High-Q Phonon-polaritons in Spatially Confined Freestanding α-MoO₃

Highly confined and in-plane anisotropic phonon-polaritons (PhPs) in orthorhombic-phase molybdenum trioxide (α-MoO3) exist in three Reststrahlen bands (RB) in the mid-infrared regime where the crystal exhibits negative permittivity along three principal axes. However, PhP behaviors in geometrically confined α-MoO3 remain enigmatic. Here, we investigated PhPs confined in freestanding α-MoO3 covering submicron-width trenches. We remarkably observed opposite trends in terms of PhP wavelengths in two RBs for PhPs propagating on supported and spatially confined freestanding α-MoO3. Due to the geometric confinement in the submicron α-MoO3 freestanding channel, we resolved ultraconfined PhPs with the record-high quality (Q) factor of up to 40 at room temperature, 2 times higher than the previously reported highest value in α-MoO3. We further demonstrated PhPs guiding along a curved trajectory and, for the first time, proved PhPs could be guided to desired angles within spatially confined freestanding α-MoO3 channels. The outcomes of this work can be exploited for creating high-Q PhP waveguides toward directional nanophotonic and polaritonic devices.