Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 58-59). Also available via the Internet.

Non-imaging Fresnel lenses have been playing an important role in improving the efficiency of the solar energy systems. Many researchers and scientists have devoted their research to optimize the design of the Fresnel lenses. Before it can contribute to energy efficiency increase, a Fresnel lens with optimized design will first need to be fabricated with the most cost-effective method as well as the best quality fabrication as possible. If targeted in a commercial market, feasibility of mass production with a minimum fabrication time would also be a consideration. To bring the design optimization of a Fresnel lens from a conceptual theory to a real-life increase in energy efficiency, the lens needs to be fabricated, tested, compared, and analyzed. This research thesis is intended to explore the performance of the lenses with optimized design through experimental investigations. The design optimization was achieved by a previous PhD student at UNT. A total of six lenses fabricated with four different methods along with two purchased lenses were tested with two different approaches. Multiple testing routes were conducted within a 10-month period to observe the effects of material decomposition and degradation on the lens performance. The resulting experimental data has provided a solid base for analyzing the performance of the lenses, in particular, the energy conversion efficiency increase of the solar cell by using each lens. The potential cause of the performance variation can be extracted from the comparison and evaluation.

This study proposes a polarization-independent liquid-crystal Fresnel lens using twisted nematic liquid crystals (TN-LCs) with two electrodes structure. A photoresist type has high diffraction efficiency, high diffraction efficiency and good polarization independence characteristic. However, its diffraction cannot be completely switched off. A patterned electrode type has the characteristics of the switch-off and good polarization independence. However, it has low diffraction efficiency. We improve the patterned electrode type with two electrodes structure. Experimental results reveal that LC Fresnel lens using the TN-LC with two electrode structures has high diffraction efficiency and a good polarization-independent characteristic.

A reflective Tiered Fresnel Zone Plate, herein called a Tiered Fresnel Mirror TFM, with a focal length on the order of a meter is studied for use as the mirror(s) in a Fabry-Perot interferometer type of laser. The relative phase transition within the individual zones (ideally smooth from zero to pi ) is stair-stepped or tiered in the longitudinal direction of the mirror. Within an individual zone the step height is constrained to a constant whereas the width of the tiers are monotonically decreased when traversing radially outward so that the overall profile follows the ideal smooth curve. The effectiveness of the number of tiers per zone, measured by the loss per pass or round-trip, varies from a Plane Mirror (zero tiers per zone) to a Spherical Mirror (an infinite number of zones per tier). The Fox and Li iterative method of determining the E-Field as the beam propagates back and forth is applied to an empty cavity resonator to determine the diffraction loss. A computer program is written to investigate the diffraction loss of various mirror configurations. The performance of the TFM is found to be not as efficient as the Spherical Mirror (the number of tiers per zone is shown to be a major variable) but may be tolerable under applications of a moderately high gain laser medium. The Gaussian Fundamental mode is easier to maintain since the higher order modes have a higher loss per round trip. The manufacture of the TFM can be incorporated easily into an IC process thereby making the cost of the novel mirror relatively cheap when produced in quantities. A major cost variable is again the number of tiers per zone which is proportional to the number of processing steps. The TFM's performance with respect to the etch depth of the steps in the mirror's stair-stepped profile is simulated and found to be a very doable etch with the current plasma etch technology.

A novel method of fabricating low reflectance mirrors has been developed based on the photosensitivity of hydrogen-loaded, germanium-doped optical fibers. Using a side-writing technique, point-wise refractive index changes have been induced in the core of Ge-doped optical fibers via ultraviolet light from a high power excimer laser. These refractive index changes cause Fresnel reflections at the boundary of the higher photoinduced index change and the lower index in the unexposed core. The boundary of the two refractive indices may be considered a low reflectance mirror. Several techniques have been explored to characterize the Fresnel reflectors including optical time domain reflectometry (OTDR) and spectral analysis. In-line, optical fiber Fresnel reflectors have applications as internal mirrors in intrinsic Fabry-Perot interferometric (IFPI) sensors and OTDR distributed strain sensors. Photoinduced IFPI sensors have performed well as temperature sensors, strain sensors, and vibration sensors. Improved manufacturing techniques are also discussed for future developments. The photoinduced IFPI sensor is inexpensive to manufacture and involves little skill, in contrast to the labor-intensive fabrication techniques of conventional IFPI sensors. The IFPI sensor has commercial applications in embedded structures, high temperature environments, and situations with large EMI conditions. / Master of Science

