Unique Design Strategy for Laser‐Driven Color Converters Enabling Superhigh‐Luminance and High‐Directionality White Light

Abstract

Robust ceramic color converters withstanding strong laser irradiations have recently drawn great attention for laser‐driven white lighting. However, the local emission within the incident laser spot usually expands to the whole surface area of the ceramics, which definitely makes it hard to achieve white light with high luminance and high directionality. Herein, a new strategy is proposed to solve the problem by elaborately introducing uniform spherical pores (diameter of ≈2 µm) into the phosphor ceramics with controlled contents from 8 to 24.6 vol%. The well‐distributed pores, acting as light scattering centers, enable reduction of the luminescent spot size greatly but without any losses in conversion efficiency. By using the high‐scattering ceramic color converters with 15% porosity, the light spot diameter is decreased by 46% and the central illuminance is increased by 156%. Moreover, laser‐driven white light with an enhanced beam directionality and uniformity is also achieved. A superhigh luminous flux of 7199 lm is realized by using this promising color converter in a rotary mode. The designed high‐scattering ceramics with controllable microstructures show great potential for use in extra‐high luminance laser‐driven lighting and projection.

Phosphor Ceramics for High-power Solid-state Lighting

“…Ce 复相荧光陶瓷，可以改善散热效果，从而提高发光 饱和阈值 [9][10][11][12] 。2)有效的热管理可以提高散热。例 如：将荧光陶瓷与高导热的 Al 基板组合利用热传 导来散热 [13][14] ，或采用旋转荧光色轮来促进散热 [14] 。3)几何结构对散热的影响较复杂。一方面，荧 光材料的吸收率随样品厚度的增加而增大， 产热随 之增加；另一方面，激光光斑的温度分布曲线随厚 度的增加而展宽，有利于散热 [8] 。 耐热与荧光材料的热淬灭直接相关， 而热淬灭 又受材料热激活能及掺杂元素的影响。热激活能 ΔE 是指激发态和基态位形曲线的交叉点与激发态 最低能级之间的能量差，热激活能越高，荧光材料 的热淬灭越小， 耐热性越佳。 Xu 等 [8] 的研究表明， 采用 Lu 取代 Y 可以降低 YAG 的热淬灭，提高其 耐热性，进而提高材料的发光饱和阈值，而采用 Gd 取代 Y 则产生相反的结果。…”

“…激光是一种具有比 LED 功率密度更高的点光 源，因此 LD 照明需要更有效的散射中心来增加光 的吸收，使蓝色激光束均匀化 [14] 。散射中心对提 高光的吸收和提取效率、 均匀混合泵入光和转换光 以及限制光斑直径实现高亮度照明具有重要作用 [3] 。未经散射和转化的入射蓝光会直接穿过荧光材 料造成"黄环效应"(Yellow Ling Effect)，影响出 光一致性 [15] 。提高光提取效率和一致性的关键在 于提高荧光材料对光的散射， 常见方法有引入气孔 [14,16] 、 引入不发光的第二相 [17][18] 、 表面粗糙化 [19][20] 及表面修饰 [21][22] 等。 Zhang 等 [16] [23][24] 。例如，Liu 等 [18] 制备 了性能优异的 MgAl 2 O 4 -GdYAG:Ce 复相荧光陶瓷， 得益于 MgAl 2 O 4 第二相粒子对光的散射，改变了 光线传播路径， 增加了荧光陶瓷对蓝光的吸收和对 黄光的提取，其示意图如图 1 所示。除此之外，更 多的研究则是添加 Al 2 O 3 作为基质来提高荧光陶 瓷的光提取效率 [10][11][12] 。 图 1 单相陶瓷(a)和复相陶瓷(b)的光传播和转化示意图 [18] Fig. 1 The schematic of light conversion and propagation in monolithic (a) and composite (b) ceramics [18] Wagner 和 Sun 等 [19][20] 研究了荧光陶瓷表面粗 糙度对发光性能的影响， 发现粗糙表面有利于减弱 "全内反射效应" ，从而提高其光提取效率。Sun 等 [19] [3] 。通过调 控基质化学成分 [25][26][27] 、改变发光中心种类 [28][29][30][31] 、 复合红色荧光材料 [32][33][34][35] [38][39] ， 得到的材料 往往存在光提取效率、 热导率和显色指数较低以及 色温偏高等缺点 [26,40] 。目前，多采用以下四种方 法来解决上述问题： 通过引入不发光第二相来解决 光提取效率和热导率偏低问题， 通过调整基质化学 组成、 发光中心种类或复合红色荧光材料等方式来 解决显色指数和色温方面的问题。 具体研究进展分 述如下。 [10] 。因此，Al 2 O 3 -YAG:Ce 复相荧光陶瓷成为目前研究最广泛和最深入的大 功率固态照明用荧光陶瓷材料 [9]…”

High Efficiency Green‐Emitting LuAG:Ce Ceramic Phosphors for Laser Diode Lighting

“…Pores also play an important role as scattering centers in strengthening light absorption and extraction of incident blue laser, making it to be possible for CPs to achieve high LE in LD lighting. [36,45] From the EDS mapping images (Figure 4), it can be seen that Lu and Al are homogeneous expect the pore region. Ba and Si are distributed homogeneously although the element ratio of Ba and Si is a little higher than the nominal composition (Figure S6, Supporting Information).…”

Phosphor material dependent spot size limitations in laser lighting

“…A similar model was used to simulate the temperature distribution in a laser excited silicone based phosphor with experimental verification [16]. Optimizing the beam shape of the laser beam to limit thermal quenching effects has been performed in a transmissive phosphor configuration with simulation and experimental verification [17] scattering on the shape of the generated luminescent spot [18]. They found that the pore structure of ceramic phosphors have great influence on the light distribution.…”