Essentially loss-less all-dielectric micro-fabricated optics can be tailored to completely eliminate the shadowing losses metallization grids create on the surface of concentrator solar cells. The nonimaging micro-concentrator exploits total internal reflection to redistribute the elevated flux from available macro-concentrators, rather than increasing overall concentration. The optical designs permit widening the metal fingers toward lessening series resistance losses, which can also finesse the need for the intricate metallization patterns of some high-flux cells. Realistic net efficiency gains of ~15% (relative) are achievable in a wide variety of concentrator cells.
Different methods of fabrication of micro-optical devices for the Infra-Red (I.R.) such as micro-lens, micro-prism, micro-mirror arrays and Fresnel lenses based on the use of chalcogenide photoresists are described. In chalcogenide photoresists, two photoinduced phenomena are observed: photoinduced structural transformations and photoinduced diffusion of some metals, primarily, silver, and both phenomena enabled the development of new types of I.R micro-optical devices. The use of a new three-component As-S-Se photoresist and a new efficient amine-based selective developer allows for the realization of soft contrast characteristics of the photolithographic process with a Xe-source of light. Recent progress in the development of devices using photostructural transformations based on these two innovations will be described. Devices using the photoinduced silver diffusion are based on the different dissolution rate (in selective etchants) of the non-doped and silver-doped chalcogenide films. Parameters and characteristics of several micro-optical devices made using this effect are compared and discussed.
A new method for solving the dispersion problem of Diffractive Optical Elements (DOEs) is suggested. By aligning two DOEs made of different dispersive materials, the optical path differences as a function of the wavelength can be controlled. Applying this concept, a combined achromatic DOE can be designed for applications that require wavefront control using different wavelengths or wideband light sources. Numerical and simulation results are presented.
A simple microlens array fabrication process based on chalcogenide glass As-Se and As-S photoresists is described. Specific properties of chalcogenide photoresists important for microlenses preparation and the parameters of fabricated spherical and cylindrical microlens arrays are measured and discussed.
A method for calculating in real time, the transmission function of a computer generated hologram (CGH) for beam shaping, is described. This method is based on the well known phase contrast method (PCM) in which phase distributions of an object are transformed to intensity distributions. In the phase contrast-like system a desired intensity distribution is obtained by using a special phase mask.
The optical quality of films and bulk elements obtained by the fast sol-gel method and the ease of preparation make this method technologically and economically attractive for micro- optical elements and arrays. Multiple replication of high-cost metal templates necessitates a two step process: initial replication onto disposable plastic, to produce negative- templates, and casting of the glass elements onto these plastic templates. Following introduction of the fast sol-gel micro-replication processes, the fabrication of refractive and diffractive micro-optical arrays is described in detail. The study focuses of the accuracy achievable by the replication of sharp curvatures by a viscous sol-gel resins. A further question addressed is how to obtain a thick crack-free self- supporting micro-optical array by such a process. Examples of replication results of Fresnel lenses and various types of lenslet arrays by this method are shown, and the various aspects of replication accuracy are discussed. The characterization of the thus produced micro-optical arrays is described, relating to the influence of mold and resin parameters on residual stresses and thence, the surface and bulk properties of the replicas. Various approaches to stress- minimization that can facilitate accurate replication in sol- gel derived matrices are discussed. Further, the preparative approach to highly patterned crack-free self-supporting thick elements is displayed, and the wide scope of new applications stemming from such elements is discussed. Finally, the new chemical approach to crack-free stress-free silica glasses by the sol-gel method is displayed.
Sol-gel processes of metal alkoxides involve hydrolysis of the alkoxy groups and condensation to a 3-D oxide glass network. Volume reduction of the drying gel typically results in cracking, unless sufficient relaxation is allowed to take place. Further, the common shrinkage by a factor of 2.5 and higher imposes great difficulty to obtain dimensional accuracy in thus prepared micro-optical elements. The new fast sol-gel method enables facile preparation of siloxane-based glassy materials in which polymerization is completed within minutes and curing within a few hours. The optical quality of thin films obtained by the fast sol-gel method and the ease of preparation makes this method technologically and economically attractive for micro-lenses and micro-optical arrays by replication. Micro-optical arrays are highly patterned, including sharp curvatures of small radii. This necessitates to study primarily two aspects of the sol-gel replication process: (1) the chemical constitution of the sol-gel and the reaction pathway that ensures prompt adhesion to the template during the process. (2) the surface chemical adaptation of the template that allows timing of adhesion and release of the produced elements. The adaptation of this process to the desired replication is described. Thence, the results of preliminary fabrication of micro-optical elements and arrays by this method are shown and their features discussed.
