Self-hybridized exciton-polaritons in multilayers of transition metal

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Self-hybridized exciton-polaritons in multilayers of transition metal dichalcogenides for efficient light absorption Battulga Munkhbat, Denis G. Baranov, Michael Stührenberg, Martin Wersäll, Ankit Bisht, and Timur Shegai ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01194 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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Self-hybridized exciton-polaritons in multilayers of transition metal dichalcogenides for efficient light absorption Battulga Munkhbat1†, Denis G. Baranov1†, Michael Stührenberg1, Martin Wersäll1, Ankit Bisht1, and Timur Shegai1* 1Department *e-mail:

of Physics, Chalmers University of Technology, 412 96, Göteborg, Sweden

[email protected]

KEYWORDS: transition metal dichalcogenides, nanocavities, strong coupling, excitonpolaritons

ABSTRACT Transition metal dichalcogenides (TMDCs) have attracted significant attention recently in the context of strong light-matter interaction. To observe strong coupling using these materials, excitons are typically hybridized with resonant photonic modes of stand-alone optical cavities, such as Fabry-Pérot microcavities or plasmonic nanoantennas. Here, we show that thick flakes of layered van der Waals TMDCs can themselves serve as low quality resonators due to their high background permittivity. Optical modes of such “cavities” can in turn hybridize with excitons in the same material. We perform an experimental and theoretical study of such self-hybridization in thick flakes of four common TMDC materials: WS2, WSe2, MoS2, and MoSe2. We observe splitting in reflection and transmission spectra in all four cases and provide angle-resolved dispersion measurements of exciton-polaritons as well as thickness-dependent data. Moreover, we observe significant enhancement and broadening of absorption in thick TMDC multilayers, which can be interpreted in terms of strong light-matter coupling. Remarkably, absorption reaches >50% efficiency across the entire visible spectrum, while simultaneously being weakly dependent on polarization and angle-of-incidence. Our results thus suggest formation of self-hybridized exciton-polaritons in thick TMDC flakes, which in turn may pave the way towards polaritonic and optoelectronic devices in these simple systems.

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INTRODUCTION Strong coupling between excitons and optical cavities has attracted significant research attention recently motivated by its remarkable ability to modify the behavior of coupled systems. Spectroscopically, such regime of light-matter interaction is manifested as appearance of vacuum Rabi splitting in e.g. absorption spectra of the coupled system, corresponding to the formation of light-matter hybrid quasiparticles called cavity-exciton polaritons

1-4.

Because polaritons are compositional in nature, it is anticipated that both

optical- 5, 6 and material-related 7-11 properties may be modified in the strong coupling regime. The latter is a subject of intense contemporary research, which, among other things, motivates a search for novel polaritonic platforms. In this context, transition metal dichalcogenides (TMDCs)

12, 13

have recently emerged

as a promising class of materials for realizing strong coupling regime. Due to their large oscillator strength, mechanical stability, large exciton binding energy and valley degrees of freedom, these materials are ideal for room temperature polaritonic devices

14, 15.

Most

previous studies of strong coupling in these materials focus on monolayers of TMDCs owning to their direct bandgap and pronounced photoluminescence signal

17, 18.

16,

However,

multilayer and even bulk TMDCs are also interesting platforms for strong light-matter coupling

19.

In contrast to a monolayer, bulk TMDCs possess an indirect bandgap, which

makes light emission inefficient

12.

Nevertheless, thick TMDC materials have strong

absorption peaks 12, which in turn can couple to and hybridize with optical cavity modes. Strong light-matter coupling involving TMDC materials was achieved in various cavity-emitter configurations, including high quality factor dielectric cavities diffractive nanoparticle arrays

21,

or single plasmonic nanoparticles

19, 22-25.

14, 15, 20,

The common

denominator here is that in all investigations mentioned above, strong coupling was achieved using well-defined resonant optical modes supported by stand-alone resonators. However, TMDC flakes of sufficient thickness can exhibit Fabry-Pérot (FP) type of resonances themselves, due to their high background refractive index in the visible range. Thus, excitons in bulk TMDC can self-hybridize with a FP mode of the flake without the necessity for an external cavity. Moreover, this interaction can reach the strong coupling regime 26. Here, we study exciton-polaritons in thick flakes of four common TMDCs possessing excitons in the visible range and thus presenting interest in the context of light-matter interactions: WS2, WSe2, MoS2, and MoSe2. We mechanically exfoliate


27

thick flakes

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(thickness in the range of 20 to 160 nm) of all four TMDCs and measure their angle-resolved reflectivity spectra. For all four materials, we observe formation of exciton-polaritons in angle-resolved dispersion measurements. Remarkably, while WS2 and WSe2 show coupling of two resonances, molybdenum-based TMDCs show hybridization of both A- and Bexcitons with FP modes of the flakes, giving rise to three exciton-polaritons in the spectra. Additionally, we observe broadband absorption over the entire visible range exceeding 50% in thick flakes, which can be interpreted in the context of strong coupling. Moreover, such high and broadband absorption turns out to be weakly dependent on the angle of incidence and polarization, making these materials highly interesting for light harvesting applications, such as photodetectors and photovoltaic devices

28-30.

Thus, our results suggest that a

strikingly simple platform – thick TMDC flakes – can be suitable for studies of excitonpolariton physics and optoelectronics applications.

