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Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride

Abstract

Phonon polaritons enable light confinement at deep subwavelength scales, with potential technological applications, such as subdiffraction imaging, sensing and engineering of spontaneous emission. However, the trade-off between the degree of confinement and the excitation efficiency of phonon polaritons prevents direct observation of these modes in monolayer hexagonal boron nitride (h-BN), where they are expected to reach ultrahigh confinement. Here, we use monochromatic electron energy-loss spectroscopy (about 7.5 meV energy resolution) in a scanning transmission electron microscope to measure phonon polaritons in monolayer h-BN, directly demonstrating the existence of these modes as the phonon Reststrahlen band (RS) disappears. We find phonon polaritons in monolayer h-BN to exhibit high confinement (>487 times smaller wavelength than that of light in free space) and ultraslow group velocity down to about 10−5c. The large momentum compensation provided by electron beams additionally allows us to excite phonon polaritons over nearly the entire RS band of multilayer h-BN. These results open up a broad range of opportunities for the engineering of metasurfaces and strongly enhanced light–matter interactions.

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Fig. 1: Measurement of vibrational spectra of h-BN in a wide momentum range.
Fig. 2: Electron energy-loss spectra of the h-BN flake with thickness of 10 nm.
Fig. 3: EELS spectra of monolayer h-BN.
Fig. 4: Ultraslow group velocity in monolayer h-BN.

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Data availability

The data presented in Figs. 14 are provided with the paper as source data. Any other relevant data are also available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by the National Key R&D Programme of China (grant numbers 2016YFA0300903, 2016YFA0201600 and 2019YFA0307801), the National Natural Science Foundation of China (grant numbers 11888101, 11974023, 51672007, 51925203, 11674073, 51972074, 11974001 and U1932153), Key-Area Research and Development Programme of Guangdong Province (grant numbers 2018B030327001, 2018B010109009 and 2019B010931001), Bureau of Industry and Information Technology of Shenzhen (grant number 201901161512), the National Equipment Programme of China (grant number ZDYZ2015-1), the ‘2011 Programme’ from the Peking-Tsinghua-IOP Collaborative Innovation Centre of Quantum Matter, the key programme of the Bureau of Frontier Sciences and Education, Chinese Academy of Sciences (grant number QYZDB-SSW-SLH021), the Key Research Programme of the Chinese Academy of Sciences (grant number ZDBS-SSW-JSC002), Youth Innovation Promotion Association CAS and CAS Interdisciplinary Innovation Team (grant number JCTD-2018-03), the European Research Council (advanced grant number 789104-eNANO), the Spanish MINECO (grant numbers MAT2017-88492-R and SEV2015-0522) and Beijing Natural Science Foundation (grant numbers 2192022 and Z190011). We thank the Electron Microscopy Laboratory at Peking University for the use of electron microscopes, C. L. Shi and T. C. Lovejoy from Nion for advice on EELS experiments and J. Feng from the International Centre for Quantum Materials, Peking University, for advice on DFT calculations.

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Contributions

The concept for the experiment was initially developed by P.G., Q.D. and X.Y. Hexagonal BN thin flakes were grown by Z.X. and monolayer h-BN by Yifei Li under the supervision of K.L. and L.L. STEM–EELS experiments were performed by N.L., assisted by R.S. and Yuehui Li under the direction of P.G. FEM simulations and s-SNOM experiments were performed by X.G. under the supervision of Q.D. and X.Y. DFT calculations of h-BN phonon dispersion curves were performed by T.Q. and MNPBEM simulations were performed by R.Q. under the supervision of P.G. Data processing and analysis were performed by N.L. and X.G. F.J.G.A. provided theoretical advice. N.L., X.G. and X.Y. wrote the manuscript with input from P.G. and Q.D. N.L. and X.G. contributed equally to this work. All authors discussed the results at all stages and participated in the development of the manuscript.

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Correspondence to Xiaoxia Yang, Qing Dai or Peng Gao.

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Supplementary information

Supplementary Information

Supplementary Figs 1–13 and Notes 1–4.

Source data

Source Data Fig. 1

DFT data points of Fig. 1c.

Source Data Fig. 2

Experimental data points of Fig. 2a,b.

Source Data Fig. 3

Experimental data points of Fig. 3a.

Source Data Fig. 4

Experimental data points of Fig. 4a,b.

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Li, N., Guo, X., Yang, X. et al. Direct observation of highly confined phonon polaritons in suspended monolayer hexagonal boron nitride. Nat. Mater. 20, 43–48 (2021). https://doi.org/10.1038/s41563-020-0763-z

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