This PDF file contains the front matter associated with SPIE Proceedings Volume 11302, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists

Flat panel displays are ubiquitous and dominated today by liquid crystal and OLED technologies. Increasingly, there is an expectation that microLED will exhibit superior performance metrics and become a new mainstream category of flat panel displays. They have the potential to be very bright, to be power efficient, and to enable new within-panel capabilities. High-throughput, high-yield, mass transfer technologies that accurately and cost-effectively integrate large arrays of wafer-fabricated microdevices onto non-native display substrates are key enablers for microLED displays. Transfer-printing with elastomer stamps is a candidate mass transfer technology for making next generation displays. A variety of microLED displays, including displays controlled with transfer-printed microICs, have been designed and fabricated using elastomer stamp transfer-printing.

Existing mobile and AR/VR headsets tether us to a display that creates a physical barrier to personal interactions. Next generation AR/VR solutions will need to fundamentally alter the way people receive and share information, a concept Mojo Vision calls “Invisible Computing”. NanoLEDs, a further shrinking of MicroLEDs to micron and sub-micron scale is expected to play a critical role in the future of Invisible Computing because of their ability to create the pixel densities required to get closer to the human eye while maintaining full visual acuity of the human visual system. Mojo Vision set the record for dynamic display NanoLED ppi at 14K (monochrome green) in May of 2019. A new record 20Kppi dynamic NanoLED display along with its application in Augmented Reality will be presented.

Recently, researches on the micro-displays using inorganic LEDs have stirred much attentions due to its various advantages such as reduced power consumption, long life time and short response time. In this study, we achieved a full-color integration of LEDs for display applications by combining growth and bonding technologies with two different approaches. First method was integrating inorganic red, green, and blue LEDs on a single substrate using metalorganic chemical vapor deposition and thin film bonding techniques. Second method was fabricating inorganic red, green, and blue LEDs by using GaN-on-Si-based blue LEDs and adhesive bonding.

Understanding recombination mechanisms in halide perovskites is of key importance to their applications in photovoltaics and light emission. We perform first-principles calculations to compute the radiative and nonradiative recombination coefficients in the methylammonium lead iodide as well as in other halide perovskites. The computed radiative recombination coefficient is as high as in typical direct-gap semiconductors used in optoelectronics. However, our first-principles calculations of nonradiative rates show that strong Auger recombination will suppress efficiency of light emitters. I will also discuss defect-assisted recombination, a problem closely coupled to the issue of degradation. Work performed in collaboration with Xie Zhang, Jimmy-Xuan Shen, and Wennie Wang, and supported by DOE.

In this work, we probe the photodegradative behaviour of CsPbBr₃ perovskite nanocrystals under illumination intensities in excess of 1 W=cm². In doing so, we uncover optical behaviours unique to this extreme form of degradation namely a pronounced period of increasing photoluminescent intensity at the outset of degradation along with a red-shifted emission lobe.

We also compare the photochemical lifetimes of CsPbBr3 to the relating organic-inorganic hybrid of FAPbBr₃ and show that FAPbBr₃ can withstand such high intensities for approximately ten times longer than CsPbBr₃. This marks out FAPbBr₃ as a potential successor to CsPbBr₃ in optoelectronic applications.

This Conference Presentation, Frontiers in LED technology for breakthrough integrated solutions was recorded at Photonics West 2020 held in San Francisco, California, United States.

As the brightness of high-power LEDs is generally limited to less than ca 200 Mnit (200 x 10⁸ lm/m²sr), and expectations are that this will stay limited to a few hundred Mnit for optimized devices, high luminance light modules have been developed during the past few years based on luminescent concentrators. With these light sources the requirements can be met for most high luminous flux applications with limited étendue, like in stage and entertainment lighting or in digital projection, where LEDs don’t meet the specifications. In this paper we report on the challenges of High Lumen Density (HLD) light engine concepts based on transparent luminescent concentrators pumped by blue LEDs and on the large improvements that were recently made with respect to luminance and module efficacy while significantly simplifying the architecture. For mainstream LCD-based front projection systems, typically a yellow-green light source with an étendue of less than 14 mm²sr and a luminous flux of more than 14 klm (DC) is requested to enable > 4k ANSI-lm while meeting a high-quality color gamut. By optimizing the pump LEDs and the light coupling configuration and by decoupling the thermal channels for converter and pump LEDs in a simplified module architecture, we have improved the efficacy from 55 lm/W to more than 70 lm/W for 15 klm yellow-green output with a luminance well over 1 Gnit while reducing the module complexity considerably. With the same concept a DC luminous flux of 19 klm was achieved within an étendue of 13.6 mm²sr (i.e., 1.4 Gnit). By design, the preferred trade-offs can be made between efficiency, luminance, luminous flux, module size, and cost. Thanks to this new architecture, further optimization for the specific applications is possible, enabling also more temperature-sensitive converter materials to be applied successfully.

