Visible-frequency long-distance subwavelength focusing via surface plasmon hyperbolic polaritons (SPHPs)

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 frequency remains challenging due to the trade-off between long propagation and strong compression. In this work, we demonstrate a mode-selection strategy and experimental approach to achieve long-distance subwavelength in-plane focusing at visible frequency. By integrating an Au metastructure with the natural hyperbolic material MoOCl2, we convert surface plasmon polaritons (SPPs) into surface plasmon hyperbolic polaritons (SPHPs), enabling selective excitation and focusing of the low-momentum (low-k) mode in the hyperbolic material. Furthermore, by modulating the thickness of the MoOCl2 layer, we leverage the intrinsic high-k component to synergistically enhance both long propagation distance and strong 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 three-fold energy enhancement. This visible-band long-focus system offers significant potential for non-contact nanoscale probing, integrated photonics, and optical interconnects. The proposed mode-selection methodology provides a versatile design paradigm for coupling hyperbolic polaritons with multiple plasmonic excitations, paving the way for advanced applications in photonic information processing.