Alan Piquette

An Investigation of Self-Absorption and Corresponding Spectral Shift in Phosphors

Many light emitting diode (LED) phosphors have overlapping emission and absorption bands, such as Ce 3+ -doped garnets and Eu 2+ -doped silicates, nitrides, and oxynitrides. These overlapping bands inevitably lead to self-absorption effects through either nonradiative energy transfer mediated via electron-electron correlation process or actual emission and radiative energy transfer which involves emission and absorption of a photon due to spectral overlap. The first process has been often discussed in the literature. The second process has been investigated in this work through a semi-quantitative formalism. Spectral shift, changes in emission peak shape, photoluminescent conversion efficiency, and phosphor characterization are discussed in light of self-absorption. The simulated results are compared to experimental data and shown to correlate well with observed data.

Light extraction from luminescent light sources and application to monolithic ceramic phosphors.

An extension of a theorem for light extraction [Adv. Opt. Technol.2, 291 (2013)] from a higher index luminescent body (LED or phosphor) through an extracting surface into a lower index output medium is derived. The result is valid for both geometric and diffractive surface structures. Using this bound and radiation transport calculations, we show that extraction from LEDs or phosphors requires a combination of cavity effects to enhance radiance behind the extracting surface and scattering or diffraction to couple trapped total-internal-reflection modes to propagating modes. The treatment applies to macroscopic luminescent sources whose thickness exceeds the longitudinal coherence length of the luminescent radiation.

Enhancement of Yellow Light Extraction Efficiency of Y 3 Al 5 O 12 :Ce 3+ Ceramic Converters Using a 2-D TiO 2 Hexagonal-Lattice Nanocylinder Photonic Crystal Layer

The effects of 2-D hexagonal-lattice TiO₂ photonic crystal layers on the directional light extraction efficiency of low-scattering (Y_1-xCe_x)₃Al₅O₁₂ (YAG:Ce ³⁺) ceramic converters have been investigated. The hexagonal-lattice TiO₂ patterns were fabricated by an electron beam lithography process. The optimized yellow directional light extraction enhancement by a factor of 4.4 is achieved with a 2-D photonic crystal layer having a diameter of 430 nm, a lattice constant of 590 nm, and a height of 350 nm. A 3-D finite-difference time-domain model has also been developed to study the geometric parameters of the photonic crystal layer, yielding results that are consistent with our measured data.

EPD of Phosphors for Display and Solid State Lighting Technologies

Electrophoretic deposition (EPD) has been used for phosphor screening for a variety of emissive information displays and more recently, for solid state lighting. EPD is well suited to deposit the fine (nanometer to micrometer diameter) phosphor particles needed for high resolution displays. The fundamentals of the EPD process in an isopropanol (IPA) bath have been characterized by the dissociation behavior of nitrate salts in IPA, measurement of the effects of pH and nitrate salt concentration on the zeta potential of the particles, studying of the processing conditions and modeling of the deposition rates. The electrochemical precipitation reactions form an adhesive agent for the particles and the adhesion strength can be enhanced by various methods to meet the requirements of these technologies.

Enhanced forward emission of YAG:Ce 3+ phosphor with polystyrene nanosphere coating

A common configuration for solid-state lighting is a blue LED combined with a phosphor platelet that emits in the yellow and green. Inherently, the light from the phosphor is emitted isotropically, but in most applications it is desirable to tailor the angular emission so that more light is emitted forward. In this work we demonstrate an effective forward emission enhancement for a YAG:Ce(3+) phosphor platelet by coating the surface with several layers of 600 nm diameter polystyrene nanospheres. After the application of the optimal concentration of three layers of nanospheres, the normal direction emission of YAG:Ce(3+) increased about 27% by focusing the emission from high angles into the forward direction through the mechanism of Mie scattering. This angular emission tailoring technique did not reduce the total emitted power. This is a convenient way to tailor the emission angles for applications like projector display and automotive forward lighting.