Laser lighting is an emerging technology to generate high luminance lighting. To achieve high luminance or high luminous exitance, the light emitter must have high flux and small size simultaneously. When laser light is focused to a small spot size on the phosphor material, the two main limitations are saturation of the phosphor material and the spot size of the generated light. Here, we investigate experimentally and numerically the spot size of laser lighting dependent on the spot size of the incident laser light and the material properties of the wavelength converting phosphor material. We find numerically that the spot size of the generated white light is significantly influenced by the phosphor properties. The spot size of the white light determines the étendue and thereby the possibility to collect and shape the light. This has important implications in applications of laser lighting.

Applications of photobiological studies and photochemical reactions are unlocking innovative results both for research and industrial fields. With this work we report on the optical, electronic and thermal design of extreme irradiance incoherent solid-state light sources. State of the art GaN (Gallium Nitride) and AlInGaP (Aluminum Indium Gallium Phosphide) LEDs have been analyzed and selected in order to achieve 450 nm and 940 nm radiation. Different optical approaches have been evaluated: i) geometric lenses, ii) TIR lenses and iii) reflectors. Lighting unit prototypes demonstrate a global efficiency (optical power vs electrical power) of up to 50% and an irradiance over an area of 100x100 mm in excess of 10 W/cm².

In recent years, there is a growing need for lighting equipment that creates complex, specific illumination and light intensity distributions, such as road surface drawing lamps, aesthetic design lighting, and direct backlighting. When designing such lighting equipment, we may often have to use a cut-off method, which is a method of projecting a partially shaded image. This method is inefficient for many reasons, as we are purposefully cutting off the light source for illumination. The development of manufacturing capabilities has made feasible the fabrication of more complex optical components, with the freeform shape as its highest candidate. This has opened up the possibility of new design approaches. We propose a design solution that meets the high demand for illumination performance in a more straightforward configuration, using complex free-form surfaces and a ray mapping approach as opposed to flooding the detector with millions of non-sequential rays. A conventional optical surface utilizes a parametric equation for the illumination lens, which can be challenging to control for higher orders of the polynomial function. The optical lens designed with conventional methods require complex parts and, in the end, has a low light efficiency due to the shading of the light source. The proposed illumination method takes advantage of a lighter computational approach via ray mapping and leverages the spatially selective surface sag over a grid of points in the OpticStudio TrueFreeForm surface.

The three-dimensional core-shell structure consisting of GaN nanowire and GaInN/GaN multi-quantum shell (MQS) is thought to be promising for high-performance light-emitting devices, because of its advantages such as non-polar surface orientation, dislocation-free and tolerance of misfit strain due to small size crystal. However, their crystal growth mechanism and its crystalline quality have not yet fully understood, and they are still under the basic considerations. The key issues in the nanowire/MQS structure are its crystalline quality and shape control, which are greatly dependent on the growth condition. Thus, in this paper, selective growth of nanowire/MQS and its optical properties are described.

Magnetron sputter epitaxy (MSE) is the standard process for the deposition of a wide range of industrially relevant coatings, operating with a low-energy ion assistance to minimize ion damage of the material, thus enabling fabrication of high-quality semiconductor materials for the optoelectronics. However, reports on the growth of GaN nanostructures by MSE are much less in comparison with molecular-beam epitaxy and metal-organic chemical vapor deposition. Here, we will present the study on the growth of single-crystal GaN nanorods by MSE using liquid Ga target. The talk will contain three parts: 1) the handling of liquid Ga target during sputtering; 2) self-assembled growth of GaN nanorods on cost-effective substrates; and 3) selective-area growth of GaN nanorods assisted by patterned substrates. Characterizations on structural and optical properties of the MSE-grown nanorods as well as the growth mechanism from nucleation stage to well-developed nanorods will be discussed.

The nanorod structure will be made by way of producing three-dimensional nano-molding process applied with a patterning and deep-etching process differentiated from the existing nano structure formation. Nano-molding process in this study enables the uniform and reproducible nano structure. For the three-dimensional structure, wavelength can be changed through the spacing of nanorods. In this way, multiple wavelengths in one device can be accomplished through the different way from the two-dimensional structure. In addition, when the light-emitting layer is grown on the side of the nanorod, the nonpolar nitride can be grown on the m-crystal plane rather than on the c-crystal plane. The formation of the light-emitting layer based on the nonpolar nitride can suppress the polarization effect generated in the nitride of the polarity, and thus it is expected that the recombination of electrons and holes can be strengthened.

