Elemental Selenium Phase‐Change Material for Scalable Ultra‐Low‐Loss Programmable Photonics

ABSTRACT Chalcogenide phase‐change photonics has revolutionized reconfigurable optics by enabling nonvolatile light‐matter control. While low‐loss phase‐change materials (PCMs) with near‐zero extinction coefficients ( k ≈ 0) unlock ideal phase‐only modulation, their growing compositional complexity hinders uniform fabrication and switching stability. Here, we introduce an elemental solution: wafer‐scale (8‐inch) selenium (Se) thin films achieving unprecedented uniformity, full near‐infrared transparency, and ultralow losses across the visible range, meanwhile sustaining one million (10 6 ) reversible cycles without degradation. First‐principles calculations reveal that the giant refractive index switching (Δ n ≈ 0.6) stems from the unique fracture‐reconfiguration dynamics of Se helical chains. Furthermore, we demonstrate a manufacturable, etch‐free platform with laser‐reconfigurable patterns for dynamic image generation and Gaussian‐vortex beam conversion. By harmonizing atomic simplicity with device‐grade scalability, this elemental PCM establishes a new paradigm for high‐performance programmable photonics.