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Development of NbN 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 $\mu W$. The LO power was coupled to the antenna via an extended hemispherical lens (1.3 mm in diameter). The design, fabrication, and measurement stages were performed by a collaborative effort between a Russian team from the Department of Physics at Moscow State Pedagogical University in Moscow, the Submillimeter Technology Laboratory at UMass/Lowell and the Department of Electrical and Computer Engineering at UMass/Amherst.^ 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 $\mu$W. A 3 dB conversion gain was demonstrated with the antenna-coupled device using a laser LO at 1.56 THz with an IF frequency of 500 KHz. A second laser was utilized as the rf source. Noise temperature for the NbN HEB mixer receiver of 5800 K has been demonstrated over the 1.25-1.75 GHz IF band. The mixer temperature was 2500 K and the total conversion loss was 27 dB.^ Further optimization of the receiver configuration and device fabrication as well as additional development in mixer modeling will gradually lead to lower noise temperatures. ^
Gerecht, Eyal, "Development of NbN hot electron bolometric mixer for terahertz frequencies: The phonon -cooled version" (1998). Doctoral Dissertations Available from Proquest. AAI9841871.