Contrary to common belief, light does not all the time propagate linearly. Hence it tends to bend when it takes the form of airy wave packets. This paper describes a first possible application of such wave packets to automotive lighting technology. After a first brief description of the historical background of the airy beams principle and their potential applications, a detailed analysis of these beams under potential-free Schrödinger equation with physical formulations is proposed. Considering that one of the most peculiar characteristic of airy waves is that they stay diffraction free when propagating, ‘diffraction’ and ‘diffraction-free propagation’ aspects and the physics behind it is then analyzed and described at the second step. In the third part of the paper, the characteristics of Bessel Beams, and their diffraction free behavior is explored and a comparison between Bessel beams and Airy beams is crosschecked. As Airy beams do accelerate during propagation, they describe a ballistic trajectory and bend. Up to now, these beams were mainly used to generate curved plasma channels in air and for particle separation in optical trapping applications. We investigate in our paper how the bending property of Airy beams could be used to achieve illumination in curved roads and corners in an automotive lighting application. Considering that so far, Airy beams were never thought to be a possible alternative to the current mechanical systems used to provide a “bending light” function, we describe how this could be achieved and what are the next steps to be investigated.
Bright white light sources are of significant importance for automotive front lighting systems. Today’s upper class vehicles mainly use HID or LED as light source. As a further step in this development laser diode based systems offer high luminance, efficiency and allow the realization of new styling concepts and new dynamic lighting functions. These white laser diode systems can either be realized by mixing different spectral sources or by combining diodes with specific phosphors. Based on the approach of generating light using a laser and remote phosphor, lighting modules are manufactured. Four blue laser diodes (450 nm) are used to activate a phosphor coating and thus to achieve white light. A segmented paraboloid reflector generates the desired light distribution for an additional car headlamp. We use high speed milling and selective laser melting to build the reflector system for this lighting module. We compare the spectral reflection grade of these materials. Furthermore the generated modules are analyzed regarding their efficiency and light distribution. The use of Rapid Prototyping technologies allows an early validation of the chosen concept and is supposed to reduce cost and time in the product development process significantly. Therefor we discuss costs and times of the applied manufacturing technologies.
We report on the achievement of relatively high power phosphor-free white light-emitting diodes (LEDs) using a new self-organized InGaN/AlGaN dot-in-a-wire core-shell nanowire heterostructure. Multiple AlGaN shell layers are spontaneously formed during the growth of the quantum dot active region. Due to the drastically reduced nonradiative surface recombination, such core-shell nanowire structures exhibit significantly increased carrier lifetime (from ~ 0.3ns to ~ 4.5ns) and massively enhanced photoluminescence intensity. Strong white-light emission was recorded for the unpackaged core-shell nanowire LEDs with an output power of >5 mW, measured under an injection current ~ 60A/cm2, with a color rendering index of ~ 95.
A twice wafer-transfer technique can be used to fabricate high-brightness p-side-up thin-film AlGaInP-based light-emitting diodes (LEDs) with an aluminum-doped zinc oxide (AZO) thin films transparent conductive layer deposited on a GaP window layer. The GaP window layer consist of the two different doping profile, the carbon doped Gap (GaP:C) window layer of 50 nm is on the top of Mg doped GaP window layer of 8 μm. The GaP:C window layer is used to improved the ohmic contact properties of GaP:C/AZO. The AZO with different cycle ratio of Zn:Al (15:1, 20:1 and 25:1) is deposited on GaP:C window layer as current spreading layer by atomic layer deposition. The AZO layer can be used to improve light extraction, which enhances light output power. The output power of p-side-up thin-film AlGaInP LED with an AZO layer of 20:1 cycle ratio has improved up to 19.2 % at injection current of 350 mA, as compared with that of LED without AZO film. The p-side-up thin-film AlGaInP LED with AZO current spreading layer exhibited excellent performance stability, the emission wavelength shift of p-side-up thin-film AlGaInP LED without and with AZO thin film(Zn:Al=20:1) are 17 nm and 3 nm under the injection current increased from 20 mA to 1000mA, respectively. This stability can be attributed to the following factors: 1) Refractive index matching, performed by introducing AZO thin film between the epoxy and the GaP window layer enhances light extraction; and 2) the favorable thermal dissipation of the silicon substrate reduces thermal degradation.
