Doctor of Philosophy, The Ohio State University, 2023, Electrical and Computer Engineering

Lithium niobate (LN) has attracted significant interest over the past few decades as a potential platform for next generation nonlinear optical devices, high speed optical interconnects and modulators, and quantum light sources. Sub-micrometer thick lithium niobate on insulator (LNOI) is a promising integrated photonic platform that provides optical field confinement and high optical nonlinearity useful for state-of-the-art electro-optic modulators, wavelength converters, and acousto-optical devices. With fabrication foundry technologies enabling realization of low loss LNOI waveguides, devices fabricated using LNOI substrates and have been able to achieve record high harmonic generation efficiencies, and low insertion and propagation losses.Fabrication of LNOI on a silicon substrate through ion-slicing is advantageous for enabling velocity matching between microwave and optical copropagating fields in electro-optic modulators and for electronic-photonic integration but is challenging because of debonding and cracking due to thermal expansion coefficient mismatch between silicon and LN. Moreover, current techniques to pattern low loss waveguides with smooth sidewalls in LNOI rely on chemical mechanical polishing and electron beam lithography. Chemical mechanical polishing can result in film etching thickness variations, while electron beam lithography is not suited for high throughput production.Lastly, current schemes for fiber to chip edge coupling rely on the use of specialty optical fibers, such as lensed fibers, and there is a requirement for an efficient packaging solution that can utilize standard single mode cleaved optical fibers. Fabrication of thin film lithium niobate on insulator on a silicon handle wafer is achieved via ion-slicing, informed by structural modeling, and facilitated by accommodating for dissimilar wafer bows using a bonding apparatus. Structural finite element analysis of strain energy and stress, due to thermal expansion coefficient (open full item for complete abstract)

Committee: Ronald Reano (Advisor); Patrick Roblin (Committee Member); Robert Lee (Committee Member); Fernando Teixeira (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Engineering; Nanotechnology; Optics

Doctor of Philosophy (Ph.D.), University of Dayton, 2023, Electro-Optics

This dissertation is concerned with two distinct applications of volume gratings recorded in photorefractive electro-optic crystals. The first of two applications involves the use of these volume gratings to non-mechanically steer laser beams. A geometric and physical-optics based analysis shows the potential for writing programmable volume gratings in lithium niobate using visible wavelengths in the transmission geometry, and subsequently probing those gratings using infrared wavelengths in a reflection geometry. By appropriate adjustments made to the writing beams, it is shown that both the grating spacing and grating tilt angle can be controlled such that the grating becomes a rotatable Bragg mirror for the incident probe beam, thus steering it to desired angles. The second application of these volume gratings is in image processing. System transfer functions determining the spatial evolution of the reference (input wave) and signal (diffracted wave) beams as they propagate inside a self-pumped volume reflection grating are derived and solved numerically. The solutions are then used to highlight the spatial filtering properties of self-pumped volume reflection gratings, with the focus being on the transmitted (un-diffracted) portion of the reference beam, which is shown to be high-pass spatially filtered. The high-pass spatial filtering manifests as programmable 2-dimensional edge enhancement in the transmitted reference beam. Contrast analysis is done for edge enhanced images, both through simulations and experiments, which show a direct proportionality between the strength of edge enhancement seen in the filtered images and the intensity of the writing beam used to record the grating.

Committee: Partha Banerjee (Advisor) Subjects: Optics; Physics

Doctor of Philosophy, The Ohio State University, 2020, Electrical and Computer Engineering

Single-crystal lithium niobate (LN) thin films have emerged as a promising platform for photonic integrated circuits with applications including quantum optics, spectroscopy, and high-speed communications. The LN films are well-suited for nonlinear optics owing to the high optical confinement compared to their bulk counterparts and their ferroelectric nature which enables quasi-phase matching by period poling. Poling of LN thin films presents new challenges due to large leakage currents and the relatively small domain size required for phase matching. Moreover, current poling techniques have not been able to reach submicrometer-scale poling periods suitable for first-order quasi-phase matched interactions with counter-propagating waves. In this dissertation, we fabricate lithium niobate thin films and demonstrate improved periodic poling techniques to enable efficient nonlinear photonic integrated circuits. Wafer-scale single-crystal LN thin films are produced by ion-slicing. The LN films are less than one micrometer thick and are bonded to a supporting oxidized LN wafer without the use any intermediate materials or adhesives. The films are chemically-mechanically polished to achieve a surface roughness less than 0.5 nm RMS. Large area void-free films are reliably produced with this process. In addition, fabrication processes to form silicon nitride strip-loaded waveguides and poling electrodes are developed based on electron beam lithography and plasma etching. A method of reducing the leakage current during electric field poling of x-cut magnesium oxide doped lithium niobate thin films is developed. The leakage current is reduced by introducing a silicon dioxide insulation layer under the co-planar electrodes. Uniform domains with a 7.5 μm period and 50% duty cycle are achieved. The poling characteristics are compared to bulk lithium niobate, with and without the silicon dioxide insulation layer. The domains are characterized on the surface by piezoresponse (open full item for complete abstract)

Committee: Ronald Reano Ph.D. (Advisor); Fernando Teixeira Ph.D. (Committee Member); Robert Lee Ph.D. (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Optics

Doctor of Philosophy (PhD), Ohio University, 2016, Physics and Astronomy (Arts and Sciences)

