THz Landau Polaritons in exfoliated graphene

Giacomo Scalari

Institute for Quantum Electronics, Department of Physics, ETH Zürich

The coupling between light and matter lies at the heart of many fundamental physical
phenomena and serves as a stepping stone for countless device applications. A particularly interesting regime is the so-called strong and ultrastrong light-matter coupling happening in optical cavities, where new quasi-particles called cavity polaritons emerge. New and intriguing quantum optical phenomena have been predicted in the ultrastrong coupling regime, when the coupling strength becomes comparable to the unperturbed frequency of the system [1].

trong light-matter coupling has been recently successfully explored in the GHz and
THz range with on-chip platforms, where metallic resonators with small cavity volumes are combined with high electron density materials to exploit the collective enhancement of the coupling [2–4]. Our laboratory developed a new platform to study ultrastrong light-matter coupling using the inter-Landau-level transitions in 2 dimensional electron gases hosted by semiconductor heterostructures strongly coupled to metallic split-ring resonators, the so-called Landau polaritons [2, 4].

n this talk we will review the state-of-the-art of Landau polaritons and we will discuss recent experimental results where a single strongly subwave-length resonator is coupled to a gated, micron-sized graphene flake and spectroscopically investigated employing a system of immersion lenses [5]. The sample features an electrical gate in order to modulate the electron density and, as a consequence, the coupling strength.
We observe clear ultrastrong coupling tunable in strength and magnetic field anticrossing as a function of the electron density.

These measurements open the way for the engineering of many-body phenomena and study the fundamental nature of light–matter interactions in quantum materials.

[1] C. Ciuti, G. Bastard, and I. Carusotto, Phys. Rev. B, 72, 11, 115303 (2005).
[2] G. Scalari et al., Science, 335, 6074, 1323 (2012).
[3] Y. Todorov et al., Phys. Rev. Lett., 105, 19, 196402 (2010).
[4] A. Bayer et al., Nano Lett., 17, 10, 6340 (2017).
[5] S. Rajabali et al., Nat. Commun., 13, 1, 2528 (2022).