Letter pubs.acs.org/journal/apchd5
Organic Exciton in Strong Coupling with Long-Range Surface Plasmons and Waveguided Modes Kevin Chevrier, Jean-Michel Benoit, Clementine Symonds, Julien Paparone, Julien Laverdant, and Joel Bellessa* Univ Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, F-69622, Lyon, France ABSTRACT: In this Letter, we demonstrate experimentally the strong interaction between organic semiconductor excitons and long-range surface plasmons. For this purpose, a thin J-aggregated cyanine dye layer is deposited on a thin silver film bounded by two poly(vinyl alcohol) layers. An anticrossing between the long-range surface plasmon and the dye exciton is measured in the dispersion relation and confirmed in luminescence experiments, showing unambiguously the hybridization between the long-range surface plasmon and the dye exciton. The extension of the polaritonic states deduced from the dispersion curves is 50 μm, which represents an increase of more than 1 order of magnitude compared to usual surface plasmon-based polaritons. We believe that the extension of the polariton coherence length can have direct applications to the energy transfer between molecules over long distances in strong coupling, as well as in the modification of the properties of materials (conductivity, second harmonic generation). Strong coupling between an aggregated dye layer and guided modes is also experimentally evidenced in a similar structure. The hybridization between dye exciton and guided modes demonstrated here opens the way to the exploitation of polariton nonlinearities and switching already evidenced in cavities for integrated optics: the strong coupling now occurs directly on the guided mode. KEYWORDS: long-range surface plasmon, strong coupling, organic exciton, waveguided mode
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with each other and hybridize. To maximize this interaction, the plasmon dispersions have to be matched in wavevector and energy, thus, requiring a symmetrical system surrounding the thin metallic film.14This hybridization results in the creation of a symmetric and an antisymmetric mode. The symmetric mode (in H field), also called LRSP, presents a reduced localization of the field in the metal and thus in a reduction of the losses. It exhibits a propagation length from 100 μm up to 250 μm depending on the experimental conditions such as silver thickness and excitation wavelength.15−17 In this paper, we will evidence the strong interaction between a LRSP and an aggregated dye exciton. The characteristic anticrossing induced by strong coupling regime and extension of the coherent region deduced from wavevector broadening will be described and discussed. We will also exploit the guided modes between the metal and the air interface to show excitons/waveguide hybridization. To create a LRP mode, based on a silver metallic layer, we produced a sample according to the following elaboration steps. To insert the metallic film in a symmetric medium, a poly(vinyl
he strong coupling between an optical mode and an exciton occurs when the interaction between light and matter prevails over the damping of the system. Strong interaction with organic materials1 has recently been applied to the modification of intrinsic properties of materials like their conductivity2,3 or chemical reactivity.4 One of the key features of organic systems in strong coupling is the coherent interaction between a set of molecules mediated by the electromagnetic mode leading to extended coherent polaritons.5−8 The extension of the coherent state is limited by the disorder of the molecular film and the losses of the electromagnetic mode.8 For the case of strong coupling with plasmons, the intrinsic losses associated with the metal are the main limitation. To reduce these losses, and thus increase the number of molecules in interaction, different types of plasmon modes with reduced losses can be used. Plasmonics grating resonances have been exploited9 and allow a condensation of the polaritons in strong coupling. The long-range surface plasmons10 that have been used for plasmonic devices11,12 or energy transfer13 could present an interesting alternative for strong coupling with increased extension of coherent states. Long-range surface plasmon (LRSP) is a surface mode appearing in a metallic slab bounded by a dielectric environment.10 If the thickness of the slab is small enough, both of the plasmons created at the metal-dielectric boundaries interact © 2017 American Chemical Society
Special Issue: Strong Coupling of Molecules to Cavities Received: May 31, 2017 Published: August 11, 2017 80
DOI: 10.1021/acsphotonics.7b00556 ACS Photonics 2018, 5, 80−84
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alcohol) (PVA) layer has been first deposited on a LASF substrate of optical index 1.8, then annealed at 230 °C. A silver film is then sputtered onto the first PVA layer and another PVA layer is spin-coated on top of the silver film. This last layer is only dried under vacuum to prevent degradation of the silver film. The thickness of the lower PVA film is 1.2 μm with a refractive index of 1.4994; the upper layer is 1.4 μm thick and has a refractive index of 1.4768. The refractive indexes have been measured by M-line spectroscopy.18 The silver thickness is a crucial parameter. On one hand, the film has to be thin enough to ensure the coupling between the plasmons propagating at both interfaces. In principle, the propagation losses of the LRSP decrease monotonously with the thickness of the metal film. On the other hand, the film has to be thick enough to ensure the formation of a continuous film. Indeed a nonplanar structure of the metal film appears typically under 15 nm.19 For our purpose, a thickness of 20 nm has been chosen for the silver film. Two types of samples have been fabricated: a sample with a silver film in between the PVA layers (sample A, used to measure the bare LRSP dispersion), and a sample with a thin dye layer embedded between the silver film and the upper PVA layer (sample B). A schematic of the sample B is presented in the Figure 1a. The aggregated dye used for this experiment is
rotating platform. In this way, the sample is illuminated with a white thermal light at a controlled angle. After reflection on the sample, the outgoing beam goes through a lens that is imaging the Fourier plane of our sample onto a spectrometer coupled with a CCD camera enabling the acquisition of dispersion images. Reflectivity dispersion images of the bare LRSP (sample A) in TM and TE polarizations are shown in Figure 2a,b. Two
Figure 2. (a, b) Experimental reflectometry images of the sample A (without dye) in TM and TE polarizations. (c, d) Reflectometry images calculated with a transfer matrix method. (e) Normalized squared H-field associated with the long-range surface plasmon (in red) and to the first guided mode (in blue). The field in red (respectively blue) corresponds to the point labeled 1 (respectively 2) in the dispersion relation of panel (c). Figure 1. (a) Structure of LRSP sample with aggregated dye. (b) Absorption spectrum of a S2275 aggregated dye in PVA matrix film deposited on a glass substrate, (inset) structural formula of the S2275 J-aggregated cyanine dye.
