Debapriya Pal, Toni López, A. Femius Koenderink December 23, 2024 ACS Nano 2024 DOI: 10.1021/acsnano.4c13472
Micro-LEDs are advanced display technologies for smart devices, such as AR-VR displays. Phosphor-converted micro-LEDs face challenges with poor light extraction. In micro-LED arrays, the phosphors must be placed directly on top of the blue-emitting GaN to prevent pixel cross-talk. However, the high refractive index of GaN causes it to absorb over 90% of emissions. We have both theoretically and experimentally demonstrated a strategy to mitigate this issue, enhancing light extraction up to four-fold. Our approach involves engineering the local density of optical states (LDOS) to create quasi-guided phosphor modes by judiciously inserting a thin low-index spacer combined with a metasurface to extract these modes. We analyze the trade-offs between blue light pumping, quasi-guided mode LDOS enhancement, and radiation pattern control using a stratified system solver for dipole emission and LDOS. Using an angle-resolved Fourier microscopy experiment, we used a plasmonic metasurface combined with a silica spacer, resulting in a 2.5-fold extraction enhancement overall. Furthermore, due to directivity enhancement, there is a four-fold increase in brightness in the forward direction.
Phosphor-converted micro-light emitting diodes (micro-LEDs) are a crucial technology for display applications but face significant challenges in light extraction because of the high refractive index of the blue pump die chip. In this study, we design and experimentally demonstrate a nanophotonic approach that overcomes this issue, achieving up to a 3-fold increase in light extraction efficiency. Our approach involves engineering the local density of optical states (LDOS) to generate quasi-guided modes within the phosphor layer by strategically inserting a thin low-index spacer in combination with a metasurface for mode extraction. We demonstrate the trade-offs between blue light pumping, LDOS enhancement at the converted emission wavelength, and radiation pattern control using a stratified system solver for dipole emission. Experimentally, the integration of plasmonic antennas and a silica spacer resulted in a 3-fold overall brightness enhancement, with nearly a 4-fold increase in forward emission. This nanophotonic metasurface waveguide design is a critical advancement for producing bright, directional micro-LEDs, particularly in augmented/virtual reality (AR/VR) devices and smartwatch displays, without the need for bulky secondary optics or reflectors.
Figure 1. Concept sketch of the current state of the art and our proposal. (a) Schematic of a proposed phosphor-converted microLED display array using InGaN/GaN, featuring a submicron-thick phosphor layer on a thin-film blue LED chip. Such architectures typically include a 6–10 μm GaN layer, InGaN multiple quantum wells, an approximately 100 nm layer of p-GaN, a conductive ITO layer, and a backside metallic mirror. (b) Close-up showing that most phosphor emission will be directed into the blue LED, with only 10% emitted into air in a Lambertian profile. (c) Inserting a spacer between the high index GaN and phosphor enhances emission into quasi-guided LDOS within the phosphor, while corrugation facilitates directional extraction.Figure 2. Calculated local density of optical states (LDOS) in stratified systems: (a) phosphor on glass, (b) phosphor on GaN, and (c) phosphor separated from GaN by a glass spacer, all with an air superstrate. Total LDOS (green dash-dotted curves) is separated into radiated (red) and (quasi-)guided (black solid curves) contributions, plotted as a function of emitter position (z, perpendicular to interfaces) for an emission wavelength of 0.60 μm, averaged over all dipole orientations. Gray, pink, blue, and white shading indicates glass, phosphor, GaN and air.Figure 3. Theoretical analysis of trade-offs in LDOS enhancement, radiation pattern, and pump efficiency for phosphor on GaN when using a low-index silica spacer to protect quasi-guided modes. (a) Sketch of the system, comprising GaN (substrate, n = 2.4), a silica spacer (dspacer thick, n = 1.46), phosphor (0.4 μm thick, n = 1.75), and air (n = 1) as superstrate. (b) Radiation pattern (dipole orientation averaged) with no spacer (green dashed curve) and 