Abstract
White light-emitting diodes (LEDs), light sources that combine blue LEDs and yellow phosphors, are equipped with bulky optics such as lenses, mirrors, and/or reflectors to shape the light into the required directions. The presence of bulky optics causes optical loss and limits the design. Here, a periodic array of metallic nanocylinders, which exhibits a high scattering efficiency owing to the excitation of localized surface plasmon resonance, is proposed as an alternative means of achieving a directional output without the limitations of bulky optics. A prototype of a directional light emitter is fabricated consisting of an Al nanocylinder array on a yellow phosphor plate and a blue laser. The array shapes the yellow luminescence into the forward direction and generates directional quasi-white light (correlated color temperature of 4900 K). The intensity enhancement reaches a factor of five in the forward direction and is further improved up to a factor of seven by the deposition of a multilayer dichroic mirror on the back side of the phosphor plate, resulting in conversion efficiencies as high as 90 lm/W. Our results pave the way toward the development of efficient and compact directional white-light-source devices without any bulky optics.
Highlights
The need for directional illumination devices is clearer in the context of next-generation high-power illumination, in which laser diodes (LDs) are used instead of light-emitting diodes (LEDs) as blue light sources to generate quasi-white light
A periodic array of metallic nanocylinders, which exhibits a high scattering efficiency owing to the excitation of localized surface plasmon resonance, is proposed as an alternative means of achieving a directional output without the limitations of bulky optics
A prototype of a directional light emitter is fabricated consisting of an Al nanocylinder array on a yellow phosphor plate and a blue laser
Summary
The need for directional illumination devices is clearer in the context of next-generation high-power illumination, in which laser diodes (LDs) are used instead of LEDs as blue light sources to generate quasi-white light. Our intention is to produce new incoherent and directional light sources which are sufficient for illumination applications by applying plasmonic nanostructures to conventional phosphors. For this purpose, we place a periodic array of Al nanocylinders on a yellow phosphor plate. We fabricated a prototype directional and compact quasi-white-light source by placing an Al nanocylinder array on a thick yellow phosphor plate (thickness = 200 μm) and exciting it with a blue LD. We demonstrated that the periodic Al nanocylinder array could enhance the PL from the 200-μm-thick phosphor plate This result is surprising given that the surface plasmon is a local phenomenon bound to the surface. A new means of achieving incoherent and directional light sources was developed in this study
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