Tunable Hyperbolic Phonon Polaritons in a Suspended van der Waals α‐MoO₃ with Gradient Gaps

Abstract Highly confined and low‐loss hyperbolic phonon polaritons (HPhPs) sustained in van der Waals crystals exhibit outstanding potential to concentrate the long‐wave electromagnetic fields deep into the subwavelength region. However, precise tuning on the HPhP propagation characteristics is a critical challenge to facilitate its practical applications in nanophotonic devices and circuits. This study, by taking advantage of the varying air gaps in a suspended van der Waals α‐MoO₃ crystal, shows the feasibility to tune wavelength and damping rate of the HPhPs propagating inside the α‐MoO₃. The results indicate that the dependence of polariton wavelength on the gap distance for HPhPs in lower and upper Reststrahlen bands contradict each other. Most interestingly, the tuning range of the polariton wavelengths for HPhPs in the lower band, which exhibits in‐plane hyperbolicity, is wider than that for HPhPs in the upper band which exhibits out‐of‐plane hyperbolicity. A polariton wavelength elongation of up to 160% and a reduction in the damping rate up to 35% is recorded. These findings can not only provide fundamental insight into the nanoscale manipulation of light by polaritonic crystals, but also open up new opportunities for tunable nanophotonic applications.