An important factor in the success of electronic technology in the last few decades is the continuous drive for miniaturization: computing power that used to require a hall full of equipment fits nowadays comfortably in a notebook sized machine. The size of basic optical instruments, however, remained almost unchanged. Today, it is the size of optical devices and subsystems that limits further miniaturization of opto-electronic systems. Here, we describe and demonstrate optical configurations that enable us to design and fabricate very compact optical systems, based on the use of multiple micro-lens arrays. One such system is the multiple lenslet array imager (MLAI) that allows the distance between an object and its full size optical image to be as small as few centimeters, regardless of the size of the object. Another system is the lenslet array holographic correlator/convolver (LAHC), where we combine multiple lenslet arrays and a holographic filter to obtain a very compact optical correlator. Issues in micro-lens arrays fabrication and selected applications are discussed, and some laboratory demonstrations are presented.
Filters used in communication systems in the atmosphere are usually of interference-filter type or of step-filter type. The major disadvantage of the interference filter is the limitation on the maximum incidence angle. In the step filter, the incidence angle is not limited. However, it has a wide spectral bandwidth. Therefore more background radiation enters the detector and as a result more noise is generated. The aim of this project is to design and fabricate an interference filter with a wide field of view. The filter is fabricated by evaporating thin films of suitable materials onto a transparent hollow hemisphere. The center of the detector will coincide with the center of the hemisphere. In this structure, the incident radiation always hits the filter at small angles independent of the transmitter position, so that the signal will always reach the detector.
Micro-optical elements, particularly microlenses, are finding growing application in different fields of modern optoelectronics. One of the most promising methods of microlens fabrication is based on photolithographic processes. Organic photoresists were used in the earlier development of microlens arrays. A new technique of microlens fabrication using inorganic chalcogenide photoresists is presented. Such photoresists have many advantages, such as very high resolution, photosensitivity in wide spectral range, high values of refractive index, transparency in the IR range, and the ability to be used as positive or negative resists depending on the developer used. These unique properties create new possibilities for the development of microlens arrays in the IR.
The use of a microlens on top of a photovoltaic solar cell working at high light concentration and the need for a very low absorption photoresist in the whole solar spectrum are explained. A process has been developed in a negative photoresist with adequate properties. The optical properties of the fabricated microlenses are described.
A novel method for controlling the chromatic aberration of kinoforms is described. The method is based on the superposition of a few kinoforms, each of which is designed for one specific wavelength. This approach can be useful for applications where elimination of chromatic aberration is necessary (for example CO2 and HeNe lasers in surgery), as well as for applications which are based on color separation (e.g. multicolor or thermal detection).
Utilization of 8 - 10 micrometers spectral band symmetrical, and 3 - 5 micrometers band asymmetrical GaAs/GaAlAs multiple quantum well (MQW) structure for a multiple color detector is discussed. A theoretical comparison between the predicted performance of a GaAs-MQW based focal plane array (FPA) operating in the 8 - 10 micrometers band, and state of the art narrow band gap (MCT, InSb) operating in the 3.2 - 4.2 micrometers band is given. We show that because operating in the 8 - 10 micrometers band even with 0.2 quantum efficiency the noise equivalent delta temperature is better than the theoretical limit at 3.2 - 4.2 micrometers IR FPA.
Microlenses can be made by refractive and diffractive optics. A tailored distribution of light creates, in exposed photoresist, a three dimensional pattern, which is then reproduced by ion beam etching in the substrate, resulting in the desired microlens. The first technique uses a Fraunhofer diffraction pattern of a hole in the focal plane of a lens. The second technique uses diffractive optics, i.e. a Dammann grating, to generate a uniform array of equally spaced beams. The theoretical background of a Dammann grating and the experimental results are presented.
The need for microlenses with a wide-range of focal lengths from 1O to 100mm and with a diameter
varying from 1Oi to 1mm lead to the development of various techniques which are able to generate these
lenses in a photoresist substrate or in special glasses . The existing techniques are reviewed and a new one
proposed. In this technique a positive or negative photoresist layer is exposed to a tailored light intensity
distribution. After development of the photoresist, its surface is identical to the spatial intensity light
distribution. Photoresist with an index of refraction of n=1.6 in the visible spectrum, can be used as a
lens. Furthermore this surface can be transferred to substrate like glass, silicon germanium etc., by etching