Metasurface Based Phosphor-Converted Micro-LED Architecture—Creating Guided Modes for Enhanced Directionality

Phosphor-converted LEDs produce white light using a blue 'pump' LED to illuminate a phosphor blend that emits longer wavelengths. This study focuses on the complex scenario of micro-LEDs, typically envisioned as high-resolution display pixels, which constrains the LED's lateral size and requires placement of the phosphor directly on the blue die (GaN), which is highly disadvantageous for light extraction. We have experimentally established that driving phosphors more efficiently in a micro-LED architecture benefits from promoting light emission into guided modes within the phosphor layer. Furthermore, using a plasmonic metasurface to outcouple the light to the far field leads to a threefold increase in brightness.<br/><br/>Micro-LEDs are the advanced display technologies for smart devices such as wearables and AR-VR displays. Blue InGaN/GaN multi-quantum well LEDs are power efficient and used for phosphor-converted micro-LEDs to produce the desired spectrum. High-resolution displays demand compactly packed pixels in chips with high pixel density. Micro-LEDs are individually addressable self-emissive pixels with lateral dimensions below approximately 50 microns. It's crucial to prevent any optical crosstalk between adjacent pixels in an array, limiting the lateral size and distance between the phosphor layer and the GaN die due to pump light spreading and scattering effects. As a result, most of the phosphor emission goes into the GaN layer, posing a significant challenge for light extraction. Common nanophotonic strategies, such as diffractive metasurfaces, enhance pump absorption and emission directivity since the phosphor is far from the blue-emitting GaN, avoiding phosphor emission disappearing into the LED. This work focuses on the more complex scenario of micro-LEDs for display pixels. In this context, the efficiency of phosphor light emission can be enhanced by channeling it into guided modes. A metasurface can achieve a threefold brightness increase by outcoupling the guided mode emission in the phosphor layer into the far field. While promoting emission into guided mode existence in the phosphor layer may diminish the coupling of blue pump photons from the GaN into the phosphor, the overall benefit lies in the directed emission enhancement.<br/><br/>We propose geometries in which the phosphor is concentrated in submicron-sized layers and separated from the blue die chip by either (a) a micron-sized dielectric spacer with a refractive index lower than that of the phosphor (low index normal dielectric spacer) or (b) by a 1D dielectric multilayer stack of materials of alternating refractive index (Bragg stack dielectric spacer). The proposed geometrical design aims to restore the waveguide mode characteristics of the phosphor layer, prevent undesired emission into the GaN die, and accelerate it into the guided modes of the phosphor through the Purcell effect. This geometry can be combined with plasmonic and dielectric metasurface to increase pump light absorption and extract the guided light into specific directions in a microLED architecture.<br/><br/>We present rigorous theoretical calculations based on the local density of optical states, LDOS, to design spacers that enhance emission into quasi-guided modes in phosphor. We perform experiments demonstrating the benefits of inserting a thin micron-sized low-index silica spacer combined with periodic metasurface to facilitate the outcoupling of guided mode emissions towards the far field using angle-resolved fluorescence (Fourier) microscopy. Notably, the emission is highly directional towards the air, resulting in a four-fold enhancement in the forward direction by using a micron-thick silica spacer in conjunction with a plasmonic particle array, as compared to a planar layer of the dye-doped polymer layer of similar thickness directly on top of GaN (blue die). These results provide concrete evidence of the effectiveness of our proposed spacer strategies in enhancing light emission in micro-LEDs.