Figure 1. Overview of the system under study. (a) Sketch of the system: a thick slab of a bulk TMDC material MX2 is illuminated by a plane wave, which excites a FP cavity mode of the slab. (b) Reflection spectra at normal incidence (calculated using the transfer matrix method

31)

from a thin 5 nm TMDC flake on a glass

substrate (blue), from a hypothetical 70 nm TMDC flake with the oscillator strength of A-exciton switched off forming the cavity mode (green), and from a 70 nm TMDC flake with the exciton resonance switched on. (c) Exemplary reflection spectra collected under normal incidence from WS2 multilayer flakes of various thicknesses. Inset shows a bright field image of the corresponding multilayers. Flake I does not support FP modes and features only a single Fano-like resonance at the A-exciton frequency, whereas Flakes II and III possess FP modes that couple to the exciton of the material. Vertical dashed lines in (b, c) show the position of A-exciton in WS2.

Results and Discussions


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The system under study is schematically depicted in Figure 1a. A thick slab of a bulk TMDC material is illuminated by a plane wave, which excites a cavity mode in the slab. All four TMDC materials studied here have very high real part of the background permittivity 32 (ranging from 15 to 20), which is comparable or even higher than the best high-index semiconductor materials, such as silicon or germanium 33. Due to such high refractive index, flakes of subwavelength (𝜆/2𝑛60-80 nm) thickness should exhibit the first order FP cavity mode in the visible range, which is indicated by the green line in Figure 1b that shows reflection from a hypothetical 70 nm thick TMDC material where the transition corresponding to the A-exciton is artificially switched off (see Methods). This mode is expected to couple to absorption bands of bulk TMDCs

26,

potentially giving rise to strong

exciton-FP mode coupling, as indicated by the red curve in Figure 1b, where the complete permittivity of a bulk TMDC was used. Figure 1c shows a bright field image, as well as exemplary reflection spectra collected from WS2 multilayer flakes of various thicknesses at ambient conditions. An optically thin flake that does not possess FP modes in the visible range (Flake I), exhibits only a single Fano-like resonance in the reflection spectrum associated with the A-exciton of the material. However, when the thickness increases up to 60-70 nm, a FP cavity mode appears close to the exciton energy, giving rise to FP-exciton hybridization and splitting. Such behavior can be easily visualized by observing the flakes in bright field, as shown in the inset of Figure 1c. Flake I with thickness of ~27 nm appears white in reflection, indicating that it absorbs only a narrow part of the spectrum due to exciton resonance. At the same time, Flakes II (thickness ~62 nm) and III (~68 nm) appear blue, due to additional absorption at the self-hybridized exciton-polariton states. TMDC multilayers in our experiments were fabricated by mechanical exfoliation from high quality bulk crystals onto polydimethylsiloxane (PDMS) stamps and transferred on glass substrates (see Methods). Thicknesses of all flakes used in the experiments were determined by atomic force microscopy (AFM, NTEGRA Prima, NT-MDT) in non-contact mode (see Methods for further details).


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Figure 2. Reflection spectra from four types of TMDC flakes at normal incidence: (a) WS2, (b) WSe2, (c) MoS2, and (d) MoSe2, respectively. Orange (top) curves in each plot show reflectivity from an optically thin flake exhibiting uncoupled exciton resonance, whereas blue, green, and red curves show reflectivity spectra from flakes of various thicknesses where exciton is coupled to the cavity modes. Vertical dashed lines indicate the resonant energies of bare A- and B-excitons. Gray curves are guides for the eye indicating evolution of excitonpolaritons in resonant flakes.

First, we measured reflection spectra under quasi-normal incidence using a long working distance 20 objective (Nikon, NA=0.45) (see Methods) for flakes of different thicknesses for all four materials. To ensure reproducibility, we characterized more than a hundred different TMDC flakes (data available upon request). The data is highly reproducible and shows smooth transition from simple absorption lines for thin flakes to hybrid broadband absorption in thick multilayers. For this reason, for each material we choose to show four exemplary samples, such that one of the flakes is optically thin and thus does not selfhybridize, whereas the other three support FP cavity modes exhibiting positive, near-zero, and negative detuning with respect to the exciton resonance(s), respectively. Figure 2


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summarizes the data collected from sixteen representative samples (four samples for each material). Thin non-resonant WS2 and WSe2 flakes exhibit a pronounced dip corresponding to their A-exciton and another weaker dip at higher energy corresponding to the B-exciton. At the same time, thin MoS2 and MoSe2 flakes show two relatively closely spaced dips with similar amplitudes corresponding to A- and B-excitons. For thicker flakes the behavior is drastically different. In particular, we observe the appearance of additional dips: two and three pronounced dips for WS2/WSe2 and MoS2/MoSe2 flakes, respectively, indicating coupling of TMDC excitons to FP modes of the same flakes. In addition, all reflectivity dips demonstrate gradual red shift with increasing thickness, which is consistent with the standard FP mode behavior. In the case of WS2 and WSe2, only A-exciton is resonant with FP mode resulting in formation of upper (UP) and lower (LP) polaritons. The reflectivity spectra do not show any signs of B-exciton coupling to the cavity mode, since in the range of thicknesses studied here the high-energy B-exciton is off-resonant with the first FP mode (see Figure 1a, b). The MoS2 and MoSe2 flakes, on the contrary, possess two spectrally close A- and B-excitons in such a way that both excitons couple to the same cavity mode simultaneously, resulting in formation of three (upper, middle, and lower – UP, MP and LP) polaritons. A remarkable feature of reflection spectra observed for all four materials is that the LP always exhibits a narrower line width compared to the UP or MP. This can be understood on account of the fact that the LP is an anti-symmetric combination of the cavity mode and the exciton

