Optical beatnote detection from a portable THz QCL comb at 80 K by direct free space mixing in a high-frequency Hot Electron Bolometer

We present optical beatnote detection from a THz QCL comb operating at 80 K in a small nitrogen-cooled dewar. The 21.7 GHz comb beatnote is detected by downconversion, directly mixing free-space signals from the QCL and a microwave synthesizer onto an NbN HEB optimized for RF frequencies. The setup constitutes a very convenient platform for the study of QCL-based optical frequency combs and a building block for compact, portable frequency comb fast spectrometers.