We present the design, fabrication, and experimental characterisation of a broadband waveguide power divider for submillimeter-wave receiver front ends requiring low-loss power splitting and high output-port isolation over the 260–380 GHz frequency range. The architecture is particularly suited to local-oscillator distribution in sideband-separating heterodyne receivers, where equal-amplitude splitting, low insertion loss, good port matching, and high isolation between receiver branches are required to minimise standing waves and preserve image-rejection performance. The proposed architecture is based on a modified E-plane waveguide T-junction incorporating an integrated thin-film titanium nitride dissipative load, which realises a virtual fourth port to absorb reflected power and suppress coupling between the output ports. The virtual fourth port is implemented using a waveguide probe connected to a lumped TiN termination through an RF choke filter that provides a broadband virtual RF ground. The divider is fabricated using split-block waveguide machining combined with thin-film microfabrication of the integrated termination chip. Room-temperature vector network analyser measurements show output-port isolation better than 15 dB across the targeted band, return loss better than 15 dB at all ports over the full band and better than 20 dB over most of it, an intrinsic excess insertion loss below 0.1 dB, and amplitude imbalance below 0.4 dB between the output arms. These results demonstrate that integrated TiN thin-film terminations provide a compact and broadband solution for LO power splitting in 2SB SIS receiver architectures.
Next-generation communication systems are poised to operate in sub-terahertz bands, unlocking ultra-high data rates through abundant spectral bandwidth. However, beyond 0.1 THz, severe ohmic losses in metallic conductors and strong absorption in polymers render conventional copper cables and polymer-based waveguides impractical. This drives an urgent need for compact, low-loss THz waveguiding solutions. In this work, we propose and experimentally validate a hollow-core fiber with hybrid cladding, featuring a simplified antiresonant structure combined with a metallic outer coating that reflects leaked fields back into the core to mitigate propagation loss. The fiber was fabricated via 3D printing and wrapped with aluminium foil, and characterized in the D-band (0.11–0.17 THz), demonstrating proof-of-concept operation. Measurements reveal six antiresonant transmission bands spanning 0.1–1 THz. With compact outer dimensions of 4.56 mm × 5.25 mm, the fiber achieves simulated and measured losses of 6.6 dB/m and 27.4 dB/m, respectively, at 0.145 THz. The measured birefringence at the frequency of interest is $10^{-3}$. Moreover, simulations predict a group delay of 4.2 $\pm$ 0.3 ns, a birefringence of $10^{-2}$ and an air-core power fraction of 86%–93% in the 0.13–0.17 THz region. This design achieves an exceptionally small $D_{\text{core}}/\lambda$, representing the smallest reported 3D-printed hollow core fiber with competitive loss performance in the sub-THz regime. Moreover, it offers a practical, compact potential alternative to copper interconnects for terahertz transmissions while overcoming the size limitations of previously reported THz hollow core fibers.
This article presents a low-loss and high-efficiency TM01-mode radial power combiner operating from 320 to 355 GHz for terahertz (THz) power-combination applications. First, a compact transducer is developed by integrating a five-step septum and a circular-to-rectangular waveguide taper for TM01 to-TE10 conversion. This configuration reduces the required mode-conversion length and consequently lowers insertion loss (IL). Subsequently, a 10-way TM01-mode radial combiner is designed and integrated with the transducer to form the complete radial power combiner. Measurements of the back-to-back combiner show a return loss better than 10 dB from 320 to 355 GHz. The average IL of a single combiner is approximately 0.69 dB, corresponding to a passive combining efficiency (PCE) of 85%. In particular, the IL reaches its minimum of 0.54 dB at 342.2 GHz, corresponding to a PCE of 88%. The proposed combiner provides a compact, high-efficiency power-combining solution for high-power terahertz systems.
We demonstrate the generation of broadband ter ahertz pulses by optical rectification in a 1-mm-thick undoped gallium phosphide (GaP) crystal pumped by 100-fs pulses from a Cr:forsterite laser operating at λ = 1.24 μm. In a collinear geometry, a conversion efficiency of ∼ 5.8 × 10−3% is achieved. At a pump fluence of ∼ 10.5 mJ/cm2, the THz pulse energy is 0.15 μJ and the peak electric field strength in the focal plane reaches 0.5 MV/cm. The emitted THz spectrum extends up to ∼ 5 THz, in good agreement with theoretical calculations, while the usable bandwidth for spectroscopic applications is limited to ∼ 3 THz. From the measured transmittance versus pump intensity, the three-photon absorption coefficient in GaP at 1.24 μm is estimated as γ ≈ 2.9 × 10−4 cm3/GW2. These results suggest that GaP crystals pumped by Cr:forsterite lasers may offer a promising route to high-field broadband THz sources.
We report the design, fabrication, and experimental characterization of a 300-GHz-band superconductor-insulator-superconductor (SIS) mixer prototype for the Black Hole Explorer (BHEX) mission. The mixer employs a three-junction series SIS array, a waveguide probe with a microstrip impedance transformer, and an on-chip IF tuning circuit. As a first implementation of the baseline RF design, the mixer chip was fabricated using an established aluminum-oxide-barrier SIS junction process. Test junctions on the same wafer showed a critical current density of approximately 12 kA/cm2. The fabricated mixer chip was mounted in a waveguide mixer block and evaluated at a mixer-block temperature of 4 K by the Y-factor method. The measured double-sideband receiver noise temperature was 26–41 K at LO frequencies of 240–307 GHz. At 285 GHz, a noise temperature of approximately 30 K was obtained over the 4–9 GHz IF band, close to the two-photon sensitivity target adopted for the BHEX SIS receiver. These results demonstrate low-noise operation of the first prototype and provide feedback for improving RF/IF bandwidth and mixer stability in subsequent designs.
