Doctor of Philosophy (Ph.D.), University of Dayton, 2019, Electro-Optics

This dissertation describes the first application, of Copper Oxide (Cu(II)O or CuO) and Zinc Oxide (ZnO) pn-junction thin films in photorefractive liquid crystal light valve applications. A novel thin film preparation technique was developed based on ball milling, spin coating, and thermal treatment that is able to produce optically transparent (>80% transmission in the visible) multilayer thin films of ZnO (~75 nm) and CuO (~6 nm). The electro-optic response of liquid crystal light valves prepared with CuO/ZnO pn-junction thin films was demonstrated using optical Freedericksz measurements and two-wave mixing. Results from the optical Freedericksz showed a forward and reverse bias response when the illumination intensity was varied, which was attributed to the photodiode behavior of the pn-junction. The results from the two-wave mixing experiment showed a photorefractive asymmetric energy exchange between two laser beam inputs of equal power. A new model was proposed to explain the origins of this asymmetric coupling response based on the anisotropy of the liquid crystal medium, the two input beam's polarization states, and k-vectors.

Committee: Dean Evans PhD (Committee Chair); Partha Banerjee PhD (Committee Member); Terrence Murray PhD (Committee Member); Andrew Sarangan PhD (Committee Member) Subjects: Electrical Engineering; Nanotechnology; Optics

Master of Science (M.S.), University of Dayton, 2017, Electro-Optics

This thesis reports on an experiment to measure charge carrier contributions from different Fe species and their effects on beam coupling efficiency using self-pumped counter-propagating two-beam coupling in iron-doped potassium niobate KNbO3:Fe. We used multiple continuous wave lasers operating across the visual spectrum to explore charge carrier creation from various transitions. Photorefractive grating formation data was acquired and analyzed using a new theoretical model which incorporates multiple charge carrier species. Initial analysis provides supporting evidence of a multiple charge carrier model and presents new insights about the effects of various charge carriers on the photorefractive periodic space-charge fields.

Committee: Dean Evans (Advisor) Subjects: Electromagnetism; Materials Science; Optics; Physics

Master of Science (M.S.), University of Dayton, 2016, Electrical Engineering

The photorefractive effect is a nonlinear optical effect that refers to change in refractive index of a material when it is illuminated by light. When illuminated by an interference pattern of coherent light source, this PR effect is responsible for two-beam coupling in PR materials, sometimes leading to energy exchange between the beams. PR materials can also be used as holographic storage media. In fact, dynamic real-time holographic interferometry can be implemented using photorefractive materials. To achieve this, two beams, one called the pump and one called the object beam, are introduced onto a photorefractive material to write the hologram of the object. During the hologram writing process, these beams can couple in intensity and/or phase which thereafter are responsible for self-diffraction of these beams, and can also give rise to Bragg and non-Bragg orders. The information from the Bragg and non-Bragg orders plays an important role in determining the 3D information of the object. In this thesis, an exact study is performed to examine the spatial evolution of Bragg and non-Bragg orders in photorefractive iron doped lithium niobate for different types of beam profiles such as Gaussian and flattops using an angular plane wave spectral decomposition technique. For Gaussian beam incidence, it has been found that higher or non-Bragg orders shows evidence of mode conversion of incident beam profiles. The numerical technique developed in this work should be useful in determining the phases of the Bragg and non-Bragg orders which have applications in dynamic phase-shifting digital holography and holographic interferometry.

Committee: Partha Banerjee Dr (Committee Chair); Monish Chatterjee Dr (Committee Member); Joseph Haus Dr (Committee Member) Subjects: Electrical Engineering; Optics

Doctor of Philosophy (Ph.D.), University of Dayton, 2015, Electro-Optics

Materials exhibiting effective nonlinearity through refractive index modulation at relatively low optical powers can be exploited for various applications. Examples of such materials include liquids where the refractive index is modified through heating, and photorefractives where the refractive index modulation is caused by the induced space charge field due to optically generated charges and their redistribution. Optical probing techniques of these and related effects include digital holography, holographic interferometry, and diffraction. First, the effect of self-phase modulation of a focused laser beam in a thermal medium such as a liquid is studied using a low power probe beam. Beyond self-phase modulation, thermal blooming occurs, due to bubbles generated in the liquid. These bubbles are characterized using the same probe and digital holography. An application of these bubbles to nanoparticle agglomeration and transport for drug delivery systems is proposed. Next, the use of recording materials such as photorefractive lithium niobate for implementing real-time phase shifting holographic interferometry is examined in detail. Holographic interferometry is a convenient tool for 3D characterization of deformations of an object. The hologram of an object is first written in the material using a reference beam, and then read out by the same reference beam and light from the deformed object. It is shown that the use of both Bragg and non-Bragg orders during conventional two-beam coupling in a photorefractive material facilitates the simultaneous generation of phase shifts necessary for this type of holographic interferometry. In certain applications involving liquid crystals, the spatial modulation of the director axis can yield improved energy coupling in hybrid liquid crystal – photorefractive devices. Nanoscale engineering of the director axis is possible using the surface corrugation in photorefractives induced by the space charge field through the piez (open full item for complete abstract)

