  We report low temperature, microwave transmission measurements on a new
switchable and tunable class of nonlinear metamaterials. A wireless two
dimensional array of Josephson junctions (JJ) is probed as a metamaterial where
each plaquette in the array is considered as a meta-atom. In the presence of
microwaves, this compact metamaterial of 30,000 connected meta-atoms
synchronizes the flow of Cooper pairs to yield a single robust resonant signal
with a quality factor of 2800 at the lowest temperature for our measurements.
The transmission signal is switched on and off and its amplitude and frequency
are tuned with either temperature, incident rf power or dc magnetic field.
Surprisingly, increasing the incident rf power above a threshold causes the
resonance to split into multiple discrete resonances that extend over a range
of 240 MHz for a wide temperature window. We posit that this effect is a new
incarnation of the inverse ac Josephson effect where the Josephson plasma
frequency locks to the rf drive frequency generating quantized frequency steps,
instead of voltage steps, in the absence of a dc bias.
