Observation of Resonant Faraday Rotation in Landau-Polaritons of a 2DEG

Strong light–matter coupling forms polariton states, which are hybridized modes of electromagnetic cavity fields and dipolar-active matter transitions. In the terahertz (THz) range, strong coupling can be achieved with solid-state quasiparticles such as phonons, magnons [1], or plasmons in two-dimensional electron gases (2DEG) [2]. Here, we experimentally study cyclotron resonance and magnetoplasmons excited in a high-mobility 2DEG based on GaAs/AlGaAs heterostructures coupled to Fabry–Perot cavity modes formed by the sample substrate itself. We demonstrate that strong coupling of 2DEG excitations occurs with Fabry–Perot cavities of hundreds of micrometers in size, and that the coupling strength can be efficiently tuned by thinning the substrate, which defines the cavity length. Experiments were performed at 10 K in an optical cryostat using THz time-domain reflection spectroscopy. We measured reflected signals in both co- and cross-polarized configurations as a function of out-of-plane magnetic field up to 2.5 T. Under these conditions, we observed clear Rabi oscillations in the time domain, corresponding to the formation of Rabi splitting in the frequency domain and respective Landau-polariton modes. Strong light–matter coupling enhances the resonant Faraday effect, allowing observation of a resonant rotation of the polarization plane by up to 90°.

[1] M. Białek, W. Knap, and J.-P. Ansermet, Phys. Rev. Applied 19, 064007 (2023).

[2] M. Geiser et al., Phys. Rev. Lett. 108, 106402 (2012).