Light-emitting Surface Research Articles

Nonwetting and optical properties of BN nanosheet films

This study begins with a brief discussion on the high-temperature chemical vapor deposition synthesis of transparent boron nitride nanosheet films on silicon/silicon dioxide substrates. The compact nanosheets grew perpendicular to the substrate surface, and the majority of them had thicknesses of less than 5 nm. Ultraviolet-visible spectroscopy measurements demonstrated a wide optical band gap of ∼5·6 eV of nanosheets, and cathodoluminescence spectroscopy showed their strong luminescence emission in the ultraviolet region. The nanorough surface morphology of the films induced nonwetting and self-cleaning features with water-contact angles reaching ∼153°. Such transparent superhydrophobic films can be utilized for the preparation of nonwetting ultraviolet light-emitting surfaces for optoelectronics applications, antifouling surfaces on marine vessels or oil–water separation equipments.

Large-Area (over 50 cm × 50 cm) Freestanding Films of Colloidal InP/ZnS Quantum Dots

We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm × 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand-alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines.

Enhancing Light Extraction in Top‐Emitting Organic Light‐Emitting Devices Using Molded Transparent Polymer Microlens Arrays

The light extraction efficiency in organic light-emitting devices (OLEDs) is enhanced by up to 2.6 times when a close-packed, hemispherical transparent polymer microlens array (MLA) is molded on the light-emitting surface of a top-emitting device. The microlens array helps to extract the waveguided optical emission in the organic layers and the transparent top electrode, and can be manufactured in large area with low cost.

Using carbon-containing field-emission structures in cathodoluminescent light sources

Results of experimental and theoretical investigation of vacuum cathodoluminescent (VCL) light sources with field-electron emission from carbon-containing structures are presented. Comparative analysis of the distribution of current on the light-emitting anode surface in VCL sources of various designs has been performed. The influence of pulsed regime parameters on the VCL source brightness and efficiency has been evaluated. Promising directions in the development of VCL light sources based on planar-edge type field-emission structures are discussed.

Luminescent Surface Quaternized Carbon Dots

Thermal oxidation of a salt precursor made from the acid−base combination of tris(hydroxymethyl)aminomethane and betaine hydrochloride results in light-emitting surface quaternized carbon dots that are water-dispersible, display anion exchange properties, and exhibit uniform size/surface charge.

Lateral cavity photonic crystal surface-emitting laser with ultralow threshold

A lateral cavity photonic crystal (PhC) surface-emitting laser is realized on a commercial epitaxial waveguide wafer without a distributed Bragg reflector first. Energy is coupled from the lateral resonance to surface-emitting light through the Γ band edge of the PhC with a square lattice. Electrically driven 1553.8 nm surface-emitting lasing action is observed at room temperature. The threshold current density of 667 A/cm2 is ultralow for the surface-emitting laser.

Enhancing light extraction in organic light-emitting devices via hemispherical microlens arrays fabricated by soft lithography

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.

Correction of self-screening effect in integrating sphere-based measurement of total luminous flux of large-area surface-emitting light sources

We present a correction method of a systematic error that arises when total luminous flux of a large-area surface-emitting light source (SLS) is measured in an integrating sphere by substitution with a reference lamp. Putting a large-area SLS into an integrating sphere is equivalent to adding a low-reflective baffle to screen the spatial distribution of radiation inside the sphere, which severely changes the sphere responsivity. To compensate this self-screening effect, we propose to use a specially designed auxiliary lamp whose illuminating area is spatially matched to that of the SLS under test. The validity of the proposed correction method is tested by numerical simulations based on the radiative transfer equation.

LEDs (λmax = 3.6 μm) with cone-shaped light-emitting surfaces

A series of light-emitting diodes (LEDs) operating at λmax = 3.6 μm are created using cone-shaped mesas with heights of 10–130 μm and concave side surfaces. The dependence of the efficiency of room-temperature (T = 298 K) emission on the mesa height at various injection currents has been studied. The character of the observed dependence agrees with the results of theoretical calculations. The output radiation power of LEDs with the maximum mesa height (130 μm) at a pumping current of 220 mA amounts to 53 μW, which is 1.5 times higher than the power of LEDs with a mesa height of 10 μm.

