White light emitting devices were fabricated using blue emitting organic light emitting diodes (OLEDs) and down-conversion phosphor mixtures. Three different thicknesses of yellow and mixtures of yellow and red luminescent phosphor films were prepared on separate glass slides using a silicone matrix. The down-conversion films were optimized by varying the thickness and phosphor to silicone weight ratio. The phosphor films with different thickness were coupled to an optimized blue emitting OLED with a refractive index matching gel. Optimized down-conversion phosphor layers integrated with blue OLEDs exhibited 2× enhancement of efficiency (lumens per electrical Watt) for white to that of the blue OLED. The International Commission of Illumination color coordinates and average color rendering index for this device were (0.43, 0.46) and >80, respectively.
High-quality downconverted white light is important for many applications ,including general illumination. Downconversion of blue light from inorganic InGaN-based light emitting diodes to produce white light is demonstrated using red- and green-emitting phosphors. After characterization, films of the phosphors are prepared by mixing the powder into a polymethyl methacrylate host. The quality of light is improved and optimized by varying the weight ratio of green to red phosphors and the thickness of the phosphor layer.
Photon sieves (PSs) are diffractive lenses with an array of pinholes that are capable of focusing light with either short, visible, or long wavelengths. PSs were fabricated using electron-beam lithography to pattern opaque silver films on a glass substrate (3 mm diam, 51.7 mm focal length). The size and spacing of pinholes was varied to create single and dual-wavelength PSs to focus 500 and/or 600 nm light, and the pinholes for the dual wavelength design were arranged in sectors, concentric with one another, or randomly. Single wavelength PSs produced focused images of a 100 µm source aperture with full width at half maximum (FWHM) of ~100 µm and high relative intensities at the design wavelength. Chromatic aberration resulted in no focused image and very low intensities when monochromatic light was 100 nm from the designed wavelength. Dual wavelength PSs produced focused images of the 100 µm source aperture with FWHMs of ~100 µm but lower relative intensities at both design wavelengths of 500 and 600 nm. The background “secondary maxima” were higher for dual wavelength designs, especially when the PS was illuminated by white light. The FWHM was smaller and the intensities higher for the random or concentric dual wavelength designs as compared to the sector design.
It is shown that 2, 4, 6-Trinitrotoluene (TNT) displays strong and distinct structures in differential reflectograms, near
420 nm and 250 nm. These characteristic peaks are not observed from approximately two dozen organic and inorganic
substances which we tested and which may be in or on a suitcase. This exclusivity infers an ideal technique for
explosives detection in mass transit and similar locations. The described technique for detection of explosives is fast,
inexpensive, reliable, portable, and is applicable from some distance, that is, it does not require contact with the
surveyed substance. Moreover, we have developed a curve discrimination program for field applications of the
technique. Other explosives such as 1, 3, 5-trinitro-1, 3, 5 triazacyclohexane (RDX), 1, 3, 5, 7-Tetranitro-1, 3, 5, 7-
tetraazacyclooctane (HMX), 2, 4, 6, N-Tetranitro-N-methylaniline (Tetryl), Pentaerythritol tetranitrate (PETN), and
nitroglycerin have also been investigated and demonstrate similar, but unique, characteristic spectra. The technique
utilizes near-ultraviolet to visible light reflected from two spots on the same sample surface yielding a differential
reflectogram corresponding to the absorption of the sample. The origin of the spectra is attributed to the highest
occupied molecular orbital to lowest unoccupied molecular orbital (HOMO-LUMO) transitions of the respective
explosive molecule. Experiments using transmission spectrophotometry have also been performed to compliment and
confirm the specific transitions. The results are supported by computer modeling of the molecular orbitals that yield
UV and visible transitions.
The performance of semiconductor radiation detectors is a function of electronic properties which are in turn related to crystallographic quality. In this paper we used devices from <100> and <110> growth regions of several different HgI2 crystals grown by the PVD method. We measured I/V characteristics of HgI2 devices over the range of +/-1000V. Voltages were ramped at different rates and at a range of temperatures (-70oC to +20oC) and the dark current decreased with temperature. Several devices exhibited negative differential resistance indicating field enhanced trapping and/or the formation of high-field domains. These devices exhibited NDR at both positive and negative voltages and it was observed that the current peak reduced with repeated cycling of positive bias indicating the reduction of carriers with time. After applying a negative bias, the current peak on the positive bias increased dramatically indicating that the traps were repopulated. These experimental results were modeled with several analytical expressions of conduction processes, considering both semiconductor and insulator models, e.g., Frenkel-Poole, Schottky, and space-charge-limited emission, toward lending insight to mechanisms resulting in HgI2 detector conditioning.