In the expanding field of 2D, hBN serves as the 2D insulator finding application as a non-interacting substrate, passivation layer, and gate dielectric for use with 2D semiconductors, as well as, for deep UV emitters and single-photon sources. This has driven research into synthesis methods for controlled growth form mono to many layer thick films. Here we present on growth of mono to few layer hBN by metal organic vapor phase epitaxy on various substrates from sapphire to transition metals. Models describing growth chemistry for these various substrates are described. The effects growth conditions on properties will also be discussed.

Challenges and opportunities of MOVPE and THVPE/HVPE for nitride light emitting devices are discussed. Attempts to grow uniform and high quality epitaxial layers for both visible and UV range are presented. In order to examine the possibility of using a bulk GaN substrate, cost parity condition of GaN on GaN LED compared with LED on sapphire is presented in terms of a lumen per dollar. It is important to improve through-put of HVPE for GaN substrate manufacturing. Tri-halide VPE (THVPE) is introduced as a newly evolving technology with a high growth rate of 300μm/h at a high growth temperature of 1250ºC, which may replace HVPE for a bulk GaN substrate. Economical consideration of the comparison of HVPE and THVPE is discussed. Regarding UVC LEDs, there is an option to use a high quality AlN template on sapphire which is fabricated by 1700ºC annealing at nitrogen with a face to face configuration. Possible cost reduction and remaining issue are described.

Transition metal dichalcogenides (TMDC) have become attractive candidates for 2D electronics and optoelectronics. While several concepts for light emitting devices have been reported, many of them realized using exfoliated TMDC flakes of micrometer size, only few approaches tackle the challenge of upscaling to relevant device sizes. We demonstrate a light emitting diode based on WS2 monolayers in a scalable design. The devices are fabricated by combining two industrially relevant deposition processes in a vertical p-n architecture: Metal organic CVD (MOCVD) is used to realize the optically active WS² monolayers, while ZnO deposited by spatial atomic layer deposition (sALD) is employed as an electron injection layer on the cathode side. Organic layers spin-coated on an ITO covered glass substrate provide hole injection and transport. The resulting devices exhibit rectifying behavior and red electroluminescence from an area of 6 mm².

In its trivalent form, Europium is well-known for its red emission at ~620 nm; however, transitions at ~590 nm and ~545 nm are also possible if additional excited states are exploited. Using intentional co-doping and energy-transfer engineering, we show that it is possible to attain all three primary colors due to emission originating from two different excited states of the same Eu3+ ion mixed with near band edge emission from GaN centered at ~430 nm. The intensity ratios of these transitions can be controlled by choosing the current injection conditions such as injection current density and duty cycle under pulsed current injection.

We will present recent findings on the physics of III-nitride recombinations, including the demonstration of defect-assisted Auger recombination as a significant droop process, and the intricacies of low-current radiative recombinations, where alloy disorder and Coulomb interaction play a key role. We will discuss implications for future LED applications, from long-wavelength devices to low-power micro-LEDs.

To study the optical degradation of InGaN-based LEDs, we designed an experiment based on color-coded devices, having two quantum-wells with different positions and emission wavelengths. We analyzed a structure (A) with 20% AlGaN EBL, a QW emitting at 495 nm close to the n-side, a QW emitting at 405 nm close to the p-side; a second structure (B) with reversed QW position (495 nm closer to the p-side). The 495 nm QW is the reference QW, whose degradation is investigated during stress time, aiming at analyzing the impact of QW position on degradation rate.
We submitted devices to 80 A/cm² constant current stress, monitoring optical power and voltage by I-V and L-I characterization at each step. All the structures showed an increase in reverse leakage and low forward bias current, possibly due to trap-assisted tunneling ascribed to an increase in trap concentration. Reverse current was found to increase with the square root of stress time, indicating the presence of a diffusion process. The intensity of both QWs decreased during stress time; remarkably, degradation rate of reference QW (495 nm) was found to be much stronger for device B, where the 495 nm QW is closer to the p-side.
The defects responsible for degradation were characterized by Steady-State Photocapacitance measurements, indicating the presence of a ~2 eV level, whose signal changes during stress time. Shallower defects were detected by C-DLTS, that identified a level with 0.284 eV activation energy, possibly related to VN, whose concentration decreases during stress, due to defect annealing.

Surface-emitting InGaAlP SLEDs that are based on in-plane amplification along a horizontal waveguide are demonstrated. Total internal reflection at tilted etched micromirrors are used for the deflection of optical modes normal to the chip surface. Applying a bonding process of the etched wafer onto a new carrier, light can be emitted through the surface that was originally covered by the growth substrate. Depending on the waveguide geometry superluminescence as well as lasing operation is observed. In superluminescence operation an emission spectrum with a FWHM of 10nm centered at 637nm is obtained. The peak output power of the amplified spontaneous emission is up to ~ 200mW which corresponds to a WPE of ~ 6 %. In case of lasing (λ ~ 639 nm) a peak output power larger than 1000mW at a WPE that exceeds 20% is achieved. In addition, an array configuration with radially aligned waveguides that contribute to a common far field is investigated as well.

