Low noise in a diffusion-cooled hot-electron mixer at 2.5 THz

The noise performance of a Nb hot-electron bolometer mixer at 2.5 THz has been investigated. The devices are fabricated from a 12-nm-thick Nb film, and have a $\mathrm{0.30\hspace{0.17em}\mu m\times 0.15\hspace{0.17em}\mu m}$ in-plane size, thus exploiting diffusion as the electron cooling mechanism. The rf coupling was provided by a twin-slot planar antenna on an elliptical Si lens. The experimentally measured double sideband noise temperature of the receiver was as low as $\text{2750±250\hspace{0.17em}}\mathrm{K}$ with an estimated mixer noise temperature of $\text{≈900\hspace{0.17em}}\mathrm{K}.$ The mixer bandwidth derived from both noise bandwidth and IF impedance measurements was $\text{≈1.4\hspace{0.17em}}\mathrm{GHz}.$ These results demonstrate the low-noise operation of the diffusion-cooled bolometer mixer above 2 THz.