Nonlinear optical processes are an excellent candidate to provide the heralded, indistinguishable, or entangled photons necessary for development of quantum mechanics based technology which currently lack bright sources of these photons. In order to support these technologies, and others, two classes of materials: traditional and novel, were investigated via optical characterization methods with goal of gaining insight into which materials and experimental conditions yield the greatest nonlinear optical effects. Optical characterization of periodically poled lithium niobate (PPLN) helped support the development of a simple, efficient photon pair source that could be easily integrated into optical networks. Additionally, an in-situ measurement of the 2nd order nonlinear optical coefficient was developed to aid in the characterization of PPLN pair sources. Lastly, an undergraduate demonstration of quantum key distribution was constructed such that students could see the primary application for PPLN photon pair sources in an affordable, approachable demonstration. A class of novel optical materials known as 2D materials has been identified as potential replacements to the traditional nonlinear optical materials discussed in Part I. Through optical characterization of second harmonic generation (SHG) the ideal conditions for spontaneous parametric downconversion were established as well as signal thresholds for successful detection. Attempts to observe SPDC produces hints that weak generate SPDC may be present in WS2 samples however this is incredibly difficult to confirm. As growth techniques of 2D materials improve, a photonic device constructed from these materials may be possible, however it will need some mechanism e.g. stacking, a cavity, etc. to help enhance the SPDC signal.

Committee: Eric Stinaff (Advisor); Alexander Govorov (Committee Member); Savas Kaya (Committee Member); Nancy Sandler (Committee Member) Subjects: 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, The Ohio State University, 2015, Electrical and Computer Engineering

A hybrid silicon and lithium noibate (LiNbO3) material system is developed to combine the high index contrast of silicon and the second order susceptibility of lithium niobate. Ion-sliced single crystalline LiNbO3 thin film is bonded to silicon-on-insulator (SOI) waveguides via Benzocyclobutene (BCB) as the top cladding. The LiNbO3 thin films are patterned to achieve desired size, shape and crystal orientations. Integrated electrodes are integrated to confine electric fields to the LiNbO3 thin film. Empowered by the linear electro-optic effect of LiNbO3, compact chip-scale hybrid Si/LiNbO3 integrated photonic devices are enabled on the SOI platform, including radio-frequency electric field sensors, tunable optical filters, high speed electro-optical modulators for optical interconnects, and high linearity modulators for analog optical links. Compact and metal-free electric field sensors based on indirect bonding of z-cut ion-sliced LiNbO3 thin film to silicon microrings are demonstrated. The demonstrated sensitivity to electric fields is 4.5 V m-1Hz-1/2 at 1.86 GHz. Tunable optical filters based on hybrid Si/LiNbO3 microring resonators with integrated electrodes are also demonstrated with a tunability of 12.5 pm/V, which is over an order of magnitude greater than electrode-free designs. By integrating metal thin film electrode and utilizing silicon as an optically transparent electrode, voltage induced electric fields in the LiNbO3 are enhanced. We also presented low power compensation of thermal drift of resonance wavelengths in hybrid Si/LiNbO3 ring resonators. A capacitive geometry and low thermal sensitivity result in the compensation of 17 oC of temperature variation using tuning powers at sub-nanowatt levels. The method establishes a route for stabilizing high quality factor resonators in chip-scale integrated photonics subject to temperature variations. Gigahertz speed hybrid Si/LiNbO3 electro-optical microring modulators ar (open full item for complete abstract)

Committee: Ronald Reano (Advisor); Joel Johnson (Committee Member); Fernando Teixeira (Committee Member); Gregory Lafyatis (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Nanotechnology; Optics

Master of Science, The Ohio State University, 2008, Electrical and Computer Engineering

A wireless strain sensing system utilizing passive, wireless, physically small and light weight sensors is desirable for measuring strain in harsh environments such as jet engine compressor and turbine blades. A cluttered and time varying environment results in high loss, blockage, multipath and modulation of the electromagnetic wave. Also, temperature changes affect the sensitivity of the strain measurement. Isolating the information signal from the reverberations in the environment requires time delays in the order of 100s of ns for jet engine environment. Therefore, a wireless strain gauge system that utilizes surface acoustic wave (SAW) strain sensors was studied and tested.SAW strain sensors are designed to operate at 2.45GHz. Electron beam lithography is used to achieve minimum required feature size at this frequency. The fabrication process is outlined and scanning electron microscope images of some results are given. A transceiver circuit is designed and constructed. The circuit is tested in free space, in the presence of signal blockage and a time varying channel. Measurements are shown to be in good agreement with predicted data. Sources of errors in the setup are identified to be leakage from transceiver circuit switches and bounce waveforms from the transceiver antenna. A General Electric J85 jet engine compressor section is analyzed for signal propagation characteristics. Minimum frequency that can propagate through the compressor section is determined to be 5.2GHz. Measurements are done to show that circumferential polarization propagates stronger than radial inside the compressor section. An analytical approximation for the compressor section is generated by modeling compressor section blades as rectangular waveguides. Good agreement on cutoff frequency is achieved for circumferential polarization with the analytical predictions and measurement. SAW temperature and strain sensors are measured in comparison to traditional gauges. This concept can be ge (open full item for complete abstract)

Committee: Roberto Rojas-Teran (Advisor); Eric Walton K. (Committee Member); Jonathan Young D. (Committee Member) Subjects: Electrical Engineering; Engineering; Experiments