Committee: Partha Banerjee Dr. (Committee Chair); Joseph Haus Dr. (Committee Member); Andrew Sarangan Dr. (Committee Member); Sergei Lyuksyutov Dr. (Committee Member); Georges Nehmetallah Dr. (Committee Member) Subjects: Engineering; Optics

PhD, University of Cincinnati, 2014, Arts and Sciences: Physics

The intensity dependent diffraction efficiency of a phase coherent photorefractive (PCP) ZnSe quantum well (QW) is investigated at 80 K in a two-beam four-wave mixing (FWM) configuration using 100 fs laser pulses with a repetition rate of 80 MHz. The observed diffraction efficiencies of the first- and second-order diffracted beam are on the order of 10-3 and 10-5, respectively, revealing nearly no intensity dependence. The first-order diffraction is caused by the PCP effect where the probe-pulse is diffracted due to a long-living incoherent electron density grating in the QW. The second-order diffraction is created by a combination of diffraction processes. For negative probe-pulse delay, the exciton polarization is diffracted at the electron grating twice by a cascade effect. For positive delay, the diffracted signal is modified by the destructive interference with a χ(5) generated signal due to a dynamical screening effect. Model calculations of the signal traces based on the optical Bloch equations considering inhomogeneous broadening of exciton energies are in good agreement with the experimental data. To study the carrier dynamics responsible for the occurrence of the PCP effect, three-beam FWM experiments are carried out. The non-collinear wave-vectors k1, k2 and k3 at central wavelength of 441 nm (~2.81 eV) were resonantly tuned to the heavy-hole exciton transition energy at 20 K. In the FWM experiment the time coincident strong pump pulses k1 and k2 create both an exciton density grating in the QW and an electron-hole pair grating in the GaAs while the delayed weak pulse k3 simultaneously probes the exciton lifetime as well as the electron grating capture time. The model calculations are in good agreement with the experimental results also providing information about the transfer delay of electrons arriving from the substrate to the QW. For negative probe-pulse delay we still observe a diffracted signal due to the long living electron density grating in (open full item for complete abstract)

Committee: Hans Peter Wagner Ph.D. (Committee Chair); Howard Everett Jackson Ph.D. (Committee Member); Rostislav Serota Ph.D. (Committee Member); L.C.R. Wijewardhana Ph.D. (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2010, Arts and Sciences: Physics

The phase coherent photorefractive (PCP) effect in different ZnSe quantum well structures and its dependence on various extrinsic and intrinsic parameters have been investigated using 90 fs laser pulse in a two-beam four-wave-mixing (FWM) configuration. At low excitation intensities the signal is dominated by the PCP effect (which is attributed to a long living electron grating formed in the QW due to coherent QW excitons) and pulse overlap (PO) effect while at high excitation intensities it is governed by Χ(3) FWM processes and the PO effect. With increasing excitation intensity the signal dip at pulse overlap (τ ≈ 0) which is characteristic for the destructive interference between the PO and PCP effect shifts to positive delay times τ > 0. The higher PCP diffraction efficiency value of ~1.5 x10-3 in QW B (Zn0.92Mg0.08Se/ZnSe) as compared to the value of ~3.5 x10-4 in QW A (Zn0.94Mg0.06Se/ZnSe) at 55 K is attributed to an increased Mg concentration in the barrier of QW B leading to a higher captured equilibrium electron density ne. Repetition rate dependent measurements on QW B show a drop of the diffraction efficiency for repetition times larger than 1.25 µs which is attributed to the reduction of the electron grating amplitude due to thermally activated electron tunneling. FWM experiments on two 10 nm ZnSe QWs with different barrier thicknesses of 20 (QW1) and 50 nm (QW2) between the QW and substrate show a redshift of the exciton line and an increased exciton dephasing rate due to increasing E-field induced tilt of the QW structure indicating an increased density of captured electrons ne. At temperatures below 35 K and laser excitation close to the exciton energy the creation of trions significantly compensates the formation of the spatially modulated electron density grating. At lower excitation energies increasing space-charge-fields significantly tilt the QW which reduces the trion binding energy leading to an enhanced thermal ionization of trions resulting (open full item for complete abstract)

