Directional Emission of Fluorescent Dye-Doped Dielectric Nanogratings for Lighting Applications.

By structuring a luminescent dielectric interface as a relief diffraction grating with nanoscale features, it is possible to control the intensity and direction of the emitted light. The composite structure of the grating is based on a fluorescent dye (Lumogen F RED 305) dispersed in a polymeric matrix (poly(methyl methacrylate)). Measurements demonstrate a significant enhancement of the emitted light for specific directions and wavelengths when the grating interface is compared to nonstructured thin films made of the same material. In particular, the maximum enhancement of photoluminescence for a given pump wavelength is obtained at an angle of incidence that is close to the Rayleigh anomaly condition for the first-order diffracted waves. In this condition, the maximum extinction of incident light is observed. Upon excitation with coherent and monochromatic sources, photoluminescence plots show that the Rayleigh anomalies confine the angular interval of the emitted light. Being the anomalies  directly related to the pitch of the diffraction grating, the system can be thus implemented as an optical device whose directional emission can be designed for specific applications. The exploitation of nanoimprinting techniques for the fabrication of the luminescent grating enables production of the device on large areas, paving the way for low-cost lighting and solar applications.

High-brightness source based on luminescent concentration.

The concept of a high-luminance light source based on luminescent conversion of LED light and optical concentration in a transparent phosphor is explained. Experiments on a realized light source show that a luminous flux of 8500 lm and a luminance of 500 cd/mm2 can be attained using 56 pump LEDs at 330 W electrical input power. The measurement results are compared to optical simulations, showing that the experimental optical efficiency is slightly lower than expected. The present status enables applications like mid-segment digital projection using LED technology, whereas the concept is scalable to higher fluxes.

Diffractive flat panel solar concentrators of a novel design.

A novel design for a flat panel solar concentrator is presented which is based on a light guide with a grating applied on top that diffracts light into total internal reflection. By combining geometrical and diffractive optics the geometrical concentration ratio is optimized according to the principles of nonimaging optics, while the thickness of the device is minimized due to the use of total internal reflection.

Directional sideward emission from luminescent plasmonic nanostructures.

Periodic arrays of metallic nanoparticles can be used to enhance the emission of light in certain directions. We fabricated hexagonal arrays of aluminium nanoparticles combined with thin layers of luminescent material and optimized period (275 nm) and thickness (1500 nm) to obtain sideward directional emission into glass for a wavelength band around 620 nm. The key physics is that the luminescent layer acts as a waveguide, from which light is emitted at preferential angles using diffractive effects. This phenomenon has applications in the field of solid-state lighting, where there is a desire for small, bright and directional sources.

Increased efficiency of luminescent solar concentrators after application of organic wavelength selective mirrors.

Organic wavelength-selective mirrors are used to reduce the loss of emitted photons through the surface of a luminescent solar concentrator (LSC). A theoretical calculation suggests that application of a 400 nm broad reflector on top of an LSC containing BASF Lumogen Red 305 as a luminophore can reflect 91% of all surface emitted photons back into the device. Used in this way, such broad reflectors could increase the edge-emission efficiency of the LSC by up to 66%. Similarly, 175 nm broad reflectors could increase efficiency up to 45%. Measurements demonstrate more limited effectiveness and dependency on the peak absorbance of the LSC. At higher absorbance, the increased number of internal re-absorption events reduces the effectiveness of the reflectors, leading to a maximum increase in LSC efficiency of ~5% for an LSC with a peak absorbance of 1. Reducing re-absorption by reducing dye concentration or the coverage of the luminophore coating results in an increase in LSC efficiency of up to 30% and 27%, respectively.

Progress in phosphors and filters for luminescent solar concentrators.

Luminescent solar concentrators would allow for high concentration if losses by reabsorption and escape could be minimized. We introduce a phosphor with close-to-optimal luminescent properties and hardly any reabsorption. A problem for use in a luminescent concentrator is the large scattering of this material; we discuss possible solutions for this. Furthermore, the use of broad-band cholesteric filters to prevent escape of luminescent radiation from this phosphor is investigated both experimentally and using simulations. Simulations are also used to predict the ultimate performance of luminescent concentrators.

Surface-relief and polarization gratings for solar concentrators.

