Superlattices and Microstructures 47 (2010) 10–15

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Strong-coupling regime at room temperature in one-dimensional microcavities containing ultraviolet-emitting perovskites G. Lanty a,∗ , J.S. Lauret a , E. Deleporte∗ , a , S. Bouchoule b , X. Lafosse b a

Laboratoire de Photonique Quantique et Moléculaire de l’École normale Supérieure de Cachan, 61 avenue du Président Wilson, 94235 Cachan Cedex, France b Laboratoire de Photonique et Nanostructures, Route de Nozay, 91460 Marcoussis, France

article

info

Article history: Available online 4 July 2009 Keywords: Polaritons Perovskite Strong coupling Microcavity Dielectric mirror

abstract We have realized several one-dimensional (1D) microcavities, containing self-assembled perovskite layers, emitting in the ultraviolet (UV) range (around 3.5 eV), working in the strong-coupling regime at room temperature. Procedures to increase the quality factor of the microcavities have been explored. Two different technologies have to be combined: the soft technologies related to the organic materials and polymers and the technologies which are more specific to inorganic materials, such as electron-beam evaporation and sputtering, used in particular to deposit dielectric mirrors. © 2009 Elsevier Ltd. All rights reserved.

One-dimensional (1D) planar microcavities, consisting of Pérot–Fabry structures containing an optically active region, have proved that they are a powerful tool to study light–matter interaction [1– 3] and may lead to applications in optical devices [4,5]. In particular, in the strong-coupling regime, the cavity photon mode and the exciton of the optically active region are not eigenmodes of the system any more: the new eigenmodes are a linear and coherent superposition of the exciton and photon states, called cavity polaritons. This particular regime is intensively studied due to the interest in coherent and stimulated effects in such systems, which can lead to the realization of low threshold polariton lasers [5–7]. In order to realize new optoelectronic devices based on polaritonic effects, it is crucial to find optically active materials allowing one to reach the strong-coupling regime at room temperature. In the field of inorganic semiconductors, the strong-coupling regime has been observed recently at room temperature in GaN and ZnO 1D microcavities [8–11], presenting Rabi splittings of several tens of meV. Alternatively, for a decade, it has been demonstrated that the strong-coupling

∗

Corresponding author. Tel.: +33 01 40 47 55 55. E-mail addresses: [email protected] (G. Lanty), [email protected] (E. Deleporte).

