Ultrafast (≈10 GHz) mid-IR modulator based on ultrafast electrical switching of the light–matter coupling

We demonstrate a free-space amplitude modulator for mid-infrared radiation (λ ≈ 9.6 μm) that operates at room temperature up to at least 20 GHz (above the −3 dB cutoff frequency measured at 8.2 GHz). The device relies on the ultrafast 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 remains linear with input radio frequency power in the 0-9 dBm range. The increase in optical powers up to 25 mW exhibit a weak beginning of saturation a little bit below.