Ultra-strong coupling with spin-split heavy-hole cyclotron resonances in strained Ge quantum wells

We report the observation of multiple polariton branches in an ultra-strongly coupled system of Landau level transitions in strained Ge quantum wells (sGe QWs) and a THz metamaterial. The sGe QW system shows narrow spin-split heavy-hole cyclotron resonances and thus leads to multiple cyclotrons. The coupled system can be described by an expanded ultra-strong coupling Hamiltonian including all oscillators in the system, leading to one polariton branch per involved oscillator.