Nano-Optical Engineering of Anisotropic Phonon Resonances in a Hyperbolic α-MoO3 Metamaterial

Low-dimensional alpha-phase molybdenum trioxide (alpha-MoO3), a layered van-der-Waals (vdW) semiconductor, has emerged as an attractive natural hyperbolic material supporting mid-infrared hyperbolic phonon polaritons (PhPs), which exhibit strong spatial confinement and low loss. With the advantages of strong in-plane optical anisotropy, many efforts have been devoted to investigating the properties of hyperbolic PhPs in alpha-MoO3 using scanning near-field optical microscopy. However, the studies of far-field controlling of hyperbolic PhPs in alpha-MoO3 have been quite limited so far. This work reports the first experimental demonstration of far-field excitation and manipulation of hyperbolic phonon resonances in metamaterial structures consisting of alpha-MoO3 nanodisks and slabs. The excited phonon resonances show a maximum spatial confinement factor (free space wavelength/period) of 32 and a quality factor of 76. It is shown that the in-plane phonon resonances in alpha-MoO3 can be selectively excited in the two Reststrahlen bands featuring hyperbolic dispersion relations along its two crystal directions, by simply controlling the incident polarization. In addition, the relative strength of resonances along different in-plane crystal directions and the resultant field distributions can be continuously reconfigured by varying the incident polarization angle. These findings pave the way for future development of novel nanophotonic devices based on hyperbolic PhPs in vdW materials.