Strong Light-Matter Coupling and Hybridization of Molecular

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Strong Light-Matter Coupling and Hybridization of Molecular Vibrations in a Low-Loss Infrared Microcavity Merav Muallem, Alexander Palatnik, Gilbert Daniel Nessim, and Yaakov R. Tischler J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00617 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 15, 2016

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The Journal of Physical Chemistry Letters

Strong Light-Matter Coupling and Hybridization of Molecular Vibrations in a Low-Loss Infrared Microcavity

Merav Muallem, Alexander Palatnik, Gilbert D. Nessim, and Yaakov R. Tischler*

The Department of Chemistry and the Bar-Ilan Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan, 52900, Israel

*E-mail: [email protected] Tel: 972-3-738-4514 Fax: 972-3-738-4053

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Abstract Hybridized polaritons are generated by simultaneously coupling two vibrational modes of two different organic materials to the resonance of a low-loss infrared optical microcavity. A thin film of poly methyl methacrylate with solvent molecules of dimethylformamide trapped inside provided two spectrally narrow, closely spaced carbonyl stretches with absorption peaks at 1731 cm-1 and 1678 cm-1. Situating this film in a microcavity based on Ge/ZnS distributed Bragg reflector mirrors produced three distinct polariton branches in the dispersion relation due to hybridization of the vibrational resonances. Two anti-crossings were observed with Rabi-splittings of 9.6 meV and 5.2 meV, between the upper-to-middle and middle-to-lower polariton branches, respectively. This system marks the first demonstration of polariton hybridization between a solid and solvent molecules and can open new paths towards chemical reaction modification and energy transfer studies in the mid-infrared spectral range.

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Strong light-matter coupling can occur when a material possessing a dipole-allowed optical transition is properly situated in a resonantly tuned optical microcavity and the rate of energy exchange between the vacuum field of the cavity and the transition dipoles of the material is larger than the competing dephasing processes.1 When strong coupling is achieved, new superpositional eigenstates of the light matter system are generated, referred to as cavity polaritons, which are spectroscopically detectable

as distinct resonances separated in energy by the Rabi splitting ℏΩR. The new states

possess remarkable optical, electronic, and material properties. Since the first

demonstration using inorganic quantum wells in planar microcavities,2 the study of microcavity polaritons has led to the realization of low threshold polariton lasing,3-5 Bose-Einstein condensation,6-8 polariton switching,9-10 long range energy transfer,11 and offer new possibilities for material science studies.12 The matter component of the cavity polariton can be composed of inorganic quantum wells,2 colloidal quantum dots,13-14 monolayers of transition metal dichalcogenide,15 and/or molecular organic excitonic materials.16-24 Recent studies have demonstrated that strong light-matter coupling can be achieved using the molecular vibrational modes of organic materials.25-31 Organic materials in solid or liquid phase have been utilized, for example the carbonyl stretch mode of PVAc25-26 and PMMA,27-28 and the cyanide stretch mode of benzonitrile solution.29 The resulting microcavity vibrational polaritons are generated in the mid-infrared spectral range and have the potential to yield new sources of infrared coherent light and to influence chemical reactions that depend on the specific energy level manifold of reagents and/or intermediates.32 Replacing one or both of the conventionally used metallic mirrors of the microcavity with a low optical-loss distributed Bragg reflector (DBR) was shown to deliver narrower polariton linewidths and lower optical absorption losses but at the expense of reduced Rabi-splitting.28,

31

Nevertheless,

DBR-based microcavities provide the flexibility to tune the cavity linewidth to match the narrow linewidths of molecular vibrational transitions and therefore generate a more optimized Rabi-splitting relative to dephasing processes. Microcavity structures in which two dipole-allowed optical transitions are coupled to a single cavity resonance result in multicomponent hybridized polariton modes.33-34 Three new eigenstates are generated that are linear combinations of the cavity photon mode and the two matter-based optical transitions. An important property of such systems is the ability to transfer energy between the component materials, because the 3 ACS Paragon Plus Environment


