  A superconducting metasurface operating in the THz range and based on the
complementary metamaterial approach is discussed. Experimental measurements as
a function of temperature and magnetic field display a modulation of the
metasurface with a change in transmission amplitude and frequency of the
resonant features. Such a metasurface is successively used as a resonator for a
cavity quantum electrodynamic experiment displaying ultrastrong coupling to the
cyclotron transition of a 2DEG. A finite element modeling is developed and its
results are in good agreement with the experimental data. In this system a
normalized coupling ratio of $\frac{\Omega}{\omega_c}=0.27$ is measured and a
clear modulation of the polaritonic states as a function of the temperature is
observed.
