Planar Focusing of Anisotropic Plasmon Polaritons in MoOCl 2 /Au
Jianqiao Zhao, Yuxin Zhang, Xiaotian Xue, Haodong Zhong, Tongzhou Xu, Yaolong Li, Jingbo Sun, Weipeng Wang, Ji Zhou, Shufeng Wang, Zhengjun ZhangABSTRACT
Hyperbolic polaritons offer a promising approach for subwavelength light confinement and manipulation. While extensively explored across mid‐infrared to terahertz bands, achieving long‐distance focusing at visible frequencies remains challenging due to the trade‐off between long propagation and strong compression. In this work, we demonstrate planar focusing of anisotropic plasmon polaritons with an extended focal length in a MoOCl 2 /Au heterostructure. By integrating an Au metastructure with the natural hyperbolic material MoOCl 2 , we convert surface plasmon polaritons (SPPs) into surface plasmon hyperbolic polaritons (SPHPs), enabling excitation and focusing of the low‐momentum hybrid anisotropic polariton in the heterostructure. Furthermore, by optimizing the thickness of the MoOCl 2 layer, we synergistically enhance both the propagation distance and the field compression. Using transmission‐mode scanning near‐field optical microscopy (SNOM) and quantitative methods, we demonstrate a focal length of 2.5λ and a focal spot with a full width at half maximum (FWHM) of λ /5 under 633 nm excitation, accompanied by a threefold energy enhancement. This visible‐frequency long‐focus system offers significant potential for noncontact nanoscale probing, integrated photonics, and optical interconnects. The proposed heterostructure engineering approach provides a versatile design paradigm for coupling hyperbolic polaritons with multiple plasmonic excitations, paving the way for advanced photonic applications.