2-4,

which gives it a sub-radiant nature and a narrower line width compared to the

super-radiant UP state that has a symmetric composition. For further quantitative analysis, the Rabi splitting values were estimated from the normal incidence reflection spectra using a simple expression Ω𝑅 = 𝜔 + ― 𝜔 ― close to zero cavity-exciton detuning, with 𝜔 ± being the frequencies of the UP and LP, respectively. The obtained values of the vacuum Rabi splitting in reflection for A-exciton in WS2 and WSe2 and A- and B-excitons in MoS2 and MoSe2 are ranging in between 150 meV for MoS2 (Aexciton) and up to 235 meV for WS2. We note that reflection spectra at normal incidence in general are not sufficient to draw conclusions on the regime of interaction based on the appearance of additional dips, as the coupling strength cannot be extracted with certainty from these data. One way to confirm


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strong coupling is to measure angular dispersions of exciton-polaritons. Since the FP cavity mode energy displays distinct dependence on the incidence angle, exciton-polaritons, being mixed light-matter states, should inherit this dependence. To provide more insight into the system response, we therefore measured angle-resolved reflection spectra from the same samples. The dispersion measurements were carried out using s-polarized incidence and total internal reflection 60 oil-immersion objective (Nikon, NA=1.49) (see Methods), which allowed collecting data above the critical angle. The results demonstrate dispersive reflection dips exhibiting anti-crossing behavior typical for strongly coupled structures for all four types of TMDC flakes: WS2, WSe2, MoS2, and MoSe2 (see Figure 3). Reflection spectra of thin non-resonant flakes (Figures 3a, e, i, m) display only flat dips corresponding to bare A- and B-excitons of different materials. Thicker TMDC flakes, clearly exhibit dispersive reflection dips due to the angle-dependence of the cavity mode. This anti-crossing behavior is a manifestation of exciton self-hybridization. WS2 and WSe2 multilayers show the most prominent anti-crossings between the UP and LP. MoX2 flakes exhibit less pronounced anti-crossings, likely due to the presence of two closely spaced A- and B-exciton transitions leading to formation of three polaritonic branches, but their dispersions can still be identified.


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Figure 3. False-color maps of experimentally measured angle-resolved reflection spectra for s-polarized incidence for sixteen selected TMDC flakes: (a-d) WS2, (e-h) WSe2, (i-l) MoS2, and (m-p) MoSe2. Horizontal lines in each case mark the spectral position of A-exciton for WS2 and WSe2, and A- and B-excitons for MoS2 and MoSe2 in bare materials, respectively. Curves are theoretical fits of the coupled oscillator model to the experimental data. Note different color scales for thin and thick flakes.

To shed more light on the problem, we performed calculations of angle-resolved reflection spectra using the standard transfer matrix method 31. Calculations were performed assuming bulk dielectric function for all flakes extracted from the literature values

32.

The

results are shown in Figure 4. The dispersion was calculated assuming s-polarized incidence, to mimic the experimental conditions. Overall, we find excellent agreement between theory (Figure 4) and experiments (Figure 3), which confirms appearance of mode anti-crossing in


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angle-resolved reflectivity for all four studied materials. The calculated data is also in agreement with the coupled oscillator model (see Methods).

Figure 4. Calculated angle-resolved reflectivity spectra for s-polarization for TMDC flakes of various thicknesses (indicated on each plot): (a-d) WS2, (e-h) WSe2, (i-l) MoS2, and (m-p) MoSe2. Horizontal lines in each case mark the spectral position of A-exciton for WS2 and WSe2, and A- and B-excitons for MoS2 and MoSe2 in bare materials, respectively. White dotted curves are fits using the coupled oscillator model. Note different color scales for thin and thick flakes.

It is important to emphasize that true strong coupling should show mode splitting in absorption in addition to transmission and reflection

34-36.

To verify this, we studied

absorption spectra for several exemplary samples. We evaluate absorption from experimentally measured transmission and reflection spectra in accordance to the standard


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expression 𝐴 = 1 ― 𝑅 ― 𝑇 (we ignored scattering contribution since it is negligible in these samples). For the full range of 𝐴, 𝑇, and 𝑅 spectra we refer the reader to Figure S1, while comparison between theoretically calculated normal incidence absorption and experiment is shown in Figure S2. The measured absorption spectra for flakes of various thicknesses are shown in Figure 5 in comparison to absorption in a corresponding TMDC monolayer. These spectra do not allow claiming splitting in absorption with certainty, although signs of broadening and appearance of additional spectral features can be observed for flakes of sufficient thickness. Such inconclusive behavior can be attributed to the fact that the cavity itself is formed by a highly dispersive Lorentz medium, which leads to the presence of multiple eigenmodes close to the resonance frequency of the material and therefore blurring the splitting in absorption

37.

Moreover, experimentally measured absorption spectra at

normal incidence show significantly enhanced absorption in a broad spectral range compared to monolayer TMDCs (black lines in Figure 5). Interestingly, for some configurations, absorption exceeds 50% in the entire visible range, which includes energies much lower than the A-exciton transition. Similar observations were recently made for selenide TMDCs, where such behavior was attributed to a breakdown of the Beer-Lambert law 38.

