Directional light emission enhancement from LED-phosphor converters using dielectric Vogel spiral arrays

Abstract

Increasing light extraction efficiency and narrowing the angular spread of light emitted from optically thick light emitting diodes (LEDs) are desirable but difficult goals to achieve. In this paper, we design, optimize, and fabricate Vogel spiral arrays of dielectric nanostructures for optical directional extraction enhancement of incoherent emission from optically thick phosphor-converted LEDs. In order to design and optimize large-scale structures, we perform a systematic numerical investigation based on full-vector three-dimensional finite difference time domain simulations using a cloud of randomly positioned and randomly phased dipoles that approximate an incoherent Lambertian source. An analytical model based on kinematic scattering is also developed and used to parametrically study changes in the emission profile as the spiral geometry is tuned. The optimal Vogel spiral arrays are fabricated as TiO2 nanodisks atop YAG:Ce3+ blue-to-white converter layers using electron-beam lithography and reactive ion etching. Photoluminescence spectroscopy is used to experimentally measure extraction enhancement up to 2× compared to a flat reference. Finally, accurate Fourier-space (k-space) fluorescence spectroscopy is used to measure the emission intensity profile up to 54° in a single snapshot image, and we find up to a 35% enhancement in power-normalized forward emission compared to a flat reference, under remote laser excitation. The integration of optimized Vogel spiral arrays of dielectric nanoparticles with phosphor-converted LEDs will increase efficiency and narrow the directional spread of light. These effects are beneficial to a variety of active device applications, including imaging, lighting, and projecting devices that require enhanced extraction efficiency combined with directional emission.