Ultrabroadband Nanocavity of Hyperbolic Phonon-Polaritons in 1D-Like α-MoO3

The exploitation of phonon-polaritons in nanostructured materials offers a pathway to manipulate infrared (IR) light for nanophotonic applications. Notably, hyperbolic phonon-polaritons (HP2) in polar bidimensional crystals have been used to demonstrate strong electromagnetic field confinement, ultraslow group velocities, and long lifetimes (up to similar to 12 ps). Here we present nanobelts of alpha-phase molybdenum trioxide (alpha-MoO3) as a low-dimensional medium supporting HP2 modes in the mid- and far-IR ranges. Through real-space nanoimaging techniques with synchrotron and tunable laser IR light, we observe HP2 FabryPerot resonances that demonstrate distinct anisotropic propagation and frequency dependence. We remark an anisotropic propagation that critically depends on the frequency range. Our findings are supported by the convergence of experiment, theory, and numerical simulations. Our work shows that the low dimensionality of natural nanostructured crystals, like alpha-MoO3 nanobelts, provides an attractive platform to study polaritonic light-matter interactions and offers appealing cavity properties that could be harnessed in future designs of compact nanophotonic devices.