A wide spectral asymmetry between the wide and narrow facets of a two-section tapered quantum dot (QD) superluminescent diode (SLD) emitting around 1240 nm was observed and investigated. This asymmetry, as characterized by the mismatch in the center wavelengths of the wide and narrow facet spectra, was found to be tunable and had some dependence on the magnitude of the difference in current densities applied to each section of the device. A maximum spectral mismatch of 14 nm was observed when the current density difference between the two SLD sections was 1.5 kAcm².This spectral asymmetry presents an unexplored degree of freedom which could be exploited via multiplexing from a single device to optimize spectral bandwidth.

Furthermore, potentially useful output powers of up to 50 mW were observed from the narrow facet of the SLD, which could again be exploited via single device multiplexing to increase output power, with little to no cost to spectral bandwidth. The experimental findings were analyzed using a rate-equation based QD model considering the QD ensemble inhomogeneous broadening, the multilayer chirped active material, the spatial distribution of the QD carriers and the spectral and spatial distribution of the photons in the SLDs. The numerical simulations were able to predict the asymmetric output powers extracted from the SLD facets, mainly to due to different equivalent material losses experienced by the forward and backward fields in the weakly gain guided tapered device. Simulations were also able to predict the spectral distribution of the optical fields at the output facets.

We present the state-of-the-art electrically pumped single transverse-mode superluminescent diodes (SLDs) emitting in the 2 – 3 µm wavelength range at room temperature. The structures were fabricated by MBE to include two GaSb-based quantum wells and a double-pass ridge waveguide architecture. A cavity suppression element was used for avoiding lasing at high current. Highest power was obtained for SLDs around 2 µm, exceeding 120 mW in CW and > 300 mW in pulsed operation, with spectral width of ~ 40 nm. SLDs around 2.60 µm emitted a record 15 mW output power and exhibited ~ 80 nm spectral width under pulsed operation. Wavelength extension towards 3 µm is discussed.

Focusing on graphene’s high thermal conductivity and high emissivity, we developed a mid-IR light source using a suspended graphene structure as the light emitter. Compared to conventional mid-IR light emitters, this new source has approximately 1000x faster response time and 95% lower power consumption. In addition, it features high brightness and an emission spectrum similar to that of a black body. We used multilayer graphene deposited on Ni foil and transferred it onto a micro-trench etched into a Si/SiO2 wafer to create the suspended structure. The thermal insulation provided by this design allowed for heating of the free standing segment to temperatures over 1000 deg. C through electrical currents. Packaging the light emitter into a vacuum-sealed TO-5 metal can further improved the insulation and ensures a long lifetime for the graphene light emitter. These characteristics make the light source ideal for many analytical applications.

We demonstrate high-speed LiFi data communication of over 20 Gbit/s using visible light from a laser-based white light emitting surface mount device (SMD) product platform that offers 10-100X the brightness of conventional LED sources. Equipped with high power blue laser diodes that offer over 3.5 GHz of 3 dB bandwidth, the laser-based white light SMD modules exhibited a signal-to-noise ratio (SNR) above 15 dB up to 1 GHz. The high SNR was combined with high order quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing (OFDM) to maximize the bandwidth efficiency. In this work, we present a laser based white light SMD module configured with a single 3W blue laser diode mounted on heat-sink, optically coupled to a collimating optic, achieving a LiFi data rate of up to 10 Gbit/s. Moreover, we demonstrate wavelength division multiplexing (WDM), from a white light SMD module configured with two blue laser diodes separated in peak wavelength to serve as separate communication channels. Using WDM, the dual laser SMD module enabled LiFi data rates of over 20 Gbit/s by simultaneously transmitting data over both channels.

