  Polar van der Waals (vdW) crystals that support phonon polaritons have
recently attracted much attention because they can confine infrared and
terahertz (THz) light to deeply subwavelength dimensions, allowing for the
guiding and manipulation of light at the nanoscale. The practical applications
of these crystals in devices rely strongly on deterministic engineering of
their spatially localized electromagnetic field distributions, which has
remained challenging. This study demonstrates that polariton interference can
be enhanced and tailored by patterning the vdW crystal {\alpha}-MoO3 into
microstructures that support highly in-plane anisotropic phonon polaritons. The
orientation of the polaritonic in-plane isofrequency curve relative to the
microstructure edges is a critical parameter governing the polariton
interference, rendering the configuration of infrared electromagnetic field
localizations by enabling the tuning of the microstructure size and shape and
the excitation frequency. Thus, our study presents an effective rationale for
engineering infrared light flow in planar photonic devices.
