Dynamic holography in semiconductors and biomedical optics
Three-dimensional scanning and display are rapidly-advancing new technologies with important commercial drivers such as 3D printing and remote imaging for big data applications. Holography is a natural approach to recording and displaying three-dimensional information because it uses phase-sensitive interferometry to record interference patterns when a reference beam encounters coherent light arriving from an object. The 3D information is contained in the values of wave optics. Holography is a broad field that goes beyond recording and displaying. For instance, holographic optical elements, which take advantage of holographic imaging principles, perform the functions of lenses, gratings or mirrors. Holographic interferometry is also widely used for non-destructive testing to reveal structural faults within materials without damaging the specimen. More recently, digital holography is being used in biomedical applications to provide details of living tissues that are missed by alternative techniques. Future photonics devices will incorporate micro-lasers and holographic optical elements for optical computers, free-space interconnects, and massive analog and digital memory systems. Over the past several decades, dynamic holography has developed on the ideas and methods of traditional holography by applying them to time-dependent processes. Dynamic holographic media have potential for non-linear image processing, optical limiting, beam steering, and adaptive interferometry. However, there is usually a trade-off between dynamic holographic speed and diffraction efficiency. For instance, photorefractive crystals and polymers can have high efficiencies, but operate at low speeds. Conversely, photorefractive quantum-well devices have high speed, but suffer from low diffraction efficiencies. This thesis describes theoretical and experimental studies of a novel quantum-well semiconductor structure that achieves high diffraction efficiencies with high dynamic recording speeds. When illuminated by spatially-modulated coherent light patterns, photon-generated electron-hole pairs create spatially modulated gratings in the structures which alter the refractive index and absorption coefficient, establishing index and absorption gratings that diffract a probe beam in a nondegenerate four-wave mixing optical configuration. The key to the performance of the asymmetric Fabry-Perot quantum-well microcavities is a tuned low-Q cavity resonance that supports wide angle-of-view while operating on the edge of stimulated light emission. A diffraction efficiency of 70% has been obtained using a phase-grating contribution that approaches the maximum π phase shift. The diffracted signal exhibits rise/fall times of 5 nsec, demonstrating the high speed capabilities of this device. Dynamic holograms are not only physical entities, such as photorefractive media, but can also be detected digitally on a detector array such as a digital camera. Digital holography offers an improvement over conventional optical coherence tomography (OCT) that was developed as an interferometric imaging technique that forms high-resolution cross-sectional images of morphological features. Digital holographic optical coherence imaging (OCI), which is an en face full-frame coherence-gated imaging approach, uses a CCD camera to record digital holograms from a fixed depth inside scattering media without the need to scan. Digital holography performs coherence-gated dynamic light scattering, recording dynamic speckle information that carries the Doppler scattering information about intracellular motions inside living tissue. Fluctuation spectroscopy is used to extract the spectral response of the scattering media that is perturbed by various environmental changes, such as applied therapeutics. This thesis describes experimental studies using biodynamic imaging to understand dynamic biological phenomena by performing phenotypic profiling of cancer tissues and biopsies. Three-dimensional tissue culture techniques are compared and contrasted in terms of their biodynamic responses to applied drugs. An important finding is the non-equivalence of multiple forms of three-dimensional growth, raising questions about the use of such tissue culture in drug discovery and development that could have important consequences for practices at pharmaceutical companies. Theoretical and numerical studies were carried out on the role of dynamic transport mechanisms and how they contribute to the phenotypic profiling signatures of biodynamic imaging.
Nolte, Purdue University.
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