A cost-effective realization of multimode exciton-polaritons in single

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Functional Nanostructured Materials (including low-D carbon)

A cost-effective realization of multimode exciton-polaritons in single-crystalline microplates of a layered metal-organic framework Dileep Kottilil, Mayank Gupta, Kapil Tomar, Feng Zhou, Cherianath Vijayan, Parimal K. Bharadwaj, and Wei Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20179 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

A cost-effective realization of multimode excitonpolaritons in single-crystalline microplates of a layered metal-organic framework Dileep Kottilil †, ‡, Mayank Gupta §, Kapil Tomar §, Feng Zhou†, C. Vijayan ‡, *, Parimal K. Bharadwaj §, *, Wei Ji †, ||, * †

Department of Physics, National University of Singapore, 3, Science Drive 3, Singapore

117542, Singapore ‡

Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India

§

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India

||

SZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology,

International Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong 518060, P. R. China

KEYWORDS: Exciton-Polaritons, Metal-Organic Framework, Strong Coupling, Multimode Coupling, Rhodamine B, Angle-Resolved Reflectivity.

ACS Paragon Plus Environment

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ABSTRACT: We report the observation of multimode exciton-polaritons in single-crystalline microplates of a two-dimensional (2D) layered Metal-Organic Framework (MOF), which can be synthesized through a facile solvothermal approach, thereby eliminating all fabrication complexities usually involved in the construction of polariton cavities. With a combination of experiments and theoretical modelling, we have found that the exciton-polaritons are formed at room temperature, as a result of a strong coupling between Fabry-Perot cavity modes formed inherently by two parallel surfaces of a microplate and Frenkel excitons provided by the 2D layers of dye molecular linkers in the MOF. Flexibility in rational selection of dye linkers for synthesizing such MOFs renders a large-scale, low-cost production of solid-state, micro- exciton-polaritonic devices operating in the visible and near infrared (NIR) range. Our work introduces the MOF as a new class of potential materials to explore polariton-related quantum phenomena in a costeffective manner.


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INTRODUCTION Exciton-polaritons are quasiparticles that are formed when a sufficiently strong interaction is maintained between electron-hole pairs (excitons) inside a material and photonic modes of an optical microcavity. Embedding an exciton-active material within a high-quality microcavity triggers a light-matter interaction at a rate faster than the average dissipation rate of light and matter components, thereby achieving a strong coupling regime. The characteristic feature of excitonpolaritons is the presence of an anti-crossing region in the dispersion curves of exciton and cavity modes near an excitonic resonance. Exciton-polaritons offer the implementation of intriguing technological applications such as inversionless lasers1–4, polariton routers5, polariton transistors6 and others. However, a cost-effective and large-scale implementation of these quantum devices at room temperature is mainly hindered by the complex and sophisticated growth techniques involved in the fabrication of a high-quality exciton-polariton microcavity. In this context, we report an extremely simple realization of exciton-polaritons in single-crystalline microplates of a layered Metal-Organic Framework (MOF), which can be synthesized through a facile solvothermal approach. Such a MOF microplate naturally consists of both an excitonic material and a high-quality optical microcavity, and hence eliminates the costly, time-consuming fabrication process of a high-quality polariton microcavity. Up to now, exciton-polaritons have been demonstrated mostly with inorganic or organic materials. With inorganic materials, exciton-polaritons have been observed mainly under cryogenic temperatures7,8 as Wannier-Mott excitons have small binding energy compared to the thermal energy of room temperature. However, room-temperature exciton-polaritons have been recently demonstrated in various inorganic structures such as 2D semiconductor layers1,4,9–13, nanowires14-16, and photonic crystals17. In organic crystals such as anthracene, the presence of


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Frenkel excitons have allowed to attain room-temperature exciton-polaritons2,3,18 because of their large binding energy. But the lack of a simple, scalable, and cost-effective production method of exciton-polariton cavities remained as one of the main challenges. MOFs are a novel class of artificial materials where a metal ion or metal ion clusters are coordinated to organic linkers in a repeating fashion forming a well-defined structure with a high degree of crystallinity and porosity. Apart from their exotic chemical properties, researchers have already demonstrated their potential in the field of photonics such as efficient multiphoton harvest19-22, two-photon lasing from a dye confined structure23, and polarized three-photon lasing24. However, the presence of quasiparticles such as exciton-polaritons in MOF has never been reported in the literature. Here, we report the realization of exciton-polaritons in a MOF microplate, resulting from a strong coupling between Fabry-Perot cavity modes developed inherently by the two parallel surfaces of a MOF microplate and Frenkel excitons provided by 2D layers of Rhodamine B (Rh B) dimers in the MOF microplate. Such a quantum device is realized at room temperature, without the support of any external mirrors made by metal coatings, or distributed Bragg reflectors (DBRs). Additionally, there are at least two more advantages in using MOFs for a strong light-matter interaction: (i) They are known for their flexibility to form multidimensional structures via simple chemical synthesis processes. Therefore, it is foreseen that the interaction between 1D, 2D or bulk excitons with cavity modes can possibly bring out new intriguing physics. (ii) The rational selection of organic linkers (such as fluorescent dyes) to synthesize MOF crystals can offer a simple realization of room-temperature, inversionless lasing which can be operated at various wavelengths in the visible and NIR region.


