  Chiral engineering of TeraHertz (THz) light fields and the use of the
handedness of light in THz light-matter interactions promise many novel
opportunities for advanced sensing and control of matter in this frequency
range. Unlike previously explored methods, this is achieved here by leveraging
the chiral properties of highly confined THz surface plasmon modes. More
specifically, we design ultrasmall surface plasmonic-based THz cavities and THz
metasurfaces that display significant and adjustable chiral behavior under
modest magnetic fields. For such a prototypical example of non-hermitian and
dispersive photonic system, we demonstrate the capacity to magnetic field-tune
both the poles and zeros of cavity resonances, the two fundamental parameters
governing their resonance properties. Alongside the observed
handedness-dependent cavity frequencies, this highlights the remarkable ability
to engineer chiral and tunable radiative couplings for THz resonators and
metasurfaces. The extensive tunability offered by the surface plasmonic
approach paves the way for the development of agile and multifunctional THz
metasurfaces as well as the realization of ultrastrong chiral light-matter
interactions at low energy in matter with potential far-reaching applications
for the design of material properties.