Visible Light communication is a data transmission technology that uses the LED lighting infrastructure to simultaneously illuminate and communicate. The ubiquitous existence of LED lamps opened a new opportunity for addressing VLC communication in many indoor communication scenarios. The motivation for the application presented in this paper is the modern, efficient management of warehouses supported by autonomous navigation robots that grab goods and deliver the items at the packaging station. This functionality demands bi-directional communication among infrastructures and vehicles. In this paper we propose links for Infrastructure-To-Vehicle (I2V), Vehicle-To- Infrastructure (V2I) and Vehicle-To-Vehicle (V2V) to perform indoors, bi-directional communication for robot navigation in automated warehouses.
In this work it is proposed a bidirectional communication system between a static infrastructure and a mobile robot (I2V). The LED lamps of the warehouse illumination system are used to lighten the space, and to transmit information about position and about racks content. The mobile robots communicate with the infrastructure (V2I) to transmit information on the items that are being removed and carried to the packaging station. The communication among the autonomous robots (V2V) provides information on the number of items intended to be collected when the vehicles are in the same lane, possibly with the purpose of collecting the same items. Different codification schemes are proposed to establish the V2I, I2V and V2V links. Tri-chromatic white LEDs with the red and blue chips modulated at different frequencies and a photodetector based on a-SiC:H/a-Si:H with selective spectral sensitivity are used at the emitter and receiver. Position information is provided by each LED lamp to the autonomous vehicle by adequate modulation of the RGB emitters. The decoding strategy is based on accurate calibration of the output signal. Different scenarios were designed and tested. Requirements related to synchronous transmission and flickering were addressed to enhance the system performance.

In harsh weather conditions contaminants like water or particles from road dirt attach on the outer surfaces of a vehicle. This applies to the sensor systems of the vehicle as well, which is why they are separated from the environment with a protection window. In today's vehicles, a rain sensor can control the windshield wiper to ensure a clear view for the driver through the windshield. This technique can be applied to automotive light detection and ranging (LiDAR) sensors, which are particularly sensitive to contaminants. However, if solid particles attach on the protection window, the wiper cannot clean the surface properly until a cleaning solution is sprayed on the surface before. We present a method for detecting contaminants on this protection window using total internal reflection techniques. The proposed system is able to separate between liquid and solid contaminants, making the automatic control of a cleaning system consisting of a wiper and nozzles possible. Furthermore, the system yields a degree of contamination on the entire surface of the sensor cover which is of high importance for the data interpretation of the underlying LiDAR system.

The epitaxial growth of UV-C-LED-structures in metal organic vapor phase epitaxy is very challenging. To control and improve the growth process, optical in-situ metrology is therefore indispensable. However, the in-situ metrology itself is also affected by some of the process related circumstances such as patterned substrates or thin layer thickness. In this talk we will provide insight into the challenges and benefits of in-situ metrology during growth of UV-C-LED-structures and we will show how these issues can be overcome. We will also show, that these advanced techniques can also be used to improve epitaxy of other optical devices besides UV-LEDs.

AlGaN UVC light-emitting diodes (LEDs) are attracting much attentions for applications of sterilization and water purification. Here, we demonstrated the improvement in LEE by employing a photonic crystal (PhC) structure in a p-GaN contact layer. FDTD Simulation showed a significant reflection of emitted light at the interface between the PhC and the active layer, which was a function of the distance between the PhC and the active layer as well as the PhC parameters. A UVC LED with a PhC structure showed an enhanced LEE by 1.9 times compared to an LED without a PhC structure.

The predicted progress and enhanced performance of UVC LED modules requires both an increased efficiency of the LED dies along with design and implementation of highly reliable and innovative package technology concepts. This includes the need for outstanding thermal conductivity, the use of non-aging materials that withstand UVC radiation, low optical losses, optics that are highly transparent for UVC radiation and useable for beam shaping, and the ability to enable reliable performance in harsh environmental conditions by implementing hermetic protection of the die. Hermetic package components from SCHOTT use proven Glass-to-Metal Sealing (GTMS) and Ceramic-to-Metal Sealing (CerTMS®) technology, offering a suitable package solution for UV LED modules. Longevity and reliability tests show clear advantages in comparison to polymer-based modules. In addition, the outcoupling efficiency can be significantly increased by using high-transmission SCHOTT Deep UV as encapsulation material

Packaging materials usable for DUV-LEDs are limited as most organic materials are affected by DUV radiation. Packages used are TO-packages or 3D-structured ceramic housings with quartz lid. Due to DUV-LEDs radiating up to 50% of their light to the sides a significant share is lost.

This paper describes a hermetic SMD-compatible packaging approach that integrates reflectors into a quartz window to maximize the light extraction. Side emitted and otherwise wasted light is redirected towards the surface. It avoids costly 3D-structured ceramics and improves the overall thermal performance. Design choices of the reflector structures allow to tailor the radiation patterns of the LEDs.

Colloidal quantum dots (QDs) are intriguing materials due to their outstanding properties like spectral tunabilty, high quantum efficiency, narrow emission spectra and solution processability. Many past and on-going researches on quantum dot light-emitting devices (QLEDs) have led to achieving efficiency levels comparable to organic LEDs and semiconductor LEDs. However, most QLED studies are based on toxic QDs, which raises concerns about environmental and health issues. Our study demonstrates the application of a new non-toxic nanomaterial, InP/ZnO QDs, to LEDs. Integrating InP/ZnO QDs into device architecture, we produced low turn-on voltage (2.8 V), saturated color devices, which have luminance levels (600 cd/m2) suitable for display technology.

