Tunable Planar Focusing Based on Hyperbolic Phonon Polaritons in α-MoO₃

Manipulation of the propagation and energy-transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonic devices in photocatalysis, biosensing, and thermal management. In this context, metamaterials are useful composite materials, although traditional metal-based structures are constrained by their weak mid-IR response, while their associated capabilities for optical propagation and focusing are limited by the size of attainable artificial optical structures and the poor performance of the available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region is reported by exploiting highly oriented in-plane hyperbolic phonon polaritons in α-MoO₃. Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 μm is demonstrated by the following three different means of control: the dimension of the device, the employed light frequency, and engineering of phonon–plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO₃ permit the focal length and focal spot size to be reduced to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons supported in α-MoO₃ are combined with tunable surface-plasmon polaritons in graphene to realize in situ and dynamical control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications.