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Thesis (Ph.D.)--Tufts University, 1995. / Adviser: M. Cronin-Golomb. Submitted to the Dept. of Electrical Engineering. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Generally, nonlinear optics studies investigate optically-induced changes in refraction or absorption, and their application to spectroscopy or device fabrication. The photorefractive effect is a nonlinear optical effect that occurs in solids, where transport of an optically-induced free-carrier population results in an internal space-charge field, which produces an index change via the linear electrooptic effect. The photorefractive effect has been widely studied for a variety of materials and device applications, mainly because it allows large index changes to be generated with laser beams having only a few milliwatts of average power.Compound semiconductors are important photorefractive materials because they offer a near-infrared optical response, and because their carrier transport properties allow the index change to be generated quickly and efficiently. While many researchers have attempted to measure the fundamental temporal dynamics of the photorefractive effect in semiconductors using continuous-wave, nanosecond- and picosecond-pulsed laser beams, these investigations have been unsuccessful. However, studies with this goal are of clear relevance because they provide information about the fundamental physical processes that produce this effect, as well as the material's speed and efficiency limitations for device applications.In this dissertation, for the first time, we time-resolve the temporal dynamics of the photorefractive nonlinearities in two zincblende semiconductors, semi-insulating GaAs and undoped CdTe. While CdTe offers a lattice-match to the infrared material HgxCd1-xTe, semi-insulating GaAs has been widely used in optoelectronic and high-speed electronic applications. We use a novel transient-grating experimental method that allows picosecond temporal resolution and high sensitivity. Our results provide a clear and detailed picture of the picosecond photorefractive response of both materials, showing nonlinearities due to hot-carrier transport and the Dember space-charge field, and a long-lived nonlinearity that is due to the EL2 midgap species in GaAs. We numerically model our experimental results using a general set of equations that describe nonlinear diffraction and carrier transport, and obtain excellent agreement with the experimental results in both materials, for a wide variety of experimental conditions.

An optical computer which performs the classification of an input object pattern into one of two learned classes is designed and demonstrated. The classifier is an optical implementation of a neural network model of computation featuring learning, self-organization, and decision-making competition. Neural computation is discussed including models for learning networks and motivation for optical implementation. A discussion of photorefractive crystal holographic storage and adaptation is presented followed by experimental results of writing and erasing gratings in several different crystals. The optical network features a photorefractive crystal to store holographic interconnection weights and an opto-electronic circuit to provide a means of competitive decision making and feedback. Results of the optical learning network and its operation as an associative memory are followed by extensions of the architecture to allow improved performance and greater flexibility.

Yuen Chi Yan = 有關光折變晶體中空間孤子、偏振轉動以及霍爾效應的研究 / 阮志仁. / Thesis submitted in: July 2004. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references (leaves 81-82). / Text in English; abstracts in English and Chinese. / Yuen Chi Yan = You guan guang zhe bian jing ti zhong kong jian gu zi, pian zhen zhuan dong yi ji Huoer xiao ying de yan jiu / Ruan Zhiren. / Acknowledgments --- p.i / Abstract --- p.ii / Table of Contents --- p.v / Chapter Chapter 1 --- Photorefractive Spatial Soliton --- p.1 / Chapter 1.1 --- Introduction --- p.1 / Chapter 1.2 --- Bright Spatial Soliton --- p.3 / Chapter 1.2.1 --- Experiment --- p.4 / Chapter 1.2.2 --- Results and Discussion --- p.6 / Chapter 1.2.2.1 --- Expansion --- p.6 / Chapter 1.2.2.2 --- Contraction --- p.10 / Chapter 1.3 --- Dark Spatial Soliton --- p.15 / Chapter 1.3.1 --- Experiment --- p.15 / Chapter 1.3.2 --- Results and Discussion --- p.20 / Chapter Chapter 2 --- Polarization Rotation --- p.23 / Chapter 2.1 --- Introduction --- p.23 / Chapter 2.2 --- Experiment --- p.24 / Chapter 2.3 --- Results and Discussion --- p.30 / Chapter 2.3.1 --- Effect of varying pump beam power --- p.30 / Chapter 2.3.2 --- Effect of different polarizations of signal beam --- p.41 / Chapter 2.3.3 --- Effect of signal beam size --- p.43 / Chapter 2.3.4 --- Effect of applied E-field --- p.46 / Chapter 2.3.5 --- Effect of signal beam and pump beam separation and perpendicularly --- p.52 / Chapter 2.3.6 --- Investigation of Δne using interferometer --- p.60 / Chapter 2.3.7 --- Computer Simulation --- p.69 / Chapter Chapter 3 --- Hall Effect --- p.72 / Chapter 3.1 --- Introduction --- p.72 / Chapter 3.2 --- Experiment --- p.75 / Chapter 3.3 --- Results and Discussion --- p.76 / Conclusion and Possible Further Works --- p.79 / References --- p.81