Narrow band (i.e. line) emitters have an enormous potential for use in LEDs, as they will allow both improvement of color rendering while maintaining high lm/W, and open up new spectral engineering possibilities for specific applications. The major bottle neck for implementation of line emitters in LEDs, is the lack of absorption in the blue spectral region of conventional trivalent lanthanide doped phosphors. In this talk, I will present a new nanoscale approach to engineer phosphor materials, which allows tuning of absorption and emission wavelengths over a wide range of the spectrum. The underlaying mechanism of these engineered phosphors is interparticle energy transfer, which will be discussed in more detail.

In this contribution, we report on the newly found red phosphor Sr[Li₂Al₂O₂N₂]:Eu²⁺ (SALON) and the closely related series of phosphors with the composition SrAl_2-xLi_2+xO_2+2x N_2-2x (0.12≤ x ≤ 0.66). The latter compounds can be regarded as unordered variants of SALON exhibiting much broader emissions that can be tuned between 581 nm (x = 0.66) and 672 nm (x = 0.12). These examples impressively illustrate the importance of ordering phenomena for structure property relationships in luminescent materials. With an emission maximum of 614 nm, the ordered SALON phosphor perfectly meets the desired emission maximum stated in the 2016 solid state lighting R&D plan while also exhibiting an ultranarrow emission band (fwhm = 46 nm). This new phosphor has been shown to enable an LER gain of 16 % in a prototype LED, relative to a state of the art CRI90 reference.

Laser pumped phosphor (LPP) technology has started to replace high intensity discharge lamps in digital projectors. LPP technology is typically based on garnet phosphors that are excited with blue laser diodes at 445 – 465 nm. The luminance of LPP light sources can well surpass 1000 cd/mm² and thus address high luminance applications that could not benefit from the advantages of solid state lighting (SSL) so far. Due to the recent cost down of blue laser diodes, digital projectors with LPP light sources that make use of fast spinning phosphor wheels and laser powers >100 Watts are now widely commercially available.

Another commercially available application of LPP technology are laser headlights. To avoid moving parts, laser headlights make use of static phosphor assemblies, but they operate at only a few Watts of laser power. A static phosphor assembly consists of a ceramic phosphor that is bonded onto a heat spreader. Due to their good thermal and optical properties, static ceramic phosphors are excellent light converters even at higher powers. They enable high luminance light sources with a luminous flux that is far beyond the flux needed for car headlights. Static converters will also improve the performance of light sources for microscopy, machine vision and any other etendue limited specialty lighting application.

We investigate factors that determine the achievable power and luminance such as irradiance limit, conversion efficacy, spot confinement and cooling. Experimental and numerical results are presented.

While LEDs have been the used in many of the high “lumen” applications, they are not “bright” enough for projectors, entertainment spotlight, etc., where the etendue of the systems are small. Laser phosphor system have been developed in the last 10 years using mostly silicone, some ceramic and glass, phosphors for low power applications. For higher power systems such as projectors, phosphor wheels are used so as to dissipate the heat in a larger area, allowing the operating temperature to be below the damage and droop threshold of the phosphor material. For silicone phosphor, the outputs are usually limited by the bonding materials. This paper presents static, without a rotating wheel, high power laser excited crystal phosphor system in which the crystal phosphor has a very high damage and drooping threshold temperature. Using 2 laser diode arrays, a total of 170 W of blue laser light is focused into an area of smaller than 2 mm in diameter, giving a power density of over 54 W/sq. mm., which is limited by the available laser power. It is expected to increase in the near future with higher power laser sources, development of homogenizing and diffusing optics at the input, and micro and photonic structures at the output surfaces. For projector applications, this high power static crystal phosphor system can replace the current phosphor wheel, in most case, directly without redesign of the other projector components in terms of mechanical, optical, and electronics. The crystal phosphor materials have been developed and manufactured by Taiwan Color Optics, Inc.

LED surface structuring has been widely used to increase light extraction[1]. Due to the high refractive index of the thick GaN epitaxy layers, most emitted light becomes trapped and reabsorbed by the epitaxial layers. While random structuring can effectively scatter trapped light out of the LED, it gives little control over the resulting beam-shapef[2]. Photonic crystals however provide a means to simultaneously improve light extraction efficiency and control beam directionality. Furthermore, P-side up LEDs normally utilize a transparent top contact layer in order to allow top light emission whilst maintaining good electrical properties. In this paper we investigate a novel photonic crystal LED configuration with a nontransparent metal top contact layer, and cylindrical holes etched through the top contact layer and deep into the underlying epitaxy. In this novel configuration light emission is only possible from the etched holes giving rise to extreme beam steering effects. We utilize broadband spectroscopic reflectometry to experimentally investigate beam shape and optical properties from fabricated devices. We observe a range of achievable beam patterns with extreme deviations from the normal Lambertian. We investigate the effect of square and triangular photonic crystal lattices on beam directionality.