Figure 5. Experimental absorption spectra for various thicknesses of (a) WS2, (b) WSe2, (c) MoS2, and (d) MoSe2, respectively. Black curves depict theoretical absorption spectra for monolayers (1L) of WS2, WSe2, MoS2, and MoSe2, respectively.


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We note that due to experimental constrains, we were able to measure absorption only at normal incidence (Figure 5). However, we complement these measurements with calculated angle-resolved absorption maps in the Supporting Information (SI) for both s- and p-polarized configurations (see Figures S3-4). The important outcome of these calculations is that for thick TMDC flakes: (a) one can obtain significant, >50%, absorption across a broad spectral range – significantly broader and more pronounced than that of the monolayer and thin multilayer TMDCs, and (b) the absorption is weakly angle- and polarization-dependent. Such combination makes thick TMDC materials studied here very promising for light harvesting applications 28-30. We also note that by placing a TMDC multilayer (or a Lorentz slab) in between two metallic mirrors, the splitting in absorption is easily observed and reaches as much as 200 meV for an 88 nm thick sample (see Figure 6). These additional experiments and calculations show that improving the quality factor of the FP resonator by using metallic mirrors facilitates observation of the vacuum Rabi splitting in this case. We also show that anti-crossing between the polaritonic bands here can be observed in the angle-dependent reflection and absorption plots (see Figure 6c-f for both s- and p-polarizations). These results were obtained using the transfer matrix method and bulk dielectric constant of TMDC materials. Similar to the case of bare TMDC multilayer flakes shown in Figures S3-4, the angle-dependence in this case is rather weak.


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Figure 6. (a) Experimental and (b) Theoretical plots for transmission (T), reflection (R) and absorption (A) spectra at normal incidence for a thick WS2 flake (d~88 nm) sandwiched between two gold mirrors. Absorption spectrum clearly shows Rabi splitting approaching 200 meV. Splitting in absorption here is observed because of higher quality factor of the FP resonator due to presence of 20 nm gold mirrors. Angle-dependent reflection (c, d) and absorption (e, f) spectra for the same FP-WS2 microcavity configuration as in (b) calculated for s- and p-polarization, correspondingly. Dispersion spectra show clear anti-crossing of polaritonic branches in both reflection and absorption with the Rabi splitting of around 200 meV in both quantities. Horizontal dotted lines show the position of uncoupled A-exciton in WS2 multilayer.

To elaborate more on the problem of mode splitting in absorption of a thick TMDC flake, we performed additional theoretical analysis using two simplified models: first, a system of a hypothetical lossless optical mode coupled to an exciton (see Figure S5) and, second, absorption in a thick slab of a Lorentzian material (see Figures S6-8). To illustrate the qualitative difference between the two situations, we refer the reader to the SI (Figures S5-6 accompanied with an additional discussion). An explicitly defined lossless cavity coupled to an exciton resonance exhibits a clear anti-crossing in absorption at sufficiently high oscillator strength contrary to the experiments. The Lorentian slab model, however, does not show a similar behavior even for large oscillator strength of the Lorentz material, although broadening and mode bending - clear signatures of hybridization - are observed in accordance with the experimental results. This indicates that the present system is more complicated than just two coupled oscillators for a reason already pointed out above: highly dispersive mirrors introduce a myriad of eigenmodes that blur splitting in absorption. Therefore, the combined effect of material dispersion and geometry must be taken into account to obtain reliable predictions. To illustrate this hybridization further, we theoretically investigate dependence of absorption spectra on the TMDC flake thickness using the bulk dielectric function data for all four materials (see Figure 7). All studied TMDC materials start exhibiting strong broadband absorption for flake thicknesses above 40-60 nm depending on the material. This strong absorption is accompanied by a complex fine structure, which appears due to exciton-FP mode self-hybridization. In particular, the results show signs of mode bending and anticrossing in qualitative agreement with a simplified Lorentz slab model (Figure S6). This behavior is especially pronounced in selenide TMDCs materials, where considerable absorption is found at energies far below A-exciton resonances. We also notice that absorption consistently increases with the thickness and saturates at about 50-60%. Such high absorption, together with its weak polarization and angle-of-incidence dependence (see Figures S3-4) is very promising for light harvesting applications.


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Figure 7. Theoretical absorption spectra of TMDC flakes (calculated at normal incidence) as a function of thickness for (a) WS2, (b) WSe2, (c) MoS2, and (d) MoSe2, respectively. Horizontal dashed lines show positions of A-excitons for WS2 and WSe2 and A- and B-excitons for MoS2 and MoSe2 in bulk materials.

Conclusions To conclude, we observed self-hybridization of excitons in 40-100 nm thick flakes of WS2, WSe2, MoS2, and MoSe2 with its own FP-like modes. We presented both normal incidence and angle-resolved reflectivity spectra from flakes of TMDCs possessing FP cavity modes and identified the strong coupling regime for some samples. Our results suggest that these simple systems may provide a feasible platform for realization of exciton-polaritons and thus may be used to study rich physics of these quasiparticles. For example, such systems could be a useful playground for studies of various effects that rely on collective excitonpolariton behavior, such as enhanced electrical conductivity 8 and exciton transport 39. In a broader context, one should emphasize that self-hybridization observed here is not a unique feature of TMDCs. Similar observations were previously made in inorganic semiconductor nanowires

40,

hybrid perovskite nanowires and plates

41,

halide perovskite

nanostructures supporting hybridized Mie – exciton modes 42, TMDC nanotubes 43, as well as in generic Lorentzian slabs (studied here in Figures S6-8). Moreover, guided polaritonic


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modes have been observed in multilayers of WSe2 and MoSe2 44, 45. This implies that excitonpolariton systems are in fact observed ubiquitously 46, which in turn suggests that improved exciton transport and other polariton-related phenomena may be within reach in a broad class of materials. The truly unique features of TMDC, however, are their layered van der Waals appearance, strong exciton absorption lines and their exceptionally high background permittivity, which all together enable facile observation of exciton-FP mode selfhybridization in this case. Such systems therefore provide broad and pronounced absorption, which may find use in light harvesting

47

and photovoltaic

28-30

applications based on strong

light-matter coupling.

