α-MoO ₃ is demonstrated as a mid-infrared biaxial hyperbolic crystal supporting in-plane phonon polaritons with concave wavefronts.

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid‐infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α‐MoO₃. The α‐MoO₃ supports strong hyperbolic PhPs in the mid‐infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ₀/120, which can be tailored by altering the thicknesses of the α‐MoO₃ 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal‐ion‐intercalation strategy. The results demonstrate α‐MoO₃ as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid‐infrared nanophotonic devices.

Graphene has great potential for enhancing light−matter interactions in a two-dimensional regime due to surface plasmons with low loss and strong light confinement. Further utilization of graphene in nanophotonics relies on the precise control of light localization properties. Here, we demonstrate the tailoring of electromagnetic field localizations in the mid-infrared region by precisely shaping the graphene into nanostructures with different geometries. We generalize the phenomenological cavity model and employ nanoimaging techniques to quantitatively calculate and experimentally visualize the two-dimensional electromagnetic field distributions within the nanostructures, which indicate that the electromagnetic field can be shaped into specific patterns depending on the shapes and sizes of the nanostructures. Furthermore, we show that the light localization performance can be further improved by reducing the sizes of the nanostructures, where a lateral confinement of λ₀/180 of the incidence light can be achieved. The electromagnetic field localizations within a nanostructure with a specific geometry can also be modulated by chemical doping. Our strategies can, in principle, be generalized to other two-dimensional materials, therefore providing new degrees of freedom for designing nanophotonic components capable of tailoring two-dimensional light confinement over a broad wavelength range.