The influence of photoluminescence (PL) spectra from two same-sized zinc oxide (ZnO) microspheres (MS), made by hydrothermal growth, coupling with each other is studied. It is shown that the difference of the PL spectra between single ZnO MS and coupling ZnO MSs can be observed. Coupled ZnO MSs leads to peak splitting of whispering gallery modes (WGMs) for both TE and TM polarization. The PL spectra were obtained by illuminating coupled ZnO MSs directly by 325 nm laser source via a micro-PL setup. First, we inject the laser into the one of the coupling ZnO MSs with 1.72 μm diameter, the PL spectra has few difference between single ZnO MS because the other MS makes less influence on the spherical resonator. Second, by injecting the laser which covers the coupling ZnO MSs, it’s surprising that the PL spectra shows various splitting peaks in the visible region. The phenomenon tells these two coupling MSs lose their symmetry which exist if they are separated, thus, the angular momentum of one ZnO MS is destroyed by another attached ZnO MS. To sum up, the PL spectra of ZnO MS is sensitive to the surface-decorated particles, which can be applied in biosensing to detect various molecules.

This paper describes a patent pending system in which the configuration, utilizing a single parabolic reflector, could allow high power reflective phosphor mode of operation, accommodate multiple laser diodes, and recycles the high angle light for increased output with low NA. In addition, the reflective phosphor and the laser diodes are both mounted on the same heat sink allowing a simple light module design with low cost. The parabolic reflector functions as the coupling optics between the laser diodes and the phosphor plate and as part of the recycling optics. With an addition of another parabolic reflector, the system produces a focused output suitable for coupling light into a fiber optic or cable. Other related recycling systems are also described, which will be useful for various applications with different requirements.

Micro-LEDs are expected to bring revolution to display technology with its high brightness, long lifetime, refined performance and low power consumption. However, the electroluminescence spectrum of a Micro-LED varies significantly depending on the driving voltage and current, and thus ensuring the uniformity in red, green, blue colors in terms of the chromaticity and luminance across a massive amount of pixels is challenging for mass production. Due to its micron-level size, color correction by mechanical treatments is limited in practice. In order to overcome this bottleneck in the development and commercialization of Micro-LED technology, we propose a 4D transformation algorithm which allows to efficiently adjust non-linear chromaticity and luminance of Micro-LEDs and acquire the true color characteristics with high precision. We evaluate the performance of our algorithm with measurements and verify its outstanding performance compared to the conventional algorithms. The standard deviation of proposed uniformity correction result is about 2% and the color and luminance uniformity ΔE is below 3.5.

Future cars with (semi-) autonomous driving enable more use of interior lighting with special visual effects. This deals as well with interior materials – like plastics, wood, metals - which are illuminated or backlighted by RGB LEDs. We performed 3D BRDF measurements from a luminance imager and compare them to traditional goniophotometers. Perception test with 37 subjects for transflective materials show that there is a narrow width of backlight intensity for “pleasant” (not bothering) intensity. Such fundamental optical examinations enable special visual effects for future vehicles.

Cesium based wide band gap inorganic perovskites evolve as a promising material for novel applications in optoelectronic devices due to high stability and band gap tunability. In this work, we have synthesized the inorganic CsPbCl₃ nanoparticles (NPs) as well as Mn-doped CsPbCl₃ NPs. Effect of Mn as dopant on the structural properties of CsPbCl₃ has been investigated by performing the X-ray diffraction (XRD). XRD results clearly indicate lattice contraction due to the incorporation of Mn as a dopant. Photoluminescence (PL) plot of Mn-doped CsPbCl₃ shows the emission of dual colour which is attributed to d to d band transition of Mn²⁺. Blue and Orange colour emission has been seen for CsPbCl₃ and Mndoped CsPbCl₃ NPs respectively. A theoretical study has been also performed within the framework of Density Functional Theory (DFT) to validate our experimental results. DFT calculated (experimentally calculated) lattice parameters for the optimized crystal structure of CsPbCl₃ and Mn-doped CsPbCl₃ are 5.608 Å (5.621 Å) and 5.563 Å (5.574 Å) respectively. These results also show a reduction in lattice parameters due to the introduction of Mn as a dopant in CsPbCl₃ which are in good agreement with our XRD results. Electronic band structure calculation for Mn-doped CsPbCl₃ shows the presence of additional energy levels around 2.06 eV in the band gap. Additionally, Blue shift phenomena have been found in the absorption coefficient plot due to the incorporation of Mn. This communication envisages the potential of these NPs in various optoelectronic devices including orange LEDs (Light-emitting diodes).

