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Abstract
Solid-state laser lighting is an emerging technology, whereby high-brightness white light can be generated using blue laser diodes combined with a yellow-emitting phosphor. In this study, Ce:YAG transparent ceramic wafers with different cerium concentrations and thicknesses are prepared and their optical characteristics are measured. A transmission mode is used, wherein the phosphor ceramic is fixed onto an oxygen-free copper sink, whose temperature is accurately controlled, and excited using the blue laser diode. The recorded spectrum shows that blue laser light is completely converted to yellow light with a wavelength of 565 nm and width of 200 nm. Moreover, with varied temperatures of 10–80 °C, the luminous flux, spectrum, and color coordinates exhibit relatively stable. More importantly, the luminous flux is highest (2690 lm) when irradiated by a 19.5-W blue laser (with a center wavelength of 454 nm) for a Ce3+ dopant concentration of 0.5 mol% and a Ce:YAG thickness of 1.6 mm. Based on these results, the Ce:YAG transparent ceramic can be used as a potential phosphor material in applications of high-power solid-state laser lighting and laser display.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Owing to advantages, such as high intensity, low electricity consumption, and long lifetime, solid-state lighting (SSL) is expected to replace conventional incandescent and fluorescent lamps in the near future. Conventional white light emitting diodes (WLEDs) normally combine InGaN blue-emitting chips with Ce3+ doped Y3Al5O12 (Ce:YAG) yellow-emitting phosphor powders. The phosphor powders are packed on the chip surface using epoxy resin [1–7]. The advantages include luminous efficacy, durability, and non-toxicity.
However, the organic resin has a major stability issue because of its poor heat resistance [8–10]. Under high blue-irradiation, the temperature of the epoxy resin increases, thereby drastically reducing the light-conversion efficiency, lifetime of the WLEDs, and color quality. Hence, it is necessary to improve the brightness, lifetime, and optical stability of lighting for applications such as displays, projectors, and automotive headlights.
Researchers have investigated novel durable phosphors without resins, such as glass phosphors [11–13], single crystals [14–16], Ce:YAG ceramics [17–23], composite phosphor ceramics [24–26], and other substitutions of Ce [27–29]. Although a transparent Ce:YAG phosphor-in-glass material has good heat resistance, it has low thermal conductivity, thus limiting its applications wherein a high-power laser is employed as the excitation light source. A single-crystal phosphor [30] is expensive, difficult to fabricate in large sizes, and easily undergoes thermal quenching [31, 32]. Regarding composite phosphor ceramics, it is difficult to obtain samples having large size as well as high quality. Moreover, it is difficult to remove the scatters inside the crystal, and due to the low separation coefficient, it is very difficult to obtain single crystal with high dopant concentration. A transparent ceramic is one of the candidates for SSL. Compared to the former material systems, a large-size sample can be easily obtained for transparent ceramics. In addition, they have good heat resistance, high thermal conduction, and simple preparation method. For transparent ceramics, the separation coefficient is exact 1, so it is easy to get high dopant samples. A high brightness, high efficiency, and stable white light, which are critical for SSL and speckle reduction in laser displays, can be realized by exciting Ce:YAG transparent ceramics with blue light.
It is well known that the emission property of Ce3+ doped materials is influenced by the doping concentration, sample thickness, and working temperature. Laser Diodes (LDs) deliver much higher light intensities than LEDs, and therefore, highly efficient, and temperature stable phosphors are in high demand [33]. In this study, a fiber-coupling blue LD with a center wavelength of 454 nm is used as the exciting light. A temperature control system is designed. Ce:YAG transparent ceramics with different thicknesses and Ce3+ doping concentrations are used as the phosphor. The optical emission properties of the different samples are carefully measured to obtain optimum parameters for high brightness, high efficiency, and stable light. Finally, a yellow light with a luminous flux of 2690 lm is generated using a 19.5-W blue laser with a dopant concentration of 0.5% and for a Ce:YAG thickness of 1.6 mm at 20 °C. This demonstrates that transparent ceramics are suitable for high-brightness lighting applications.
