Ultrastrong coupling in hexagonal Boron Nitride microcavities*

When the interaction between light and matter is very efficient, it is possible to attain the regime of ultrastrong coupling (USC), where a variety of quantum effects of considerable interest can be revealed [1]. In this contribution, we study an infrared Fabry-Perót microcavity formed by metallic mirrors, filled with hexagonal Boron Nitride (hBN) acting as a phononic material [2]. The optical response obtained within a transfer-matrix approacimation can be interpreted with the use of a semi-classical model of coupled harmonic oscillators. As a consequence of the large Reststrahlen band of hBN, the Rabi splitting obtained is of the order of the frequencies of the bare cavity and the phonons. The system is thus in the USC regime. We also show that the calculations are in excellent agreement with experimental measurements, which suggests the interest of this configuration as a practical setup to explore USC.
[1] A. F. Kockum et al., Nat. Rev. Phys. 1, 19-40 (2019).
[2] M. Autore et al., Light: Science and Applications 7, 17172 (2018).