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Receivers based on superconducting Hot-Electron Bolometers (HEBs) are widely used for terahertz (THz) sensing
owing to their advantages of high sensitivity, low noise, and low LO power requirement. Balanced HEB mixers are
superior to single-element ones since the thermal noise and AM noise from the LO injection can be effectively
suppressed. Although a 1.3 THz balanced waveguide HEB mixer has been reported, waveguide mixer configurations
offer relatively narrow RF bandwidths. We report on the development, fabrication and characterization of a THz quasioptical
balanced superconducting HEB mixer utilizing a dual-polarization sinuous antenna that can potentially achieve
both multiband operation and ultra-high sensitivity. In the balanced mixer configuration, a lens-coupled four-arm
sinuous antenna was designed for operation from 0.2-1.0 THz with a nearly frequency-independent embedding
impedance of ~106 Ω. Two identical superconducting niobium HEB devices have been integrated at the antenna
feedpoints, connecting each opposing pair of antenna arms to form a balanced mixer configuration. An air-bridge was
also fabricated to separate the two mixer branches. The HEB devices were fabricated from 10 nm thick niobium film
sputtered on semi-insulating silicon substrates. Each HEB device has dimensions of 80 nm × 240 nm (3 squares) for
approaching a resistance of 105 Ω for impedance matching. Mixer properties including antenna radiation patterns,
broadband operation and polarization isolation have been characterized. Finally, in order to achieve multiband mixer
operation, electronically reconfigurable THz quasi-optical mesh filters are needed. Frequency-tunable antenna elements
using Schottky varactor diodes suitable for the above applications have been designed, simulated and demonstrated at Gband
(140-220 GHz) showing 50 GHz tuning range.
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