  As the field of terahertz (THz) photonics advances, we present a monolithic
gallium arsenide (GaAs) disk-shaped whispering gallery mode resonator that has
potential as a component in THz nonlinear optics. GaAs is a material with
significant optical nonlinearity which can be enhanced when the crystal is
shaped into a microdisk resonator. A 4-mm-disk-resonator was fabricated using
single-point diamond turning and was characterised to obtain a quality (Q)
factor of 2.2k at ~150 GHz and 1.4k at ~300 GHz. We also demonstrated the
blue-shifting of up to ~0.3 GHz of the THz modes using a block of metal. This
post-fabrication degree of freedom could be useful for phase-matching
requirements for nonlinear optical processes, such as detection based on
optical up-conversion of THz radiation. Such a compact, tunable and efficient
device could be integrated into nonlinear photonic platforms for THz
generation, manipulation and detection.
