The utilization of blue lasers to excite phosphor materials holds great potential for the development of high-brightness laser-driven light sources. However, phosphor materials that can simultaneously constrain light spot expansion and enhance the maximum luminous flux have been elusive, thereby limiting the output luminance. In this study, optical fiber-inspired core–cladding phosphor ceramics (CCPC) of YAG:Ce@Al2O3 wafers were engineered using a gel–casting technique to restrict light spot expansion. The smaller refractive index of Al2O3, combined with the dense and sharp core–cladding interface of these CCPC, effectively confines the light spot area. The sample with a 1.0 mm core diameter has a small spot size nearly identical to that of the incident blue laser beam. Furthermore, the high thermal conductivity of the non-luminescent Al2O3 cladding endows the CCPC with an impressive luminance saturation threshold of 30 W·mm−2 and a maximum luminous flux of 2100 lm for white light within a straightforward transmissive optical setup. The combination of a confined light-spot area and elevated luminous flux results in an ultra-high luminance of 3900 lm·mm−2, surpassing current reports. This research presents a pioneering approach to the design of phosphor materials, targeting the realization of light sources with unprecedented luminance for broad frontier applications.
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Laser-remote activated phosphor (LARP) converted solid state lighting is now developing towards high power density and super-brightness, and phosphor ceramic converters with high efficiency, high thermal conductivity, acceptable transmittance and suitable spectra are thus required. Y3Al5O12:Ce (YAG:Ce)-based ceramics are promising color converters to produce white light with a color temperature of 6000 K for vehicle headlamps, but the brightness and luminous efficiency are not well optimized. In this work, two series of Al2O3-YAG:Ce and Al2O3-(Gd,Y)AG:Ce transparent ceramics were fabricated by vacuum sintering, and their microstructure, thermal and optical properties were controlled by changing the Ce3+ or Al2O3 content as well the thickness of the ceramics. Both Al2O3–Y2.925Al5O12:Ce0.0175 (AY0.0175) and Al2O3-(Gd0.1Y2.89)Al5O12:Ce0.01 (AGY) ceramics containing 70% (in mass) Al2O3 show a luminance saturation threshold of 30.3 W/mm2 and 38.4 W/mm2, enabling to produce white light with a color temperature of 6000 K, luminous flux of 1928 lm and 3101 lm, luminous efficiency of 135.0 lm/W and 161.1 lm/W when pumped by blue laser diodes, respectively. This work provides a solution to finely control the composition, microstructure, and optical properties of transparent ceramics for super-high brightness laser-driven solid-state lighting.
As a fundamental physical parameter reflecting the microscopic motion within matter, the measurement of temperature holds a critical significance in industrial production, biomedical fields, and aerospace engineering. Conventional contact-based thermometry, requiring direct interaction with measured objects, demonstrates limitations in extreme environments due to the susceptibility to external interference and inherent measurement perturbations, thus failing to meet modern technological demands for high sensitivity, non-invasiveness, and rapid response. These challenges propel the development of non-contact thermometric technologies featuring enhanced measurement sensitivity, robust anti-interference capability, non-invasive characteristics, and fast response. Fluorescence-based temperature sensing has emerged as an innovative solution via leveraging the distinctive energy level configurations of rare-earth ions and their temperature-dependent luminescent properties. This technique has attracted substantial research attention due to its rapid response kinetics, exceptional sensitivity, and remarkable adaptability to harsh operational environments.
Fluorescence thermometry based on the correlation between fluorescence intensity and temperature represents one of the earliest developed methods in temperature measurement technology. This approach primarily falls into two categories, i.e., single-energy-level fluorescence intensity thermometry and fluorescence intensity ratio (FIR) thermometry. The single-energy-level method, as the most straightforward technique, determines temperature via monitoring the intensity variation of a specific emission peak with temperature changes, directly demonstrating the relationship between fluorescence intensity and temperature. However, its applications reduce in recent years due to the inherent limitations such as susceptibility to fluorescence loss during detection and strong dependence on excitation light intensity, hindering precise control in measurement processes.
