Cavity resonance effects on dynamic holography in semiconductor thin films
Abstract
Photorefractive quantum wells are highly sensitive thin film holographic devices that utilize the enhanced electro-optic effects of quantum-confined excitons. They have recently gained popularity for use in systems such as laser-based ultrasound detection, femtosecond autocorrelators, and ultrafast pulse-shapers. When illuminated by a spatially modulated coherent light pattern, photogenerated charges in the quantum wells drift under the application of an applied field and trap at defects. This spatially modulated space-charge creates a spatially modulated space-charge field which alters the refractive index and absorption of the photorefractive quantum well device, setting up index and absorption gratings which then diffract a probe beam. The low intensity ability for hologram writing in photorefractive quantum well devices makes them ideal candidates for use in in vivo biomedical imaging. For many applications, including biomedical imaging, enhancement of the diffraction efficiencies of photorefractive quantum wells above the typical value of 0.01%, is required. The principle objective of this work was to examine the diffraction enhancement effects of cavity resonances. Light within the thin film photorefractive devices experiences multiple-beam interference effects. By exploring various cavity geometries, including hybrid transmission/reflection devices and reflection devices with one or two mirrors, we have achieved a diffraction efficiency of 0.45%, well into the range for use with CCD imaging technology. A new chemical etching technique that minimized surface scatter was also developed and led to the realization of real-time direct-to-video imaging. Gratings in semiconductor thin films may also be generated by sufficient densities of spatially modulated free carriers. The absorption bleaching effects and diffraction from gratings of free carriers in GaAs based thin films were investigated. The realization that strong exciton-photon effects are present even in low finesse cavities led to the design and testing of a novel device, the holographic vertical-cavity surface-emitting laser, which uses laser-induced band filling to bleach the device absorption. Significant cavity enhancements, even with low reflectivity mirrors, as well as diffraction saturation at high pump intensities were seen, a strong indication of the presence of laser gain.
Degree
Ph.D.
Advisors
Nolte, Purdue University.
Subject Area
Condensation|Optics
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