0749-6036/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2009.06.006

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regime can be obtained at 300 K in 1D microcavities containing thin layers of organic materials [12–15], or hybrid organic–inorganic materials [16–18]. Due to the relatively large oscillator strengths of the exciton in these materials, Rabi splittings of several hundreds of meV have been reported at room temperature. Such large Rabi splittings are of particular significance for current efforts to achieve polariton lasing in microcavities at room temperature. In this paper, we focus our attention on materials absorbing and emitting in the UV range. Up to now, the molecular structures demonstrating the strong-coupling regime at room temperature when inserted in a Pérot–Fabry cavity emit light in the near-IR range (J-aggregates of cyanine dyes) or in the visible range (sigma-conjugated polysilane, perovskite): the shortest emitted wavelength is 413 nm [15]. We use here the two-dimensional layered perovskite-type semiconductor (C6 H5 C2 H4 –NH3 )2 PbCl4 [bis-(phenethylammonium) tetrachloroplumbate] to realize different kinds of 1D microcavities showing the strong-coupling regime at room temperature and emitting light in the UV range. Two-dimensional layered perovskite compounds such as (R–NH3 )2 MX4 (R = alkyl chain, M = metal, X = halogen) have been shown to have a self-organized multiple quantum well structure when the organic solution is deposited by spin-coating on a substrate: inorganic wells of thickness around 0.5 nm alternate with organic barriers of thickness around 1.0 nm [19]. Because the band gap of the MX4 layers is smaller than that of organic layers, the lowest exciton is confined in the MX4 layer, so the quantum wells consist of MX4 inorganic monolayers (thickness around 0.5 nm) and the barriers consist of the organic alkylammonium layers (thickness around 1.0 nm). By virtue of the high contrast in dielectric constants between the organic layers and the MX4 layers, the Coulomb interaction in the well layer is hardly screened by the presence of the barrier layers, so the interaction between an electron and a hole in an exciton is strengthened, resulting in very large exciton binding energies of a few hundred meV and huge oscillator strengths, [16]; these are one order of magnitude higher than in conventional inorganic semiconductor quantum wells. Because of this strong binding energy of the exciton, the optical features can be observed at room temperature: the absorption spectrum of a (C6 H5 C2 H4 –NH3 )2 PbCl4 layer (thickness around 30 nm) deposited on a quartz substrate exhibits a relatively sharp peak (width ' 100 meV) at 3.64 eV, and a relatively sharp photoluminescence peak can be observed at 3.61 eV when the sample is excited at 3.82 eV by a He–Cd laser. The perovskite molecule (C6 H5 C2 H4 –NH3 )2 PbCl4 has been embedded in a λ Pérot–Fabry microcavity constituted with a dielectric mirror and a semi-transparent metallic one. The bottom dielectric Bragg mirror is deposited onto a fused silica substrate by plasma-enhanced chemical vapor deposition (PECVD) and is composed of 7.5 λ/4 pairs of silicon oxide (d = 64 nm, n = 1.49) and silicon nitride (d = 46 nm, n = 1.96). Its stop-band is then centered at 3.4 eV at normal incidence and at 3.6 eV under 40◦ incidence. The reflectivity spectrum at normal incidence of the PECVD mirror shows a maximal reflectivity of 96% and the stop-band extends from 3.1 eV to 3.7 eV. The dielectric mirror ends with a thin film (21 nm) of SiO2 in order for the perovskite to be centered near an antinode of the stationary electric field in the cavity. A thin film of (C6 H5 C2 H4 –NH3 )2 PbCl4 perovskite is deposited on top of this dielectric mirror by spin-coating a 5 wt% solution of C6 H5 C2 H4 –NH3 Cl and PbCl2 dissolved in stoichiometric amounts in DMF (N,N-dimethylformamide); the thickness of the film is around 30 nm. Then a PMMA (polymethylmethacrylate) layer is spin-coated, acting as a spacer layer in order to tune the cavity photon mode energy at 40◦ incidence close to that of the exciton. The top mirror of the microcavity is then produced by electron-beam evaporation of aluminium (thickness of 13 nm) on the PMMA layer. The energies of the two minima observed in Fig. 1a are reported in Fig. 1c as a function of k// = E sin θ , the wavevector parallel to the surface, where θ is the incident angle. The experimental results h¯ c are fitted to the dispersion using a standard two-level model [20] (solid lines in Fig. 1c). The very good agreement between the experimental and calculated results shows unambiguously that strong coupling leads to an anticrossing between the exciton and the photon mode, with a value of the Rabi splitting equal to 230 meV. The two transitions observed in Fig. 1a are then identified as the lower polariton branch (LPB) and the upper polariton branch (UPB). In order to confirm this result, photoluminescence experiments have been performed, the energy position of the photoluminescence (PL) signal being directly linked to the polariton dispersions [21]. The 325 nm He–Cd laser beam is focused on the microcavity through the dielectric mirror, at normal

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a

LPB

b

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30 35 40 45

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c UPB

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Fig. 1. (a) Reflectivity spectra of the microcavity for different incidence angles. The dotted lines are guides to the eyes showing the angular dispersion of the upper (UPB) and lower (LPB) polariton branches. (b) Photoluminescence spectra for different collection angles (excitation at 3.82 eV, through the dielectric mirror at normal incidence). (c) Polariton dispersions measured from reflectivity spectra (squares) and calculated (solid lines). The dispersions of the uncoupled perovskite exciton Eper (dotted line) and cavity photon mode Eph (dashed line) are also shown. The stars represent the energy positions of the photoluminescence peaks.

incidence. Fig. 1b shows a series of photoluminescence spectra obtained for different collection angles, ranging from 5◦ to 50◦ . For the lower angles, two peaks are present in the spectra. The position of the high energy peak is independent of the detection angle, whereas the position of the lower energy peak varies as the detection angle is tuned. For the higher angles, the lower energy peak becomes increasingly intense, suggesting a bottleneck effect [22]. The energy positions of the luminescence peaks have been reported as stars in Fig. 1c, superimposed to the fitted dispersion curves and reflectivity positions of the upper and lower polaritonic branches. The dispersionless data correspond to the non-coupled part of the perovskite exciton, since the energy position of this peak corresponds to that of the photoluminescence spectrum of the perovskite layer. The variation of the low energy photoluminescence peak as a function of k// coincides with the dispersion relation of the low energy polaritonic branch. This clearly indicates that this photoluminescence arises from the polaritonic emission and confirms the demonstration of the strong-coupling regime at room temperature in the UV range. The emission of the upper polaritonic branch has not been observed, probably because of the relaxation towards uncoupled excitonic states [23] or because of the fast emission of optical phonons between the upper and the lower branches [20].

