We report on the design, fabrication, and characterization of the first photonic crystal (PhC)-based red multiple-quantum-well (MQW) color converters fully optimized for augmented reality (AR) microdisplays through a quasi-3D light harnessing principle. This principle leverages an aluminum (Al) bottom reflector and a silicon dioxide (SiO2) gap to harness the bottom-emitted light, along with copper (Cu) lateral mirrors and a silicon nitride (SiN) phase-matcher for Bloch-mode replication. These structures were designed using 3D-FDTD simulations. As a proof-of-principle, we fabricated corresponding devices that exhibit promising characteristics, including record light extraction efficiencies over 40% for 4µm pixels and emission patterns with high directionality. Time-resolved photoluminescence (TRPL) analyses, along with a four-wave intensity model developed in this work, indicate that there is still room for improvement. We believe that the guidelines established in this study could pave the way for the use of MQW color converters in the next generation of very bright, high-resolution RGB microdisplays for AR glasses and beyond.
While state-of-the-art RGB microdisplays rely on quantum-dot color-converters (CC), the next generation of very bright and high-resolution AR microdisplays could be based on MQW CC due to their better photostability and higher blue-light absorption. However, their practical implementation is still hindered by their low light extraction efficiency (LEE), their Lambertian-like emission patterns, as well as the challenges related to their integration on on-Silicon blue µLED arrays (transfer and pixelization). In this work, we use photonic-crystals (PhCs) as light extractors and investigate all the challenges related to the design, integration & fabrication as well as testing of novel MQW based CC.
State-of-the-art RGB microdisplays use quantum-dot color converters (CC), which suffer from photostability issues and low blue-light absorption. Inorganic MQW based CC offer higher blue-light absorption and better photostability. However, their practical use for microdisplays has not been implemented yet because of their low light extraction efficiency (LEE) inherent to their high refractive indices. In this paper, we investigate the use of photonic crystals (PhC) with different optogeometrical parameters to fully optimize AlGaInP/InGaP MQW CC for blue-to-red and green-to-red color conversions in microdisplays. A 600nm-thick CC was successfully bonded on a transparent substrate using oxide-mediated molecular bonding. By using optimized photonic-crystal designs, we obtained a large LEE enhancement (x 9) within ultra-short extraction lengths (~2μm), which shows quasi-perfect light outcoupling and compatibility with pixel lateral sizes under 5μm. Experimental results are in agreement with 3D-FDTD simulations, showing that those unique characteristics are paired with highly directional emission. A phenomenological model derived from the standard coupled-mode theory has been proposed and used to determine the mean coupling strengths between the guided Bloch modes and radiated modes. We believe that the design guidelines set in this work could pave the way for the use of inorganic MQW CC to achieve monolithic integration for full-color microdisplay applications.
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