In AlGaN, the dominating emission polarization depends on the Al content. Generally speaking, a higher Al content leads to a stronger TM-polarized emission. Normally, the dominating emission polarization of an AlGaN layer changes from the TE polarization into the TM polarization when the emission wavelength is shorter than 300 nm. Because a TM-polarized photon propagate along the lateral dimension of a c-axis grown LED sample, its light extraction efficiency is lower, when compared with a TE-polarized photon. In this study, the material characterization techniques of transmission electron microscopy observation, reciprocal space mapping and omega-2theta scan in X-ray diffraction measurement, and geometric phase analysis are used for first identifying the existence of the high-Al layers (HALs) on both sides of a quantum well (QW) in three 3-period AlGaN QW structures of different deep-UV emission wavelengths. Then, optical analyses, including transmission and photoluminescence (PL) measurements, particularly the PL measurements under an applied stress along the sample c-axis, are undertaken for understanding the effects of such HALs on the band structures and hence the polarized emission behaviors of the samples. Simulation studies are also performed for providing the favorable comparisons with the experimental data. Basically, the HALs produce an extra compressive strain in the c-plane for lowering the heavy-hole (HH) band edge (lower than the edge of the split-off band) such that the TE-polarized emission through the electron transition between the conduction and HH band becomes dominating. In this situation, the light extraction efficiency of such a deep-UV light-emitting diode can be enhanced.
Due to the worldwide portable devices and illumination technology trends, researches interest in laser diodes applications are booming in recent years. One of the popular and potential LDs applications is near-eye display used in VR/AR. An ideal near-eye display needs to provide high resolution, wide FOV imagery with compact magnifying optics, and long battery life for prolonged use. However, previous studies still cannot reach high light utilization efficiency in illumination and imaging optical systems which should be raised as possible to increase wear comfort. To meet these needs, a waveguide illumination system of near-eye display is presented in this paper. We focused on proposing a high efficiency RGB LDs light engine which could reduce power consumption and increase flexibility of mechanism design by using freeform TIR reflectors instead of beam splitters. By these structures, the total system efficiency of near-eye display is successfully increased, and the improved results in efficiency and fabrication tolerance of near-eye displays are shown in this paper.
Nowadays, light emitting diodes (LEDs) have been widely used in backlight module for display technology. Most of researches tend to improve optical performance in specific applications, such as sufficient efficiency, desired intensity distribution and high illuminance uniformity. However, most of phosphor converted white LEDs have the problem of inducing impure white light. The undesired phenomenon of yellow ring or blue ring becomes more serious through incorrect secondary optical design. In this paper, we emphasize on enhancing the spatial color and illuminance uniformity of LED direct-lit backlight using nonimaging achromatic lens design. We propose a new design method to re-distribute and uniform the ratio of blue and yellow light on the target surface. Moreover, we further apply it in direct-lit LED backlight lens design, in which the uniformity of illuminance on the out coupling surface can be as much as 83.7% and the color uniformity triangleu'v') is improved to 0.0039. Therefore, the result of high color and illumination uniformity can be achieved simultaneously.
Research interests on sunlight applications are booming in recent years, due to the worldwide green-energy trends. Either using PV cells to store sunlight then convert to electricity, or to use sunlight for direct illumination source are among the many research projects which deserve investigation.
In this research, we focus a design combined the above two features together: direct sunlight illumination, and store the sunlight for later usage. Our design structure is as follows: 1. On the surface of outer layer, we use the liquid-prism structure to increase the angle tolerance range of solar concentrator; 2. Combine the micro structure of the solid-state prism and aspheric surfaces to produce a planar light guide structure, which compresses the plane light source into line light source, then guide the light into solar cells area; 3. Design a light switch using the liquid-prism of inside layer, and guides the sunlight into solar cells channel or indoor illumination channel.
We apply it in the NLIS® developed at NTUST, not only retain the advantages of the static concentrator modules, but also eliminate the complex procedure of transmitting and emitting, reduce the loss and cost of energy transfer.
The concept of the remote phosphor is proven to be one of the effective solutions for improving luminous efficacy of pc-WLEDs by solving the problem of phosphor thermal and scattering loss. However, most of them need to use larger packaging design to enhance their performance. Such development is adverse to market trends, which also resulted in higher manufacturing costs and the difficulties in luminaire design. In this paper, we present the analysis of pc-WLEDs as the function of the packaging size and figure out its limitation, so that we can apply to reduce the device size but keep the luminous efficacy as high as possible.