Development of niobium nitride hot electron bolometric mixer for terahertz frequencies: The phonon-cooled version

NbN HEB mixers represent a promising approach for achieving receiver noise temperatures of a few times the quantum noise limit at frequencies above 1 THz. NbN HEB devices have been shown to have sufficient bandwidth for applications in astronomy, remote sensing, and plasma diagnostics in the FIR range. The NbN HEB is a phonon cooled bolometer in which the energy is transfered from the hot electrons to the substrate via inelastic collisions with phonons.;The development of an NbN HEB mixer contained two steps: (1) implementing mixing in a comparatively large "direct-coupled" prototype device which required LO power of a few milliwatts, and (2) optimization of the first step by the development of an "antenna-coupled" (quasi-optically coupled) device with an LO power level of less than one ;Mixing at 2.5 THz was demonstrated for the first time using the direct-coupled device achieving an intrinsic conversion loss of 23 dB. Sufficient level of LO power coupling at four different frequencies was demonstrated with the antenna-coupled device. The antenna/lens configuration has performed as well as expected insuring coupling to LO power of less than one ;Further optimization of the receiver configuration and device fabrication as well as additional development in mixer modeling will gradually lead to lower noise temperatures.