We report on green (550–560 nm) electroluminescence (EL) from (Al0.5Ga0.5)0.5In0.5P–(Al0.8Ga0.2)0.5In0.5P double p–i–n heterostructures with monolayer–scale tensile strained GaP insertions in the cladding layers and light–emitting diodes (LEDs) based thereupon. The structures are grown side–by–side on high–index and (100) GaAs substrates by molecular beam epitaxy. Cross–sectional transmission electron microscopy studies indicate that GaP insertions are flat, thus the GaP–barrier substrate orientation–dependent heights should match the predictions of the flat model. At moderate current densities (~500 A/cm2) the EL intensity of the structures is comparable for all substrate orientations. Opposite to the (100)–grown strictures, the EL spectra of (211) and (311)–grown devices are shifted towards shorter wavelengths (~550 nm at room temperature). At high current densities (>1 kA/cm2) a much higher EL intensity is achieved for the devices grown on high–index substrates. The integrated intensity of (311)–grown structures gradually saturates at current densities above 4 kA/cm2, whereas no saturation is revealed for (211)–grown structures up to the current densities above 14 kA/cm2. We attribute the effect to the surface orientation–dependent engineering of the GaP band structure which prevents the escape of the nonequilibrium electrons into the indirect conduction band minima of the p– doped (Al0.8Ga0.2)0.5In0.5P cladding layers.
We have developed a low-cost alternating current (AC) direct light-emitting diode (LED) chip in which bridge rectifiers are implemented within a multi-cell array. The chip was designed and fabricated to form a ladder type electrical circuit of integrated multi-cells for direct operation with high voltage AC power source. Through a new isolation process technique, the luminous flux in the multi-chip LEDs increased by 5%, which is ascribed to the increase of active area in the chip. In this paper, we report on the effects of a cell array design on the luminous efficiency, and a new process to improve the device performance. The 2 W device exhibited a typical luminous efficiency of 85 lm/W at a color temperature of 3000 K and color rendering index (CRI) 80. Furthermore, an advanced design to overcome the optical and electrical degradation by the high reverse voltage applied to a bridge rectifier are also discussed.
This paper reviews the properties of the defects which limit the performance and the reliability of LEDs based on InGaN. More specifically we discuss: (i) the origin and properties of the defects responsible for SRH recombination; (ii) the role of defects in favoring the degradation of InGaN-based LEDs. Original data are compared to previous literature reports to provide a clear understanding of the topic.
A flexible large area lighting devices have been demonstrated by PDMS films. The (polydimethylsiloxane) PDMS films doped with organic/inorganic materials. The PDMS film is favorable due to its heat stability, good transparency, and flexibility. This study aimed to combine both organic and inorganic materials for flexible large area lighting applications. The architecture consists of blue LEDs coupled to a leaky waveguide that is covered with the PDMS film. The white light was generated with the poly (9, 9-dioctylfluorene-co-benzothiadiazole)F8BT blended into the PDMS slurry. Organic wavelength conversion materials were chosen owing to their ability to decompose in nature. The more conventional inorganic phosphors such as YAG are difficult to decompose and may present environmental issues which can bring concerns in many lighting applications. These flexible PDMS films had thicknesses of 100μm, 440μm, and 980μm. The resulting white light devices had color temperatures of 8944K, 4863K, and 4429K, respectively. In this study, we have also compared the performance of the organic versus conventional YAG phosphor embedded films.
The current crowding effect in AlGaN 275 nm deep UVLEDs has been investigated by using 2D drift diffusion solver and the Monte Carlo ray-tracing method. Optimized conditions for both lateral and vertical structure have been presented in this paper by changing the spacing between 2 fingers. To improve the light extraction efficiency, using graphene as the contact layer and removing the p-GaN layer have also been discussed here. Thus with the markable increasing of LEE, the external quantum efficiency can be improved to 37.8% in the textured vertical structure without p-GaN layer.