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Encapsulation of fluorescent dyes as guest molecules in the porous structure of a MOF has been reported23,24 previously, although the realization of such dyes as organic linkers has never been achieved. Here, we synthesized MOF microplates with Rh B dye as the organic linker, which forms a stabilized 2D layered sheets through π-stacking. This ensures an ordered and uniform distribution of dye molecules to form a single-crystalline MOF microplate. We have found that the dimerization of Rh B molecules in a MOF unit cell lifts degeneracy of monomer-exciton states of Rh B to form a new set of non-degenerate dimer-exciton states. The mechanism behind such an energy level modification has been successfully modelled by employing the theory of Davydov splitting25 combined with additional inter-molecular Coulomb interactions. The predicted energy levels of new dimer-excitons are in good agreement with both photoluminescence (PL) and angleresolved reflectivity measurements. One of these dimer-excitons is strongly coupled with multiple cavity modes to form multimode exciton-polaritons. Angle-resolved reflectivity measurements have confirmed the formation of such quasiparticles inside the MOF microplate. The dependence of exciton-polaritons on the microplate thickness has revealed a highest Rabi splitting energy, ħΩRabi = 35 meV, for the thinnest microplate.

RESULTS AND DISCUSSION MOF microplate structure: Microplates of various thicknesses were synthesized using relatively simple solvothermal method (see Methods). The as-synthesized microplate is a single-crystalline MOF {Zn2 (C23 H13 NO8) (C28 H30 N2 O3) ·5(C3 H7 NO)} n, consisting of several π-stacked 2D layers with an inter-layer separation of 10 Å. Within each layer, there are dinuclear metal centers which are interconnected through H4L linkers, as shown in Figure 1a. These metal atoms are also coordinated to the spectrally active Rh B linkers, which essentially enable the π-stacking through


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their π-electron rich xanthene rings. As shown in Figure 1b, each unit cell consists of two Rh B molecules which are separated by a π − π interaction length of 3.53 Å. Such a π-stacking is formed along the a-axis of the unit cell. Lattice constants a, b and c are shown. The crystal quality is confirmed by comparing the powder X-ray diffraction (PXRD) data of as-synthesized MOF with the simulated data, (see Section 1 in Supporting Information for details). The material characterization of the as-prepared MOF microplates with analytical techniques such as 1H-NMR, 13C-NMR,

ESI-MS and IR spectroscopy can be found elsewhere26. Figure 1c shows the schematic

of the MOF microplate cavity containing a stack of 2D layers of dye molecules. The top and bottom reflecting surfaces of this microplate inherently form a Fabry-Perot cavity. For clarity a strong coupling between a single Frenkel exciton and a photonic mode is only indicated. In fact, many excitons from each 2D layer are coupled with any given photonic mode to form exciton-polaritons as described in the following sections. Theoretical modelling: It was carried out based on the molecular structural information obtained from single-crystalline XRD experimental data (Table S1 in Supporting Information). The intermolecular interactions between two Rh B chromophores inside a unit cell were theoretically modelled based on the theory of Davydov splitting combined with additional Coulomb interactions. Such an energy level modification is heavily dependent on inter-molecular separation, relative orientation and magnitude of transition dipole moments of the monomers participating in the process of dimerization. Our modelling suggests that every unit cell is occupied by an Rh B chemical dimer whose modified exciton levels determine the properties of light-matter interaction. A detailed theoretical treatment is given in Section 2 in Supporting Information, by considering the Hamiltonian of a unit cell containing two coupled two-level molecules. The magnitudes of Coulomb interaction potential (Uij) and Davidov splitting (∆ij) were calculated as -0.85 eV and -


