Phosphor Materials for Extended NIR-III Window: Pushing Emission Boundaries beyond 1700 nm

High Resolution Image Download MS PowerPoint Slide The extended near-infrared third window (NIR-III, 1700–2500 nm) is attracting growing interest for bioimaging, sensing, and infrared photonics. Selected longer-wavelength windows can reduce scattering and background autofluorescence and provide access to vibrationally informative spectral regions, although practical penetration depth is also governed by wavelength-dependent absorption from water and biomolecules. For phosphor research, crossing 1700 nm is only an entry point because useful NIR-III emission must remain efficient, excitable, and thermally stable at low transition energies. Here, we discuss the major limitations of lanthanide-based NIR-III phosphors, including weak excitation, enhanced multiphonon relaxation, thermal quenching, and limited device compatibility. We further propose a host-centered design framework based on lattice rigidity, phonon energy, and thermomechanical stability, highlighting negative-thermal-expansion frameworks, ceramics, and glass-based composites as promising routes toward practical high-power NIR-III phosphor-converted light sources.