Recently, near-ultraviolet light-emitting diodes (NUV-LEDs) have been used in many applications such as light sources for ultraviolet curing, environmental cleaning, biomedical instrumentation, counterfeit bill detection and phosphor-based white LEDs. However, it is difficult to fabricate NUV-LEDs with high emission efficiency. As the wavelength of NUVLEDs decreases, the most dominant emission will be photons with transverse-magnetic (TM) polarization. For LED structures grown on a c-plane substrate, TM-light propagates mainly in the lateral direction, and it suffers strong effects of total internal reflection (TIR) due to the large incident angle on the interface. Therefore, light extraction efficiency (LEE) of NUV-LEDs is still lower than that of visible LEDs. In this study, a spin coating process in which the grating structure comprises the metallic nanoparticle layer coated on a p-GaN top layer was developed. Various sizes of metallic nanoparticles forming a suspended nanoparticle layer (SNL) embedded in a transparent conductive layer were clearly observed after the deposition of indium tin oxide (ITO). The SNL enhanced the light extraction efficiency of NUVLEDs. Light output power was 1.4 times the magnitude of that of conventional NUV-LEDs operating at 350 mA, but retained nearly the same current-voltage characteristic. Unlike in previous research on surface-plasmon-enhanced LEDs, the metallic nanoparticles were consistently distributed over the surface area. Device performance can be improved substantially by using the three-dimensional distribution of metallic nanoparticles in the SNL, which scatters the propagating light randomly and is coupled between the localized surface plasmon and incident light internally trapped in the LED structure through TIR.
In the exposure process, the intensity and uniformity of light in the exposure area directly influenced the precision of products. UV-LED (Ultraviolet Light-Emitting Diode) exposure system was established to reduce the radiation leakage and increase the energy efficiency for energy saving. It is a trend that conventional mercury lamp could be replaced with UV-LED exposure system. This study was based on the law of conservation of energy and law of refraction of optical field distributing on the target plane. With these, a freeform lens with uniform light field of main exposure area could be designed. The light outside the exposure area could be concentrated into the area to improve the intensity of light. The refraction index and UV transmittance of Polydimethylsiloxane (PDMS) is 1.43 at 385 nm wavelength and 85-90%, respectively. The PDMS was used to fabricate the optics lens for UV-LEDs. The average illumination and the uniformity could be obtained by increasing the number of UV-LEDs and the spacing of different arrangement modes. After exposure process with PDMS lens, about 5% inaccuracy was obtained. Comparing to 10% inaccuracy of general exposure system, it shows that it is available to replace conventional exposure lamp with using UV-LEDs.
We have built a novel oblique angle scatterometer designed and optimized for measurements of rough surfaces having a root mean square roughness value (RMS Roughness) on the order of 100 nm – 1000 nm or larger. The majority of existing techniques for measurement of such surfaces are slow, sensitive to vibration, provide short or no working distance, may result in generation of particles and have very small throughput.1 In this paper, we novel metrology addressing the above limitations.
The influence of nitride heterostructures on efficiency droop is presented. It was developed a special method based on simulation for investigating the changes in the semiconductor devices characteristics due to different influencing factors. The cause of efficiency droop was detected - large difference in carrier lifetimes. The simulation results are used to suggest several ways for improving LED efficiency about 12 %.
In the study, the Polyol reduction process was used to fabricate silver nanowires (AgNWs). In the experiment, the ratio of PVP/Ag, silver seed, AgNO3 and the amount of ethylene glycol (EG) were adopted to design orthogonal array with a constant temperature and heating time and the synthesis parameters of AgNWs were obtained. Therefore, the optimal AgNWs solution was obtained, followed by centrifuging to obtain AgNWs which were used to fabricate AgNWs film. The scanning electron microscope (SEM), Fourier Transform Infrared Spectroscope (FTIR), Energy Dispersive Spectrometer (EDS) and four-point probe were used to measure the sheet resistant and transmittance of AgNWs film. Moreover, the AgNWs film was adopted to be the conductive wires of LED. From the experiment results, the synthesis parameter of 15ml EG, 0.01g AgCl, ratio 2 of PVP/Ag and 0.22g AgNO3 could be used to fabricate optimal AgNWs with 45nm average diameter, 5μm average length and aspect ratio of 110. The sheet resistance and transmittance of film fabricated by centrifuged AgNWs was 0.1252 Ω/sq and 70%, respectively. Furthermore, the luminance of LED with conductive wires made of AgNWs film was better than that made of commercial silver plastic. In the future, the AgNWs film can be broadly applied to the conductive films of touch electric products, LCD display and solar panels.