1.5 μm spacer (red solid curve). Upper hemisphere (toward air) is magnified 30× for clarity. Gray solid and dash-dotted lines denote the air–GaN and phosphor–GaN light lines, respectively. (c) Emitted power (dipole orientation averaged) for dipole at phosphor midheight plotted as a function of the in-plane wave vector and spacer thickness. (d) Crosscut of (c) at spacer thickness dspacer = 1.5 μm. As a function of the spacer thickness, plots of (e) normalized emission LDOS contribution of the phosphor layer into various channels, (f) absorption of blue photons in the phosphor layer, (g) efficiency of blue-to-red conversion into different channels, (h) figure of merit (FOM). In panels (d) and (f), red dash-dotted, green dashed, and blue solid curves represent the radiation to the GaN side, airside, and quasi-guided into the phosphor layer, respectively.Figure 4. System performance for diverse phosphor choices and spacer configurations. (a) Fraction of emission coupled into guided LDOS within the phosphor layer as a function of emitter position (z) and phosphor thickness (dPhosphor) for an air–phosphor–glass system. For the remaining panels, the layer configuration is identical to that shown in Figure 3, where the phosphor is separated from the GaN by a silica spacer. (b) Figure of merit (FOM) as a function of phosphor and silica spacer thickness. For varying phosphor thickness (dPhosphor) and refractive index (nPhosphor), assuming 80% of guided LDOS is extracted (c) minimum spacer thickness needed to achieve this, and (d) corresponding potential figure of merit.Figure 5. Theoretical analysis of a Bragg stack spacer design between GaN and phosphor. (a) Sketch of the system with N repeating alternating layers of silica (0.118 μm thick, n = 1.46), and titania (n = 2.45), forming a periodic structure of 0.189 μm period, terminated with silica between GaN and phosphor (0.4 μm thick, n = 1.75), capped with air. (b) Wavevector resolved reflectivity for N = 8 unit cells for light impinging from the GaN side. Multilayer structure is optimized for almost full reflection around the emission wavelength at high angles (blue dotted box). (c) Radiation pattern as a function of polar angle for randomly oriented dipoles, comparing no spacer (green dashed curve) to N = 8 unit cells (red solid curve). Upper hemisphere is magnified 20× for clarity. (d) Figure of merit (FOM) as a function of the number of unit cells (N), with N = 0 indicating no spacer.Figure 6. Experimental results for directional photoluminescence extraction from phosphors on GaN with varying heights of a low-index silica spacer. (a) White light image of an example staircase evaporated on silicon, showing the height differences through colors (etaloning). (b) Schematic: stepped silica spacer atop 5 μm thick GaN on sapphire, allowing multiple height measurements from a single sample. Each step includes a flat section and a corrugated section with a fabricated plasmonic lattice and is topped with a ∼400 nm thick spin-coated phosphor layer. (c) White light image of one of the samples used for measurements, showing a pink tinge from the phosphor dye. For the flat configuration, panels (c) and (d) present the back focal plane (Fourier) photoluminescence enhancement and normalized photoluminescence dispersion plots as the spacer thickness increases. Panels (e) and (f): corrugated configuration. For the 805 nm spacer thickness, the theoretical isofrequency projection of repeating circles is overlaid as dotted white lines on the dispersion relation.Figure 7. Performance comparison as a function of spacer height. (a) Total photoluminescence counts plotted against spacer thickness for flat (green) and corrugated (red) cases. The right vertical axis indicates the performance boost, with the black dashed line at unity representing the reference case (no spacer, no corrugation), i.e., phosphor directly on top of blue GaN die. (b) Azimuthally averaged directional photoluminescence enhancement plotted against the polar angle for various configurations. For the flat configuration, green dashed and solid curves denote cases with no spacer and a 1.55 μm spacer, respectively. Red dashed and solid curves represent the same spacer conditions for the corrugated configuration.
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