Yasushi Hattori

A revised Kubelka–Munk theory for spectral simulation of phosphor-based white light-emitting diodes

We developed a simulation method for designing the luminescence profiles of white light-emitting diodes (LEDs) with multicolor phosphor blend. By taking into account the effect of the light scattering by phosphor particles as well as the reabsorption effect by phosphor species, we found a model of white LEDs to calculate the light output as a function of various parameters such as the thickness of the resins, the concentration of phosphors, and the spatial distribution of phosphor particles. This method is widely applicable to phosphor-based white LEDs with either blue or UV excitation.

Device Simulator for Designing High-Efficiency Light-Emitting Diodes

A general device simulator has been constructed for designing high-efficiency light-emitting diodes (LEDs). The optical characteristics, including the spatial profiles of the light extracted from LEDs, are important for practical LED applications such as display and illumination. The radiation patterns are affected by device structures such as electrodes, which determine the current distribution, and by the light-absorbing layers, which intercept the internal light emission. The simulator analyzes light extraction efficiency and the radiation pattern, both of which are very important in designing an optical system using LEDs. The simulator is useful for designing high-efficiency LEDs and package configurations for various applications.

Numerical Simulation on Light Output of UV-based White Light-Emitting Diodes with Multicolor Phosphor Blends

We developed a new simulation method for designing the luminescence profiles of phophor-based white light-emitting diodes (LEDs). By combining the rate equations for absorption/emission processes by phosphors with a differential equation for spatial distribution of light intensity, we take into account the cascade process of phosphor emission due to the reabsorption of photons. We found that our model is suitable for a systematic design method of white light sources with multicolor phosphor blends.

Cation and anion ordering in Sr2Si7Al3ON13 phosphors

A series of photoluminescent Ce3+ doped samples with compositions close to Sr2Si7Al3ON13:Ce have been studied by neutron powder diffraction to determine the Si4+/Al3+ and N3−/O2− site ordering. Contrary to a commonly held assumption that the edge sharing tetrahedral sites in this structure are occupied exclusively by Al3+, we find a partial occupancy of Al3+ on these site but also an unexpected preference for Al3+ to occupy 2 other tetrahedral sites which are only corner sharing. From the crystal structures and local structures, as determined by pair distribution function (PDF) analysis, we also find evidence for alternating Si–Al site ordering within the edge sharing chains as well as dimerization of the Si4+ and Al3+ cations within these chains. The O2− are found to be partially ordered onto 2 of the anion sites, although small amounts of O2− are found on other sites as well. The cation and anion ordering found by neutron diffraction is supported by theoretical calculations. Understanding cation and anion ordering is essential for optimizing the photoluminescence properties of this promising class of phosphor materials.

High-efficiency GaN-based laser diodes for solid-state lighting

GaN-based laser diodes (LDs) with a slope efficiency of 2.6 W/A are presented. The white light source was fabricated by using 405 nm LDs and phosphors. The luminous flux was estimated to be 200 lm.