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cavity mode hybridizes the two matter-derived optical transitions, thus suggesting advanced applications in light-emitting devices, lasers, and photovoltaics.35-36 Examples of hybridized exciton-polariton systems include microcavities containing organic and inorganic quantum wells (QW) when the energies of the 2D Frenkel exciton in the organic and the Wannier exciton in the inorganic are resonant with cavity optical mode.33-35, 37-40 Simultaneous strong coupling of the excitations of two different molecular dyes,11, 36, 41 or different excitonic transitions in a polymer film,42 to a single cavity mode can lead to this type of hybridization as well. To observe hybridization, two key requirements are necessary:35 (1) the separation in energy between the dipole transitions of the two materials should be comparable to the Rabi splitting achievable in a microcavity from either of the individual materials alone, and (2) the Rabi splitting of the hybridized modes must be greater than the individual linewidths. Here, we show polariton-mediated hybridization of the vibrational modes of two different organic materials that are simultaneously situated in a resonantly tuned infrared microcavity. Such structures can lead to an improved understanding of energy transfer between organic materials with different vibrational state properties and the dynamics of vibrational state relaxation. Moreover, the realization of vibrational polariton hybridization can be an enabling step to chemical reaction modification via polaritonic interactions. The two organic materials that we used are poly methyl methacrylate (PMMA) and dimethylformamide (DMF), both of which have strong and narrow absorption peaks due to the carbonyl group (C=O) stretching band (Figure 1a and b). Since the functional groups in which the carbonyl is contained are different, amide and ester in the case of DMF and PMMA respectively, the spectral position of the their carbonyl peaks is shifted by 6 meV. This separation in energy is 2.3 times smaller than the Rabi splitting we observed in our previous work28 for the PMMA transition alone (14.3 meV), which satisfies the requisite conditions to observe hybridization. The PMMA molecules act like a host matrix for the trapped DMF solvent guest molecules. When the combined PMMA:DMF film is placed between two DBRs composed from Ge and ZnS (Figure 1d), the vibrational transitions simultaneously couple to the vacuum field of the cavity and produce hybridized polaritonic states. For the polaritonic device, Ge and ZnS were chosen as the high and low refractive index materials for the DBRs, with n = 4.01 and n = 2.24 respectively, since these 4 ACS Paragon Plus Environment

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materials are transparent in the mid-infrared spectral range. Each DBR consisted of 3 pairs of Ge and ZnS, and the center wavelength of the DBRs was designed to be λc = 5.78 µm. The first DBR was thermally evaporated onto a CaF2 substrate, then the PMMA:DMF film was spin coated on top, and finally, the structure was capped with the second DBR. Please see the Methods Section for more details about the deposition of the DBRs and PMMA:DMF layer. The transmission spectra of DMF, PMMA, and a combined PMMA:DMF thin film are shown in Figure 1. The thin film sample of neat PMMA was prepared by spin coating polymer dissolved in DMF onto a CaF2 substrate and then removing residual solvent. The DMF sample was prepared by sequestering some solvent (1%) in a KBr pellet. Spectrally narrow and intense absorption is observed from the stretching modes of the carbonyl bonds in all three samples. For the DMF sample, the absorption peak occurs at 0.206 eV (1662 cm-1) with a linewidth of 4.7 meV (Figure 1a), and for the PMMA film at 0.214 eV (1729 cm-1) with a linewidth of 3.7 meV (Figure 1b). The spectral position of the stretching mode appears at a lower energy in DMF than in PMMA, which is typical for carbonyl bonds that are contained in amide functional groups versus esters. The transmission spectrum of the combined film shows the DMF and PMMA carbonyl stretches occurring at 0.208 eV (1677 cm-1) and 0.214 eV (1729 cm-1) with linewidths of 3.1 meV and 3.8 meV, respectively (Figure 1c). These linewidths are smaller than in the uncombined samples possibly due to greater bond stabilization, since the DMF molecules are now in an organic surrounding instead of KBr and the PMMA is now partially solvated by the DMF.



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Figure 1. Transmission spectra of DMF contained in a KBr pellet (a), neat film of PMMA spin coated onto a CaF2 substrate (b), and the combined PMMA:DMF thin film spin coated on a CaF2 substrate (c). The circled region in (c) marks the vibrational energies for the carbonyl stretches in DMF (EDMF = 0.208 eV) and PMMA (EPMMA = 0.214 eV). Schematic of the microcavity structure with the combined PMMA:DMF layer situated between two Ge/ZnS DBRs, three pairs each, on a CaF2 substrate (d).


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When the combined PMMA:DMF film is incorporated into the microcavity structure of Figure 1d, the spectral signatures of strong light-matter coupling become manifest. To study the optical properties of the polaritonic structure, we measured its transmission spectrum in the mid-infrared spectral range using a Fourier transform infrared spectroscometer (FTIR). Three transmission peaks were observed at 0.199, 0.209, and 0.216 eV for light incident normal to the sample plane. Then, we performed angle-resolved transmission measurements to adjust the energy of the cavity photon relative to the DMF and PMMA vibrational modes and thus to control their relative coupling. The results are shown in Figure 2a. The bare cavity photon resonance depends on the angle of incidence according to the equation:1

