Direct detection effect in small volume hot electron bolometer mixers

We measure the direct detection effect in a small volume ͑0.15 m ϫ 1 m ϫ 3.5 nm͒ quasioptical NbN phonon cooled hot electron bolometer mixer at 1.6 THz.We find that the small signal sensitivity of the receiver is underestimated by 35% due to the direct detection effect and that the optimal operating point is shifted to higher bias voltages when using calibration loads of 300 K and 77 K. Using a 200 GHz bandpass filter at 4.2 K the direct detection effect virtually disappears.This has important implications for the calibration procedure of these receivers in real telescope systems.