In contrast, FIR thermometry has attracted much attention due to its insensitivity to external disturbances, high measurement accuracy in complex environments, and excellent reproducibility. This technique is further divided into thermally coupled energy level FIR thermometry and non-thermally coupled energy level FIR thermometry. The former operates based on the principle that particles in thermally coupled energy levels (denoted as Level 1 and Level 2) reach a thermal equilibrium within a short timeframe, where the population redistribution between these levels induces measurable changes in fluorescence intensity ratio. While a larger energy level spacing generally enhances temperature sensing performance, the inherent limitation of thermal coupling energy gaps poses some challenges for improving relative sensitivity. To address this constraint, non-thermally coupled FIR thermometry through co-doping dual luminescent centers is porposed. This strategy enhances relative sensitivity and measurement reliability via leveraging the distinct temperature-dependent luminescence characteristics of two different centers. The intensity ratio between dual centers provides more accurate temperature determination through mutual calibration and effectively compensates for measurement errors caused by external factors. Common dual-center configurations include rare-earth ion pairs and rare-earth/transition metal ion combinations, showing significant improvements in relative sensitivity for advanced thermometric applications.
The application of rare-earth-doped fluorescence intensity-based thermometry focuses on two primary domains, i.e., temperature measurement within the physiological range and in ultra-high-temperature environments. Distinct requirements for the upper temperature limit and sensitivity arise across these applications. For instance, in physiological temperature monitoring, slight temperature variations can lead to significant biological effects, necessitating exceptionally high relative sensitivity in low-temperature regimes to ensure sufficient measurement accuracy. Conversely, in high-temperature scenarios, the primary objective is to achieve an upper measurement limit compatible with extreme thermal conditions. Consequently, the design and development of novel materials should be closely aligned with specific application contexts, emphasizing performance optimization tailored to operational demands, such as tunable temperature thresholds, enhanced thermal stability, and environment-specific signal responsivity. This approach ensures that material systems balance sensitivity, durability, and temperature range adaptability for targeted technological implementations.
The development of fluorescence intensity-based thermometry faces two major challenges, i.e., enhancement of temperature measurement performance and expansion of application domains. Regarding performance improvement, the existing fluorescence intensity thermometric methods primarily focus on fluorescence intensity ratio (FIR) techniques involving thermally coupled and non-thermally coupled energy levels. However, several critical issues have emerged.
Firstly, rare-earth ions and certain transition metal ions exhibit significant thermal quenching effects at elevated temperatures, thus leading to substantial attenuation of fluorescence signals in high-temperature environments. Secondly, in thermally coupled energy level-based FIR thermometry, despite the abundant energy levels of rare-earth ions, some studies on potential thermally coupled energy levels for specific rare-earth ions remain insufficient. Moreover, systematic investigations into performance variations among different thermally coupled energy level pairs for FIR thermometry are notably lack. Thirdly, the fundamental mechanisms underlying non-thermally coupled FIR thermometry, particularly the operational principles of dual-luminescent-center systems, have yet to establish universally accepted theoretical explanations. The energy transfer processes and interaction mechanisms between different luminescent centers’ energy levels require a further elucidation. In terms of luminescent center selection, more combinations such as rare-earth–rare-earth ion pairs and rare-earth–transition metal ion systems warrant a comprehensive exploration. Lastly, although numerous temperature-sensitive materials demonstrate either high upper measurement limits or superior relative sensitivity, materials simultaneously with a high relative sensitivity in a broad temperature range remain scarce. Therefore, the design and development of novel temperature-sensitive materials will constitute a crucial research frontier.
Concerning application expansion, fluorescence intensity thermometry has attracted considerable attention due to its unique characteristics. The existing thermometric materials exhibit diverse advantages, each possesses inherent limitations. A critical challenge lies in rationally designing application scenarios based on material properties to maximize their functional advantages for practical implementations. Current research predominantly focuses on material development itself, with insufficient exploration of potential application fields. Note that raw materials cannot be directly employed in industrial production but require integration into temperature sensor architectures coupled with complete testing systems. However, the research framework spanning from material development to practical applications, encompassing complete testing systems and material optimization, remains underdeveloped. Furthermore, although applications have extended to biomedical and aerospace fields, investigations under extreme conditions such as ultra-low temperatures and highly corrosive environments remain inadequate.
It is anticipated that these challenges above will be progressively addressed with sustained research efforts. The development of advanced temperature-sensitive materials with enhanced performance and their subsequent integration into industrial applications are expected to drive significant advancements in fluorescence intensity-based thermometry technology.
