A mid-infrared biaxial hyperbolic van der Waals crystal

TLDR

Hyperbolic media can confine light to arbitrarily small volumes, and two‑dimensional hyperbolic metasurfaces can support in‑plane hyperbolic guided modes, but nanopatterning imposes fabrication challenges that limit confinement. α‑MoO3 is a biaxial hyperbolic crystal with three distinct Reststrahlen bands, each aligned with a different crystalline axis, enabling naturally in‑plane hyperbolic polariton modes at mid‑infrared frequencies without patterning. Thin α‑MoO3 flakes support naturally in‑plane hyperbolic polariton guided modes at mid‑infrared frequencies, offering a new paradigm for manipulating and confining light in planar photonic devices.

Abstract

Hyperbolic media have attracted much attention in the photonics community due to their ability to confine light to arbitrarily small volumes and their potential applications to super-resolution technologies. The two-dimensional counterparts of these media can be achieved with hyperbolic metasurfaces that support in-plane hyperbolic guided modes upon nanopatterning, which, however, poses notable fabrication challenges and limits the achievable confinement. We show that thin flakes of a van der Waals crystal, α-MoO3, can support naturally in-plane hyperbolic polariton guided modes at mid-infrared frequencies without the need for patterning. This is possible because α-MoO3 is a biaxial hyperbolic crystal with three different Reststrahlen bands, each corresponding to a different crystalline axis. These findings can pave the way toward a new paradigm to manipulate and confine light in planar photonic devices.