2. Experimental procedure
A typical preparation process of Ce:YAG transparent ceramics and x-ray diffraction (XRD) spectra are explained elsewhere [27]. The blue laser diode employed in this study has a center wavelength of 454 nm and a diameter of 6 mm. The photoluminescence (PL) and PL excitation (PLE) spectrum were measured using a spectrometer (HR4000CG-UV-NIR, Ocean Optics, USA). Figure 1 shows the experimental setup. The Ce:YAG ceramic was fixed onto an oxygen-free copper heat sink and irradiated using the blue laser diode. The fluorescence is collected by an integrating sphere with a diameter of 500 mm (CGJFQ500, Hefei Xingyue Luminous Technology Applications Institute, China) in a 2π forward detection geometry. The brightness, color coordinates, and color temperature were measured using an illuminance spectrophotometer (CL-500A, KONICΛ MINOLTΛ, Japan). A temperature controller is designed to vary the temperature from 10 to 80 °C. The temperature was controlled throughout the experimental process. The main objective of this research is to apply phosphor ceramics to solid-state laser lighting. Hence, it is necessary to determine the appropriate parameters (of the ceramic) that would yield the significantly high luminous flux under blue laser excitation. As shown in Fig. 1, a transmission mode is used, wherein the phosphor ceramic is fixed onto an oxygen-free copper sink and excited using the blue laser diode.
3. Results and discussion
The dopant concentration of Ce3+ is an important parameter of transparent ceramics. Highly doped materials are associated with a fluorescent quenching problem, whereas the absorption factor is low for less doped materials. The dopant concentration of transparent ceramics can be significantly higher than that of single crystals, allowing thinner samples of transparent ceramics to absorb an appropriate amount of exciting light [34–36]. In this study, 1-mm thick samples with dopant concentrations of 0.5, 0.8, 1, 1.2, and 1.5% were prepared, and their fluorescence intensities were measured. The experimental temperature was set at 20 °C.
Figure 2(a) displays the XRD pattern of Ce:YAG ceramics with a Ce3+ concentration of 0.5mol%. YAG phase was identified and no peak was assigned to other crystalline phases. Figure 2(b) shows the transmission spectrum of 1-mm thick Ce:YAG with Ce concentration of 0.5mol%. Figure 2(c) shows the variation in the luminous flux as a function of the exciting power. The sample with a Ce3+ concentration of 0.5 mol% emits the highest luminous flux, i.e., 2087 lm, when irradiated by a 14.6-W blue laser, with an efficiency of 143 lm/W. The luminous flux of phosphor ceramics with higher Ce concentrations is strongly limited. For example, the luminous flux of the sample with a Ce3+ concentration of 1.5% was only 775 lm when irradiated by an 8.2-W exciting laser. With the increase in the exiting laser power, the emitting luminous flux reduces. The reduction in the emitting luminous is mainly due to fluorescence quenching. When the samples were irradiated by high power lasers, only part of the energy was emitted out as fluorescence. All the other energy was finally transport into heat by means of lattice vibration.
Fig. 2 (a) the XRD pattern of Ce:YAG ceramics with a Ce3+ concentration of 0.5mol% ; (b) the transmission spectrum of 1-mm thick Ce:YAG with Ce concentration of 0.5mol%; (c) Variation in the luminous flux of 1-mm thick Ce:YAG ceramics with respect to the power for Ce concentrations of 0.5, 0.8, 1, 1.2, and 1.5%; (d) Variation in the luminous flux of the ceramics at a fixed blue light power of 7.3 W for Ce concentrations of 0.8, 1, 1.2, and 1.5%. The insets in the top show images of the 0.5 and 1.5% Ce: YAG ceramics
Figure 2(d) shows the luminous flux as a function of the dopant concentration for a fixed exciting laser power of 7.3 W. The curve shows a disordered trend. Nevertheless, the luminous flux of the sample with a dopant concentration of 0.5% is clearly higher than those of the other samples. With the increase in the Ce3+ dopant concentration, more defects occur in the ceramics, particularly scattering points. The scattering points lead to significant optical loss (for the exciting laser) and deterioration in the emitting fluorescence, thus dramatically reducing the fluorescence efficiency. The inset images (in Fig. 2(d)) are optical images of the ceramics with Ce3+ dopant concentrations of 0.5 and 1.5%. The ceramic with a Ce concentration of 0.5% has better light transmittance and could absorb all the exciting light completely. Hence, it is not advisable to use YAG transparent ceramics with a Ce3+ dopant concentration greater than 0.5% as a phosphor light converter.
