Maxim Tchoul

Laser-activated remote phosphor conversion with ceramic phosphors

Direct laser activation of a remote phosphor, or LARP, is a highly effective approach for producing very high luminance solid-state light sources. Such sources have much smaller étendue than LEDs of similar power, thereby greatly increasing system luminous fluxes in projection and display applications. While several commercial products now employ LARP technology, most current configurations employ phosphor powders in a silicone matrix deposited on rotating wheels. These provide a low excitation duty cycle that helps limit quenching and thermal overload. These systems already operate close to maximum achievable pump powers and intensities. To further increase power scaling and eliminate mechanical parts to achieve smaller footprints, OSRAM has been developing static LARP systems based on high-thermal conductivity monolithic ceramic phosphors. OSRAM has recently introduced a static LARP product using ceramic phosphor for endoscopy and also demonstrated a LARP concept for automotive forward lighting1. We first discuss the basic LARP concept with ceramic phosphors, showing how their improved thermal conductivity can achieve both high luminous fluxes and luminance in a static configuration. Secondly, we show the importance of scattering and low optical losses to achieving high overall efficiency and light extraction. This is shown through experimental results and radiation transport calculations. Finally, we discuss some of the fundamental factors which limit the ultimate luminance achievable with ceramic converted LARP, including optical pumping effects and thermal quenching.

Radiance limits of ceramic phosphors under high excitation fluxes

Ceramic phosphors, excited by high radiance pump sources, offer considerable potential for high radiance conversion. Interestingly, thermodynamic arguments suggest that the radiance of the luminescent spot can even exceed that of the incoming light source. In practice, however, thermal quenching and (non-thermal) optical saturation limit the maximum attainable radiance of the luminescent source. We present experimental data for Ce:YAG and Ce:GdYAG ceramics in which these limits have been investigated. High excitation fluxes are achieved using laser pumping. Optical pumping intensities exceeding 100W/mm2 have been shown to produce only modest efficiency depreciation at low overall pump powers because of the short Ce3+ lifetime, although additional limitations exist. When pump powers are higher, heat-transfer bottlenecks within the ceramic and heat-sink interfaces limit maximum pump intensities. We find that surface temperatures of these laser-pumped ceramics can reach well over 150°C, causing thermal-quenching losses. We also find that in some cases, the loss of quantum efficiency with increasing temperature can cause a thermal run-away effect, resulting in a rapid loss in converted light, possibly over-heating the sample or surrounding structures. While one can still obtain radiances on the order of many W/mm2/sr, temperature quenching effects ultimately limit converted light radiance. Finally, we use the diffusion-approximation radiation transport models and rate equation models to simulate some of these nonlinear optical pumping and heating effects in high-scattering ceramics.

Chemical wave characterization of self-oscillating gelatin and polyacrylamide gels

Self-oscillating hydrogels driven by the Belousov-Zhabotinsky (BZ) reaction provide a unique foundation for the mimicry of autonomic biological systems. One of the key challenges for assessing practical performance limits of these materials is detailed knowledge of the chemical and mechanical characteristics of the BZ gels at various states of autonomic behavior. Recently we developed two BZ gel systems based on gelatin and polyacrylamide. The desired chemical response for effective swelling-deswelling oscillation and mechanical force production involves a delicate balance of chemical wave period, amplitude, and gel swelling properties. The chemical performance of gelatin and polyacrylamide BZ gels according to this criteria is discussed.