dispersion lines are present in the TM dispersion image, and the upper one vanishes in TE polarization. This suggests that the upper line can be attributed to LRSP. Transfer matrix simulations have been performed with the sample A geometry, and are shown in Figure 2c,d. The parameters used for the simulation are the thickness and refractive index of the layers. The only adjustable parameter is the thickness of the aggregated dye and has been modified to fit the anticrossing. A good agreement between experiments and simulations is obtained. The normalized squared magnetic fields associated with both modes in TM polarization at 650 nm are shown in Figure 2e. The H field associated with the upper mode presents the characteristic symmetrical shape of the LRSP and is maximized at the metal interfaces. The one associated with the lower mode corresponds to the first guided mode in the upper PVA layer, confined between the metal and air. Higher order guided modes are also present in this structure at lower angles.
an organic molecule called S2275 (5-chloro-2-[3-[5-chloro-3(4-sulfobutyl)-3H-benzothiazol-2-ylidene]-propenyl]-3-(4-sulfobutyl)-benzothiazol-3-ium hydroxide, inner salt, triethylammonium salt) from Few Chemicals. It is mixed with PVA in water for a better structuration of the layer and spin coated to form a thin film. The ratio in weight between the J-aggregate and the PVA is 1:20. The aggregated dye absorption spectrum is shown in Figure 1b and presents a maximum at 650 nm and a width of 14 nm (γexc = 41 meV). The dispersion relation of sample A has been characterized in Otto geometry.20 The sample is mounted onto the flat side of a prism of index 1.8 with a liquid of the same refractive index to ensure continuity with the prism index. The prism sits on a 81
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An estimation of the propagation length LLRSP of the LRSP can be deduced from the dispersion line width Δk. Indeed, this dispersion (recorded in the Fourier plane of the collection lens) is the Fourier transform of the direct image. The Fourier transform of an exponential decay exp(−|x|/LLRSP) is a Lorentzian of full width at half-maximum Γ, with Γ = 2/ LLRSP. It must be noted that an inhomogeneous broadening of the dispersion line would result in an underestimated propagation length. The experimental dispersion width at 650 nm is Γ = 0.034 μm−1, which gives a propagation length of LLRSP = 59 μm (80 μm at 700 nm). To couple dye molecule excitations to LRSP, a layer of aggregated dye is inserted between the silver film and the upper PVA layer (sample B, Figure 1a). The layer is thin (of the order of 15 nm) compared to the LRSP extension in order to prevent a symmetry breaking of the refractive index in the structure. The dispersion image of sample B is shown in Figure 3a. The
branches occurs at 57.78° with a splitting of 120 meV. The simulation of the system by transfer matrix calculations performed with a Lorentzian absorption for the dye layer is in good agreement with the experimental results (see Figure 3b). The oscillator strength used in the simulation has been deduced from the transmission spectra of the dye layer (Figure 1b) and the thickness of the dye layer adjusted at 15 nm to account for the Rabi splitting. The data extracted from the polaritonic dispersion lines have been converted in k-space and fitted with a two level model. The value of the Rabi splitting evaluated in k-space is ℏΩ = 26 meV, 5× smaller than the value found at constant angle. This can be related to the flat slope of the long-range dispersion lines. The simulations performed in k-space have been transposed in angle, and reported on the experimental dispersion of Figure 3c. The width of the bare LRSP is γLRSP = 4.4 meV (deduced in k-space). The strong coupling can be characterized by the cooperativity C = (ℏΩ)2/ 2γLRSPγexc, which gives in our case C = 1.9 (the limit of the strong coupling is usually considered at C = 1). We deduce the extension of the low energy polaritonic coherent states from the width of the dispersion lines. At fixed angle, the minimal energy separation between both polariton corresponds to a lower polariton wavelength of 675 nm. At this point, we measured Δk = 0.039 μm−1 leading to an extension of 50 μm. Compared to conventional surface plasmon/exciton polariton where an extension of the polaritonic states of a few microns has been demonstrated,5 an increase of 1 order of magnitude is observed with LRSP. Closer to the resonance at 660 nm, the extension deduced from Δk is 33 μm. Far from the resonance, at 700 nm, the measured extension is 78 μm, close to the one of the bare LRSP (80 μm). The Hopfield coefficient representing the magnitude of the plasmon and exciton contribution to the low energy polariton state are at 660 nm, αplasmon = 0.89 and αexciton = 0.45; at 675 nm, αplasmon = 0.98 and αexciton = 0.19; at 700 nm αplasmon = 0.99 and αexciton = 0.1. The dye layer inserted in the LRSP structure also affects the waveguide dispersion but no clear anticrossing can be observed in the experiments as well as in the simulations. This reduced interaction can be related to the low electric field of the guided mode near the metallic interface, that is, at the dye layer location. Luminescence