Ken Sakuma

Eu2+-doped Ca-α-SiAlON: A yellow phosphor for white light-emitting diodes

In this letter, a yellow oxynitride phosphor α-SiAlON with compositions of Ca0.625EuxSi0.75−3xAl1.25+3xOxN16−x (Ca-α-SiAlON:Eu, x=0–25) was prepared by gas pressure sintering. The diffuse reflection spectrum, photoluminescence spectrum, and chromaticity of the powder phosphors were presented. It absorbs light efficiently in the UV–visible spectral region, and shows a single intense broadband emission at 583–603nm. This phosphor may become a good candidate for creating white light, typically warm white light, when coupled to a blue light-emitting diode (λem=450nm).

2-phosphor-converted white light-emitting diodes using oxynitride/nitride phosphors

Green α-sialon:Yb2+ and red Sr2Si5N8:Eu2+ oxynitride/nitride phosphors have been demonstrated as potential downconversion luminescent materials for white light-emitting diodes (LEDs). In this letter, the authors attempt to fabricate white LEDs by combining α-sialon:Yb2+ and Sr2Si5N8:Eu2+ with a blue LED die and report their optical properties. These two phosphors lend themselves for use in 2-phosphor-converted white LEDs with promising properties: a wide range of tunable correlated color temperature (2700–6700K), acceptable color rendering index (82–83), and luminous efficacy (17–23lm∕W). These LEDs are acceptable for general lighting.

Wavelength-tunable and thermally stable Li-α-sialon:Eu2+ oxynitride phosphors for white light-emitting diodes

Eu2+-activated Li-α-sialon is a promising yellow phosphor for white light-emitting diodes (LEDs). This letter reports that the emission of Eu2+ in Li-α-sialon can be tuned widely (563–586nm) by tailoring the composition or controlling the Eu2+ concentration. The thermal stability of Li-α-sialon:Eu2+, relying a little on the composition and the Eu2+ concentration, remains high in a wide temperature range (25–200°C). Moreover, the chromaticity of Li-α-sialon:Eu2+ does not shift with changes in temperature. Using a single Li-α-sialon:Eu2+ phosphor, highly efficient white LEDs (46–55lm∕W) with different color temperatures (3000–5200K) can be fabricated.

Extrahigh color rendering white light-emitting diode lamps using oxynitride and nitride phosphors excited by blue light-emitting diode

The blue-light-excitation-type white light-emitting diode (LED) lamps are considered to be very suitable for lighting for art objects, shop window displays, and medical applications because they do not give infrared ray and ultraviolet ray. But their color rendering indices are needed to be improved for such applications. In this letter, the authors have fabricated white LED lamps with a broad range of color temperatures, and realized extrahigh color rendering index Ra values of 95–98 in them, using four oxynitride/nitride phosphors and a blue LED die. It means UV LED die is not always necessary for high color rendering white LED lamps. The luminous efficacies of white LED lamps are 28–35lm∕W, which are sufficiently high for extremely high color rendering white LED lamps.

Highly efficient white-light-emitting diodes fabricated with short-wavelength yellow oxynitride phosphors

We have already reported orangish yellow Ca–α-SiAlON:Eu2+ phosphors, and applied them to fabricate warm white light-emitting diodes (LEDs). In this letter, we report on greenish yellow Li–α-SiAlON:Eu2+ phosphors, and use them to create daylight when coupled to an InGaN blue LED chip (460nm). The newly discovered Li–α-SiAlON:Eu2+ phosphors emit at shorter wavelengths of 573–577nm under the 460nm excitation, and exhibit a smaller Stokes shift than Ca–α-SiAlON:Eu2+ does. By using this short-wavelength yellow oxynitride phosphor, bright daylight emissions from white LEDs can be generated. Thus, highly efficient white LEDs with tunable white light can be fabricated with α-SiAlON:Eu2+ phosphors, enabling them for a wider range of applications.

Warm-white light-emitting diode with yellowish orange SiAlON ceramic phosphor

A warm-white light-emitting diode (LED) without blending of different kinds of phosphors is demonstrated. An approach that consists of a blue LED chip and a wavelength-conversion phosphor is carried out. The phosphor is a newly developed yellowish orange CaEuSiAlON ceramic phosphor with high efficiency. The CIE1931 chromaticity coordinates (x, y) are (0.458, 0.414), the color temperature is 2750 K, and the luminous efficacy of this LED is 25.9 lm/W at room temperature and with a forward-bias current of 20 mA. The chromaticity of the assembled LED is more thermally stable than that of a LED with a conventional oxide phosphor (YAG:Ce) because of the better thermal stability of the oxynitride phosphor.

White light-emitting diodes (LEDs) using (oxy)nitride phosphors

(Oxy)nitride phosphors have attracted great attention recently because they are promising luminescent materials for phosphor-converted white light-emitting diodes (LEDs). This paper reports the luminescent properties of (oxy)nitride phosphors in the system of M–Si–Al–O–N (M = Li, Ca or Sr), and optical properties of white LEDs using a GaN-based blue LED and (oxy)nitride phosphors. The phosphors show high conversion efficiency of blue light, suitable emission colours and small thermal quenching. The bichromatic white LEDs exhibit high luminous efficacy (~55 lm W−1) and the multi-phosphor converted white LEDs show high colour rendering index (Ra = 82–95). The results indicate that (oxy)nitride phosphors demonstrate their superior suitability to use as down-conversion luminescent materials in white LEDs.

White Light-Emitting Diode Lamps Using Oxynitride and Nitride Phosphor Materials

White-light emitting diode lamps for general illumination can be realized by a combination of a blue light-emitting diode semiconductor die and phosphors. Newly developed oxynitride and nitride phosphors are promising candidates for this application because they have suitable excitation and emission wavelengths and stable optical properties in a high temperature environment. High brightness warm-white LED lamps have been realized using a yellowish-orange α-SiAlON oxynitride phosphor. High color-rendering index white LED lamps have been also realized using three color oxynitride/nitride phosphors.

Low-loss optical waveguides written by femtosecond laser pulses for three-dimensional photonic devices

Femtosecond laser microfabrication attracts much attention due to its ability to write three-dimensional photonic devices into various transparent materials. By optimizing laser processing parameters and annealing at high temperature, low-loss straight optical waveguides are written in a pure silica glass. The minimum propagation-loss is 0.05 dB/cm at the wavelength of 1550 nm. The utility of the femtosecond laser processing is demonstrated by writing a low-loss three-dimensional 1×8 optical splitter.

Low insertion-loss and high isolation polymeric Y-branching thermo-optic switch with partitioned heater

A thermo-optic switch with low insertion-loss and high isolation was realized using a polymeric Y-branching waveguide. The partitioned heater greatly improves the isolation. Over 40dB isolation was achieved with the low insertion loss of 1.8dB.