Search

An exciton is an electrically neutral quasiparticle that consists of an electron and a holeattracted to each other by the Coulombic force of attraction due to the opposite charge ofthe constituent pair. Exciton can form with the absorption of a photon into a material(insulator or semiconductor) as an intermediate state which can soon dissociate andproduce photocarriers i.e., electron and hole under suitable condition. Normally a photonwith above bandgap energy would get absorbed into any material of interest, but under anapplied electric field a photon with below bandgap energy can be absorbed due to bandbending as the wave function of the constituent electron and hole leak respectively fromthe Conduction Band and Valance Band into the forbidden region. This absorption processis known as the Franz-Keldysh (FK) effect. In Wide Bandgap Material (WBM)s such asGallium Nitride () and Beta-phase of Gallium Oxide ( − 23) the FK effect isdominated by the formation and later dissociation of exciton. The below bandgap photonabsorption via intermediate exciton state can be a process of general interest to explain thebelow bandgap photoresponsivity of the WBM under study. The understanding of excitonphysics in this context can be utilized to engineer important applications such as toaccurately detect and quantify the local electric field and the onset of breakdown behaviorof the material which can help in designing Radio Frequency (RF) and power electronicdevices with high reliability and stability. This dissertation looks at the excitonic physicsof Wide Bandgap Materials i.e., and − 23 with the objective to explain theexperimentally observed photoresponsivity characteristics through eXciton Franz Keldysh(XFK) effect. Application of the exciton mediated below bandgap photon absorptionphysics in mapping out the electric field variation of p-n diode and − 23Schottky diode with applied external voltage are also discussed to demonstrate theimportance of such studies. In addition to these unusual electric field dependent absorptionand photocarrier generation processes, − 23 also displays a striking polarizationdependent optical absorption that is a far cry from any previously explored wide bandgapsemiconductors. The anisotropy in optical absorption of − 23 has been demonstratedexperimentally by varying light’s polarization orientation and measuring the anisotropicphotocurrent. The origin of the polarization anisotropy are the non-zero off-diagonal termswithin the complex dielectric tensor of this monoclinic crystal. There exist non-negligibleoff-diagonal components in both the real and imaginary parts of the dielectric tensor,especially in the near bandgap region. As a result, not only is the absorption polarizationdependent, but so is the refractive index, resulting in a material that is both dichroic andbirefringent. These physical properties have their origin in the energy shift of the excitonictransitions and the orientation anisotropy of the exciton dipole moments that change theabsorption behavior and photon flux decay in − 23. The decay behavior of theincident photon flux in this material is modified away from the well-known isotropic BeerLambert law; giving rise to depth dependent absorption behavior which itself arises froma depth dependent polarization rotation as well as optical dispersion. Understanding thehighly selective polarization dependent solar-blind behavior of − 23 from theperspective of raw photoresponsivity calculated from Electro-Magnetic wave equationsolution can confirm the observed experimental results and provide basis for engineeringultra-sensitive narrowband UV photodetectors.

Knowledge of cerebral oxygenation is critical to an understanding of brain status in normal physiology and especially in development of pathophysiological conditions, such as cerebral amyloid angiopathy. This thesis describes development and evaluation of a simple multispectral reflectance imaging device. The device was modeled after a previously published paper by Dunn et al., and consists of a 3-D printed filter wheel, a CCD camera, a motor driving circuit, a photodiode, an Arduino, a light source and six bandpass filters. Two approaches to calculate differential pathlength factor (DPF) values were investigated and compared with published values from Dunn et al., and a surprising finding was that a DPF algorithm derived originally for point spectroscopy was the most accurate approach of the three to calculate changes in oxy- and deoxyhemoglobin. Further studies are warranted.