Christo G. Stojanoff

Determination of the mean refractive index and thickness of dichromated gelatin holographic films using the thin film resonance method

The precise knowledge of the mean refractive index and the thickness of holographic films is important in applications such as polarization-sensitive holographic optical elements or substrate-mode holography. We present results of the measurements of the mean refractive index and the thickness of dichromated gelatin films before exposure, uniformly exposed films, and holograms. The measurements are based on the thin film resonance method. The interference between the two waves reflected at the air-film surface and the film-substrate interface modulates the reflectivity of the holographic film as a function of the angle of incidence. The frequency and the amplitude of this modulation are analyzed in order to determine the optical parameters. With the measured mean refractive index and the thickness as input we simulated the angular response of volume gratings and compared the results of the modelling with experimental data. The excellent agreement between the simulated and measured diffraction efficiencies confirm the applicability of the method to holographic films.

Bandwidth and angle selective holographic films for solar energy applications

The objective of this research program is the development of the technology for the industrial fabrication of large format holographic optical elements (HOEs) with predetermined spectral characteristics and angular selectivity. HOEs of this type are used in a variety of technical applications, such as: holographic concentrators for photo-voltaic energy conversion and solar photo- chemistry or as integrated holographic stacks compromising several holograms operating in different ranges of the solar spectrum for daylighting, glazing and shading in buildings. The latter are required for the effective control of the transmission of solar radiation through the windows or the glass curtain wall envelopes of buildings. The HOEs (reflective or transmissive) are recorded in dichromated gelatin layers deposited on glass or plastic substrates. This material and the corresponding thermochemical development process facilitate the achievement of bandwidths, spectral ranges and angular selectivity that match accurately the design spectral and geometrical properties of a particular application.

Conceptual design and practical implementation of dichromated gelatin films as an optimal holographic recording material for large-format holograms

The commercial manufacturing of large format holographic optical elements (HOE) -- these are used in the fabrication of holographic solar concentrators or for daylighting applications in buildings -- requires inexpensive materials exhibiting high diffraction efficiency, bandwidth and controlled shift of the operating wavelength. Hence, the ideal recording material must possess adequate spectral sensitivity at the wavelengths of present day high power lasers and permit the desired shift of the operating wavelength by means of process control. The material should manifest a predictable diffraction efficiency as a function of the layer fabrication technique, of the exposure, and of the development process and display high spatial resolution and low noise. The properties of dichromated gelatin (DCG) as a recording material for volume holograms are close to ideal. It provides a large refractive index modulation, high resolution, negligible absorption, and low scattering. The holographic film is prepared in the laboratory and extensively tested. The processing of the film after exposure is a sequence of chemical reactions and physical treatments. We report in this paper our experience with large format DCG films on glass substrates and present the dependence of the holographic properties upon the layer preparation procedures and upon the exposure energy. The results for the film development and after-treatment are presented in a forthcoming paper.

Design and optimization of a holographic concentrator for two-color PV operation

The subject matter of this research project is to develop, manufacture and field test a spectrally dispersing solar collector system using a holographic solar concentrator in conjunction with spectrally matched advanced solar cells for photovoltaic power generation. The advantage of a holographic solar concentrator as compared to a conventional one is seen in the overall reduction of investment cost and in the possibility to generate inexpensive solar electric power. In this paper we present the techniques specifically developed for the design and manufacturing of efficient holographic optical elements and holographic lens stacks that are used in the fabrication of bandwidth matched solar concentrators for VIS and NIR photovoltaic operation. The lens stack separates the white light radiation into several spectral ranges that are focussed onto photocells possessing corresponding spectral characteristics. Contrary to previously published arrangements, we present here the concept and the design characteristics of a holographic concentrator that allows positioning of the cell in a plane parallel to the lens aperture. The initial idea of using two lenses recorded in the same aperture or same holographic layer focussing onto two off-axis foci proved to be of limited value due to the off-axis focussing that introduces strong reflection and aberration. Here we present a new concept in which the two lenses are shifted in the plane of the aperture so that each lens-cell configuration exhibits axial geometry. Both lenses are designed as axially corrected holographic stacks that include a lens and a correction grating. The design minimizes the cross coupling between the two holographic systems. Stack layouts for AlGaAs/GaAs and GaAs/Si combinations are discussed. Cross-coupling effects and aberrations involving the IR lens are minimized. Experimental diffraction efficiencies are fitted with non-cosinusoidal refractive index modulation showing best performance for 100 by 100 mm2 aperture. The theoretical predictions are compared with the first experimental results.

Fabrication and test of a holographic concentrator for two-color PV operation

The dispersion of the solar radiation into different spectral bands which are focused onto spectrally matched solar cells improves the electric efficiency of photovoltaic collectors in comparison with conventional systems. Holographic lenses are able to disperse and to focus solar radiation at the same time. They may be reproduced from a master and are suitable for cheap mass production. The paper presents the fabrication and test of a 50 X 50 cm2 PV-concentrator composed of a stack of two holographic lens arrays. Each lens array consists of 49 single lenses of dimension 7 X 7 cm2. One array operates in the long wave spectral band, the other in the short wave range. The solar radiation is focused on spectrally matched solar cells of size 1 X 1 cm2.

