Implementation of ultra-broadband optical null media via space-folding

Optical null medium (ONM) has garnered significant attention in electromagnetic wave manipulation. However, existing ONM implementations suffer from either narrow operational bandwidths or low efficiency. Here, we demonstrate an ultra-broadband ONM design that simultaneously addresses both challenges - achieving broad bandwidth while preserving perfect impedance matching with air for near-unity transmittance. The proposed space-folding ONM is realized by introducing precisely engineered folds into a metal channel array, creating an effective dispersion-free medium that enables independent phase control in each channel. The design incorporates optimized boundary layers implemented through gradually tapered folding structures, achieving perfect impedance matching with the surrounding medium. Beam bending effect and broadband beam focusing effect are experimentally verified using the proposed space-folding ONM. Due to its simple material requirements, broadband characteristics, and high transmittance, the proposed space-folding ONM shows potential for applications in electromagnetic camouflage, beam steering devices and ultra-compact microwave components.