We report a systematic, comprehensive set of measurements on the dynamics and noise processes in diffusion and phonon-cooled superconducting hot-electron bolometer mixers which will serve as ultralow noise detectors in THz heterodyne receivers. The conversion efficiency and output noise of devices of varying lengths were measured with radio frequency between 8 and 40 GHz. The devices studied consist of 100-Å-thin film Nb bridges connected to thick ͑1000 Å͒, high conductivity normal metal ͑Au͒ leads. The lengths of the devices studied range from 0.08 to 3 m. For devices longer than the electron-phonon interaction length L e-ph ϵͱD e-ph , with D the diffusion constant and e-ph Ϫ1 the electron-phonon interaction rate, the hot electrons are cooled dominantly by the electronphonon interaction, which in Nb is too slow for practical applications. If the device length is less than L e-ph (Ϸ1 m at 4.2 K͒, then out diffusion of heat into the high conductivity leads dominates the cooling process. In this limit, the intermediate frequency ͑IF͒ bandwidth is found to vary as L Ϫ2 , with L the bridge length, as expected for diffusion cooling. The shortest device has an IF bandwidth greater than 6 GHz, the largest reported for a low-T c superconducting bolometric mixer. The dominant component of the output noise decreases with frequency in the same manner as the conversion efficiency, consistent with a model based on thermal fluctuations. The noise bandwidth is larger than the gain bandwidth, and the mixer noise is low, ranging from 100 to 530 K ͑double sideband͒. The crossover from phonon dominated to diffusion dominated behavior is also demonstrated using noise thermometry measurements in the normal state. Scalar measurements of the device differential impedance in the intermediate state agree with a theoretical model which takes into account the thermal and electrical dynamics. We also present detailed comparisons with theoretical predictions of the output noise and conversion efficiency.

We are developing hot-electron superconducting transition-edge sensors (TES) capable of counting THz photons and operating at = 0 3 K. We fabricated superconducting Ti nanosensors with Nb contacts with a volume of 3 10 3 m 3 on planar Si substrates and have measured the thermal conductance in the material, = 4 10 14 W K at 0.3 K, caused predominantly by the weak electron-phonon coupling. The corresponding phonon-noise = 3 10 19 W Hz 1 2 . Detection of single optical photons (1550 nm and 670 nm wavelength) has been demonstrated for larger devices and yielded the thermal time constants of 30 s at 145 mK and of 25 s at 190 mK. This Hot-Electron Direct Detector (HEDD) is expected to have a small enough energy fluctuation noise for detecting individual photons with 1 THz where NEP 3 10 20 W Hz 1 2 is needed for spectroscopy in space.

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 0.30 pmxO. 15 urn 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 (DSB) noise temperature of the receiver was as low as 2750+ 250 K, with an estimated mixer noise temperature of = 900 K. The mixer bandwidth derived from both noise bandwidth and IF impedance measurements was =1.4 GHz. These results demonstrate the low-noise operation of the diffusion-cooled bolometer mixer above 2 THz.