Laser Phosphors for Next-Generation Lighting Applications

ConspectusLaser diodes (LDs), free of "efficiency droop", can bear a superhigh power density. Laser-driven white light sources (blue LD + laser phosphors), which promise super brightness and high directionality, have emerged for various applications, including lighting, displays, communications, and endoscopy. Laser phosphors are critical components of this technology, which determine the luminous efficacy, luminance, and color quality of the device. However, most phosphors suffer from serious luminance saturation when excited by a high-power-density blue laser. The synergetic effect of thermal quenching and photoexcitation quenching causes the luminance saturation, named thermally and optically induced luminance saturation, respectively. To avoid luminance saturation, laser phosphors should have the following merits to withstand a high-power-density laser excitation: (i) high internal quantum efficiency; (ii) small Stokes shift; (iii) low thermal quenching; (iv) high thermal conductivity; (v) short decay time. Robust inorganic phosphor bulks have been developed for laser excitation. Among them, phosphor ceramics are the best choice for high-power-density laser excitation owing to their excellent reliability and rich microstructure tunability, and phosphor films act as a complementary component to realize flexible color tuning with low fabrication cost. Moreover, property evaluation methods for LED phosphors are not all suitable for laser phosphors, as the light conversion process occurs only within a small light spot area. First, the color-conversion process in laser-driven lighting occurs at a small interface between the excitation blue laser and the phosphor, and the output luminous flux is collected from either the transmissive or the reflective side for practical applications. Therefore, when evaluating the luminous flux of a laser phosphor, the sample should be placed at the entrance or exit of the integrating sphere rather than in the center of the integrating sphere as in LED lighting. Second, the incident blue laser spot tends to broaden during the color-conversion process. Comparison of the incident laser spot with the real light spot provides a descriptor to evaluate the light confinement ability of the laser phosphor. The real light spot area is also a key parameter for calculating the luminance of the light source. Therefore, the precise measurement of the light spot area is very important, but most researches have ignored it. Third, the excitation blue light is highly collimated with a Gaussian-distributed energy, whereas the phosphor-converted yellow light energy follows a Lambertian distribution. This intrinsic difference leads to uneven mixing of blue and yellow light, and the light uniformity of laser-driven white light sources must be measured to evaluate the color quality. Furthermore, this account also proposes future research priorities of discovering high-performance red-emitting laser phosphors and extending the applications to communications. This Account will promote scientific and technological development of laser phosphors and laser-driven lighting applications.