  We present a complementary THz metasurface realised with Niobium thin film
which displays a quality factor Q=54 and a fully switchable behaviour as a
function of the temperature. The switching behaviour and the high quality
factor are due to a careful design of the metasurface aimed at maximising the
ohmic losses when the Nb is above the critical temperature and minimising the
radiative coupling. The superconductor allows the operation of the cavity with
an high Q and inductive elements with an high aspect ratio. Comparison with
three dimensional finite element simulations highlights the crucial role of the
inductive elements and of the kinetic inductance of the Cooper pairs in
achieving the high quality factor and the high field enhancement.