Cavity Effect in Nanocrystalline Porous Silicon Ballistic Lighting Device

A planar light-emitting device was developed using a nanocrystalline porous Si layer as an energetic hot electron generator and a ZnS:Mn film as a luminescent film. Uniform orange light emission was observed at relatively low bias voltages. Furthermore, a cavity built using two Ag mirrors on both sides of the ZnS:Mn film operated well and effectively narrowed both the photoluminescence and electroluminescence (EL) spectra. The emission characteristics under the device operation in a three-electrode configuration clarified the EL mechanism based on the ballistic electron generation in the nanocrystalline Si layer followed by the direct excitation of the fluorescent film. The device, which provides a ballistic lighting concept for inorganic EL with a low driving voltage, is a very attractive all-thin-film surface-emitting light source for various applications.

Fundaments in Luminance and Retroreflectivity Measurements of Vertical Traffic Signs Using a Color Digital Camera

This paper is a study of the influences of the different parameters which affect the photometric evaluation of light-emitting surfaces (due to reflection or self-emission) when a conventional color digital camera is used. The overall purpose of this paper is to evaluate the luminance and the reflectivity of the vertical traffic sign with the camera in order to provide an automatic recognition of deteriorated reflective sheeting material of which the traffic signs were made. This paper describes how the A/D converter output signal given by a pixel of the digital camera can be related to the luminance and the reflectivity of the corresponding surface element whose image is formed on a pixel. Thus, each surface element of the traffic sign's surface can be separately evaluated. By photometrically calibrating the camera, we have been able to prove this relationship in our experiments.

Viewing Zones in PLS Based Multiview 3-D Imaging Systems

In 3D imaging systems, with a point light source (PLC) array as a backlight illumination and viewing zone forming, the finite size of the light-emitting surface of each PLC in the array and mismatch between the array period and pixel cell pitch limits the region of individual view image to a small portion of the viewing zone cross section (VZCS). The portion becomes smaller as the size approaches that of a pixel for an individual view image in the pixel cell, and/or the fractional size of the pixel becomes smaller. As a consequence, the VZCS becomes filled with viewing regions for mixed images of two adjacent view images. Hence, the effective number of different images which can be viewed at VZCS could be increased around two times that of the total different view images displayed on the display panel.

The Freeform Reflector for Uniform Rectangular Illumination

Energy from the source was rearranged through reflection by a freeform reflector, in order to get uniform rectangular illumination. The numerical results of partial differential equation sets were investigated to obtain the freeform reflector and these equations were obtained upon the determination of the characters of source and the desired illumination. As an example, a light emitting diode (LED) with a Lambertian light-emitting surface of 1 ×1 mm2 and a viewing angle of 120° was applied as the source, and the target plane was a 4:3 rectangle with uniform illumination. The projective length of the reflector on x-axis is about 23 mm, and on y-axis is about 21 mm. Thus the illumination system is very compact.

Three-dimensional reconstruction of in vivo bioluminescent sources based on multispectral imaging

A new method is described for obtaining a 3-D reconstruction of a bioluminescent light source distribution inside a living animal subject, from multispectral images of the surface light emission acquired on charge-coupled device (CCD) camera. The method uses the 3-D surface topography of the animal, which is obtained from a structured light illumination technique. The forward model of photon transport is based on the diffusion approximation in homogeneous tissue with a local planar boundary approximation for each mesh element, allowing rapid calculation of the forward Green's function kernel. Absorption and scattering properties of tissue are measured a priori as input to the algorithm. By using multispectral images, 3-D reconstructions of luminescent sources can be derived from images acquired from only a single view. As a demonstration, the reconstruction technique is applied to determine the location and brightness of a source embedded in a homogeneous phantom subject in the shape of a mouse. The technique is then evaluated with real mouse models in which calibrated sources are implanted at known locations within living tissue. Finally, reconstructions are demonstrated in a PC3M-luc (prostate tumor line) metastatic tumor model in nude mice.