Methods Fabrication. Multilayer flakes of four different materials were mechanically exfoliated onto a PDMS stamp using a scotch tape method and transferred onto a glass substrate

27.

To

determine thicknesses of multilayer flakes, we used atomic force microscopy (AFM, NTEGRA Prima, NT-MDT) in non-contact mode. AFM measurements were performed using gold-coated single crystal silicon tips with a tip curvature radius of 6 nm (NSG01) at resonance frequency of typical 150 kHz. First, the whole flake has been scanned for an overview with either 512 or 256 pixels at scan rate of 0.75 Hz. Afterwards, a more detailed scan of the area of interest has been done with a step of ~20-30 nm. Later, AFM images were analyzed by the Gwyddion software to extract the thickness. Reflectivity measurements. Reflection spectra at normal incidence were collected using a 20 objective (Nikon, NA=0.45), directed to a fiber-coupled spectrometer and normalized with reflection from a standard dielectric coated silver mirror. Dispersion relations in reflection were measured using a 60 oil-immersion objective (Nikon, NA=1.49) in the back focal imaging setup. Spectral images were obtained using a visible and near-infrared tunable liquid crystal filters (LCF) combined with an electron-multiplying charge-coupled device (EMCCD) camera (Andor, iXon) 48. Artificial permittivities. In order to calculate the exemplary reflection spectra presented in Fig. 1(b) we introduced the following artificial permittivity approximating the typical bulk TMDC: 𝜀art = 𝜀0 + 𝑓0𝜔2

𝑒𝑥

𝜔2𝑒𝑥 ― 𝜔2 ― 𝑖𝛾𝑒𝑥𝜔

, where 𝜀0 = 20 is the background permittivity due to


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higher energy transitions, 𝑓0 = 0.2 is the oscillator strength, 𝜔𝑒𝑥 = 2 eV, and 𝛾𝑒𝑥 = 50 meV is the exciton full width. For background index-only material, 𝑓0 was set to zero.

AUTHOR INFORMATION The authors declare no competing financial interest. †

These authors contributed equally to this work.

ACKNOWLEDGMENTS The authors acknowledge financial support by Knut and Alice Wallenberg Foundation, Engkvist foundation and the Swedish Research Council (VR, grant number 2016-06059).


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REFERENCES

1. Scully, M. O.; Zubairy, M. S., Quantum optics. Cambridge University Press: Cambridge, 1997. 2. Khitrova, G.; Gibbs, H. M.; Kira, M.; Koch, S. W.; Scherer, A. Vacuum Rabi splitting in semiconductors. Nat Phys 2006, 2, (2), 81-90. 3. Törmä, P.; Barnes, W. L. Strong coupling between surface plasmon polaritons and emitters: a review. Reports on Progress in Physics 2015, 78, (1), 013901. 4. Baranov, D. G.; Wersäll, M.; Cuadra, J.; Antosiewicz, T. J.; Shegai, T. Novel nanostructures and materials for strong light-matter interactions. ACS Photonics 2017. 5. Birnbaum, K. M.; Boca, A.; Miller, R.; Boozer, A. D.; Northup, T. E.; Kimble, H. J. Photon blockade in an optical cavity with one trapped atom. Nature 2005, 436, (7047), 87-90. 6. Englund, D.; Faraon, A.; Fushman, I.; Stoltz, N.; Petroff, P.; Vuckovic, J. Controlling cavity reflectivity with a single quantum dot. Nature 2007, 450, (7171), 857-861. 7. Hutchison, J. A.; Schwartz, T.; Genet, C.; Devaux, E.; Ebbesen, T. W. Modifying Chemical Landscapes by Coupling to Vacuum Fields. Angewandte Chemie International Edition 2012, 51, (7), 1592-1596. 8. Orgiu, E.; George, J.; Hutchison, J. A.; Devaux, E.; Dayen, J. F.; Doudin, B.; Stellacci, F.; Genet, C.; Schachenmayer, J.; Genes, C.; Pupillo, G.; Samori, P.; Ebbesen, T. W. Conductivity in organic semiconductors hybridized with the vacuum field. Nat Mater 2015, 14, (11), 1123-1129. 9. Galego, J.; Garcia-Vidal, F. J.; Feist, J. Suppressing photochemical reactions with quantized light fields. Nature Communications 2016, 7, 13841. 10. Herrera, F.; Spano, F. C. Cavity-Controlled Chemistry in Molecular Ensembles. Physical Review Letters 2016, 116, (23), 238301. 11. Munkhbat, B.; Wersäll, M.; Baranov, D. G.; Antosiewicz, T. J.; Shegai, T. Suppression of photo-oxidation of organic chromophores by strong coupling to plasmonic nanoantennas. Science Advances 2018, 4, (7), eaas9552. 12. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nat Nano 2012, 7, (11), 699-712. 13. Mak, K. F.; Shan, J. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nature Photonics 2016, 10, 216. 14. Sun, Z.; Gu, J.; Ghazaryan, A.; Shotan, Z.; Considine, C. R.; Dollar, M.; Chakraborty, B.; Liu, X.; Ghaemi, P.; Kéna-Cohen, S.; Menon, V. M. Optical control of room-temperature valley polaritons. Nat Photon 2017, 11, (8), 491-496. 15. Chen, Y.-J.; Cain, J. D.; Stanev, T. K.; Dravid, V. P.; Stern, N. P. Valleypolarized exciton–polaritons in a monolayer semiconductor. Nature Photonics 2017, 11, 431. 16. Liu, X.; Galfsky, T.; Sun, Z.; Xia, F.; Lin, E.-c.; Lee, Y.-H.; Kéna-Cohen, S.; Menon, V. M. Strong light–matter coupling in two-dimensional atomic crystals. Nat Photon 2015, 9, (1), 30-34. 17. Gan, X.; Gao, Y.; Mak, K. F.; Yao, X.; Shiue, R.-J.; Zande, A. v. d.; Trusheim, M. E.; Hatami, F.; Heinz, T. F.; Hone, J.; Englund, D. Controlling the spontaneous emission rate of monolayer MoS2 in a photonic crystal nanocavity. Applied Physics Letters 2013, 103, (18), 181119.