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Discovery of new phosphors with desired properties is of great significance for developing high optical quality solid-state lighting. The single-particle-diagnosis approach is an effective way to search novel phosphors by analyzing tiny single crystals screened from the fired powder mixtures. In this work, a broadband orange-emitting phosphor of Sr3Si8O4N10:Eu2+ for solid state lighting was discovered by this method. The new oxonitridosilicate crystallizes in the monoclinic space group of P21/n (No. 14) with cell parameters of a = 4.8185 Å, b = 24.2303 Å, c = 10.5611 Å, β = 90.616°, and Z = 4. The crystal structure of Sr3Si8O4N10 was determined from the single-crystal X-ray diffraction (XRD) data of a single crystal, which is made up of a three-dimensional framework consisting of vertex-sharing SiN4 and SiN3O tetrahedra. Sr2+ ions occupy five crystallographic sites and have coordination numbers between 6 and 8 with one ordered Sr and other four disordered Sr atoms. The multiple Sr sites lead to a broadband emission centered at 565–600 nm and a bandwidth of 128–138 nm. The internal and external quantum efficiencies (IQE/EQE) of the title phosphor are 48.6% and 29.1% under 450 nm excitation, respectively. To improve the accuracy and speed of distinguishing phosphor particles in fired powder mixtures, a microscopic imaging spectroscopy is developed and demonstrated to modify the single-particle-diagnosis method.
The mechanism of mechanoluminescent(ML) materials is a highly interdisciplinary research field involving the composite processes of mechanics, electricity, magnetism, and optics. At present, some special phenomena that cannot be fully covered by the existing mechanism models indicate that the process mechanism of ML is not fully clarified. This review represented the process mechanism of ML and the existing mechanism models and development strategies. In addition, some challenges and research directions of ML materials were also described.
The model of developing luminescent materials has transferred from the conventional “trial and error” to “experience-guided experiments”, and further to a novel paradigm of “theoretical prediction and experimental verification”. Efficient theoretical prediction and rapid experimental verification are a key to this transformation. The theoretical prediction methods such as high-throughput calculations and machine learning are becoming more and more mature, and the corresponding experimental methods such as single-particle diagnosis are more efficient, laying a theoretical and experimental foundation for the development of novel luminescent materials. This review briefly summarizes recent research progresses on discovering new rare earth-activated luminescent materials based on single-particle diagnosis and high-throughput computation approaches.
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Mini-LED backlights, combining color conversion materials with blue mini-LED chips, promise traditional liquid crystal displays (LCDs) with higher luminance, better contrast, and a wider color gamut. However, as color conversion materials, quantum dots (QDs) are toxic and unstable, whereas commercially available inorganic phosphors are too big in size to combine with small mini-LED chips and also have strong size-dependence of quantum efficiency (QE) and reliability. In this work, we prepare fine-grained Sr2Si5N8:Eu2+-based red phosphors with high efficiency and stability by treating commercially available phosphors with ball milling, centrifuging, and acid washing. The particle size of phosphors can be easily controlled by milling speed, and the phosphors with a size varying from 3.5 to 0.7 μm are thus obtained. The samples remain the same QE as the original ones (~80%) even when their particle size is reduced to 3.2–3.5 μm, because they contain fewer surface suspension bond defects. More importantly, SrBaSi5N8:Eu2+ phosphors show a size-independent thermal quenching behavior and a zero thermal degradation. We demonstrate that red-emitting mini-LEDs can be fabricated by combining the SrBaSi5N8:Eu2+ red phosphor (3.5 μm in size) with blue mini-LED chips, which show a high external quantum efficiency (EQE) of above 31% and a super-high luminance of 34.3 Mnits. It indicates that fine and high efficiency phosphors can be obtained by the proposed method in this work, and they have great potentials for use in mini-LED displays.
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A three-layered phosphor structure was designed and prepared by the spin coating of BaSi2N2O2:Eu (cyan-emitting) and (Sr,Ca)AlSiN3:Eu (red-emitting) phosphor films on the yellow- emitting Y3Al5O12:Ce (YAG:Ce) phosphor ceramic synthesized by the solid-state reaction under vacuum sintering. In order to achieve high color rendering lighting, the influence of the composition and structure of the three-layered phosphors on the optical, thermal, and electrical properties of the chip-on-board (COB) packaged white-light-emitting diodes (WLEDs) was studied systematically. The WLED with the structure of "red+cyan+yellow" (R+C+Y) three-layered phosphor generated neutral white light and had a luminous efficacy of 75 lm/W, the fidelity index (Rf) of 93, the gamut index (Rg) of 97, and the correlated color temperature (CCT) of 3852 K. Under the excitation of laser diode (LD), the layer-structured phosphor yielded the white light with a luminous efficacy of 120 lm/W, color rendering index (CRI) of 90, and CCT of 5988 K. The result indicates that the three-layered phosphor structure is a promising candidate to achieve high color rendering and high luminous efficacy lighting.
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