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

openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/965124/EU//FEMTOCHIP | openaire: EC/H2020/872049/EU//IPN-Bio | openaire: EC/H2020/834742/EU//834742 Funding Information: The authors acknowledge Dr. Pablo Alonso‐González, Dr. Javier Martín‐Sánchez, and Dr. Jiahua Duan (Departamento de Física, Universidad de Oviedo) for valuable discussions and constructive comments, and acknowledge Nanofab Lab @ NCNST for helping with sample fabrication. This work was supported by the National Key Research and Development Program of China (Grant No. 2020YFB2205701), the National Natural Science Foundation of China (Grant Nos. 51902065, 52172139, 51925203, U2032206, 52072083, and 51972072), Beijing Municipal Natural Science Foundation (Grant No. 2202062), and Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB36000000 and XDB30000000). F.J.G.A. acknowledges the ERC (Advanced Grant 789104‐eNANO), the Spanish MICINN (PID2020‐112625GB‐I00 and SEV2015‐0522), and the CAS President's International Fellowship Initiative (PIFI) for 2021. Z.P.S. acknowledges the Academy of Finland (Grant Nos. 314810, 333982, 336144 and 336818), The Business Finland (ALDEL), the Academy of Finland Flagship Programme (320167,PREIN), the European Union's Horizon 2020 research and innovation program (820423,S2QUIP; 965124, FEMTOCHIP), the EU H2020‐MSCA‐RISE‐872049 (IPN‐Bio), and the ERC (834742). Publisher Copyright: © 2022 Wiley-VCH GmbH. Manipulation of the propagation and energy-transport characteristics of sub-wavelength 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 alpha-MoO3. Specifically, an unprecedented change of effective focal length of polariton waves from 0.7 to 7.4 mu m 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 alpha-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 alpha-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.