Ultrawideband Emission of Bi ³⁺ Ions Spanning Visible to Near‐Infrared Spectral Regions (400 nm–1700 nm)

Abstract Although Bi 3+ ions exhibit exceptional luminescent properties and spectral tunability, their potential as activators for near‐infrared (NIR) ultrawideband emission remains underexplored, while transition metal ions such as Ni 2+ , Fe 3+ , and Cr 3+/4+ have long been the predominant candidates in this demanding research area. Herein, we demonstrate ultrawideband emission from 400 to 1700 nm in K 2 MgGeO 4 :Bi 3+ (KMGO:Bi 3+ ), achieving an internal quantum efficiency (IQE) of 88.02% and external quantum efficiency (EQE) of 66.41%. The emission spectrum features interconnected visible and NIR bands, peaking at 614 and 1125 nm, respectively. Notably, the full width at half‐maximum (FWHM) of the NIR band exceeds 340 nm, significantly broader than most of the conventional phosphors doped with transition metal ions. Through a comprehensive combination of experimental investigations and crystal structure analysis, we elucidate the underlying mechanism of this ultra‐broadband emission, attributing it to Bi 3+ centers formed by the substitution of K + and Mg 2+ sites. This work expands the role of Bi 3+ ions as activators in the second NIR (NIR‐II) region, offering new insights into the design of ultra‐broadband‐emitting materials and introducing the only known phosphor capable of spanning the full 400 nm to 1700 nm spectrum, thereby filling a longstanding gap in this field.