Thermodynamics of Light Extraction from Luminescent Materials

Abstract

Light extraction from luminescent materials, where luminescence generated in a high refractive index medium must be coupled to a lower index medium, is a complex problem with significant ramifications for efficient LED and phosphor converted LED lighting. We derive thermodynamic arguments which show that light transmission for incident Lambertian light through arbitrary structured or non-structured surfaces is always limited by the ratio of incident to output étendues. Numerical simulations of various strongly scattering surfaces in the wave-optic regime are made to confirm the surface extraction results. The results are also extended to arbitrary angular radiance distributions of incident light as in Lenef, et al. [Opt. Lett., 39, 3058 (2014)], but also generalized to cases of restricted angular output emission. Furthermore, we show how cavity effects and scattering are both needed to overcome the thermodynamic surface extraction limits. Examples of both surface scattering and a converted LED with volume scattering are given. Finally, a derivation of the Lambertian light transmission theorem is developed from scalar wave-optics to highlight the physics of the thermodynamic result and to provide limits of applicability of the surface extraction limit.

ZnO nanowires/YAG:Ce functional heterostructure coatings with tunable optical properties

“…Even if WLEDs-based display and lighting devices exhibit good photometric parameters, their performances in terms of external quantum efficiency and service life can be further impro ved [2][3][4]. This latter could be lengthened thanks to a better thermal dissipation whereas the former relies on both Internal Quantum Efficiency (IQE) and Light-Extraction Efficiency (LEE), which represents the ratio between the number of photons extracted to the air and the number of photons generated in the active layer [2,4,5]. Actually, the emitted light is hard to extract into the air since there is a gap between the high refractive index of the phosphor (n1.8 for the most common used Y3Al5O12:Ce 3+ ̶ YAG:Ce phosphor) and the air (n1).…”

“…Actually, the emitted light is hard to extract into the air since there is a gap between the high refractive index of the phosphor (n1.8 for the most common used Y3Al5O12:Ce 3+ ̶ YAG:Ce phosphor) and the air (n1). Then, the device suffers from a loss of light because a large fraction of the produced photons are backscattered to the chip or trapped in dielectric structures inherent to the substrate (luminescent coatings) due to Total I nternal Reflection (TIR) and the presence of a waveguide mode [5][6][7]. Several approaches have been reported to boost LEE value.…”

Unique Design Strategy for Laser‐Driven Color Converters Enabling Superhigh‐Luminance and High‐Directionality White Light

“…It was observed that: i) strong backscattering occurs in HSYAGs samples, and the degree of backscattering increases with the porosity; ii) the side emission ratio remarkably decreases with increasing porosity; and iii) the top emission ratio could reach >95% in all HSYAGs samples. So, the high light scattering contributes to the strong backscattering, which is mainly ascribed to the small scattering length . This means the light emission is somehow engineered to be directional, namely, the backward direction, which is very favored in the reflection mode.…”

“…So, the high light scattering contributes to the strong backscattering, which is mainly ascribed to the small scattering length. [54][55][56] This means the light emission is somehow engineered to be directional, namely, the backward direction, which is very favored in the reflection mode. Since strong backscattering also hinders the light propagation in the lateral direction and confines the light emission close to the surface, the side emission ratio due to the TIR is greatly decreased.…”

Large and Versatile Plasmonic Enhancement of Photoluminescence Using Colloidal Metallic Nanocubes

“…In the case of solid-state lighting, the emitted light is hard to extract into air because there is a gap between the high refractive index of the phosphor (around 1.8 for the most common used Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) phosphor) and the air ( n = 1). This kind of device usually suffers from a loss of light because a large fraction (typically at least 80% , ) of the produced photons are backscattered to the chip or trapped in dielectric structures due to total internal reflection (TIR), under the form of guided modes. ,, Light, in that case, can eventually escape through the edges. Allowing these guided modes to be extracted can thus significantly enhance the LEE and ultimately the EQE.…”