Design and characterisation of terahertz Schottky diode harmonic mixers

Licentiate thesis, 2021

Efficient, compact, and reliable terahertz frequency converters preferably operating at ambient temperature are crucial for stabilising far-infrared optical sources. This thesis focuses on the design and characterisation of terahertz Schottky diode harmonic mixers for frequency stabilisation of quantum-cascade lasers.
First, the influence of idler terminations on harmonic mixer performance and diode embedding impedance at radio and local oscillator frequencies was
studied. Based on this, a 3.5-THz, ×6-harmonic mixer was designed. A planar, single-ended Schottky diode with a sub-µm contact area was realised on a
2-µm suspended GaAs substrate. The integrated mixer circuit was assembled in an E-plane split-block with the aid of beam leads. The Schottky diode anode pad was designed to act as a radio frequency E-plane probe and extended as a beamlead to provide dc ground, resulting in an electrically compact circuit. The design was validated by conducting a sensitivity analysis of critical parameters that are susceptible to fabrication tolerances. Two integrated circuits with Schottky contact areas of 0.11 and 0.14 µm2 were assembled on mixer modules and dc/RF characterisation was carried out. Both harmonic
mixers exhibited a conversion loss of about 60 dB at an intermediate frequency of 200 MHz. When evaluated in a phase-locked loop, it resulted in a
signal-to-noise ratio of 40 dB, which was more than sufficient to phase lock the quantum-cascade lasers.
This work presents uncooled, efficient THz harmonic mixers for stabilising THz optical sources. It opens up many opportunities for building a THz heterodyne spectrometer with a high spectral resolution to detect gas species such as hydroxyl radical and atomic oxygen in the frequency range of 3-5 THz.