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0.15 eV respectively, where interaction occurs between molecules at ith and jth sites of a unit cell. The modified exciton energy levels for a molecular dimer were calculated as 1.54 eV and 1.24 eV. The ground state energy shift of the dimer was also calculated by considering the dipole-dipole interaction between ground-state Rh B monomers within a unit cell. This calculation yielded a redshift (Vg) to the ground state of the dimer by an amount of -0.58 eV. Figure 1d schematically shows these energy level modifications of a dimer unit cell. The theoretically predicted optical transition energies of upper and lower dimer-exciton states are 2.12 eV and 1.82 eV respectively. It is known that the polarization axes for the transitions to such excited dimer levels are mutually perpendicular27. The magnitudes of transition dipole moments of upper and lower dimer-exciton levels are proportional to,

sin β and

cos β, respectively27

where β = 87.5° (see Section 2 in Supporting Information for details) is the angle made by the line of molecular centers with the polarization axis of a monomer transition. Therefore, an optical transition to the upper dimer level is relatively higher. Our PL and angle-resolved reflectivity measurements convince the formation of lower- and upper dimer-exciton levels whose observed values are 5% and 9% greater than their theoretically predicted value. Disregarding the interactions involving H4L ligand and Zn atoms inside a unit cell, were the main source of errors in our modelling. Photoluminescence (PL) measurements: We first measured PL spectra from MOF microplates of three different thicknesses (4.1 µm, 8.5 µm, and 16.1 µm), and from Rh B molecules in solution. The microscopy setup employed for obtaining both PL spectrum and bright field imaging is shown schematically in Figure 2a (see Methods). The bright field and PL images of these microplates are presented in Figure 2b (and Figure S3 in Supporting Information). For comparison, Figure 2c shows the normalized PL spectra of liquid (aqueous solutions) and crystalline (microplates) phases


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of Rh B molecules, excited with a CW laser (403 nm, power density: 235 W/cm2). A spectral redshift (2.16 eV → 1.98 eV) is observed for a higher concentration aqueous solution (6 × 1025 molecules/m3) as compared to a lower concentration solution (6 × 1020 molecules/m3). Therefore, the chromophores in the lower concentration solution are regarded as monomers, while dimers are present in the higher concentration solution. On the other hand, the PL spectrum of the MOF microplate consists of several unequally spaced peaks, among which one peak (1.98 eV) coincides with the emission peak of the dimer solution, see an arrow at 1.98 eV in Figure 2c. To further confirm this, we compare the PL spectra of MOF microplates with different thicknesses (Figure S4 in Supporting Information). Many thickness-dependent PL peaks are observed for microplates. Among these peaks, only the peak at 1.98 eV is independent of the thickness, and hence, it is attributed to radiative decay of lower dimer-exciton level of the dye-dimer. The energy of this exciton is also in good agreement with the theoretically-predicted lower dimer-exciton energy level of 1.82 eV (5% error). No emission from the upper dimer-exciton manifests itself in the PL spectra because β > 54.7°27. It is expected as excited electrons at the upper (or any higher energy) dimer-exciton levels relax quickly to the lower dimer-exciton level by non-radiative decay. Excitation power dependence of PL spectra of the three MOF microplates are shown in Figure 3a-c. A slight spectral blue shift (black dotted arrows) is observed for the thickness-dependent PL peaks as the excitation laser power density increases, however the peak at 1.98 eV (lower dimerexciton) is unshifted as indicated by the red arrow. Such a blueshift in the PL is a characteristic of exciton-polariton modes. These modes became completely unresolved for the thickness of 16.1 µm. Angle-resolved reflectivity measurements: These measurements were performed on the same microplate areas the above-discussed PL experiments. The setup is shown schematically in Figure


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4a (and see Methods for more details). The in-plane (k||) and out-of-plane (k⊥) components of the excitation white light (TM mode) is shown on the MOF microplate. Mutually perpendicular polarizations (green color double sided arrows), corresponding to the transition dipole moments of upper and lower dimer-exciton levels, are represented by M’ and M’’, respectively. The observation of a well-resolved anti-crossing region, near the resonance of an exciton and a cavity mode, is a solid evidence to the formation of exciton-polaritons. As shown in Figure 4b-d, all the three microplates exhibit anti-crossing when the polarization axis corresponds to the electronic transition of upper dimer-exciton was aligned parallel to the k|| of TM mode. This configuration ensures a population enhancement in the upper dimer-exciton level. It, however, does not populate the lower dimer-exciton level because polarization axes of upper and lower dimer-exciton transitions are mutually perpendicular. The anti-crossing is observed when the cavity modes come close to an energy value of 2.238 eV and therefore, the existence of an active Frenkel exciton at this energy is concluded. This value (2.238 eV) is in good agreement with the theoretically predicted upper dimer-exciton level of 2.12 eV (error: 9%). At angles (≤ 25°), two prominent low-reflectivity regions are observed for the 4.1-µm-thick microplate (Figure 4b). These are identified as the lower polariton (P2) and upper polariton (P1) branches. Low-reflectivity regions are also observed for the 8.5-m-thick and 16.1-m-thick microplates, see Figures 4c and 4d, respectively. They are attributed to polariton branches, resulting from multimode coupling. Hamiltonian of the coupled oscillator model was utilized to simulate these data (see Methods). The simulated curves (polariton branches) are denoted as green dashed curves in the reflectivity maps. These polariton branches are labelled as P1, P2, P3 etc. in Figures 4b-d. The bare cavity modes (orange dotted curves) close to the exciton energy with negative detuning 3 meV, 2 meV and 15 meV are designated as i, j and k respectively for the three