As the computer industry grows, so will the requirements for data storage. Magnetic memory has been the most stable method in terms of capacity and recording/reading speed. However, we have reached the point where a substantial increase in the capacity cannot be produced without increasing the size of the system. When compact discs (CDs) were introduced in the 1980�s they revolutionized the concept of data storage. While the initial force behind compact discs could easily be said to be the music industry, once recordable and rewritable discs became available they quickly found more use in the computer industry as backup devices. Since their inception, the capacity requirements have far exceeded what is available on a compact disc, and they are now following the same path as magnetic memories. Following this trend, it could be assumed that digital versatile discs or digital video discs (DVDs) have a limited lifetime as a storage medium. In fact it has been noted (Higuchi et al., 1999) that the maximum capacity of digital video discs will be reached in 3 � 5 years. The question then is, what comes next? The efficiency of conventional optical data storage is extremely poor. For an optically thick recording medium, both CDs and DVDs use less than 0.01% of the total volume to store the information. Three-dimensional bit optical data storage endeavors to increase the efficiency by recording information in a volume that is greater than 90% of the total volume. The concept of three-dimensional bit optical data storage was first proposed by Parthenopoulos and Rentzepis in 1989, where they demonstrated that capacities far exceeding that of compact discs could be achieved. Three-dimensional bit optical data storage relies on creating a highly localised chemical or physical change within a recording medium, such that further layers can be recorded without causing interference. Ideally the chemical/physical change in the material should be reversible to enable erasable/rewritable data storage. In order to create a highly localised effect nonlinear excitation can be used; whereby the excitation is limited to a small region around the focal spot. Depending on the material and recording method there are several techniques for reading the information such as transmission imaging or reflection confocal microscopy. However, all the recording and reading methods require focusing to a deep position within a recording medium, such focusing encounters spherical aberration as a result of the difference in the refractive indices between the immersion and recording media. This thesis has concentrated on several areas to understand and develop the concept of three-dimensional bit optical data storage. The photorefractive effect in crystals has been studied for many years and is now widely used in optoelectronic devices. The use of photorefractive polymers is a relatively new and exciting development in optical data storage. Until now they have been used solely in the area of holographic data storage. The research in this thesis was conducted using photorefractive materials that were fabricated in two polymer matrices, poly(N-vinylcarbazole) (PVK) and poly(Methyl Methacrylate) (PMMA). The recording samples also consisted of the following compounds in various proportions, 2,5-dimethyl-4-(p-nirtophenylazo)anisole (DMNPAA), 2,4,7-trinitro-9-fluorenone (TNF) and N-ethylcarbazole (ECZ). In this project two-photon excitation was used as the recording mechanism to achieve erasable/rewritable data storage in a photorefractive polymer. As a result of two-photon excitation, the quadratic dependence of excitation on the incident intensity produces an excitation volume that is confined to the focal region in both the transverse and axial directions. Therefore, focusing the laser beam above or below its previous position provides a method by which layers of information can be recorded in the depth direction of a material, without causing interference from neighbouring layers. The feasibility of two-photon excitation in photorefractive polymers is demonstrated in this thesis. The quadratic relationship between excitation and incident light in two-photon excitation requires high photon density to ensure efficient excitation. The use of ultra-short pulsed lasers, while effective, is not a practical solution for an optical data storage system. This thesis demonstrates the ability to produce three-dimensional erasable/rewritable data storage in a photorefractive polymer using continuous wave illumination. Using this technology it has been possible to achieve a density of 88 Gbits/cm3, which corresponds to a capacity of 670 Gbytes on a compact disc sized recording medium. This is an increase of 1000 times the capacity of a CD and 130 times the capacity of current DVDs. While erasable optical data storage is an exciting prospect there are problems associated with the deterioration of the information. For long term information storage a permanent recording process would be more practical. It is demonstrated that there is a point after which further increases in the recording power result in the formation of a micro-cavity. While two-photon excitation is the recording method for erasable data storage, the increase in power results in an increase in ultra-violet absorption such that multi-photon excitation may occur. This thesis demonstrates the ability to record multi-layered arrays of micro-cavities. The change in refractive index associated with an erasable bit is less than 1%. As a result only phase sensitive reading methods (transmission imaging or differential interference contrast (DIC) microscopy) can be used to image a recorded bit. Both transmission and DIC imaging systems have poor axial resolution and therefore limit the density of the recording system, as well as being large optical systems. The introduction of a split or quadrant detector reduces the size of the optical reading system and is demonstrated to be sensitive enough to detect the phase changes of a recorded bit. However, the change in refractive index across a micro-cavity is large enough that reflection confocal microscopy can be used to detect a bit. It is demonstrated in this thesis that multi-layered micro-cavity arrays can be read using reflection confocal microscopy. Focusing of light to deep positions within an optical thick recording medium has the effect of increasing spherical aberration resulting from the refractive index mismatching between the immersion and recording media. The work in this thesis illustrates the effect of spherical aberration on the performance of both the recording and reading systems. The work conducted in this thesis shows the ability to record multi-layered erasable/rewritable information in a photorefractive polymer using pulsed and continuous wave two-photon excitation. It has also been demonstrated that through multi-photon excitation multi-layered micro-cavity arrays can be fabricated. It has also been illustrated that while spherical aberration deteriorates the performance of the recording and reading systems it is possible to achieve a density of greater than 88 Gbits/cm3.

Thesis (Ph. D.)--University of Chicago, Department of Chemistry, 2002. / Includes bibliographical references. Also available on the Internet.

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Thesis (M. Eng.)--Carleton University, 1992. / Also available in electronic format on the Internet.

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