Gate tunable magneto-plasmon ultrastrongly coupled to LC cavity

We couple magneto-plasmons ultra-strongly to subwavelength split-ring resonator cavities, probed with THz time domain spectroscopy (TDS). In contrast to cyclotron-polaritons, the ultrastrong coupling regime is reached already at zero magnetic field. Further, the plasmon frequency ω
_ρ
is widely tunable using a gate and by changing the sample geometry. This thus allows to observe the transition from a cyclotron-polariton to a magneto-plasmon-polariton all the way to a new regime, where ω
_ρ
> ω
_cavity
. In the latter, the cavity couples again to the cyclotron resonance, but with a significantly lower coupling strength.