LEDs technology
with exceptionally high transmission across the interface. At close proximity the lens is also able to capture most of the emitted light, focusing this to a tight spot with high power density. Such an assembly is ideal for optical coupling to a fibre (see Figure 3(a)).
To test its efficiency, the aperture of a 2mm-diameter plastic optical fibre was aligned to the focal spot of the LED assembly. Measurements determined a coupling efficiency of 53.8 percent, the highest value reported to the best of our knowledge. fibre-coupled LED sources find uses in a diverse range of applications from optical microscopy to short-range optical communications. Apart from the integration of optics, the monolithic integration of LED chips has also been demonstrated as a viable solution for building colour-tuneable LED assemblies.
Three LEDs chips of the primary colours, cut into truncated pyramidal geometries, were vertically stacked on top of each other in the order of decreasing wavelength from the bottom to the top, as illustrated in Figure 3(b). The inclined sidewalls are mirror-coated to suppress lateral leakage.
With this stacking arrangement, light emitted from chips at the bottom of the stack emits through the upper chips; the top blue LED chip serves as the output window. As the optical paths of the three chips overlap with each other, their colours are naturally mixed along the optical path without requiring additional optics. By individually controlling the intensities of the three colours via bias current, a wide range of colours can be produced with excellent homogeneity and performance (see Figure 4 for examples).
Such stacked devices are obviously useful for building high-resolution LED display panels. However, they are also excellent candidates as plain white light LEDs. Being free of phosphor, the stacked LEDs emit white light with high efficiency, excellent homogeneity, high colour- rendering index and long-term stability; all of this is achieved without colour-conversion losses.
So let’s get moving and get in shape! With smart tweaks in design, extended functionalities and enhanced efficiencies can be achieved with amazing results, without the need for additional components.
© 2011 Angel Business Communications. Table 1. Light extraction efficiencies of LED chips with different geometries
Further reading X.H. Wang et al. Journal of Vacuum Science and Technology B 27 2048 (2009) X.H. Wang et al. Journal of Applied Physics 10 023110 (2010) W.Y. Fu et al. IEEE Photonics Technology Letters 21 1078 (2009) L. Zhu et al. IEEE Photonics Technology Letters 22 513 (2010) K.N. Hui et al. OSA Optics Express 17 9873 (2009)
January / February 2011
www.compoundsemiconductor.net 27
With this stacking arrangement, light emitted from chips at the bottom of the stack emits
through the upper chips; the top blue LED chip serves as the output window. As the optical paths of the three chips overlap with each
other, their colors are naturally mixed along the optical path without requiring additional optics
Figure 4. Wide range of colors emitted by a stacked LED
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