Design and Characterisation of Terahertz Schottky Barrier Diode Mixers

Doctoral thesis, 2024

Efficient and compact terahertz frequency converters operating at ambient temperatures are essential for long lifetime space/air-borne missions. Yet, realising mixers in the 3–5 THz range remains challenging due to the lack of stable local oscillator sources, complexities in circuit fabrication, and the need for high-precision machining of E-plane split blocks.

This dissertation addresses these challenges with a focus on developing 3.5-THz and 4.7-THz harmonic and fundamental mixers. First, the effect of idler terminations on the harmonic mixer's performance was evaluated. Based on this theoretical study, 3.5-THz, x6-harmonic mixers were designed, featuring a planar, single-ended Schottky-barrier diode with a sub-micron anode on a 2-um GaAs membrane. The mixers exhibited a conversion loss of 60 dB at 3.5 THz and 76 dB at 4.7 THz. This was followed by a successful demonstration of phase-locking of quantum-cascade lasers, representing the state-of-the-art wide-band harmonic mixers using planar diodes in the 3-5 THz range.

In addition, this research focused on the design and experimental characterisation of 3.5-THz and 4.7-THz fundamental mixers. The initial video detection and noise temperature measurements indicated significant losses and poor RF coupling. To address these issues, a scaled-down experiment was performed to analyse the impact of E-plane misalignment on diagonal horn antennas in the WM-570 frequency band. An E-plane misalignment of approximately 8% of the wavelength led to a 3-dB degradation in the optical coupling to a Gaussian beam (Gaussicity) in the middle of the waveguide band. Based on this, an optimised design for a 4.7-THz, x8-harmonic mixer with an integrated pyramidal horn, less susceptible to E-plane asymmetry, is presented. Finally, an alternate measurement technique which enables broadband characterisation of THz harmonic mixers is demonstrated. These results represent the first broadband characterisation of THz harmonic mixers from 2.2 to 3 THz.

These results present essential findings that will enable the realisation of THz heterodyne receivers for detecting atomic and molecular species such as hydroxyl radical (OH) and atomic oxygen (OI), which are crucial for atmospheric and planetary science research.