982-3988. (11) Kéna-Cohen, S.; Forrest, S. R. Room-temperature polariton lasing in an organic single-crystal microcavity. Nature Photonics 2010, 4, 371-375. (12) Plumhof, J. D.; Stöferle, T.; Mai, L.; Scherf, U.; Mahrt, R. F. Room-temperature Bose–Einstein condensation of cavity exciton–polaritons in a polymer. Nature Materials 2014, 13, 247-252. (13) Daskalakis, K. S.; Maier, S. A.; Murray, R.; Kéna-Cohen, S. Nonlinear interactions in an organic polariton condensate. Nature Materials 2014, 13, 271-278. (14) Dietrich, C. P.; Steude, A.; Tropf, L.; Schubert, M.; Kronenberg, N. M.; Ostermann, K.; Höfling, S.; Gather, M. C. An exciton-polariton laser based on biologically produced fluorescent protein. Sci. Adv. 2016, 2, e1600666. (15) Cookson, T.; Georgiou, K.; Zasedatelev, A.; Grand, R. T.; Virgili, T.; Cavazzini, M.; Galeotti, F.; Clark, C.; Berloff, N. G.; Lidzey, D. G.; Lagoudakis, P. G. A yellow polariton condensate in a dye filled in microcavity. Adv. Opt. Mater. 2017, 5, 1700203. (16) Lidzey, D. G.; Fox, A. M.; Rahn, M. D.; Skolnick, M. S.; Agranovich, V. M.; Walker, S. Experimental study of light emission from strongly coupled organic semiconductor microcavities following nonresonant laser excitation. Phys. Rev. B 2002, 65, 195312. (17) Somaschi, N.; Mouchliadis, L.; Coles, D.; Perakis, I. E.; Lidzey, D. G.; Lagoudakis, P. G.; Savvidis, P. G. Ultrafast polariton population build-up mediated by molecular phonons in organic microcavities. Appl. Phys. Lett. 2011, 99, 143303. 13


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(44) Virgili, T.; Lidzey, D. G.; Bradley, D. D. C.; Cerullo, G.; Stagira, S.; De Silvestri, S. An ultrafast spectroscopy study of stimulated emission in poly(9,9-dioctylfluorene) films and microcavities. Appl. Phys. Lett. 1999, 74, 2767-2769.

Acknowledgements K.Y. thanks to JSPS KAKENHI Grant No. 15H03974. M. N. is also thankful to JSPS KAKENHI Grant No. 15H03678. K.Y. thanks Dr. F. Sasaki for fruitful discussions. The authors are grateful to Ms. L. Eyre for her support in ultrafast TRPL measurements.

Author Information Corresponding Author * E-mail: [email protected] The authors declare no competing financial interests.

Associated Content Supporting Information Detailed description of BP1T-CN crystal structure, fundamental optical properties of the materials, and analyses of the experimental data are shown in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figures

Figure 1

BP1T-CN single crystal microcavity. a Molecular formula of BP1T-CN. Transition dipole

moment lies in the direction of the long molecular axis. b Molecular packing of vapor grown BP1T-CN crystal with projection on (001) plane. c Schematic of organic microcavity structure. Dielectric thin film multilayers were deposited by the magnetron sputtering as the top and bottom high reflectivity DBRs. d Fluorescent micrograph of BP1T-CN crystal. A rhomboid single crystal with ~100 × 100 µm2 can be seen. The image was taken before the deposition of top DBR.

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a

b

Figure 2

2.4 2.6 2.8 Energy (eV)

Energy (eV)

LPB3

Transmission (arb. units)

LPB2

2.2

c 3

LPB1

PL Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Energy (eV)

UPB3

2.8

UPB1

UPB2

2.8 UPB2

LPB3

2.6

LPB2

2.4

2.6 LPB2

LPB1

2.2

0

10 20 30 40 50 Angle (degree)

-50

0 50 Angle (degree)

Steady state optical properties of BP1T-CN microcavity. a Transmission and PL spectra at

low fluence in the linear regime. Three lower polariton branches (LPBn) can be seen in both the results. b Dispersion characteristics of polariton modes. Closed circles plot the peak energies of transmission spectra. Dashed curves show dispersions of the upper and lower polariton branches (UPBn and LPBn) obtained from the least-squares fit of the transmission peak energies using a coupled oscillator model. c A schematic of energy dispersion of polariton (solid curves), cavity photon (dashed curve), and exciton (dash-dotted curve) modes. This schematic is drawn for the LPB2 and UPB2 modes. In this depiction the detuning energy ∆ is in negative.

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LPB2

LPB1

LPB3

θ = 20 o

θ = 10 o

c

LPB3

LPB2

Figure 3

2.4 2.6 Energy (eV)

2.8

LPB3

LPB2

θ = 20 (o)

θ=0o

2.2

Normalised Intensity (arb. units)

b


a


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10 0

0

1 2 Delay (ps)

LPB1

3

LPB1

0

1

2 Delay (ps)

3

Ultrafast TRPL spectroscopies and polariton population dynamics. a Emission angle θ

dependence of time-resolved PL spectra under non-resonant pumping. The intensities are temporally integrated from 0 to 8 ps in the delay time and normalized by the peak intensities of LPB2 modes. b TRPL kinetics for the LPB3, LPB2, and LPB1 modes at various θ. The intensities are normalized to their peak values. c Comparison of the experimental decay profiles at θ = 0 º (red circles) with numerical simulation results based on the rate equation including the stimulated polariton cooling process. All the LPBn emissions can be accounted for by superposition (red curves) of fast (pink shadow) and slow (grey shadow) components, which are dynamics of polariton density at aboveand below-threshold conditions, respectively. See the Supporting Information for more details.

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Figure 4

TT spectroscopies and polariton renormalization dynamics. a Time-resolved ∆/

spectra for the LPB2 mode. Solid, dashed, and dotted spectra show the data temporally integrated in ranges of 0.3 – 1.0, 3 – 10, and 30 – 100 ps, respectively. b Time-resolved transmission spectra reproduced from ∆/. The vertical axis shows transmission intensity under the pumping, , normalized by , which is the maximum value in no-pumped transmission spectrum . c Top: ∆/ kinetics detected at different energies. Middle and bottom: kinetics of energy shift ∆ and relative intensity / , respectively, evaluated for spectra. and are maximum values in the and spectra, respectively. d Enlarged kinetics of ∆/, ∆, and / in early times. e and f Contour plots of ∆/ and / signals, respectively. g Comparison of / between two renormalization kinetics as a function of ∆.

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Table of Contents Graphic Ultrafast Dynamics of Polariton Cooling and Renormalization in an Organic Single Crystal Microcavity under Non-Resonant Pumping

Kenichi Yamashita, Uyen Huynh, Johannes Richter, Felix Deschler, Akshay Rao, Kaname Goto, Takumi Nishimura, Takeshi Yamao, Shu Hotta, Hisao Yanagi, Masaaki Nakayama, and Richard H. Friend

Organic single crystal microcavity with strong coupling.

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Ultrafast Dynamics of Polariton Cooling and Renormalization in an

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