Fast Programming of in‐Plane Hyperbolic Phonon Polariton Optics through Van der Waals Crystals Using the Phase‐Change Material In 3 SbTe 2

The high directionality of hyperbolic phonon polaritons (HPhPs) has opened radically new ways to route and steer the flow of energy at the nanoscale. However, launching HPhPs requires fabricating efficient and precisely aligned polariton launching structures, what remains time‐consuming and expensive with conventional nanofabrication approaches. Recently, using visible laser pulses, polariton launching structures have been programmed into the plasmonic phase‐change material In 3 SbTe 2 . Here, we leverage this approach to tailor in‐plane hyperbolic polaritons by programming launching and confining nanostructures through α‐MoO 3 flakes deposited onto In 3 SbTe 2 . Importantly, optical programming after flake deposition enables alignment of launching stripes to the flake's [001]‐axis, essential to control the directional polariton propagation. We showcase i) an optically programmed focusing disk, showing similar tuning ranges and confinement as focusing by gold disks; and ii) a first experimental realization toward a nanocavity for in‐plane HPhPs created by reconfiguring the single disk to a double disk structure, tailoring the confinement by simply reprogramming the disk distance. Our fabrication scheme offers fast turnaround times, flexible alignment, and the opportunity to reversibly reconfigure structures within 30 s. Thus, it is a fast, efficient, and versatile way to tailor propagation and confinement of highly directional polaritons on demand.