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microplates in Figures 4b-d. The multiple Rabi splitting (ħΩRabi) are indicated as white doublesided arrows. These values for the microplates of thickness 4.1 µm, 8.5 µm and 16.1 µm are 35 meV, 22 meV and 10 meV respectively, as shown in Figure 4e. Ideally the Rabi splitting value should be a constant for all microplates because the density of the Rh B molecules (6 x 1022 molecules/m3) is a constant (see Figure 2c) irrespective of the microplate thickness. However, the slight imperfections practically occurred in our single crystalline microplates can reduce the quality of the cavity, which in turn influences the value of the Rabi splitting for different microplates12. The regime of strong coupling is achieved when the sum of the exciton decay rate (ħΓexc) and cavity mode decay rate (ħΓc) is less than ħΩRabi. From the time-resolved PL and reflectivity measurements (at an incident angle of 0°), the values of ħΓexc and ħΓc are found to be 0.05 meV and 8 meV, respectively (see Section 5 in Supporting Information). Thus, the minimum ħΩRabi required for strong coupling can be calculated as 0.05 meV + 8 meV ≈ 8 meV. Hence, a strong light-matter coupling is achieved in every MOF microplate. More angle-resolved reflectivity maps covering negative and positive angle values can be found in Section 6 in Supporting Information. Furthermore, the observed Rabi splitting energies were spectrally resolved because the sum of the HWHM of either upper or lower polariton branches is less than the Rabi splitting energy. A single-mode exciton-polariton coupling exists in the 4.1-µm-thick microplate, while multimode exciton-polariton coupling happens for both the 8.5- and 16.1-µm-thick microplates. The reason is attributed to the lower value of the free spectral range (FSR) of bare cavity modes of these two microplates, as compared to the FSR of the 4.1-µm-thick microplate. For the 8.5-m- and 16.1m-thick microplates, their FSR are 49 meV and 25 meV, respectively, less than 101 meV for the 4.1-m-thick microplate. Additionally, multimode coupling is numerically verified by calculating


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mixing fractions (Hopfield coefficients) of each polariton branch for the three microplates. Results shown in Figure 5 (and Section 7 in Supporting Information) reveal that single-mode coupling is evident for the 4.1-m-thick microplate as only one bare cavity mode contributes to both P1 and P2 branches. Multimode coupling is clear for both 8.5-µm- and 16.1-µm-thick microplates. Multimode coupling is more pronounced at emission angles such as 100 and 200, where the exciton is on resonance with cavity modes. However, no signature of exciton-polaritons was observed when k|| of TM mode was made parallel to the polarization axis of the lower-dimer exciton level, although cavity modes are expected to couple with the lower dimer-exciton level in this configuration. The reason is attributed to the weak transition dipole moment of the lower-dimer exciton, as mentioned previously. Hence, no strong and continuous energy exchange is maintained between the cavity modes and the lowerdimer exciton. The formation of exciton-polaritons in such a 2D layered MOF microplate can be physically explained as follows: Two Rh B monomers from any two adjacent 2D layers within a microplate interact to each other through a dipole-dipole interaction to form a Rh B dimer. Such a Rh B dimer contributes two Frenkel excitons among which the exciton with an energy of 2.238 eV (experimentally obtained value from angle-resolved reflectivity) couples with any given photonic mode. Each unit cell of the MOF microplate contributes one active Frenkel exciton (2.238 eV) for the polariton formation. Therefore, many Frenkel excitons from these unit cells couple to any given photonic mode to form exciton-polaritons. The resulting exciton-polaritons are localized because of the Frenkel nature of the participating excitons. Stability: One challenge towards the realization of practical polariton devices is the stability of the material involved. A material with a low chemical and thermal stability is unattractive in terms