Monte-Carlo based simulation of surface light emission profiles from AlGaInP light emitting diodes

Light emission profiles from AlGaInP LEDs are compared with those obtained via Monte-Carlo simulation and a lossy transmission line model. Results show good agreement for devices with thin current spreading layers. However, for thicker layers, geometric effects dominate the output profile, resulting in discrepancies between experimental data and the transmission line model. The simulation of emission profiles confirms this discrepancy, caused by light escaping from below the p-contact.

Improved characteristics of organic light-emitting devicesby surface modification of nickel-doped indium tin oxide anode

This letter presents the optoelectrical performance of an organic light-emitting diode (OLED) through the elevation of indium tin oxide (ITO) anode work function by Ni co-sputter surface doping and additional O2 plasma treatment. The turn-on voltage of OLED devices can be reduced by 2.3V for Ni atomic concentration greater than 1.8% and by 2.7V for the additional O2 plasma treatment. Devices with Ni(2.6%)-doped and O2 plasma treated ITO anodes perform the highest luminance efficiency (0.91lm∕W), three times larger than undoped ITO (0.31lm∕W) at 250cd∕m2.

38.4: Invited Paper: Backlight Unit With Double Surface Light Emission Using A Single Micro‐Structured Light‐Guide Plate

AbstractFor simultaneous illumination of front and rear color LCDs of a cellular phone a double surface light emission backlight using single micro‐structured light‐guide has been developed. The light guide employed two prism sheets each for front and rear to direct the controlled forth and back emergent light toward the LCDs. Only three optical components, that is, one light guide and two prism sheets, and four LEDs were used in the new structure compared with ten components and six LEDs of the current type.

Method to evaluate the enhancement of luminance efficiency in planar OLED light emitting devices for microlens array

A method based on the variation of the area ratio of the base surface of microlenses to the light-emitting surface of the planar OLED device has been demonstrated. This method can evaluate the performance of microlens arrays with a desired geometrical shape on the improvement of the luminance efficiency for the planar light emitting devices. The maximum enhancement of the luminance efficiency of the devices in the studied microlens arrays is 56%. It is also found that the improvement of the luminance efficiency of the devices increases linearly with decreasing base length of the microlens array.

Low resistance high reflectance contacts to p-GaN using oxidized Ni/Au and Al or Ag

We report on a high reflectance low resistance multilayer contact to p-GaN composed of a thin oxidized Ni/Au bilayer overcoated with a thick Al or Ag layer and then capped by Ni/Au. Measurements on 500 μm diameter light emitting diode-like test structures show an operating voltage below 4 V at 20 mA that is comparable to identical devices fabricated with a conventional Ni/Au contact. Back surface light emission is about 70% greater than that from devices with the Ni/Au contact due to lower light absorption by the Al or Ag. Performance for both Al and Ag based contacts is stable at temperatures of up to around 100 °C.

A solid-state light-emitting device based on ballistic electron excitation using an inorganic material as a fluorescent film

It has been demonstrated that a solid-state light-emitting device based on ballistic electron excitation generated in nanocrystalline porous silicon (nc-PS) layer can be fabricated using an inorganic material as a fluorescent film. This device is composed of a semitransparent thin Au film, an inorganic fluorescent film (in this case, ZnS: Mn), a nc-PS layer, n-type Si substrate and an ohmic back contact. When a positive bias voltage of higher than 20 V is applied to the Au contact, electrons injected into the nc-PS layer are accelerated towards the outer surface, and then hit a fluorescent film as quasi-ballistic or ballistic electrons. The spectra of emitted light correspond to the original emission band of the deposited fluorescent material. These results show that it is possible to use a variety of materials (such as organic materials and inorganic ones) as fluorescent films for the novel solid-state luminescent devices. This is applicable to multicolor surface-emitting light source and flat panel display.