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

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 (10⁶) reversible cycles without degradation. First-principles calculations reveal that the giant refractive index switching (Δn ≈ 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.