Transmission gratings that combine a large diffraction angle with a high diffraction efficiency and a low angular and wavelength dispersion could be used to collect sunlight in a light guide. In this paper we compare the diffractive properties of polarization gratings and classical surface-relief gratings and explore their possible use in solar concentrators. It is found that polarization gratings and surface-relief gratings have qualitatively comparable diffraction characteristics when their thickness parameters are within the same regime. Relatively large grating periods result in high diffraction efficiencies over a wide range of incident angles. For small grating periods the efficiency and the angular acceptance are decreased. Surface-relief gratings are preferred over polarization gratings as in-couplers for solar concentrators.

Photoanisotropic polarization gratings beyond the small recording angle regime.

Polarization gratings can be realized by polarization holographic recording in photoanisotropic materials. In this paper, we study two types of polarization gratings. One is recorded with two orthogonally circularly (OC) polarized beams and the other one with two orthogonally linearly (OL) polarized beams. The interference of both cases is explored beyond the small recording angle regime. A novel method is proposed to represent the polarization states of the modulation. The diffraction by polarization gratings is studied with rigorous diffraction theory. Simulations based on the Finite Element Method are performed for both OC and OL polarization gratings at small and large recording angles.

Characterization of the diffraction efficiency of polymer-liquid-crystal-polymer-slices gratings at all incidence angles.

Recently, a novel holographic diffraction grating made of polymer slices alternated to homogeneous films of nematic liquid crystal (POLICRYPS) was realized. We study the optical performance of the POLICRYPS gratings by both numerical simulations and experiments. Characterization of the grating at normal and conical reading mount are performed. The diffraction efficiency depends strongly on the angles of incidence. Besides, the characterization of the diffraction efficiency at Bragg angle incidence is studied. A uniform high diffraction efficiency is achieved when the incident wave satisfies the Bragg condition.

General polarized ray-tracing method for inhomogeneous uniaxially anisotropic media.

Uniaxial optical anisotropy in the geometrical-optics approach is a classical problem, and most of the theory has been known for at least fifty years. Although the subject appears frequently in the literature, wave propagation through inhomogeneous anisotropic media is rarely addressed. The rapid advances in liquid-crystal lenses call for a good overview of the theory on wave propagation via anisotropic media. Therefore, we present a novel polarized ray-tracing method, which can be applied to anisotropic optical systems that contain inhomogeneous liquid crystals. We describe the propagation of rays in the bulk material of inhomogeneous anisotropic media in three dimensions. In addition, we discuss ray refraction, ray reflection, and energy transfer at, in general, curved anisotropic interfaces with arbitrary orientation and/or arbitrary anisotropic properties. The method presented is a clear outline of how to assess the optical properties of uniaxially anisotropic media.

Optimization of light extraction from OLEDs.

In recent years, many methods were developed to improve the efficiency of an OLED. In this paper, we investigate the effects of some main factors contributing to the extraction efficiency. These factors include the polarization of the source, the distance between the source and the metal, and the thickness of the layers. We will also discuss the effect of periodic patterns such as a grating structure.

Simulations of birefringent gratings as polarizing color separator in backlight for flat-panel displays.

A color and polarization separating backlight can be obtained by using a surface-relief grating made of birefringent material as an out-coupling structure on top of the lightguide. A rigorous finite element diffraction model was applied to study the polarization effect of such a grating. The diffraction of plane waves by the anisotropic grating was studied for general conical incidence.

Short period holographic structures for backlight display applications.

The use of holographic structures is promising for the realization of efficient systems in backlight applications for displays. By applying surface relief gratings on top of a side-lit lightguide we realize a backlight that avoids the use of color filters. The grating is used as a light outcoupling and color-separating element. The demands for this grating are stringent and calculations have been performed to meet them. A prototype backlight, including the grating structure, has been assembled and characterized. Results of experiments are discussed.

Reduced-order model technique for the analysis of anisotropic inhomogeneous media: application to liquid-crystal displays.

Electromagnetic wave propagation in anisotropic inhomogeneous media is computed by a novel reduced-order model technique, which is based on the restriction of the Marcuvitz-Schwinger equations on Krylov subspaces and on the application of the singular-value decomposition. The model is derived from the standard coupled-wave method and includes both wide-angle diffraction and light scattering at dielectric interfaces. The method, currently implemented for two-dimensional problems, was applied to the analysis of different liquid-crystal test cells. Numerical results are compared with those obtained through the application of the coupled-wave method and the Jones method and with experimental microscopic measurements.