This document presents the modelling and characterization of novel photonic devices based on periodic arrays of multiwalled carbon nanotubes. Multiwalled carbon nanotubes are mostly metallic in nature and interesting plasmonic effects are observed when nanotubes are grown close together, with spacing of about 400 nm. The effective electronic mass on the nanotubes changes, due to mutual coupling between them and they start displaying dielectric properties which are inherently different from the their own, forming metamaterials. We present a plasmonic high pass filtering application of carbon nanotube based metamaterials. Some promising modelling and experimental results are demonstrated showing a strong cut-off filtering effect at the plasma frequency displayed by the periodic arrays of multiwalled carbon nanotubes. The artificial negative dielectric constant displayed by the nanotube arrays was also successfully utilised for producing micron-scaled applications like optical waveguides and negative lenses for overcoming the diffraction limit. The fabrication of these optical devices using the arrays of silicon nanopillars was also considered. These arrays when fabricated at nano-scaled dimensions (of about 400 nm) present a greater degree of periodicity and require a simpler fabrication process compared to carbon nanotubes. We report the detailed computational analysis on silicon nanopillars based photonic crystals, waveguides and metamaterials which operate well within in the optical regime. However, due to the fabrication limitations, the fabricated Si nanopillars presented an inverted cone shape profile along their lengths. These inverted nanocone structures were successfully utilised for enhancing reflection from Si surfaces for applications in photovoltaic devices. Lastly we present a novel application of carbon nanotube arrays for producing micron-scale Fresnel lens arrays. Forests of carbon nanotubes were utilised as absorbing media on top of a bare silicon substrate. Optical diffraction of light across the nanotube forests produced strong focusing of light, at focal lengths of order 125 microns. Numerical simulations were in excellent agreement with the measured results.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: This study describes the design and construction of a low-cost linear Fresnel solar concentrator. Ray-trace simulation models that analyse optical performance were developed and then used to perform sensitivity analyses of various characteristics of linear Fresnel concentrators. The design of a small-scale concentrator was optimised using the simulation models, after which the concentrator was constructed in the solar laboratory. The concentrator consists of a single-motor tracking system, flat primary mirrors and a low-cost secondary concentrator that approximates a compound parabolic concentrator. Testing revealed satisfactory performance that was comparable to the simulation models’ prediction. The construction of a low-cost solar concentrator that can replace existing thermal sources for the generation of power and process heat is thus achievable. / AFRIKAANSE OPSOMMING: Die ontwerp en konstruksie van 'n laekoste- lineêre Fresnel-sonkonsentreerder word in hierdie studie beskryf. Stralingsimulasiemodelle wat optiese werksverrigting analiseer is ontwikkel en gebruik om sensitiwiteitsanalises van die verskillende eienskappe van lineêre Fresnel-konsentreerders te doen. Die modelle is verder gebruik om die ontwerp van 'n kleinskaalse konsentreerder te optimeer, waarna die konsentreerder in die sonlaboratorium gebou is. Die konsentreerder bestaan uit 'n enkelmotorvolgingstelsel, plat primêre spieëls en 'n laekoste- sekondêre konsentreerder soortgelyk aan 'n saamgestelde, paraboliese konsentreerder. Toetsing dui bevredigende werksverrigting aan, vergelykbaar met wat die simulasiemodelle voorspel het. Dit is dus moontlik om 'n laekoste-sonkonsentreerder wat bestaande termiese bronne vir kragopwekking en proseshittegenerasie kan vervang, daar te stel.