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18. Wang, G.; Chernikov, A.; Glazov, M. M.; Heinz, T. F.; Marie, X.; Amand, T.; Urbaszek, B. Colloquium: Excitons in atomically thin transition metal dichalcogenides. Reviews of Modern Physics 2018, 90, (2), 021001. 19. 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.; Baumberg, J. J. Strong-coupling of WSe2 in ultra-compact plasmonic nanocavities at room temperature. Nature Communications 2017, 8, (1), 1296. 20. 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.; Novoselov, K. S.; Smith, J. M.; Skolnick, M. S.; Krizhanovskii, D. N.; Tartakovskii, A. I. Exciton– polaritons in van der Waals heterostructures embedded in tunable microcavities. Nature Communications 2015, 6, 8579. 21. Liu, W.; Lee, B.; Naylor, C. H.; Ee, H.-S.; Park, J.; Johnson, A. T. C.; Agarwal, R. Strong Exciton–Plasmon Coupling in MoS2 Coupled with Plasmonic Lattice. Nano Letters 2016, 16, (2), 1262-1269. 22. Wen, J.; Wang, H.; Wang, W.; Deng, Z.; Zhuang, C.; Zhang, Y.; Liu, F.; She, J.; Chen, J.; Chen, H.; Deng, S.; Xu, N. Room-Temperature Strong Light–Matter Interaction with Active Control in Single Plasmonic Nanorod Coupled with Two-Dimensional Atomic Crystals. Nano Letters 2017, 17, (8), 4689-4697. 23. Zheng, D.; Zhang, S.; Deng, Q.; Kang, M.; Nordlander, P.; Xu, H. Manipulating Coherent Plasmon–Exciton Interaction in a Single Silver Nanorod on Monolayer WSe2. Nano Letters 2017, 17, (6), 3809-3814. 24. Cuadra, J.; Baranov, D. G.; Wersäll, M.; Verre, R.; Antosiewicz, T. J.; Shegai, T. Observation of Tunable Charged Exciton Polaritons in Hybrid Monolayer WS2−Plasmonic Nanoantenna System. Nano Letters 2018. 25. Stührenberg, M.; Munkhbat, B.; Baranov, D. G.; Cuadra, J.; Yankovich, A. B.; Antosiewicz, T. J.; Olsson, E.; Shegai, T. Strong Light–Matter Coupling between Plasmons in Individual Gold Bi-pyramids and Excitons in Mono- and Multilayer WSe2. Nano Letters 2018. 26. Wang, Q.; Sun, L.; Zhang, B.; Chen, C.; Shen, X.; Lu, W. Direct observation of strong light-exciton coupling in thin WS2 flakes. Optics Express 2016, 24, (7), 7151-7157. 27. Andres, C.-G.; Michele, B.; Rianda, M.; Vibhor, S.; Laurens, J.; Herre, S. J. v. d. Z.; Gary, A. S. Deterministic transfer of two-dimensional materials by all-dry viscoelastic stamping. 2D Materials 2014, 1, (1), 011002. 28. Jariwala, D.; Davoyan, A. R.; Tagliabue, G.; Sherrott, M. C.; Wong, J.; Atwater, H. A. Near-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics. Nano Letters 2016, 16, (9), 5482-5487. 29. Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. High Photovoltaic Quantum Efficiency in Ultrathin van der Waals Heterostructures. ACS Nano 2017, 11, (7), 7230-7240. 30. Jariwala, D.; Davoyan, A. R.; Wong, J.; Atwater, H. A. Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook. ACS Photonics 2017, 4, (12), 2962-2970. 31. Born, M.; Wolf, E., Principles of Optics. Cambridge University Press: Cambridge, 1999. 32. Li, Y.; Chernikov, A.; Zhang, X.; Rigosi, A.; Hill, H. M.; van der Zande, A. M.; Chenet, D. A.; Shih, E.-M.; Hone, J.; Heinz, T. F. Measurement of the optical dielectric function of monolayer transition-metal dichalcogenides: ${\mathrm{MoS}}_{2}$, $\mathrm{Mo}\mathrm{S}{\mathrm{e}}_{2}$, ${\mathrm{WS}}_{2}$, and $\mathrm{WS}{\mathrm{e}}_{2}$. Physical Review B 2014, 90, (20), 205422.