Ecav θ=

E0

1- sinθ⁄neff 2

, 1

where E0 is the cavity resonance at θ = 0°, and neff is the effective refractive index of the cavity spacer taking into account penetration of the electric field into the DBR layers. Here, θ is defined relative to the normal of the cavity surface. For the micrcavity of Figure 2a, the cavity photon mode was negatively detuned at θ = 0° relative to the vibrational modes, hence when we increased the angle, the energy of the cavity photon also increased, first becoming resonant with the DMF vibrational and then with the PMMA vibrational mode. As the cavity photon mode is tuned, the three transmission peaks tune as well. The set of lower energy peaks forms a lower branch (LB) of states, which blue shifts with increasing angle, approaching the uncoupled carbonyl resonance of the DMF but not crossing it. The set of higher energy transmission peaks starts at an energy greater than the carbonyl resonance of the PMMA and also blue shifts with angle, forming an upper branch (UB) of states. Between the LB and UB, we observed an intermediate set of transmission peaks. The set of the middle peaks starts at an energy higher than the resonance of the DMF, blue shifts with increasing angle, and then approaches the PMMA resonance, thus forming a middle branch (MB) of states.



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Figure 2. Room-temperature angle-resolved transmission spectra (a). The vertical red dashed lines mark the vibrational energies of the carbonyl stretches in the DMF (EDMF = 0.208 eV) and PMMA (EPMMA = 0.214 eV). Dispersion relations of the microcavity vibrational polartion states (b) extracted from angle-resolved transmission spectra. The circles are the polariton energies obtained from the transmission peaks; orange, green, and blue represent upper, middle, and lower branches of the dispersion relation, respectively. The solid curves are the theoretical fits using a three-level coupled oscillator model. Broken curves correspond to the dispersion relations of the uncoupled photon (dotted curve) and vibrational (horizontal dashed lines) resonances.

From the angular variation of the transmission spectra, we extracted the dispersion relation of the microcavity polaritons shown in Figure 2b (circles). In the dispersion relation, the three branches are apparent, namely LB, MB, and UB, with anticrossings between LB and MB and between MB and UB, as expected for a hybridized polariton system. For θ < 25°, the dispersion relation of the LB exhibits curvature resembling the cavity photon mode, while the MB and UB present a nearly flat vibrational-like dispersion. For θ > 55°, the dispersion of the UB most closely follows the curvature of the cavity photon mode, while the MB and LB appear vibronic. In the range of angles, 25° < θ < 55°, the MB exhibits photonic character. We fit the dispersion relation of the hybridized polaritons using a coupled threeoscillator Hamiltonian.33-34,

39, 42

The superposition states are composed of two

vibrational modes and the cavity photon and are determined by the matrix equation: E E 0 V

0 E E V

α V V " β" = 0 , E ! E γ

(2)

where EDM and EPM are the energies of the vibrational modes associated with the carbonyl stretches in DMF and PMMA, respectively, Ecav is the energy of the uncoupled cavity photon, and E is an eigenenergy of the coupled system. V1 and V2 8 ACS Paragon Plus Environment

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are the interaction potentials between the photon and each of the two vibrational modes. α, β, and γ are the coefficients of the polariton states in the basis of the uncoupled states, and can be used to derive the fractional contribution of each component. The best fit of the dispersion relation for the polariton branches was obtained with the parameters of V1 = 4.8 meV, V2 = 2.6 meV, Ecav = 0.203 eV, and neff = 2.12. The agreement between the model and the measured data is excellent, demonstrating the applicability of the coupled three-oscillator model to describe the hybridized microcavity vibrational polariton system. According to the fit parameters, the empty cavity was negatively detuned to λc = 6.12 µm at θ = 0°. The fitted value of neff is nearly the average of the refractive indexes of the cavity spacer layer and ZnS because the cavity mode extends beyond the PMMA:DMF region into the adjacent ZnS DBR layers which have a higher refractive index. In general, the magnitude of the Rabi splitting is proportional to twice the

interaction energy, i.e., ℏΩR = 2V. In the case of hybridized polaritons, there are two

interaction potentials, and hence two Rabi splitting parameters of ℏΩR1 = 9.6 meV and ℏΩR2 = 5.2 meV for the anti-crossings between the UB and MB and between the MB

and LB, respectively. For the combined system, the effective Rabi splitting is given

