With the goal of achieving ultrahigh luminance performance by using blue lasers to excite phosphor materials, a team at Xiamen University, led by professor Rong-Jun Xie, fabricated core-cladding phosphor ceramics (CCPC). The team’s design for CCPC was inspired by the architecture of optical fibers. High-brightness light sources, generated by exciting phosphor materials with high-power-density blue laser diodes, show strong potential for powering applications like long-distance searchlights, projection displays, and long-range night vision systems. However, phosphor materials with the properties that can support ultrahigh luminance are difficult to fabricate. The luminance of a high-brightness source depends on two factors — maximizing luminous flux and restricting the light spot area. Most phosphor materials are unable to simultaneously constrain light spot expansion while achieving maximum luminous flux. A team at Xiamen University developed a strategy to address the inherent trade-off between light spot confinement and luminous flux maximization through the design of optical fiber-like core-cladding phosphor ceramics (CCPC). Courtesy of the Journal of Advanced Ceramics, Tsinghua University Press. The maximum luminous flux is determined by the phosphor material’s luminous efficacy and its capacity to withstand the highest blue laser power density without reaching luminance saturation. The blue laser light incident upon the phosphor material tends to diffuse within the bulk, causing the light spot to expand. This expansion can negatively affect the luminance of the light source. To restrict the light spot expansion, the researchers engineered CCPC of cerium-doped yttrium aluminum garnet embedded in aluminum oxide (YAG:Ce@Al2O3) wafer using a gel-casting technique. The researchers prepared CCPC green bodies with different core diameters. They sintered the green bodies at 2023 K for five hours in a vacuum environment maintained at one millipascal (1 mPa). The interface between the core and cladding of CCPC was tightly bonded to eliminate porosity. “The difference in refractive index between YAG:Ce and Al2O3, coupled with the non-luminescent properties of Al2O3, ensures that the light spot is mainly confined to the core region,” professor Shuxing Li said. “This allows for precise control over the light spot area by adjusting the core dimensions.” In contrast, conventional phosphor ceramics permit the diffusion of both blue laser and yellow light, resulting in an enlarged light spot area. The superior thermal conductivity of Al2O3 cladding, compared to YAG:Ce, enabled more efficient heat dissipation and elevated the luminance saturation threshold of the material. “Leveraging these dual advantages, the engineered CCPC of YAG:Ce@ Al2O3 wafer can confine the expansion of the light spot area and concurrently enhance the maximum luminous flux,” Li said. A CCPC sample with a 1.0-millimeter (mm) core diameter demonstrated a small spot size — nearly identical to that of the incident blue laser beam — with a light spot expansion ratio of only 1.04. The high thermal conductivity of the Al2O3 cladding provided the CCPC with a high luminance saturation threshold, enabling it to withstand a blue laser power density of 30 watts per square millimeter (30 W·mm2), and achieve a peak luminous flux of 2100 lumens (lm) for white light within a transmissive optical setup. The combination of a confined light spot area and an elevated luminous flux resulted in an ultrahigh luminance of 3900 lumens per square millimeter (3900 lm·mm 2) in a transmissive optical mode, surpassing current records of maximum luminance. This approach to the design of phosphor materials could provide a new research direction for the development of high-brightness laser lighting technology. “Conventional phosphor ceramics usually rely on the introduction of second-phase particles to enhance maximum luminous flux or limit the light spot area,” professor Jiaochun Zheng said. “However, there is a trade-off between these two aspects: improving one often compromises the other, which severely restricts the improvement of the output luminance of the light source. The research was published in the Journal of Advanced Ceramics (www.doi.org/10.26599/JAC.2025.9221137).