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 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. 
Curdin Maissen and Giacomo Scalari and Federico Valmorra and Sara   Cibella and Roberto Leoni and Christian Reichl and Chris...
cond-mat.mes-hall