  We demonstrate a free-space amplitude modulator for mid-infrared radiation
(lambda=9.6 um) that operates at room temperature up to at least 20 GHz (above
the -3dB cutoff frequency measured at 8.2 GHz). The device relies on the
ultra-fast transition between weak and strong-coupling regimes induced by the
variation of the applied bias voltage. Such transition induces a modulation of
the device reflectivity. It is made of a semiconductor heterostructure enclosed
in a judiciously designed array of metal-metal optical resonators, that -
all-together - behave as an electrically tunable surface. At negative bias, it
operates in the weak light-matter coupling regime. Upon application of an
appropriate positive bias, the quantum wells populate with electrons and the
device transitions to the strong-coupling regime. The modulator transmission
keeps linear with input RF power in the 0dBm - 9dBm range. The increase of
optical powers up to 25 mW exhibit a weak beginning saturation a little bit
below.