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of its practical applications. Hence, investigation on the stability of our MOF material was conducted on the 4.1-µm-thick MOF microplate and results are shown in Section 8 in Supporting Information. The ability of the material to maintain exciton-polariton modes by overcoming the thermal effects induced by a laser excitation, was tested by shining a CW laser (403 nm) with varying excitation power density. The PL spectra in Figure S12a in Supporting Information show exciton-polariton modes up to an excitation of 3.8 kW/cm2. These modes became spectrally unresolvable above 3.8 kW/cm2, indicating a thermally-induced damage to the microplate. The heat generated inside the microplate alters the unit cell structure, resulting in a non-recoverable structural change to the microplate. Additionally, the thermogravimetric analysis was performed and concluded that the microplate can withstand a temperature up to 676 K (Figure S12b in Supporting Information). These findings reveal that our MOF microplate exhibits a large thermal withstanding capacity. Moreover, the chemical stability of the same microplate in ambient conditions (at 298 K, RH: 45%, 1 atm) was studied by comparing the reduction in PL intensity over 120 days. Figure S13 in Supporting Information shows that the PL intensity is decreased by 41% after 120 days without losing the spectral resolvability of exciton-polariton modes. Hence a prolonged chemical stability is ensured for these MOF microplates in ambient conditions at least for 6 months. Normally, a strong light-matter coupling demands the implementation of external mirrors such as metal mirrors or DBRs whose fabrication usually involves complicated etching, coating or other growth techniques. The as-presented MOF microplate avoids all such complications by acting itself as a cavity, which can be synthesized through a simple solvothermal approach. The two parallel surfaces of a microplate inherently form an exciton-layer-embedded optical microcavity. The Q-factor of the microplate is extracted to be ~149 at a photon energy of 2.239 eV from the


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reflection spectrum recorded at an excitation angle of 00 (see Section 5 in Supporting Information). It is previously demonstrated24 that even a larger value of Q-factor (~1691) can be achieved in a single crystalline MOF without any external mirrors. Furthermore, the Q-factor presented in this work is greater than the minimum value (~10) reported28 to achieve a strong coupling regime.

CONCLUSION In conclusion, for the first time, we demonstrate multimode exciton-polaritons in singlecrystalline microplates of a 2D layered Metal-Organic Framework (MOF), which can be synthesized through a facile solvothermal approach, thereby eliminating all fabrication complexities usually involved in the construction of polariton cavities. With a combination of experiments and theoretical modelling, we have found that the exciton-polaritons are formed at room temperature because of a strong coupling between Fabry-Perot cavity modes formed inherently formed by two parallel surfaces of a microplate and Frenkel excitons provided by the 2D layers of dye molecular linkers in the MOF. Flexibility in rational selection of dye linkers for synthesizing such MOFs renders a large-scale, low-cost production of solid-state, micro-scale exciton-polaritonic devices operating in the visible and near-infrared (NIR) range. Our work introduces the MOF as a new class of potential materials to explore polariton-related quantum phenomena, such as Bose-Einstein condensation, superfluidity and polariton lasers, in a costeffective manner.

METHODS: Synthesis of MOF microplates {Zn2 (C23 H13 NO8) (C28 H30 N2 O3) ·5(C3 H7 NO)}n: The metal salts and other reagent grade chemicals were purchased from commercial suppliers (Sigma-


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Aldrich, Alfa Aesar, TCI, and others) and used without further purification. All the solvents were from S. D. Fine Chemicals, India. These solvents were purified following standard conventional methods prior to use. Zn(NO3) 26 H2O (60 mg, 0.20 mmol), Rh B (5 mg, 0.01 mmol) and H4L (20 mg, 0.045 mmol) were dissolved in 2 mL DMF and 1 mL H2O. The mixture was placed in a Teflon-lined stainless-steel autoclave and heated under autogenous pressure to 90° C for 3 days and then allowed to cool to room temperature. Long dark red colored microplates of the MOF were collected by filtration and washed with DMF and methanol. Finally, the microplates were dried in the air. The lateral area of each microplate was ~50 µm2. Thermogravimetric analysis: Thermogravimetric analyses (TGA) (heating rate of 5° C/min under nitrogen atmosphere) were performed with a Mettler Toledo Star System. Photoluminescence (PL) and microplate imaging: A commercial microscope (Nikon, Eclipse Ti2) with a laser-scanning head was used to carry out PL and imaging measurements. A CW laser beam of wavelength 403 nm (Melles Griot Solid State Laser) was focused on the sample through a bottom objective (20x) with a spot diameter of 4 µm. The transmitted PL signal was collected using an in-line objective (20x) placed on the top of the sample. The PL spectrum was directly channelled to a fiber-coupled spectrometer through an optical filter (500 Long Pass). PL (or Bright field optical) imaging was carried out under the same conditions, replacing spectrometer with a PMT (or CCD). Bright field optical images were taken using a Halogen lamp instead of the laser. All measurements were controlled by respective software. Angle-resolved reflectivity measurements: Measurements were carried out with a home-made Fourier imaging set-up with epi-illumination. A white light beam was passed through a linear polarizer and a translational slit (width: 25 µm) before focusing it on the back-Fourier plane of the objective (20x, 0.45 NA) using a convex lens (f = 12 cm). Light source must be focused on the