All inorganic Cesium based Lead halide perovskites exhibit unique and interesting photophysical properties arousing huge interest in the domain of photovoltaic cells and light emitting diodes (LEDs). In this communication, we present a deep theoretical insight employing Density Functional Theory (DFT) to understand the structural, electronic and photophysical properties of CsPbBr₃ quantum dots (QDs). Structural analysis of CsPbBr₃ QDs results in an optimized lattice parameter of 5.885 Å. Electronic properties of CsPbBr₃ QDs have been investigated by means of electronic band structure, the partial density of states (PDOS) and total density of states (TDOS) calculations. A direct band gap of 2.38 eV has been observed at the Gamma (Γ) point in its Brillouin Zone. Analysis of PDOS shows upper valence band and conduction band of the CsPbBr₃ are due to the Br (4p) orbital and Pb (6p) orbitals respectively. Moreover, the optical absorption coefficient has been also calculated using Kubo-Greenwood formula in a frame work of DFT. The optical absorption spectrum lies in both the visible and ultraviolet region. To confirm the feasibility of the above theoretical results, experiments have been also done. X-ray diffraction (XRD) pattern show consistent result in the lattice parameter with a calculated value of 5.841 Å. Photoluminescence (PL) has been performed to understand the optical behaviour of our prepared QDs. A high luminescence peak around 2.28 eV can be seen in the PL plot which confirms the emission of Green light from CsPbBr₃ QDs. These experimental results are in a good agreement with the DFT calculated results.

Eu-doped GaN is a promising material with a wide array of potential applications in optoelectronics, optogenetics, micro displays and quantum computing. While this system has been the subject of intense investigation for the last two decades, several questions still remain about certain aspects of its optical properties, such as the polarization dependence of the optical transitions, and the coupling between the 4f-electron configuration and bulk phonons, as well and the appearance of local phonon modes. Moreover, the origin of certain emission peaks remains under debate in the literature. In this proceeding, the results of a systematic series of “site-selective” photoluminescence measurements are presented, where the properties of pulsed and continuous-wave laser excitation, such as polarization and intensity, were controlled.

In this paper, we present a structural and optical study on micro-pixelated InGaP/AlGaInP quantum well structures with different pixel sizes down to 6 μm and a red emission at 636 nm. Temperature-dependent photoluminescence and cathodoluminescence cartographies were coupled to observe the emission homogeneity at the pixel scale and to study the impact of non-radiative recombination from sidewall defects. We deduced that micro-LEDs are impacted by surface recombination and we estimated the thermal quenching of photoluminescence related to defects. At low temperatures, a stronger luminescence was also observed from the pixel edges due to the diffusion coefficient or a geometric effect. Finally, the study was completed by a TOF-SIMS analysis in order to provide information about material composition homogeneity.

The refractive index fields of several media, say, transparent solids, transparent liquids, and air are often crucial when using these media as optical elements, probing optical properties, and deriving other physical information related to the refractive index. For example, in an optical system using a high-power laser, the thermal lens effect of optical elements is one of great issues because the laser focal position will be changed by the effect [1]. Thus, a method to measure a gradient-index of the optical element should be useful to take a step to reduce the thermal lens effect.

Basically, light rays passing through a medium with an inhomogeneous refractive index field will be deflected toward the area of higher refractive index. This way, it should be possible to obtain information about the refractive index field from measurement of such deflection. Consequently, there have been several methods for visualizing gradient-index fields, including schlieren photography [2] and shadowgraphy [3]. Although these visualization methods are powerful tools for qualitatively analyzing the gradient-index field, quantitative analysis requires additional methods. Rainbow schlieren photography is one candidate. In this method, the strength of deflection of light rays is obtained by using a rainbow aperture [4-6]. The quantitative phase-contrast imaging has also developed with incoherent-light [7, 8], coherent-light [9, 10], and asymmetric illumination [11]. There is also background-oriented schlieren (BOS) technique for measuring gradient-index field quantitatively [12-20]. In the BOS technique, displacements of dot-patterns that are printed on a background are caused by deflection of light rays passing through a gradient-index field. The optical flow algorithm can be used to obtain the displacements of the background-dot-patterns. However, obtaining depth information of the gradient-index field with the BOS technique is often difficult because the dot-pattern displacement results from the light ray deflected all along its path, requiring integration over the path. To address this, a method of reconstructing the depth information of a gradient-index field is proposed here on the basis of the Lagrangian optics with the BOS technique.