  Applications relying on mid-infrared radiation (Mid-IR, $\lambda\sim$ 3-30
$\mu$m) have progressed at a very rapid pace in recent years, stimulated by
scientific and technological breakthroughs. Mid-IR cameras have propelled the
field of thermal imaging. And the invention of the quantum cascade laser (QCL)
has been a milestone, making compact, semiconductor-based mid-IR lasers
available to a vast range of applications. All the recent breakthrough advances
stemmed from the development of a transformative technology. In addition to the
generation and detection of light, a key functionality for most photonics
systems is the electrical control of the amplitude and/or phase of an optical
beam at ultra-fast rates (GHz or more). However, standalone, broadband,
integrated modulators are missing from the toolbox of present mid-IR photonics
integrated circuits and systems developers. We have developed a free-space
amplitude modulator for mid-IR radiation ($\lambda\sim$ 10 $\mu$m) that can
operate up to at least 1.5 GHz (-3dB cut-off at $\sim$ 750 MHz) and at
room-temperature. The device relies on a semiconductor hetero-structure
enclosed in a judiciously designed metal-metal optical resonator. At zero bias,
it operates in the strong light-matter coupling regime up to 300K. By applying
an appropriate bias, the device transitions to the weak coupling regime. The
large change in reflectivity due to the disappearance of the polaritonic states
is exploited to modulate the intensity of a mid-IR continuous-wave laser up to
speeds of more than 1.5 GHz.
