ONLINE - PhD Mid-term Seminar Series: Probing and steering bulk and surface phonon polaritons in hexagonal boron nitride using fast electrons

CIC nanoGUNE Seminars

Carlos Maciel, Nanooptics Group
nanoGUNE online Webinar
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ONLINE - PhD Mid-term Seminar Series: Probing and steering bulk and surface phonon polaritons in hexagonal boron nitride using fast electrons **Probing and steering bulk and surface phonon polaritons in hexagonal boron nitride using fast electrons** Carlos Maciel _Nanooptics Group, CIC nanoGUNE_ The excitation and control of phonon polaritons (coupled excitations formed by optical phonons and light) in polar materials has been the object of intense study during the last years in Material Science and Nanophotonics. The ability of these quasi-particles to concentrate and enhance the electromagnetic field (with low intrinsic losses) deep below the diffraction limit promises the development of novel photonic technologies in the mid-infrared range [1-3]. Thus, extensive effort is being made to find new materials that support phonon polariton excitations. Hexagonal boron nitride (h-BN) is a promising candidate for that purpose, which exhibits mid-infrared phonon polaritons with remarkably low losses within the range of 90 – 200 meV in the so-called Reststrahlen bands (defined as the region between the transverse and longitudinal optical phonon frequencies) [2,4]. As a result, efficient design and utilization of h-BN structures require spectroscopic studies with adequate spatial resolution. This can be provided, for instance, by electron energy loss spectroscopy (EELS) using electrons as localized electromagnetic probes [5-9]. In this talk we present the excitation of phonon polaritons by fast electron beams in h-BN. To that end, we analyze the electron energy losses (EEL) experienced by a fast electron in two different scenarios: (i) when the fast electron travels through the bulk of h-BN and (ii) when it travels in aloof trajectories above an extended h-BN surface. In both cases, as revealed in the EEL spectra, the excitation of phonon polaritons is strongly related to the electron velocity and is highly dependent on the orientation between the electron beam trajectory and the h-BN optical axis. Our work provides a systematic study for understanding bulk and surface polaritons excited by a localized electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electrons velocity and its direction. **References** 1. [1] S. Dai, et. al., “Tunable Phonon Polaritons in Atomically Thin van der waals Crystals of Boron Nitride,” Science **343** , 1125–1129 (2014). 2. [2] J. D. Caldwell, I. Aharonovich, G. Cassabois, J. H. Edgar, B. Gil and D. N. Basov. “Photonics with hexagonal boron nitride”, Nat. Rev. Mater., s41578-019-0124-1 **** (2019). 3. [3] D. N. Basov, M. M. Fogler and F. J. García de Abajo. “Polaritons in van der Waals materials”, Science **354** , 6309 (2016). 4. [4] P. Li, I. Dolado, F. J. Alfaro-Mozaz, A. Y. Nikitin, F. Casanova, L. E. Hueso, S. Vélez and R. Hillenbrand. “Optical Nanoimaging of Hyperbolic Surface Polaritons at the Edges of van der Waals Materials”, Nano Lett. **17** , 228-235 (2016). 5. [5] F. J. García de Abajo. Optical excitation in electron microscopy, Rev, Mod. Phys. **82** , 1 (2010). 6. [6] A. Polman, M. Kociak and F. J. García de Abajo. “Electron-beam spectroscopy for nanophotonics”, Nat. Matter., (2019). 7. [7] F. S. Hage, R. J. Nicholls, J. R. Yates, D. G. McCulloch, T. C. Lovejoy, N. Delby, O. L. Krivanek, K. Refson and Q. M. Ramsse. “Nanoscale momentum-resolved vibrational spectroscopy”, Sci. Adv. **4** , eaar7495 (2018). 8. [8] A. A. Govyadinov, A. Konečná, A. Chuvilin, S. Vélez, I. Dolado, A. Y. Nikitin, S. Lopatin, F. Casanova, L. E. Hueso, J. Aizpurua and R. Hillenbrand. “Probing low-energy hyperbolic polaritons in van der Waals crystals with an electron microscope”, Nat. Commun. **8** (95), 1-10 (2017). 9. [9] O. L. Krivanek, et. al. “Vibrational spectroscopy in the electron microscope”, Nature **514** , 209-2012 (2014). 10. \---------- Please **REGISTER** for PhD Mid-term Seminar Series on Jun 29, 2020 11:00 AM CEST at: []( After registering, you will receive a confirmation email containing information about joining the webinar. Thank you very much for your participation!