  The ultrastrong light-matter interaction regime was investigated in metallic
and superconducting complementary split ring resonators coupled to the
cyclotron transition of two dimensional electron gases. The sub-wavelength
light confinement and the large optical dipole moment of the cyclotron
transition yield record high normalized coupling rates of up to
$\frac{\Omega_R}{\omega_c}=$ 0.87. We observed a blue-shift of both polaritons
due to the diamagnetic term of the interaction Hamiltonian.