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Fig. 2. Reflectivity spectra measured (solid line) and calculated (dashed line) of an empty microcavity in which the top mirror is a 50 nm thick Al mirror. The photon mode has a linewidth around 70 meV, which corresponds to a quality factor of around 46.

The quality factor of this microcavity can be evaluated from the lower energy dip in the reflectivity spectrum at low incidence angles (see Fig. 1a): we find a quality factor of 15. Because of this low quality factor, the polaritons will have an extremely short lifetime. However, to reach the polariton lasing regime we need to facilitate the polariton relaxation towards the k// = 0 state of the LPB. This can be done by increasing the polariton lifetime and thus the quality factor of our microcavity. We thus focus our efforts on the top mirror and we propose two different kinds of mirror to replace the low reflectivity Al semi-transparent mirror: a thick Al mirror or a dielectric mirror. Fig. 2 shows the experimental reflectivity spectra and the theoretical ones, calculated using the transfer matrix method, of an empty microcavity (i.e. without an embedded perovskite layer), in which the top mirror is a 50 nm thick Al mirror deposited by evaporation. Because of the thickness of the aluminium mirror, the reflectivity measurements were made through the silica substrate. The quality factor of this empty microcavity is quite promising, since we obtain a value of 46. But when a (C6 H5 C2 H4 –NH3 )2 PbCl4 perovskite layer is embedded inside this microcavity, the quality factor decreases to 22 because of the roughness of the perovskite layer. As a result, we think that some progresses will be obtained by reducing the thickness of this perovskite layer. We have also attempted to replace the semi-transparent metallic top mirror by a dielectric one. It is not possible to deposit this dielectric mirror by PECVD because the too high temperature will damage the organic layers. So we have chosen to proceed with lower temperature methods: electron-beam evaporation or sputtering, using either SiO2 /Si3 N4 or SiO2 /Ta2 O5 Bragg pairs. The first tests made on a microcavity (see inset of Fig. 3) with a top mirror composed of 3.5 SiO2 /Si3 N4 pairs deposited by sputtering are very encouraging. Figs. 3 and 4 respectively exhibit angle-resolved reflectivity measurements and an anti-crossing diagram of this microcavity. Strong coupling between the exciton of the perovskite and the confined photon is observed; this is evidence that neither the perovskite nor the PMMA layers have suffered degradation. The next stage will be to increase the number of pairs of the top dielectric mirror to obtain a thinner photon mode. In summary, we have realized several 1D microcavities emitting in the UV range (around 3.5 eV), working in the strong-coupling regime at room temperature. Procedures to increase the quality factor of the microcavities have been explored. We have to combine two different technologies: the soft technologies related to the organic materials and polymers and the technologies which are more specific to inorganic materials, such as electron-beam evaporation and sputtering, used in particular to deposit the dielectric top mirror. The difficulty is that the organic layers are much more sensitive to high temperature than inorganic materials. We have succeeded in realizing microcavities

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20 30 40 50 60 3.0

3.2

3.4

3.6

3.8

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Fig. 3. Reflectivity spectra of the microcavity for different incidence angles. The dotted lines are guides to the eyes showing the angular dispersion of the upper (UPB) and lower (LPB) polariton branches. Inset: sketch of the microcavity.

UPB

LPB

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Fig. 4. Polariton dispersions measured from reflectivity spectra (squares) and calculated (solid lines). The dispersions of the uncoupled perovskite exciton Eper (dotted line) and cavity photon mode Eph (dashed line) are also shown.

containing perovskite molecules showing improved performances. This work opens the way to opto-electronic devices emitting in the UV range and to more complex structures where the perovskite UV excitons can be coupled via the cavity photon to GaN excitons, following the suggestion of Agranovich et al. [24]. Acknowledgements The authors thank D. Byrne and A. Vella for their helpful work on the angle-resolved reflectivity experiment. This work is supported by Agence Nationale pour la Recherche (grant PNANO MICHRY), by CNANO from ’région Ile-de-France’ (grant MICRORG) and by Triangle de la Physique (grant MOSKITO). ’Laboratoire de Photonique Quantique et Moléculaire’ is a ’Unité mixte de recherche associée au CNRS’ (UMR8537).

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