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back focal plane of the microscope objective. This is to ensure that the beam of light incident at an angle on a MOF microplate is parallel. Therefore, all photons incident on the MOF microplate are at a certain angle. The slit was then translated along the cavity axis (range: ± 2.5 mm) to change the position of the focused spot on the back-Fourier plane. Depending on the spot position, the angle of incidence was changed. The reflected light from the microplate was collected using a convex lens (attached on another translational stage) and channelled to a fiber-coupled spectrometer (Oceanoptics QEPro, resolution: 0.78 nm). The angle calibration was conducted by translating the slit and noting down the corresponding distance (D) of the white light spot (formed on the sample stage) from the objective axis. Then, arctan of the ratio, D: L was taken to find out the angle of incidence, where L is the distance between the bottom end of the objective lens and the sample stage. Reflectivity map was plotted after normalizing the reflectivity data from zero to one. Time-resolved PL spectroscopy: Measurements were made using a commercial microscope (Nikon Ti2). The second harmonic (SH) of a femtosecond laser (ND: YAG, original wavelength: 1030 nm, SH wavelength: 515 nm, 150 fs, 1 MHz) was used to excite the sample through an objective (20x, 0.45 NA). The PL signal was collected by the same objective and then channelled to a single-photon avalanche diode (SPAD: manufactured by Micro Photon Devices) through a beam splitter (1: 1 @515 nm) and a 532 nm long-pass filter. The time-resolved signal was then fed into a time-correlated single photon counting machine (PicoHarp 300), and eventually displayed on a computer screen. Numerical simulation on angle-resolved reflectivity: Experimentally obtained dispersion curves from angle-resolved reflectivity measurements were modelled using coupled oscillator


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Hamiltonian. The modelling consists of coupling between a single exciton with n cavity modes. The Hamiltonian matrix for a certain thickness can be written as

(

ħ𝜔𝑖 𝑉𝑡 𝑉𝑡 𝐸1 H = 𝑉𝑡 0 ⋮ ⋮ 𝑉𝑡 0

)

⋯ 𝑉𝑡 … 0 … 0 ⋱ ⋮ ⋯ 𝐸𝑛

(1)

The exciton energy is ħωi = 2.238 eV, uncoupled cavity mode energies are E1, E2, ..., En and coupling strength between exciton and cavity modes is Vt, which is thickness-dependent. A quantitative agreement between the experimental data and theoretical simulation was achieved by taking 6 cavity modes close to the exciton resonance for the three microplates. every thickness. Simulations were carried out by choosing background refractive index of the microplate as 1.51. Hopfield coefficients were calculated from the eigenvectors of the above matrix.


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Figures:

Figure 1. Structure and energy diagram of a MOF microplate. (a) Schematic of the layered structure of the MOF microplate. (b) Unit cell representation of the layered MOF. (c) Schematic of the MOF microplate cavity, which consists of a stack of 2D layers of Rh B molecules. For clarity a strong coupling between a single Frenkel exciton and a photonic mode is only indicated. In fact, many excitons from each 2D layer are coupled with any given photonic mode to form exciton-polaritons. (d) Simplified energy level diagram of two Rh B monomers and a Rh B dimer.


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Figure 2. Photoluminescence (PL) spectroscopy of MOF microplates. (a) Microscopy setup employed for PL spectroscopy and imaging. (b) Pairs of PL and bright field optical images. All scale bars are 50 µm. White circles (radius ~2 µm) are the regions for both PL and reflectivity measurements. (c) Normalized PL spectrum of Rh B molecules in liquid and crystalline phases. The emission at 1.98 eV originates from the lower dimer-exciton.


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Figure 3. Dependence of exciton-polariton peaks on the excitation fluence. (a-c) Excitation power dependence and blueshift of photoluminescence (PL) spectra for the three microplates. A spectral blueshift is observed with the excitation power density.

Figure 4. Angle-resolved reflection spectroscopy. (a) Schematic of home-made Fourier imaging configuration with epi-illumination. The microplate on the Ag mirror is enlarged for clarity. (b-d)


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Reflectivity maps from the three MOF microplates with different thickness. Exciton (2.238 eV, blue line), bare cavity modes (orange dotted curves) and polariton branches (green dotted curves) are fitted with the coupled oscillator model. Bare cavity modes which are close to the exciton energy (with negative detuning) are represented by the letters i, j and k for microplates with thickness 4.1 µm, 8.5 µm and 16.1µm respectively. Polariton branches are labelled by P1, P2, P3, etc. (e) Rabi splitting energy dependence of microplate thickness.

Figure 5. Verification of multimode exciton-polariton coupling. (a-c) Fractional contribution of exciton and bare cavity modes to form polariton branches (P1 and P2) of three microplates with different thickness. Each column consists of mixing fractions at three different emission angles (00, 100, 200). Multimode coupling is more pronounced at certain emission angles (such as 100 and 200), where the exciton is on resonance with bare cavity modes.


