Room Temperature Heterodyne Detection up to 70GHz With Antenna-Coupled Quantum-Well Photodetectors Operating at 10μm

III-V semiconductor mid-infrared photodetectors based on intersubband transitions hold a great potential for ultra-high-speed operation up to several hundreds of GHz. In this work we exploit a ~350nm-thick GaAs/Al 0.2 Ga 0.8 As multi-quantum-well heterostructure to demonstrate heterodyne detection at $\lambda\sim 10\mu \mathrm{m}$ with a nearly flat frequency response up to 70GHz at room temperature, solely limited by the measurement system bandwidth. To the best of our knowledge this is the broadest RF-bandwidth reported to date for a quantum-well mid-infrared photodetector. Responsivities of 0.15A/W and 1.5A/W are obtained at 300K and 77K respectively. To allow ultrafast operation and illumination at normal incidence, the detector consists of a $50\Omega$ coplanar waveguide, monolithically integrated with a 2D-array of patch antennas, electrically interconnected by suspended wires. With this device architecture we obtain a parasitic capacitance of $\sim 30\text{fF}$ , corresponding to the static capacitance of the antennas, yielding a RC-limited 3dB cutoff frequency >150GHz at 300K, extracted with a small-signal equivalent circuit model. Using this model, we quantitively reproduce the detector frequency response and find intrinsic roll-off time constants as low as 1ps at room temperature.