LAUSR

Manipulation of the propagation and energy transport characteristics of subwavelength infrared (IR) light fields is critical for the application of nanophotonics 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 and their optical propagation including focusing is limited by the size of artificial optical structure and poor performance of available active means of control. Herein, a tunable planar focusing device operating in the mid-IR region was 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 was demonstrated by the following three different means: the dimension of the device, light frequency, and phonon-plasmon hybridization. The high confinement characteristics of phonon polaritons in α-MoO3 permit the focal length and focal spot size to reduce to 1/15 and 1/33 of the incident wavelength, respectively. In particular, the anisotropic phonon polaritons in α-MoO3 were combined with the tunable surface plasmon polaritons in graphene to realize in situ and dynamic control of the focusing performance, thus paving the way for phonon-polariton-based planar nanophotonic applications. This article is protected by copyright. All rights reserved.