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SUPPORTING INFORMATION The Supporting Information (PDF) is available free of charge via the Internet at http://pubs.acs.org This file includes the details of experimental techniques, theoretical modelling, additional images (CCD, PMT), PL spectra, angle-resolved reflectivity spectra and the material stability data (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected] *E-mail address: [email protected] *E-mail address: [email protected] Author Contributions D.K. performed the experiments, theoretical modelling and data analysis. M.G. and K.T. performed synthesis and material characterization. Z.F. helped with the theoretical calculations. D.K., C.V., P.K.B. and W.J. jointly wrote the manuscript. C.V., P.K.B. and W.J. conceived and supervised the project. All authors discussed the results and commented on the manuscript. ACKNOWLEDGMENT D.K. is thankful for his research scholarships funded by National University of Singapore and Indian Institute of Technology Madras. M.G. is thankful for his research scholarships funded by UGC, New Delhi, India. We thank our colleagues from National University of Singapore, Indian Institute of Technology, Madras who provided insight and expertise that greatly assisted the


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research. We are also grateful for a research grant funded by both Singapore Government and National University of Singapore: MOE Tier 1 #R144-000-401-114. P.K.B gratefully acknowledge the ﬁnancial support received from the DST and MNRE, New Delhi, India and YSF to K.T. from SERB (YSS/2015/001088/CS), New Delhi, India. REFERENCES (1)

Schneider, C.; Rahimi-Iman, A.; Kim, N. Y.; Fischer, J.; Savenko, I. G.; Amthor, M.;

Lermer, M.; Wolf, A.; Worschech, L.; Kulakovski, V. D.; Shelykh, I. A. An Electrically Pumped Polariton Laser. Nature 2013, 497, 348-352. (2)

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. (3)

Kéna-Cohen, S.; Forrest, S. R. Room-Temperature Polariton Lasing in an Organic Single-

Crystal Microcavity. Nat. Photon. 2010, 4, 371–375. (4)

Bhattacharya, P.; Xiao, B.; Das, A.; Bhowmick, S.; Heo, J. Solid State Electrically Injected

Exciton-Polariton Laser. Phys. Rev. Lett. 2013, 110, 206403. (5)

Marsault, F.; Nguyen, H. S.; Tanese, D.; Lemaître, A.; Galopin, E.; Sagnes, I.; Amo, A.;

Bloch, J. Realization of an All Optical Exciton-Polariton Router. Appl. Phys. Lett. 2015, 107, 201115. (6)

Ballarini, D.; De Giorgi, M.; Cancellieri, E.; Houdre, R.; Giacobino, E.; Cingolani, R.;

Bramati, A.; Gigli, G.; Sanvitto, D. All-Optical Polariton Transistor. Nat. Commun. 2013, 4, 1778.


22

Page 23 of 26 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


(7)

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. (8)

Dufferwiel, S.; Schwarz, S.; Withers, F.; Trichet, A. A. P.; Li, F.; Sich, M.; Del Pozo-

Zamudio, O.; Clark, C.; Nalitov, A.; Solnyshkov, D. D.; Malpuech, G. Exciton-Polaritons in van Der Waals Heterostructures Embedded in Tunable Microcavities. Nat. Commun. 2015, 6, 8579. (9)

Liu, X.; Galfsky, T.; Sun, Z.; Xia, F.; Lin, E. Strong Light-Matter Coupling in Two-

Dimensional Atomic Crystals. Nat. Photon. 2014, 9, 30-34. (10) Amo, A.; Lefrère, J.; Pigeon, S.; Adrados, C.; Ciuti, C.; Carusotto, I.; Houdré, R.; Giacobino, E.; Bramati, A. Superfluidity of Polaritons in Semiconductor Microcavities. Nat. Phys. 2009, 5, 805–810. (11) Kleemann, M. E.; Chikkaraddy, R.; Alexeev, E. M.; Kos, D.; Carnegie, C.; Deacon, W.; De Pury, A. C.; Große, C.; De Nijs, B.; Mertens, J.; Tartakovskii, A.I. Strong-Coupling of WSe2 in Ultra-Compact Plasmonic Nanocavities at Room Temperature. Nat. Commun. 2017, 8, 1296. (12) Hennessy, K.; Badolato, A.; Winger, M.; Gerace, D.; Atatüre, M.; Gulde, S.; Fält, S.; Hu, E. L.; Imamoğlu, A. Quantum nature of a strongly coupled single quantum dot–cavity system. Nature, 2007, 445, 896. (13) Waldherr, M.; Lundt, N.; Klaas, M.; Betzold, S.; Wurdack, M.; Baumann, V.; Estrecho, E.; Nalitov, A.; Cherotchenko, E.; Cai, H.; Ostrovskaya, E.A. Observation of Bosonic Condensation in a Hybrid Monolayer MoSe2-GaAs Microcavity. Nat. Commun. 2018, 9, 3286.