Thermal management is one of the key issues in solid-state laser lighting. The main negative effect of temperature on fluorescence emission is reflected in the poor fluorescence lifetime. A Ce:YAG ceramic wafer with a thickness of 1.8 mm and a Ce3+ dopant concentration of 0.5% was chosen to study the influence of temperature on the fluorescence emission. Figure 3(a) shows the emitting luminous flux as a function of temperature. The exciting laser power was increased from 0.3 W to 6.7 W. The interval of laser power taken between two curves is about 0.5 W, which is controlled by electric current driver. At low exciting laser powers, the variation in the emitting luminous flux is negligible with respect to the temperature. However, for higher exciting powers, the emitting luminous flux decreases with the increase in the temperature from 10 to 80 °C. The minor decrease from 20°C to 10°C is mainly caused by the absorption center shift in the ceramics. This is because of photoionization (which is a thermally activated electron transfer process to the conduction band). The temperature directly aggravates the collisions and increases the internal loss. Moreover, the fluorescence lifetime reduces with the increase in the temperature and is unconducive to the production of high-efficiency fluorescence. Figure 3(b) shows that the color coordinate x slightly increases with the increase in the temperature, whereas the color coordinate y exhibits an opposite trend. The regularity in which x increases and y decreases at various blue laser powers is the same. This implies that a higher exciting power or working temperature causes a red-shift in the emitting fluorescence. Figure 3(c) shows the shift between working at 10 and 80 °C for an exciting laser power of 6.732 W. Moreover, with the change in the temperature, the emission spectra basically remain unchanged, as shown in Fig. 3(d). This indicates that the ceramics have outstanding luminescence stability and good environmental adaptation, which are beneficial for future applications.
Fig. 3 (a) Temperature dependence of luminous flux; (b) color coordinates of the emitting yellow light under different controlled temperatures of 10–80 °C for the sample with 1.8 mm thickness and 0.5% dopant concentration; (c) shift between working at 10 and 80 °C when the exciting laser power is 6.732 W; (d) emission spectra with respect to the change in the temperature.
Six samples of transparent ceramics with thicknesses ranging from 1 to 1.8 mm were prepared with a Ce3+ dopant concentration of 0.5%. Figure 4 shows the emitting luminous flux as a function of the exciting laser power for the six samples. With the increase in the ceramic thickness from 1.1 to 1.6 mm, the light-conversion efficiency slope does not show a clear change; nevertheless, the highest emitting luminous flux increases. For each sample, there is a quenching point. After this quenching point, the emitting luminous flux does not increase with the increase in the exciting laser power; this is because of the thermal quenching behavior of Ce:YAG ceramics. However, when the thickness increases to 1.8 mm, the emitting brightness reduces dramatically. The temperature of the copper sink is controlled, and the temperature of the ceramics varies with respect to the copper sink temperature because of high thermal conductivity of the YAG ceramics and the indium foil between the ceramics and the copper sink. The sample itself serves as a thermal sink. Hence, a thicker ceramic sample can endure more heat. However, when the thickness is 1.8 mm, the reduction in the emitting luminous flux is mainly due to the absorption, scattering, and inner total reflection loss. Hence, for samples with a Ce3+ dopant concentration of 0.5%, the thickness should be set below 1.6 mm.
Fig. 4 Luminous flux as a function of the exciting laser power for samples with thicknesses ranging from 1 to 1.8 mm.
Based on the above experimental results, a 1.6-mm thick Ce:YAG transparent ceramic with a Ce3+ dopant concentration of 0.5% was chosen for the analysis. The working temperature was set to be 20 °C. Figure 5 shows the emitting flux and light-conversion efficiency as a function of the exciting laser power. Yellow light with the highest luminous flux, i.e., 2690 lm, is generated by the 19.5-W laser. The light-conversion efficiency is 137.9 lm/W. The color coordinate is (0.481, 0.524), which corresponds to pure yellow. To the best of the author’s knowledge, both the fluorescence generated by an SSL system and the conversion efficiency are significantly higher than that previously reported [24]. If a blue light is added, a white light (0.3444, 0.3134) can be realized.
Fig. 5 Emitting flux and light-conversion efficiency of a 1.6-mm Ce:YAG transparent ceramic (with a Ce3+ dopant concentration of 0.5%) as a function of the exciting laser power.
4. Summary
In this study, Ce:YAG transparent ceramics were fabricated and their optical characteristics, including fluorescence spectra, luminous flux, and color coordinates, were studied by varying the Ce concentration, thickness, and temperature. A luminous flux of 2690 lm was obtained when excited by a 19.5-W blue laser, and the measured photoluminescence spectrum remained largely the same with the variation in the temperature. To the best of our knowledge, this is the highest luminous fluorescence generation using Ce:YAG transparent ceramic exciting with high power blue LD. Based on these results, the proposed Ce:YAG ceramic is found to be a promising candidate for realizing solid-state laser lighting with characteristics such as lack of resin, stable temperature, high brightness, and long lifetime. In the future, we plan to improve temperature controller and research more samples to obtain more stable and higher-brightness fluorescent light for SSL and speckle reduction in laser displays [37].
Funding
National Key Research and Development Program of China (Grant No. 2016YFB0402001); National Natural Science Foundation of China (NSFC) (51502308); and Science and Technology Major Project of Ningbo Municipality (2017C110028).
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