Engineering applications of HOEs manufactured with enhanced performance DCG films.

The objective of this research program was the development of the technology for the industrial manufacturing of high efficiency holographic optical elements (HOEs) with predetermined spectral characteristics ranging in format from few square millimeters to square meters. The desired optical properties of holographic materials for specific engineering applications are determined during the making of the film and are modified during the exposure, the development and the post-treatment of the HOE. This technology includes the machine fabrication of precision holographic films with 1 to 50 micron thickness on glass or plastic substrata, the use of filler material to modify the spectral characteristics of HOEs, multiple exposure techniques, contact-copying procedures and chemically and thermally adapted hologram development and post-treatment processes. The technology extends the use of dichromated gelatin (DCG) into the blue and infrared spectral domains and is a viable tool in the control of the holographic properties of the manufactured HOEs. The usefulness of the technology is illustrated with results obtained from existing HOE installations. Design and performance information is presented for manufactured reflection and transmission HOEs that are used in a variety of technical applications, such as: holographic concentrators for photo-voltaic and thermal energy conversion, special collectors for solar photo-chemistry, holographic stacks for day-lighting, glazing and shading in buildings, optical interconnects in multi-chip modules, robotic sensors and holographic beam forming optics for LED applications. Multiple exposures technique is used to record up to four holograms in the same DCG film that are used to generate simultaneously several monochrome or RGB beams.

Development and fabrication of a hybrid holographic solar concentrator for concurrent generation of electricity and thermal utilization

The efficiency of photovoltaic generators that are based on different semiconductor materials with optimized band-gaps can achieve considerably higher values than those obtained from single junction devices, e.g. Si-based solar cells. Hence, the splitting of the solar spectrum for use with the different band-gap cells is a desired characteristic of the solar collector. An enhanced efficiency is realized with the concentration of the incident solar radiation onto the corresponding solar cell. The optical characteristics of the holographic solar concentrator satisfy these requirements. The undiffracted solar radiation should be collected by an absorber that also cools the solar cells. This is the concept of the hybrid collector for electricity and thermal utilization that is presented in this paper. It can achieve an electrical efficiency above 22% and a thermal efficiency of 35% with a temperature of 100 degree(s)C.

Plasmon spectroscopy for high-resolution angular measurements

A plasmon surface wave is excited in a thin metallic layer on a dielectric, if the layer is illuminated at an incident angle larger than the critical angle of total reflection. At certain resonance angles, the major part of the incident energy is coupled into the surface wave and the total reflection is attenuated. We probed the resonance peak with a spectrum of incident waves and detected the reflected energy at two incident angles near the resonance peak simultaneously. The ratio of he detected energies is a function of the incident angle, thus facilitating angular measurements. First experimental results indicate a resolution better than 0.001 degrees in a measuring range of 1 degree.

Design optimization and manufacturing of holographic windows for daylighting applications in buildings

The function of holographic optical elements in daylighting applications is to redirect sunlight from the immediate window area into the rear of a room in order to illuminate the darker regions and to reduce glare. A prerequisite for the successful application of holograms in daylighting systems is the solution of the problems of white light diffraction and of uniform holographic properties across a large aperture. This paper presents theoretical and experimental investigations of these two problems. It will be shown that white light diffraction is possible and that uniform diffraction efficiencies over large apertures are attainable.

Design, fabrication, and integration of holographic dispersive solar concentrator for terrestrial applications

Dichromated gelatin layers facilitate the design and fabrication of large format (1 m2) holographic optical elements (HOE) that exhibit high optical quality and diffraction efficiency. In this paper we present the results achieved in the development and fabrication of such layers and elucidate upon their applicability as holographic solar concentrators. The objective of this report is the presentation of the experience gained in the design and manufacturing of large format spectrally selective solar concentrators. The holographic lens diffracts the white sunlight into various spectral ranges outfitted with solar cells that have appropriately selected band gaps. The purpose of the holographic concentrator is the spectral and spatial separation of the incident solar radiation in order to achieve an improved overall conversion efficiency. A number of manufacturing techniques were especially developed for the design, optimization and fabrication of the specialized holographic concentrators. The HOEs needed for the construction of the integrated collector optics are: lenses and/or lens arrays and phase-correction plates. The HOES are designed by means of computer programs that facilitate the optimization of the recording geometry and provide information for the correction procedures needed for optimized performance. The HOEs are recorded in dichromated gelatin films (DCG) and are subsequently subjected to chemical and thermal treatment processes in order to promote the desired characteristics and suppress the undesired properties. These procedures guarantee the realization of HOEs with high diffraction efficiencies and low scattering losses. The optimized holographic process: exposure, development and after-treatment, was described explicitly in previous publications. The emphasis is placed on the development of a novel copying technique for the batch reproduction and manufacturing of large format holographic lenses for solar concentrators.