23


Page 24 of 26

(14) Shang, Q.; Zhang, S.; Liu, Z.; Chen, J.; Yang, P.; Li, C.; Li, W.; Zhang, Y.; Xiong, Q.; Liu, X.; Zhang, Q. Surface Plasmon Enhanced Strong Exciton-Photon Coupling in Hybrid InorganicOrganic Perovskite Nanowires. Nano Lett. 2018, 18, 3335–3343. (15) Graf, A.; Tropf, L.; Zakharko, Y.; Zaumseil, J.; Gather, M. C. Near-Infrared ExcitonPolaritons in Strongly Coupled Single-Walled Carbon Nanotube Microcavities. Nat. Commun. 2016, 7, 13078. (16) Takazawa, K.; Inoue, J. I.; Mitsuishi, K.; Takamasu, T. Fraction of a Millimeter Propagation of Exciton Polaritons in Photoexcited Nanofibers of Organic Dye. Phys. Rev. Lett. 2010, 105, 067401. (17) Yoshle, T.; Scherer, A.; Hendrickson, J.; Khitrova, G.; Gibbs, H. M.; Rupper, G.; Ell, C.; Shchekin, O. B.; Deppe, D. G. Vacuum Rabi Splitting with a Single Quantum Dot in a Photonic Crystal Nanocavity. Nature 2004, 432, 200–203. (18) Lerario, G.; Fieramosca, A.; Barachati, F.; Ballarini, D.; Daskalakis, K. S.; Dominici, L.; De Giorgi, M.; Maier, S. A.; Gigli, G.; Kéna-Cohen, S.; Sanvitto, D. Room-Temperature Superfluidity in a Polariton Condensate. Nat. Phys. 2017, 13, 837–841. (19) Quah, H. S.; Chen, W.; Schreyer, M. K.; Yang, H.; Wong, M. W.; Ji, W.; Vittal, J. J. Multiphoton Harvesting Metal – Organic Frameworks. Nat. Commun. 2015, 6, 7954. (20) Liu, M.; Quah, H. S.; Wen, S.; Yu, Z.; Vittal, J. J.; Ji, W. Efficient Third Harmonic Generation in a Metal-Organic Framework. Chem. Mater. 2016, 28, 3385–3390.


24

Page 25 of 26 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


(21) Medishetty, R.; Nalla, V.; Nemec, L.; Henke, S.; Mayer, D.; Sun, H.; Reuter, K.; Fischer, R. A. A New Class of Lasing Materials: Intrinsic Stimulated Emission from Nonlinear Optically Active Metal–Organic Frameworks. Adv. Mater. 2017, 29, 1605637. (22) Medishetty, R.; Zare, J. K.; Mayer, D. Nonlinear Optical Properties, Upconversion and Lasing in Metal–organic Frameworks. Chem. Soc. Rev. 2017, 46, 4976-5004. (23) Yu, J.; Cui, Y.; Xu, H.; Yang, Y.; Wang, Z.; Chen, B.; Qian, G. Confinement of Pyridinium Hemicyanine Dye within an Anionic Metal-Organic Framework for Two-Photon-Pumped Lasing. Nat. Commun. 2013, 4, 2719. (24) He, H.; Ma, E.; Cui, Y.; Yu, J.; Yang, Y.; Song, T.; Wu, C.-D.; Chen, X.; Chen, B.; Qian, G. Polarized Three-Photon-Pumped Laser in a Single MOF Microcrystal. Nat. Commun. 2016, 7, 11087. (25) Davydov, A. S. The Theory of Molecular Excitons (Translation). Sov. Phys. Uspekhi 1964, 82, 393–448. (26) Gupta, M.; Kottilil, D.; Tomar, K.; Lu, S.; Vijayan, C.; Ji, W.; Bharadwaj, P. K. TwoPhoton Absorption and Fluorescence in Micrometer-Sized Single Crystals of a Rhodamine B Coordinated Metal–Organic Framework. ACS Appl. Nano Mater. 2018, 1, 5408-5413. (27) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. The Exciton Model in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371–392. (28) Hobson, P. A.; Barnes, W. L.; Lidzey, D. G.; Gehring, G. A.; Whittaker, D. M.; Skolnick, M. S.; Walker, S. Strong Exciton-Photon Coupling in a Low-Q All-Metal Mirror Microcavity. Appl. Phys. Lett. 2002, 81, 3519–3521.


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Table of content figure:


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A cost-effective realization of multimode exciton-polaritons in single

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