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III-V semiconductor mid-infrared photodetectors based on intersubbandtransitions hold a great potential for ultra-high-speed operation up to severalhundreds of GHz. In this work we exploit a ~350nm-thick GaAs/Al0.2Ga0.8Asmulti-quantum-well heterostructure to demonstrate heterodyne detection at 10umwavelength with a nearly flat frequency response up to 70GHz at roomtemperature, solely limited by the measurement system bandwidth. This is thebroadest RF-bandwidth reported to date for a quantum-well mid-infraredphotodetector. Responsivities of 0.15A/W and 1.5A/W are obtained at 300K and77K respectively. To allow ultrafast operation and illumination at normalincidence, the detector consists of a 50Ohm coplanar waveguide, monolithicallyintegrated with a 2D-array of sub-wavelength antennas, electricallyinterconnected by suspended wires. With this device architecture we obtain aparasitic capacitance of ~30fF, corresponding to the static capacitance of theantennas, yielding a RC-limited 3dB cutoff frequency >150GHz at 300K, extractedwith a small-signal equivalent circuit model. Using this model, we quantitivelyreproduce the detector frequency response and find intrinsic roll-off timeconstants as low as 1ps at room temperature.

Ultra-fast quantum-well infared photodetectors operating at 10{\mu}m with flat response up to 70GHz at room temperature