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ACS Photonics

33. Baranov, D. G.; Zuev, D. A.; Lepeshov, S. I.; Kotov, O. V.; Krasnok, A. E.; Evlyukhin, A. B.; Chichkov, B. N. All-dielectric nanophotonics: the quest for better materials and fabrication techniques. Optica 2017, 4, (7), 814-825. 34. Savona, V.; Andreani, L. C.; Schwendimann, P.; Quattropani, A. Quantum well excitons in semiconductor microcavities: Unified treatment of weak and strong coupling regimes. Solid State Communications 1995, 93, (9), 733-739. 35. Antosiewicz, T. J.; Apell, S. P.; Shegai, T. Plasmon–Exciton Interactions in a Core–Shell Geometry: From Enhanced Absorption to Strong Coupling. ACS Photonics 2014, 1, (5), 454-463. 36. Zengin, G.; Gschneidtner, T.; Verre, R.; Shao, L.; Antosiewicz, T. J.; MothPoulsen, K.; Käll, M.; Shegai, T. Evaluating Conditions for Strong Coupling between Nanoparticle Plasmons and Organic Dyes Using Scattering and Absorption Spectroscopy. The Journal of Physical Chemistry C 2016, 120, (37), 20588-20596. 37. Nechepurenko, I. A.; Baranov, D. G.; Dorofeenko, A. V. Lasing induced by resonant absorption. Optics Express 2015, 23, (16), 20394-20401. 38. Stevens, C. E.; Stroucken, T.; Stier, A. V.; Paul, J.; Zhang, H.; Dey, P.; Crooker, S. A.; Koch, S. W.; Karaiskaj, D. Superradiant coupling effects in transition-metal dichalcogenides. Optica 2018, 5, (6), 749-755. 39. Feist, J.; Garcia-Vidal, F. J. Extraordinary Exciton Conductance Induced by Strong Coupling. Physical Review Letters 2015, 114, (19), 196402. 40. Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Single-nanowire electrically driven lasers. Nature 2003, 421, 241. 41. Shuai, Z.; Qiuyu, S.; Wenna, D.; Jia, S.; Zhiyong, W.; Yang, M.; Jie, C.; Fengjing, L.; Yuanzheng, L.; Mei, L.; Qing, Z.; Xinfeng, L. Strong Exciton–Photon Coupling in Hybrid Inorganic–Organic Perovskite Micro/Nanowires. Advanced Optical Materials 2018, 6, (2), 1701032. 42. Tiguntseva, E. Y.; Baranov, D. G.; Pushkarev, A. P.; Munkhbat, B.; Komissarenko, F.; Franckevičius, M.; Zakhidov, A. A.; Shegai, T.; Kivshar, Y. S.; Makarov, S. V. Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles. Nano Letters 2018. 43. Yadgarov, L.; Višić, B.; Abir, T.; Tenne, R.; Polyakov, A. Y.; Levi, R.; Dolgova, T. V.; Zubyuk, V. V.; Fedyanin, A. A.; Goodilin, E. A.; Ellenbogen, T.; Tenne, R.; Oron, D. Strong light–matter interaction in tungsten disulfide nanotubes. Physical Chemistry Chemical Physics 2018, 20, (32), 20812-20820. 44. Fei, Z.; Scott, M. E.; Gosztola, D. J.; Foley, J. J.; Yan, J.; Mandrus, D. G.; Wen, H.; Zhou, P.; Zhang, D. W.; Sun, Y.; Guest, J. R.; Gray, S. K.; Bao, W.; Wiederrecht, G. P.; Xu, X. Nano-optical imaging of $\mathrm{WS}{\mathrm{e}}_{2}$ waveguide modes revealing light-exciton interactions. Physical Review B 2016, 94, (8), 081402. 45. Hu, F.; Luan, Y.; Scott, M. E.; Yan, J.; Mandrus, D. G.; Xu, X.; Fei, Z. Imaging exciton–polariton transport in MoSe2 waveguides. Nature Photonics 2017, 11, 356. 46. Low, T.; Chaves, A.; Caldwell, J. D.; Kumar, A.; Fang, N. X.; Avouris, P.; Heinz, T. F.; Guinea, F.; Martin-Moreno, L.; Koppens, F. Polaritons in layered twodimensional materials. Nature Materials 2016, 16, 182. 47. Eizner, E.; Brodeur, J.; Barachati, F.; Sridharan, A.; Kéna-Cohen, S. Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light–Matter Coupling. ACS Photonics 2018. 48. Zengin, G.; Wersäll, M.; Nilsson, S.; Antosiewicz, T. J.; Käll, M.; Shegai, T. Realizing Strong Light-Matter Interactions between Single-Nanoparticle Plasmons and Molecular Excitons at Ambient Conditions. Physical Review Letters 2015, 114, (15), 157401.


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Theory

X M

Experiment Flake I (d ~ 27 nm)

(b)

Cavity

Coupled

Reflectivity (a.u.)