by the expression, ℏΩR=ℏ (Ω2R1 +Ω2R2 *

1/2

from which we find ℏΩR = 10.9 meV. This

Rabi splitting is comparable to the values reported for other carbonyl based microcavity vibrational polariton systems, yet lower because in the hybridized system the oscillator strength is distributed over two energetically distinct vibrational modes. Comparing the Rabi-splitting to the polariton linewidths, one finds that the hybridized system is deep within the strong coupling limit. In particular, the linewidths for polariton states at θ = 40°, i.e., for the resonant condition of minimum energy spacing between the polariton branches, are 2.2 meV, 1.6 meV, and 1.9 meV for the LB, MB, and UB respectively, giving an average polariton linewidth of Γ = 1.9 meV. Consequently, we find ℏΩR/Γ = 5.7, suggesting that underdamped polariton dynamics such as Rabi-oscillations should be observable on resonance using sub-

picosecond pulsed mid-infrared excitation. To find the contributions from each of the three components, namely the DMF and PMMA vibrational and the cavity photon mode, in the hybridized polariton states, we calculated the values of |α|2, |β|2, and |γ|2 for the LB, MB, and UB. The results are 9 ACS Paragon Plus Environment


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shown in Figure 3. The composition of the LB and UB mainly contain contributions from the cavity photon and one of the two vibrational modes. In particular, the LB is composed mainly from the cavity photon, at small angles (θ < 25°), and from the DMF vibrational modes above θ = 55°, with considerable state mixing for θ ~ 35°. Similarly, the UB is composed largely from the PMMA vibrational modes at small angles and from the cavity photon at large angles, with the greatest degree of superposition occurring for θ ~ 40°. In contrast, the MB contains substantial fractions from all three components. At small angles, the MB is predominantly composed of the DMF vibrational modes (|β|2 = 0.85 at θ = 0°). As the cavity photon resonance is tuned closer to the PMMA vibrational energy, the cavity mode mediates hybridization between the DMF and PMMA vibrational modes generating coupling among all three resonances. Maximun hybridization is observed at θ = 40°, where the MB contains equal contributions from the DMF and PMMA vibrational modes (|α|2 = |β|2 = 0.38) and a correspondingly high contribution from the cavity photon (|γ|2 = 0.24).

Figure 3. The calculated values for the polariton composition fractions, |α|2, |β|2, and |γ|2, for the upper, middle, and lower branches extracted from Eq. 2. The upper branch is composed largely of the PMMA carbonyl vibrational mode (red, circles) and cavity photon (blue, triangles). The middle branch contains considerable mixing of all three components. The lower branch consists mostly of the DMF carbonyl vibrational mode (green, squares) mixed with the cavity photon.

The anti-crossing behavior of the dispersion relations is a clear signature of hybridized microcavity polaritons. To gain greater insight into the hybridization, we analyzed the transmission spectra of both the polaritonic structure and the combined 10 ACS Paragon Plus Environment

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PMMA:DMF thin film using T-matrix simulation. The transmission data and simulation results for the microcavity tuned to θ = 40° are shown in Figure 4a. At θ = 40°, where the energy spacing between the branches of the dispersion relation is a minimum, three well-separated polariton resonances are observed at 0.206, 0.210, and 0.218 eV, in both the data and simulation. The clear visibility of all three resonances is consistent with the nearly equal distribution of photonic character among the polariton modes at θ = 40°, as presented in Figure 3. For the simulation, we modelled both the DMF and PMMA carbonyl stretches as Lorentzian oscillators with the cavity resonance set to Ecav(θ = 40°). Close agreement between T-matrix simulation and data was achieved for the polaritonic structure and the thin film sample using a single common set of parameters. From the simulation, we derived that in both the microcavity and the thin film, the percentage of carbonyls from PMMA and DMF is 86 ± 2% and 14 ± 2%, respectively, assuming the transition dipole moment for the carbonyl stretching mode is the same in both PMMA and DMF. To check our model, we measured the mass of the film before and after heating it above the evaporation temperature of DMF. We found that a film weighing 1 mg lost 0.19 mg upon heating. Attributing this mass-loss solely to DMF evaporation, we obtain a PMMA to DMF ratio for the combined film of 81 ± 2% to 19 ± 2%, in qualitative agreement with the ratio determined by T-matrix simulation. To obtain further confirmation of the PMMA to DMF ratio, more detailed elemental analysis would be required.

a

b

Figure 4. Expanded view of the transmission spectra for the hybridized polariton system at θ = 40° (a). The blue circles are the data and the solid green curve is the fit based on T-matrix simulation. The vertical red dashed lines mark the vibrational energies of the carbonyl stretches in DMF (EDMF = 0.208 eV) and PMMA (EPMMA = 0.214 eV). Three distinct polariton peaks are observed. Transmission data (red circles) and T-matrix simulation (solid green) for the PMMA:DMF film (b).



