Control of the optical behavior of active materials through manipulation of their microstructure has led to the development of high-performance photonic devices with enhanced integration density, improved quantum efficiencies and controllable color output. However, the achievement of robust light-harvesting materials with tunable, broadband and flattened emission remains a long-standing goal owing to the limited inhomogeneous broadening in ordinary hosts. Here, we describe an effective strategy for the management of photon emission by manipulating the mesoscale heterogeneities in optically active materials. Importantly, this unique approach enables control of dopant-dopant and dopant-host interactions on the extended mesoscale. This allows the generation of intriguing optical phenomena such as a high activation ratio of the dopant (close to 100%), dramatically inhomogeneous broadening (up to 480 nm), notable emission enhancement and, moreover, simultaneously extension of the emission bandwidth and flattening of the spectral shape in glass and fiber. Our results highlight that the findings connect the understanding of and manipulation in the mesoscale realm to functional behavior on the macroscale, and the approach to manage the dopants based on mesoscale engineering may provide new opportunities for the construction of a robust fiber light source.

earth (RE) ion-doped materials have been developed as significant gain matrices due to their high photoluminescence quantum yield (PLQY) and multiple-wavelength luminescence. [4,5] Recent decades have witnessed extensive investigations in doping methods and network structure design to obtain more efficient gain materials. [6,7] However, the requirements for high luminescence efficiency and excellent thermodynamic stability of optical materials are always contradictory, greatly restricting their applications in, e.g., high-temperature, high-humidity, and high-power laser pumping environments. Generally, the luminescence efficiency of an optical material is inversely proportional to the multiphonon nonradiative transition probability (W p ) of the intermediate state energy level for RE ions. [8] The luminescence process, especially for the upconversion (UC) process, is normally associated with a large number of intermediate state energy levels. Furthermore, its value (W p ) depends on the maximum phonon energy (ћω) of the elastic structure of condensed matter, and it increases dramatically with increasing ћω (as described in Equations (S1)-(S3) of the Supporting Information). [9] Traditionally, one class of soft material with extremely low ћω values, including fluorides, chalcogenides, and halogenides, has been widely Optical gain materials are of fundamental importance for various applications, such as lasers, lighting, optical communication, microscopy, and spectroscopy. However, the requirements for high luminescence efficiency and excellent thermodynamic stability of materials are always contradictory. As a result, wide applications of optical materials in high-temperature, high-humidity, and high-power laser-irradiated environments are restricted. Here, a facile approach based on phase-separation engineering is proposed to modulate the thermodynamic stability and enhance the luminescence efficiency of optical gain materials. It is shown that the thermodynamic stability and luminescence efficiency of the phase-separated fluorosilicate (FS) gain glass are both enhanced dramatically when the SiO 2 concentration is optimized. Owing to the confinement effect of phase-separation network structure on active ions, the upconversion (UC) luminescence efficiency of the designed glass is 150 times higher than that of traditional FS glasses and even seven times higher than that of ZBLAN (ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF) glass, which is the most commonly used material for UC fiber lasing applications. These intriguing properties of the glass indicate that phase-separation engineering not only provides a powerful solution to conquer the conventional contradiction between thermodynamic stability and luminescence efficiency but also offers significant opportunities for manufacturing a wide range of optical composites with multiple functions.

Glass ceramic fibers containing Ni(2+) doped LiGa(5)O(8) nanocrystals were fabricated by a melt-in-tube method and successive heat treatment. Fiber precursors were prepared by drawing at high temperature where fiber core glass was melted while fiber clad glass was softened. After heat treatment, LiGa(5)O(8) nanocrystals were precipitated in the fiber core. Excited by 980 nm laser, efficient broadband near-infrared emission was observed in the glass ceramic fiber compared to that of precursor fiber. The melt-in-tube method can realize controllable crystallization and is suitable for fabrication of novel glass ceramic fibers. The Ni(2+)-doped glass ceramic fiber is promising for broadband optical amplification.

Glass-ceramic fibers containing Cr 3+ -doped ZnAl 2 O 4 nanocrystals were fabricated by the melt-in-tube method and successive heat treatment. The obtained fibers were characterized by electro-probe micro-analyzer, X-ray diffraction, Raman spectrum and high-resolution transmission electron microscopy. In our process, fibers were precursor at the drawing temperature where the fiber core glass was melted while the clad was softened. No obvious element interdiffusion between the core and the clad section or crystallization was observed in precursor fiber. After heat treatment, ZnAl 2 O 4 nanocrystals with diameters ranging from 1.0 to 6.3 nm were precipitated in the fiber core. In comparison to precursor fiber, the glass-ceramic fiber exhibits broadband emission from Cr 3+ when excited at 532 nm, making Cr 3+ -doped glass-ceramic fiber a promising material for broadband tunable fiber laser. Furthermore, the melt-in-tube method demonstrated here may open a new gate toward the fabrication of novel glass-ceramic fibers.