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. A planar polariton focusing device based on the natural in‐plane hyperbolic van der Waals material α‐MoO3 is developed, which achieves ultrahigh field compression and wide‐range tunable performance. Moreover, a graphene/α‐MoO3 heterostructure is constructed, which supports hybrid modes consisting of α‐MoO3 phonon polaritons and graphene plasmons to achieve in situ dynamical control of the focal length.