measurements have also been performed on sample B, excited with a continuous nonresonant laser at 532 nm. The result is shown in Figure 3d. The luminescence image follows the low energy polariton dispersion line with an anticrossing at 650 nm. The observation of the polariton emission in luminescence is an unambiguous evidence of strong coupling. It should be noted that the high energy polariton is barely visible on the experimental luminescence image. This could be associated with the very low visibility of the high energy polaritonic lines in the reflectivity dispersion or to relaxation phenomena. To strengthen the interaction with the guided modes, a thicker dye film (150 nm) is now deposited on the metallic film (sample C) with the same PVA and silver layer thicknesses used for the previous samples. In this case, the LRSP can no longer be observed due to the dissymmetry created by the dye film. A dispersion image has been recorded and is shown in Figure 4a. We are interested in the first four guided modes, the first mode lying at a higher angle in the figure and the angle decreasing while increasing mode number. For the first guided mode, the anticrossing is not clear on the dispersion image. For the three following modes, the dispersion lines present an
Figure 3. Reflectometry images of sample B (thin dye layer): (a) experiments and (b) simulation with a transfer matrix method. (c) Reflectivity image in logarithmic scale. The dashed white lines correspond to the two-level model calculations. (d) Fluorescence dispersion image of the sample with a dye layer.
LRSP line is modified compared to the bare LRSP (sample A). An anticrossing appears around 650 nm, more clearly visible in the magnified image of Figure 3c. This anticrossing is characteristic of the hybridization between the LRSP and the dye exciton with the formation of two mixed, namely polaritonic, branches. The high energy polariton is barely visible at lower wavelength. This can be explained by a residual absorption in the PVA layer due to the 230° anealing. Indeed it has already been observed that a high energy absorption tail can drastically reduce the polariton visibility.21 It has to be noticed that the residual absorption of the PVA layer does not affect the guided waves as only the lower PVA film presents absorption. The lowest energy separation between the two polaritonic 82
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Joel Bellessa: 0000-0002-9525-6898 Notes
The authors declare no competing financial interest.
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REFERENCES
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Figure 4. Reflectometry image of sample C (thick dye layer): (a) experiments and (b) simulation with a transfer matrix method.
anticrossing at the dye absorption wavelength of 650 nm indicating hybridization between the dye exciton and each guided mode. Starting from the second guided mode, the measured Rabi splitting (extracted in k-space) are 41, 53, and 61 meV. Moreover, at 675 nm, the polaritonic extension is 20.7 μm for the first guided mode, 9.2 μm for the second, 5.4 μm for the third, and 2.5 μm for the fourth guided mode. A simulation with the transfer matrix method is presented in Figure 4b and is in good agreement with the experimental results. It should be noted that the guided modes obtained in this structure are lossy in the sense that the thin metallic film allows a non-negligible coupling to radiative modes in the prism through the metal layer. These losses appear in the width of the guided modes far from the resonance, where narrower lines are expected for nonlossy guided modes. Nevertheless, the strong coupling observed here can be generalized to usual guiding structures. In conclusion, we investigate the inclusion of a thin aggregated dye film in a structure supporting LRSP. The strong coupling between a LRSP and excitons in aggregated dyes has been evidenced with a Rabi splitting of 26 meV and confirmed by luminescence experiments. The extension of the polariton deduced from dispersion relation is 50 μm, resulting in an increase of a polariton coherence length of 1 order of magnitude compared to usual surface plasmon. The reduction of losses in surface plasmon directly affects the polariton coherence length and thus the number of molecules coherently coupled through the plasmon. These highly extended polaritonic states could result in a drastic modification in the physical and chemical behavior of materials in strong coupling, like conductivity and reactivity. We also demonstrate that strong coupling can be achieved with waveguide modes. This could open perspective in the use of polariton switching22 or nonlinearities already demonstrated in microcavities, in the integrated guided optics. 83
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(21) Bonnand, C.; Bellessa, J.; Plenet, J. C. Properties of surface plasmons strongly coupled to excitons in an organic semiconductor near a metallic surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 245330. (22) Schwartz, T.; Hutchison, J. A.; Genet, C.; Ebbesen, T. W. Reversible Switching of Ultrastrong Light-Molecule Coupling. Phys. Rev. Lett. 2011, 106, 196405.
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