Ultrastrong coupling in the near field of complementary split-ring resonators

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 subwavelength light confinement and the large optical dipole moment of the cyclotron transition yield record high normalized coupling rates of up to ? R/ ? c =0.87 . We observed a blue-shift of both polaritons due to the diamagnetic term of the interaction Hamiltonian.