Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

A review on the advancements in phosphor-converted light emitting diodes (pc-LEDs): Phosphor synthesis, device fabrication and characterization

DOI: 10.1002/adom.201801433 Scientific American selects plasmonic sensing as the top 10 emerging technologies of 2018.[15] Almost every single new plasmonic or photonic structure would be explored to test its sensing ability.[16–29] These works tend to report the sensing performance of their own structure. Some declare that their sensitivity breaks the world record. However, there is still a missing literature on what the world record really is, the gap between the experiments and the theoretical limit, as well as the differences between metal-based plasmonic sensors and dielectric-based photonic sensors. To push plasmonic and photonic sensors into industrial applications, an optical sensing technology map is absolutely necessary. This review aims to cover a wide range of most representative plasmonic and photonic sensors, and place them into a single map. The sensor performances of different structures will be distinctly illustrated. Future researchers could plot the sensing ability of their new sensors into this technology map and gauge their performances in this field. In this review, we focus on optical refractive index (RI) sensors with no fluorescent labeling required. We will utilize two parameters to characterize and compare the performance of optical RI sensors: sensitivity to RI change (denoted by symbol SRI) and figure of merit (in short, FoM). For simplicity, we restrict our discussions to bulk RI change, where the change in RI occurs within the whole sample. There is another case where the RI variation occurs only within a very small volume close to the sensor surface. This surface RI sensitivity is proportional to the bulk RI sensitivity, the ratio of the thickness of the layer within which the surface RI variation occurs, and the penetration depth of the optical mode.[6] The bulk RI sensitivity defines the ratio of the change in sensor output (e.g., resonance angle, intensity, or resonant wavelength) to the bulk RI variations. Here, we limit our discussions to the spectral interrogations and the bulk RI sensitivity SRI is given by[3,5–7,30]

Optical Refractive Index Sensors with Plasmonic and Photonic Structures: Promising and Inconvenient Truth

Abstract The discovery of inorganic phosphors that can be excited by GaN light emitting diodes in the wavelength range 380–460 nm is essential for improving the efficiency and light quality of solid state lighting devices. Garnets doped with Ce 3+ are of particular interest for this application. We have studied the luminescence of Ce 3+ in garnet phosphors and established a relationship between the excitation/emission wavelengths and the deviation from cubic symmetry around the rare-earth ion. Data are presented for both known yttrium aluminum and yttrium gallium garnet systems, as well as newly synthesized inverse garnets of the general formula Mg 3 Y 2− y Gd y Ge 3− z Si z O 12 :Ce (0 ⩽  y  ⩽ 2, z  = 0, 1).

http://ma.ecsdl.org/content/MA2007-02/33/1520.full.pdf

Structure-Property Correlations in Ce-Doped Garnet Phosphors for Use in Solid State Lighting

We briefly review the development of gain-switched rare-earth-doped fiber lasers and their applications in wavelength conversion to mid-IR, supercontinuum generation, and medicine in recent years. We illustrate the similarities between gain-switching and Q-switching techniques that will provide tools for the design and optimization of the gain-switched fiber lasers. From the nature of the gain-switched fiber lasers, benefits of this kind of lasers to 2-μm region and in-band-pumped (two-level system) laser systems are obvious. Advantages of in-band-pumped 2-μm lasers are discussed and analyzed with a simple numerical simulation in terms of Tm-doped fiber lasers. We also propose the key factors in the development of the gain-switched fiber lasers and predict the future tendency.

Development and applications of gain-switched fiber lasers [Invited]

A compact and high sensitivity sensor with a fiber-tip structure is proposed and demonstrated for simultaneously liquid refractive index (RI) and temperature sensing. The device is fabricated by inserting a tiny segment of capillary tube between single-mode fibers (SMFs) to form two cascaded Fabry-Perot interferometers (FPIs). The theoretical and experimental results demonstrate that the ambient liquid RI and temperature can be simultaneously determined by the intensity and shift of the resonant wavelength in the reflection spectrum. Our proposed device has the highest RI sensitivity of ~216.37 dB/RIU at the RI value of 1.30; a high spatial resolution owing to its compact size (with dimension <400 μm) makes it promising for high precision bio/chemical sensing applications.

Hybrid Fabry-Perot interferometer for simultaneous liquid refractive index and temperature measurement

Gain-switched by a 1.914-µm Tm:YLF crystal laser, a two-stage Tm(3+) fiber laser has been achieved 100-W level ~2-µm pulsed laser output with a slope efficiency of ~52%. With the 6-m length of Tm fiber, the laser wavelength was centered at 2020 nm with a bandwidth of ~25 nm. Based on an acousto-optic switch, the pulse repetition rate can be modulated from 500 Hz to 50 kHz, and the laser pulse width can be tuned between 75 ns and ~1 µs. The maximum pulse energy was over 10 mJ, and the maximum pulse peak power was 138 kW. By using the fiber-coiling-induced mode-filtering effect, laser beam quality of M2 = 1.01 was obtained. Further scaling the pulse energy and average power from such kind of gain-switched fiber lasers was also discussed.

High-power gain-switched Tm 3+ -doped fiber laser

White light-emitting diodes using blue and yellow–orange-emitting phosphors

Resonantly gain-switched by a 1.9-μm Tm:YLF crystal laser, over 11 W ~ 2.1-μm laser output has been achieved from a Ho:LuAG rod laser with a slop efficiency of 83% with respect to absorbed pump power. The high quantum efficiency (~ 92%) was attributed to the compact cavity configuration, excellent mode overlap between the pump and laser beam, and efficient crystal cooling. Near the highest output level, the laser wavelength was centered at 2130 and 2101 nm, with the T = 8% and T = 20% output couplers, respectively. At 10-W power level, the laser beam quality was measured to be M2 ~ 1.02. The narrowest pulse width achieved was 20 ns and maximum pulse energy was over 4 mJ (2 kHz).

High‐power gain‐switched Ho:LuAG rod laser

Series of Ca(La1−xEux)4Si3O13 red emitting phosphor were synthesized by solid-state reaction method. Photoluminescence excitation and emission spectra showed that the phosphors could be efficiently excited by near ultraviolet to blue light from 350 to 470 nm to give bright red emission. There were four emission bands peaking at 591, 615, 655, and 700 nm, due to the transition of the Eu3+ (5D0→7Fj (j=0, 1,2,3,4)), respectively. After using the 3.5% Li2CO3 as the flux, the sample's emission intensity increased obviously. White LED was obtained by combining blue LED chip (InGaN-based 460 nm emitting) with Ca(La1−xEux)4Si3O13 and YAG:Ce3+. As the x has the value of 0.5, the InGaN-based Ca(La0.5Eu0.5)4Si3O13 WLED presented intense white emitting and good color rendering of over 89.

Yi Yang

Papers

Hybrid Fabry-Perot interferometer for simultaneous liquid refractive index and temperature measurement

High-power gain-switched Tm 3+ -doped fiber laser

White light-emitting diodes using blue and yellow–orange-emitting phosphors

High‐power gain‐switched Ho:LuAG rod laser

Ca(La1−xEux)4Si3O13 red emitting phosphor for white light emitting diodes