Room-temperature, continuous-wave terahertz generation in free-space with an intersubband mid-infrared photomixer

We demonstrate a terahertz photomixer pumped by mid-infrared photons at a wavelength of 10 μm. The device is based on a dual-antenna architecture, in which a two-dimensional array of mid-infrared patch-antenna resonators is connected to the electrodes of a log-spiral terahertz antenna. By exploiting intersubband absorption inside an AlGaAs–GaAs multi-quantum-well heterostructure, a photocurrent is coherently generated at the difference frequency between two quantum cascade lasers. The photocurrent drives the antenna electrodes allowing tunable, continuous-wave generation in free-space up to 1 THz at room temperature. By experimentally studying the photomixer frequency response and dark impedance, we demonstrate that the observed power roll-off at high-frequency is limited by the combined effect of the photoexcited carrier’s intrinsic lifetime and of the device RC time constant. In the spectral range investigated, we obtain, in continuous-wave, mid-infrared → terahertz conversion efficiencies exceeding by orders of magnitude those of unipolar devices exploiting χ(2) processes.