Harnessing Nanophotonics for High Brightness White Micro-LEDs

The invention of the blue light-emitting diodes (LEDs) enabled the solid-state lighting (SSL) revolution. SSL typically achieves white light via phosphor-based down-conversion, where blue or UV LEDs excite phosphors that re-emit at longer wavelengths. In remote phosphor systems, nanophotonic metasurfaces leverage waveguide local density of optical states (LDOS) to induce spatial coherence and direct spontaneous emission into specific modes, which are then outcoupled via engineered periodic arrays. However, translating these strategies to compact phosphor-converted (pc) micro-LEDs is challenging due to strict pixel-scale constraints. This thesis develops nanophotonic approaches for efficient, directional light extraction in pc micro-LEDs, targeting high-resolution displays in AR glasses and smartwatches.

We introduce a Python-based solver, PyRAMIDS, to model dipole emission in multilayer structures with multiple dipole scattering, enabling efficient design of nanophotonic devices. To address the “extraction challenge” in micro-LED — 90% of converted red emission is directed back into the high-index blue LED (GaN) die — we propose a counterintuitive strategy of introducing a spacer geometry that restores waveguiding in the phosphor layer. This design, validated experimentally, enables metasurface-enhanced directional outcoupling with a 3x increase in brightness. Fourier-plane emission maps also reveal unexpected dark straight lines interlaced with bright parabolic waveguide bands, arising from TE/TM interference and exhibiting polarization singularities, which we confirm via k-space Stokes polarimetry and reciprocity-based T-matrix modeling.

To meet stringent pixel-size constraints, we implement an evolutionary optimization algorithm to design sub-3 microns metasurfaces compatible with 104 pixel-per-inch (PPI) displays, achieving a 45% improvement in forward emission over truncated periodic designs. Finally, we investigate angularly resolved chiral emission from helicene-doped polymers, revealing strong angular dependence and demonstrating a striking reversal of intrinsic handedness due to metasurface-driven spin–orbit interactions and waveguide reabsorption effects.