Committee: Hans Peter Wagner PhD (Committee Chair); Michael Sokoloff PhD (Committee Member); Young Kim PhD (Committee Member); Howard Everett Jackson PhD (Committee Member) Subjects: Condensation

PhD, University of Cincinnati, 2007, Arts and Sciences : Physics

We study the dynamics of excitons in bimodal CdSe quantum dots. The effect of exciton localization is investigated by identifying transfer mechanisms due to thermalization and redistribution of excitons. We observe an exciton emission from low energy (QDs1) and weaker emission from high energy (QDs2) at low excitation levels at 10 K. Temperature-dependent photoluminescence (PL) studies reveal a thermally activated exciton transfer from QDs1 to QDs2. Time-resolved PL estimate the characteristic radiative and nonradiative decay rates as well as the trapping rate from the QD-precursor layer. The observed PL is reasonably reproduced using a coupled rate equation model. We investigate 10 nm Zn0.94Mg0.06Se/ZnSe quantum well (QW) with two-beam four-wave mixing (FWM) using 90 fs pulses. At high intensity the signal is dominated by χ(3) FWM processes and at low intensity it reveals an exciton resonant phase coherent photorefractive (PCP) effect that is attributed to the formation of an electron grating within the QW by the interference of coherent QW excitons. The observed traces and spectra are reproduced by the model based on a 15-level system and a phenomenological PCP model. The dynamical properties of the electron grating responsible for the PCP is further studied reducing the pulse repetition at 45, 55 and 65 K. The PCP diffraction reveals a nearly constant efficiency up to 1 μs that implies a constant average equilibrium electron density. With increasing temperature, the efficiency decreases due to QW electron escape back to the substrate reducing the grating lifetime. The observed PCP efficiency is studied with a model that considers the equilibrium density dynamics in the QW. We further report on PCP effect in Zn0.9Mg0.1Se/ZnSe QW by performing intensity dependent and polarization dependent two-beam FWM experiments using 30 fs pulses at 2.79 and 2.84 eV. The PCP effect is attributed to the formation of an electron grating within the QW by the interference of exc (open full item for complete abstract)

Committee: Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter

PhD, University of Cincinnati, 2006, Arts and Sciences : Physics

Spectrally resolved and time-integrated degenerate four-wave mixing (FWM) experiments with ultra-short pulses have been used to investigate the relaxation times and coherent interactions of excitons in the Boltzmann and in the quantum kinetic regime in ZnSe single quantum wells (QWs). The interband and interexciton coherences has been investigated in three-pulse FWM experiments using 100 fs pulses in a 10 nm ZnSe QWs. The FWM signals obtained in three-beam FWM experiment are explained by optical Bloch equations (OBEs) for a ten-level model including excitation induced dephasing (EID). Two-beam FWM experiments using 30 fs pulses on a 10 nm ZnSe quantum well reveal marked quantum beats and a pronounced non-exponential decay for delay times smaller ~500 fs. The three-beam FWM trace shows a bi-exponential decay which is attributed to a nonlinear polarization that is caused by the interaction of polarization of third beam with both an exciton grating and an eh-pair grating. Model calculations using the OBEs of a two level system that includes EID at both types of density grating provide information about exciton formation processes involved. We observe coherent exciton-LO-phonon polarons in the FWM spectrum of a 3 nm ZnSe single quantum well using 30 fs pulses. The formation of the new quasi-particles is attributed to a strong phonon coupling that is caused by strong Frohlich interaction, a huge exciton binding that exceeds the LO-phonon energy and a close resonance between the 2s exciton state and LO-phonon energy. The rapid decay of these coherent quasiparticles is attributed to the disintegration into zero-phonon excitons and free LO-phonons as well as to the inhomogeneous broadening of LO-phonon energies due to disorder and k-space dispersion A novel phase coherent photorefractive (PCP) effect has been observed in ZnSe single QWs using ultra-short pulses that do not overlap in time and its spectral and thermal dependence have been investigated. The observed PCP effec (open full item for complete abstract)

Committee: Dr. Hans-Peter Wagner (Advisor) Subjects: Physics, Condensed Matter