Solid state lighting using LED-dies is a rapidly growing market. LED-dies with the needed increasing luminous flux per chip area produce a lot of heat. Therefore an appropriate thermal management is required for general lighting with LEDdies. One way to avoid overheating and shorter lifetime is the use of many small LED-dies on a large area heat sink (down to 70 μm edge length), so that heat can spread into a large area while at the same time light also appears on a larger area. The handling with such small LED-dies is very difficult because they are too small to be picked with common equipment. Therefore a new concept called collective transfer bonding using a temporary carrier chip was developed. A further benefit of this new technology is the high precision assembly as well as the plane parallel assembly of the LED-dies which is necessary for wire bonding. It has been shown that hundred functional LED-dies were transferred and soldered at the same time. After the assembly a cost effective established PCB-technology was applied to produce a large-area light source consisting of many small LED-dies and electrically connected on a PCB-substrate. The top contacts of the LED-dies were realized by laminating an adhesive copper sheet followed by LDI structuring as known from PCB-via-technology. This assembly can be completed by adding converting and light forming optical elements. In summary two technologies based on standard SMD and PCB technology have been developed for panel level LED packaging up to 610x 457 mm2 area size.
The energy-efficient use of LED light requires the development of compact illumination systems for the customized homogenization and shaping of partially-coherent LED light. Therefore a design concept which is based on arrays of aperiodic micro structures, namely cells, for primary or secondary optics is introduced. Each cell of the array deflects locally the light into predefined directions and results in a light spot in the target plane. The light spots of all array cells together form the desired light pattern. The performance of three different cell geometries (linear gratings, micro prisms andmicromirrors) on the homogenization and shaping ofmonochromatic as well as white light LEDs is demonstrated. For the realistic evaluation of the illumination system an LED model including power spectrum, polarization, spatial and temporal coherence is chosen. Furthermore wave-optical effects like diffraction at the cell apertures are taken into account. For the grating cells arrays a rigorous analysis of the diffraction efficiencies is included.
We’ve developed NIR-LED array light source module for photoacoustic imaging system, which is ultra-small and consumes extremely low power. Conventional photoacoustic imaging system uses solid state laser light source, which consumes large amount of electricity. Instead, we’ve developed high intensity NIR-LED chips on a 1cm x 6cm board, which produces approx. 1.0kW output power and obtained photoacoustic signal by driving NIR-LED light source module with about 100 ns pulse. Comparing to the laser light source, our module is much smaller than 1/15000 the volume and less than 1/1000 the power consumption approximately. We’ve achieved penetration depth of 30mm.
In this paper, we investigated the failure mechanisms of blue InGaN LEDs grown on patterned sapphire substrates and demonstrated the influence of patterned sapphire substrates on the reliability of GaN LED by comparing with conventional LEDs grown on planar sapphire substrates. From experimental results, we found that InGaN LEDs grown on patterned substrates had a higher turn-on voltage but a smaller series resistance compared with conventional LEDs owing to rough inner patterns and small threading dislocation density. Both samples were then acceleratedly aged under a high DC current for two hours. Failure modes were studied with various measurements taken before and after aging. From the power evolution performance, we found that output power of LEDs with patterned substrates increased slightly due to fewer defects while output power of conventional LEDs decayed. This can be inferred from small reverse leakage currents and tunneling currents observed from Log I-V characteristics and EMMI measurement of P-LEDs. A slight redshift in emission wavelength was also found during aging because of possible leakage shunt paths caused by defect generation. Moreover, operation voltage increased slightly after aging which was caused by contact degradation induced by thermal annealing.
Blue nitride-based LEDs have been grown hetero-epitaxially on sapphire, SiC and Si substrates. Homo-epitaxial growth of LEDs grown on GaN substrate is getting popular to improve the reliability and to increase the power density per chip. Laser lighting also would be a promising lighting technology to improve the power density per chip in the future.