Dissertations and Theses

Date of Award


Document Type




First Advisor

Robert R. Alfano


Photonics, semiconductors, orbital angular momentum, imaging, cancer, Alzheimer's


Light possesses characteristics such as polarization, wavelength and coherence. The interaction of light and matter, whether in a semiconductor or in a biological sample, can reveal important information about the internal properties of a system. My thesis focuses on two areas: photocarriers in gallium arsenide and biomedical optics. Varying the excitation wavelength can be used to study both biological tissue and condensed matter. I altered the excitation wavelengths to be in the longer near-infrared (NIR) optical windows, in the shortwave infrared (SWIR) range, a wavelength region previously thought to be unusable for medical imaging. With this method, I acquired high contrast images of hidden abnormalities. I also reached deeper penetration depths through media like the breast, prostate, bone and brain, as well as cancerous tissues. By changing the wavelength of the incident light, select transitions from the sublevels can be obtained and by utilizing light beams carrying spin angular momentum (SAM) and orbital angular momentum (OAM), the electron's orbital motion in a semiconductor can be altered, resulting in the formation of photogenerated spin-polarized electrons. OAM and SAM can be used to enhance communications, information and computers.

Understanding the mechanism of electron spin relaxation is important for engineering devices which rely on long spin relaxation times. GaAs is a key direct bandgap semiconductor used for photoemitters such as LEDs, laser diodes and photodetectors. I investigated the interaction of twisted photons (light carrying SAM and OAM) with a p-type bulk GaAs sample and a GaAs photocathode with a layer of cesium (Cs) (Cs-GaAs). The transfer of SAM and OAM light to polarized electrons were evaluated using a Cs-GaAs photocathode optical device. This study merges two important fields: condensed matter and complex light. The degree of photogenerated electron polarization was calculated based on the photocathode signals and varied due to the wavelength of the excitation beam, the handedness of the beam and the value of l. I also acquired spin relaxation times from bulk p-GaAs and Cs-GaAs using time-resolved photoluminescence spectroscopy and twisted photons. I used a streak camera to acquire direct time-resolved measurements, giving important information on the carrier dynamics during ultrafast photoexcitation and photo-ejection of electrons. Lifetime results demonstrate that the Cs-GaAs device can be utilized as an efficient source of spin polarized electrons.

Just as GaAs is the key material for studying spin-polarized electrons with optical techniques, tryptophan, an essential amino acid, is key to studying the interaction of light with biological tissues. I used label-free optical techniques to evaluate the relative tryptophan content in normal and diseased tissues. Fluorescent spectra from breast samples were analyzed in terms of relative tryptophan content using a ratio of emission peaks maxima. Results showed that these tools can be used to distinguish normal from malignant tissues. When these results were evaluated in terms of patient histologies, I found that there exists a extremely strong correlation between increased tryptophan content and high cancer grade. This data shows that there is a profound correlation between increased tryptophan content in a cancer and its aggressiveness and potential to metastasize. This also demonstrated that fluorescence of tryptophan can be useful as part of the evaluation of patients with breast cancer. I also explored the role of tryptophan in the neurodegenerative disease, Alzheimer’s, using fluorescence. I discovered that tryptophan to tryptophan metabolite (kynurenine) ratios in the hippocampus and in Brodmann’s area 9 were markedly decreased compared to ratios in normal individuals but these results were not seen in Brodmann's area 17, an area minimally affected in patients with Alzheimer's disease. These results highlight the importance of tryptophan in biologic tissue such as cancer and neurodegnerative disease. This is further evidence that abnormal tryptophan metabolism is involved in the causation of Alzheimer’s disease.



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