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Our hybridized polariton system demonstrates the feasibility of enhancing the strong coupling character of a lower concentration guest molecule by virtue of hybridization to a host molecular species. Consider if the PMMA host matrix were absent and only the trapped DMF were present in the cavity used here. The cavity:DMF structure would show strong light-matter coupling, without hybridization, with ℏΩR = 5.2 meV. At θ = 40°, the angle at which the hybridized PMMA:DMF system has maximum state mixing, the LB state of the cavity:DMF structure would be

only 17% photon due to the energy mismatch between cavity and DMF resonances. In contrast, including the PMMA host raises the photon fraction of the LB state to 30%, with 58% remaining DMF and 12% becoming PMMA. This change is discernible from the dispersion relation of Figure 2b. The inclusion of PMMA to the dispersion relation lowers the LB from the flat DMF asymptote, thereby dressing in more band curvature and an increase in photon character. Thus, for the cavity tuning condition of maximum state mixing (θ = 40°), inclusion of the PMMA host dramatically changes the superpositional composition of the polariton states compared to the strong coupling that would occur if the DMF alone were coupled to the cavity.


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We have demonstrated a hybridized vibrational polariton system in which the vibrational transitions from two different organic materials are strongly coupled to the vacuum field of a low-loss DBR based mid-IR microcavity. The observed Rabi splitting compared to the polariton linewidths, ℏΩR/Γ = 5.7, indicates that the

hybridized states are well within the strong coupling limit, where underdamped polariton dynamics should be observable. Hybridization of vibronic transitions can lead to an improved understanding of energy transfer and relaxation dynamics associated with molecular vibrational modes. Our system combining PMMA and DMF can serve as a powerful model for strongly coupling solid and solvent molecules. It demonstrates the feasibility of enhancing the strong coupling of a lower concentration guest molecule by virtue of hybridization to a host molecule that in essence boosts the overall degree of polaritonic superposition. The realization of microcavity vibrational polariton hybridization in a host-guest solid-solvent system can be an enabling step to studying chemical reaction modification via polaritonic light-matter interactions where now smaller amounts of guest material can be coaxed into strongly coupled states via hybridization.



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Methods The polariton structure consisted of a CaF2 substrate, bottom DBR, spin-casted film of PMMA:DMF, and top DBR. The DBRs were composed of 3 pairs of Ge/ZnS layers and were thermally evaporated in high vacuum (base pressure < 5 x 10-6 Torr) using thermal evaporation (Nano 36 system, K.J. Lesker) without substrate heating. Ge and ZnS, which are transparent in the mid-IR spectral range, were chosen as the high and low refractive index materials for the DBRs, with n = 4.01 and n = 2.24, respectively. The DBR center wavelength was designed to be λc = 5.78 µm, and the thickness of each layer was grown to be a quarter wavelength (di =λc /4ni) as required to satisfy the DBR Bragg condition: namely 360 nm for Ge and 645 nm for ZnS. The Ge and ZnS layers were deposited at rates of 2 Å/s and 5 Å/s, respectively. Prior to calibration and deposition, it was necessary to outgas the source boats for several hours. The bottom DBR coating was deposited directly onto a pre-cleaned CaF2 substrate. Poly methyl methacrylate (PMMA), 12 % in weight, was dissolved in dimethylformamide (DMF) and the PMMA:DMF solution was spin-coated on the bottom DBR to form a film of controlled thickness in the range of 2 µm. High molecular weight PMMA was used for the solution (MW ≈ 996,000 gr/mol) in order to ensure sufficient film thickness. The solution and film were prepared in a N2 filled glove box (MBRAUN LabMaster 130) with less than 1 ppm O2 and H2O, in order to prevent agglomeration of the PMMA during spin coating. Depositing the top DBR without substrate heating prevented thermal damage to the PMMA layer43 and prevented evaporation of the entrapped DMF molecules. The vacuum alone did not remove the trapped DMF molecules apparently because of the solvents low vapor pressure and high affinity to the PMMA matrix. Optical transmission spectra were performed at room temperature using a Fourier transform infrared spectrometer (Magna 550, Nicolet). The theoretical fitting was carried out in MATLAB. The Tmatrix formalism was implemented to model transmission spectra through the polaritonic structure and the PMMA:DMF thin film. A scale (AT21 comparator, Mettler Toledo) with 1 µg readability was used to determine the mass fraction of DMF in the PMMA:DMF Film.


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Acknowledgment This research was supported by the Israel Science Foundation (ISF No. 206738). We are grateful to Ronen Tirrer for his help with the FTIR system, and to Yafa Pinchas for her help with the scale.

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Strong Light-Matter Coupling and Hybridization of Molecular

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