Exciton


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

ACS Photonics

Flake II (d ~ 62 nm)

Flake III (d ~ 68 nm)

1

X

M = W, Mo X = S, Se

2

(a)

3

1.5

2.0 2.5 Energy (eV)


1.5

(c) 2.0 2.5 Energy (eV)

ACS Photonics

WSe2

WS2 Reflectivity (a.u.)

d ~ 13 nm

d ~ 35 nm d ~ 71 nm

d ~ 53 nm

d ~ 80 nm

d ~ 68 nm

d ~ 105 nm d ~ 76 nm

(a) 1.6

1.8

2.0

2.2

2.4

(b) 1.6

1.8

2.0

2.2

2.4

MoSe2

MoS2 Reflectivity (a.u.)

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

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d ~ 25 nm

d ~ 45 nm

d ~ 52 nm

d ~ 64 nm

d ~ 79 nm d ~ 65 nm d ~ 100 nm d ~ 75 nm

(c) 1.6

1.8

2.0

2.2

2.4

(d) 1.4

Energy (eV)


1.6

1.8

2.0

Energy (eV)

2.2

Page 23 of 28

0.2

Energy (eV)

2.4

1.0

(a)

WS2

0.1

1.0

(b)

WS2

(c)

WS2 (d)

WS2

2.2 2 1.8 1.6

d ~ 13 nm

Energy (eV)

2.4

(e)

WSe2

(f)

WSe2

d ~ 35 nm

2.2

d ~ 68 nm

d ~ 53 nm

(g)

d ~ 76 nm

WSe2 (h)

WSe2

d ~ 80 nm

d ~ 71 nm

d ~ 105 nm

2 1.8 1.6

Energy (eV)

2.4

(i)

MoS2

(j)

MoS2

(k)

MoS2 (l)

MoS2

2.2 2 1.8 1.6

d ~ 25 nm 2.4

Energy (eV)

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

ACS Photonics

(m)

MoSe2

(n)

MoSe2

d ~ 45 nm

2.2

d ~ 65 nm

d ~ 52 nm

(o)

d ~ 75 nm

MoSe2 (p)

MoSe2

d ~ 79 nm

d ~ 64 nm

d ~ 100 nm

2 1.8 1.6 -1

-0.5

0

sinq

0.5

1

-1

-0.5

0

sinq

0.5

1

-1

-0.5

0

sinq


0.5

1

-1

-0.5

0

sinq

0.5

1

ACS Photonics

0.4

Energy (eV)

2.4

WS2

(a)

WS2

(b)

WS2

(c)

WS2

(d)

2.2 2 1.8 1.6

20 nm

Energy (eV)

2.4

(e)

WSe2

65 nm

(f)

WSe2

20 nm

2.2

(g)

75 nm

70 nm

76 nm

WSe2 (h)

WSe2

80 nm

90 nm

2 1.8 1.6

Energy (eV)

2.4

(i)

MoS2 (j)

MoS2

(k)

MoS2 (l)

MoS2

2.2 2 1.8 1.6

25 nm 2.4

Energy (eV)

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

Page 24 of 28

(m)

MoSe2

55 nm

(n)

MoSe2

20 nm

2.2

60 nm

(o)

MoSe2

65 nm

(p)

MoSe2

80 nm

65 nm

85 nm

2 1.8 1.6 -1

-0.5

0

sinq

0.5

1

-1

-0.5

0

sinq

0.5

1

-1

-0.5

0

sinq


0.5

1

-1

-0.5

sinq

0.5

Page 25 of 28

1-T-R

1.0

(a)

1L 27 nm 62 nm 76 nm

Ws2

1.0

(b)

0.5

0.5

0.0 1.4

0.0 1.4

1.0

1-T-R

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

ACS Photonics

1.6

(c)

1.8

2.0

2.2

2.4 1L 25 nm 65 nm 75nm

MoS2

1.0

(d)

0.5

0.5

0.0 1.4

0.0 1.4

1.6

1.8 2.0 2.2 Energy (eV)

2.4

1.6


1.6

1L 35 nm 80 nm 105nm

WSe2

1.8

2.0

2.2

MoSe2

1.8 2.0 2.2 Energy (eV)

2.4 1L 15 nm 100 nm 125 nm

2.4

ACS Photonics

Experiment 1.0

Absorption

Reflection

Gold mirror

0.5

R T A

Energy (eV)

Gold mirror

s-polarization

WS2

(a)

2.4

2.4

2.2

2.2

2

2

1.8

1.8

(с)

1.6

(e)

1.6

0.0 -1

Theory 1.0

Gold mirror

0

0.5

R T A 0.0 1.4

1.6

(b) 1.8

2.0

Energy (eV)

2.2

2.4

Energy (eV)

Gold mirror

-1

1

2.4

2.4

2.2

2.2

2

2

1.8

1.8

88 nm WS2

p-polarization

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

Page 26 of 28

1.6 -1

(d) 0 Sinq


0

(f)

1.6

1

-1

1

0 Sinq

1

Page 27 of 28

WS2

(b)

WSe2

(c)

MoS2

(d)

MoSe2

Energy (eV)

(a)

Energy (eV)

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

ACS Photonics

thickness (nm)


thickness (nm)

ACS Photonics

Exciton

”Cavity”

Coupled

X M

X

M = W, Mo X = S, Se

1.5


2.0 2.5 Energy (eV)


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

Page 28 of 28

Self-hybridized exciton-polaritons in multilayers of transition metal

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