With the increasing use of night vision goggles and night missions, new methods to display information in the infrared region is of interest. We have developed both inorganic and organic electroluminescent thin films which emit at wavelengths between 700 nm and 1.8 μm. These thin films have been incorporated into simple devices and the feasibility of a NIR flat panel display has been demonstrated. Both inorganic zinc sulfide and organic polymers doped with rare earth lanthanide ions have been demonstrated. The wavelength of emission can be varied by choosing the appropriate lanthanide ion, such as dysprosium, erbium, thulium or neodymium. Power densities of ~30 μW/cm2 have been achieved with these devices.
The fabrication, testing and performance of a new device for the protection of optical sensors will be described. The device consists of a transparent substrate, a transparent conducting electrode, insulating polymers, and a reflective top electrode layer. Using standard fabrication techniques, arrays of apertures can be created with sizes ranging from micrometers to millimeters. A stress gradient resulting from different coefficients of thermal expansion between the top polymer layer and the reflective metal electrode, rolls back the composite thin film structure from the aperture area following the chemical removal of a release layer, thus forming the open condition. The application of a voltage between the transparent conducting and reflective metal electrodes creates an electrostatic force that unrolls the curled film, closing the artificial eyelid. Fabricated devices have been completed on glass substrates with indium tin oxide electrodes. The curled films have diameters of less than 100micrometers with the arrays having fill factor transparencies of over 70%. Greater transparencies are possible with optimized designs. The electrical and optical results from the testing of the artificial eyelid will be discussed.
The fabrication, testing and performance of a new device for the protection of optical sensors will be described. The device consists of a transparent substrate, a transparent conducting electrode, insulating polymers, and a reflective top electrode layer. Using standard integrated circuit fabrication techniques, arrays of apertures can be created with sizes ranging from micrometers to millimeters. A stress gradient resulting from different thermal coefficients of expansion between the top polymer layer and the reflective metal electrode, rolls back the composite thin film structure from the aperture area once a release layer is chemically etched away, forming a tightly curled film at one side of the aperture - the open condition. The application of a voltage between the transparent conducting and reflective metal electrodes creates an electrostatic force which unrolls the curled film, closing the artificial eyelid. Fabricated devices have been completed on glass substrates with indium tin oxide electrodes. The curled films have diameters of less than 100micrometers with the arrays having mechanical transparencies of over 80%. Greater transparencies are possible with optimized designs. The electrical and optical results from the testing of the artificial eyelid will be discussed including the optimization of the design and fabrication for applications in systems that extend into the IR spectrum. A primary area of investigation is the choice of the transparent conducting electrode.
A novel concept for protection of optical sensor will be described. The device consist of a transparent substrate, a transparent conducting electrode, insulating polymers, and a reflective top electrode layer. Using thin film deposition and photolithographic fabrication techniques commonly available for manufacture of integrated circuits, plus spin coating as commonly used for polymers, the layers can be placed on the substrate and arrays of apertures created with sizes ranging from micrometers to millimeters. Due to the stress gradient between the polymer dielectric and the reflective metal electrodes, the composite thin film structure will open over the aperture area once a 'release layer' is removed by chemical treatment. This is the 'open' condition for the 'eyelid'. By applying a voltage between the transparent conducting the metal electrodes, an electrostatic force is created which closes the 'eyelid'. Upon elimination of the voltage, the stress gradient opens the 'eyelid' again. Preliminary devices have been fabricated and operated up to a frequency of 4kHz and at lifetimes of over 1010 cycles. The power consumption is extremely low. The potential of this technology for a variety of applications will be discussed.
Auger electron spectroscopy is one of the most commonly used techniques for surface analysis. It has evolved from its first common use for solid surfaces in the late 1960s to become a tool for routine analysis of materials. This article provides a concise review of the use of Auger electron spectroscopy, with emphasis on its use for optical materials. The fundamentals of the technique are reviewed, along with a discussion of data analysis and the equipment commonly used to perform the technique. Several examples of the use of AES for optical materials are discussed.
Current research on optoeletronics is reviewed with particular attention given to thin film devices from II-VI and III-V compound semiconductors. Major achievements made since 1988 include development of world-class growth facilities for ZnSe and III-V compound semiconductor epitaxial layers and processing of these materials into devices with world-record performance characteristics.
Thin films of Y-Ba-Cu-O have been deposited on barrier layers of SrTiO3 and ZrO2 by laser ablation or RF planar magnetron sputter deposition. The barrier layers are required to prevent interactions of the deposited superconducator with the underlying semiconductor and at the same time enhance the texture in the superconductor to increase the critical current density. The ability of various processing steps to enhance these properties and simultaneously allow scale-up to coat larger areas successfully are discussed.