
**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 nitrideâ€, Nat. Rev. Mater., s41578-019-0124-1 **** (2019).
3. [3] D. N. Basov, M. M. Fogler and F. J. GarcÃa de Abajo. “Polaritons in van der Waals materialsâ€, 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 Materialsâ€, Nano Lett. **17** , 228-235 (2016).
5. [5] F. J. GarcÃa de Abajo. Optical excitation in electron microscopy, Rev, Mod. Phys. **82** , 1 (2010).
6. [6] A. Polman, M. Kociak and F. J. GarcÃa de Abajo. “Electron-beam spectroscopy for nanophotonicsâ€, 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 spectroscopyâ€, Sci. Adv. **4** , eaar7495 (2018).
8. [8] A. A. Govyadinov, A. KoneÄná, 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 microscopeâ€, Nat. Commun. **8** (95), 1-10 (2017).
9. [9] O. L. Krivanek, et. al. “Vibrational spectroscopy in the electron microscopeâ€, Nature **514** , 209-2012 (2014).
10.
\----------
Please **REGISTER** for PhD Mid-term Seminar Series on Jun 29, 2020 11:00 AM
CEST at:
[https://attendee.gotowebinar.com/register/1415037787868604427](https://attendee.gotowebinar.com/register/1415037787868604427)
After registering, you will receive a confirmation email containing
information about joining the webinar.
Thank you very much for your participation!