Advisor: B. Tittmann
Committee: A. Segall, C. Lissenden, J. Todd
Design and Development of a Piezoelectric Cable Socket Contact Force Measurement Instrument
As an electrical socket ages, it wears out and tends to lose its grip on a plug. To avoid failure, periodic measurements can be taken to determine how well the socket is holding the plug. Sockets with weak contact forces will be replaced while sockets with strong contact forces will continue to be used. The aim of this research is to design a hand-held ultrasonic force sensor to measure the socket contact force.
The ultrasonic force sensor works by inserting a special tube made of a piezoelectric crystal into the socket. A piezoelectric material's electrical and mechanical properties are linked together. When the piezoelectric tube is inserted into the socket, the socket grips the tube by applying a force to the outside of the tube. This mechanical force causes a change in electrical resonance frequency. Resonance occurs when the applied frequency equals the natural frequency of the material.
As the force on the outside of the tube increases, the change in electrical resonance frequency also increases. The socket's grip on the plug was calculated by measuring the change in resonance frequency and relating it to the value of the applied force. Socket contact forces were roughly measured through a friction-based method. The instrument was calibrated by applying weights on top of the tube and noting the change in resonance frequency.
NI hardware and LabVIEW software were used to create a program for the operation of the instrument. This program includes diagnostic tools as well as a simple user-interface. A tube holder was designed and fabricated in order to make the socket testing process easier.
Advisor: A. Lakhtakia
Committee: S. Copley, M. Horn, J. Todd
Reflection and Transmission of Elastodynamic Plane Waves by a Piezoelectric, Continuously Twisted, Structurally Chiral Medium
A specially created piezoelectric material is studied to determine its reflection and trans-mission characteristics. Piezoelectric materials change shape when an electric field is ap-plied across them. For example, suppose there is a small cube, about 1 inch on all sides, of a piezoelectric material and two metal contacts are glued on two opposite sides of the cube. Now if one of those contacts is grounded and the other is connected through an electrical wire to a wall socket, the cube will change shape, imperceptibly, with the 60 cycles per second wall voltage. If it were possible to increase the wall voltage, then the vibration of the cube would become more noticeable; and if it were possible to change the frequency of the wall voltage, one would see the cube vibrate either slower or faster with the change in voltage.
By using the piezoelectric effect, it is possible to transform electrical energy to mechanical energy-like an electric motor, except nothing here is rotating. What simply occurs is that the material either gets slightly thinner or fatter in the direction of the applied voltage due to internal movement at the molecular level. The converse also occurs: if one attaches a multimeter to opposite sides of the cube and then proceeds to place a heavy weight upon it, perhaps by standing on it, the multimeter will record a voltage difference between the sides of the cube. Hence, it is possible to both create mechanical vibrations and to sense mechanical vibrations. Such a device is called a piezoelectric transducer.
From experience, we know that it is possible for sound waves to travel not only through air but also through water, metal, wood, etc. We have all put our ears up to a piece of wood or metal when we were younger and pounded on it to listen to what the noise sounded like. If you could create a sophisticated noise-making and listening system, it would be possible to study the structural health of a piece of metal. For example, if you have played baseball or softball and come across an aluminum bat that has a crack, it is easy to identify that bat by the "thud" that it makes when you hit a ball with it, instead of the usual "ping". Just by the sound you know the bat is cracked. Essentially, this is the concept behind acoustic (sound) testing methods. Parts change their "sound"(this is sound of a much higher frequency than a human can perceive) when they develop cracks in them. It follows that such testing methods are dependant upon the sensitive generation and sensing of acoustic waves in materials. Acoustic waves are more properly called elas-todynamic waves and are of two types: longitudinal waves and shear waves.
This thesis examines the characteristics of a certain type of piezoelectric material, a piezoelectric, continuously twisted, structurally chiral medium (PCTSCM), studied principally for its elastodynamic wave filtering capabilities. These materials are have an intrinsic twist to their structure at the microscale and they thus have the ability to filter a special kind of elastodynamic wave-circularly polarized shear waves.
Near-perfect reflection of circularly polarized shear waves was observed when the waves were polarized with the same handedness of the twist of the PCTSCM, providing the waves were of a specific vibrational frequency. This is known in the literature as the circular Bragg phenomenon. It was deduced from calculations that the regime of high reflectance, i.e. the Bragg regime, moves toward higher frequencies while the intensity of the Bragg reflection decreases, as the angle of incidence increases. The reflection and transmission characteristics were compared to those of a similar, but not twisted, piezo-electric material to determine that the intrinsic twist, i.e. structural chirality, crucially affects the passage of shear waves, but not of longitudinal waves.
The passage of elastodynamic waves through two identical PCTSCMs sandwiched together with a 90 degree twist between them were also simulated. This twist allows waves of the same handedness as the PCTSCMs and of a very specific frequency to be transmitted through the Bragg regime, rather than reflected. It was also deduced that by choosing two different PCTSCMs, i.e. two different materials with a specific relation between their properties, the same effect can be produced without a twist. These conclusions are expected to be significant for the development of novel piezoelectric transducers.
Advisor: J. Xu
Committee:
Efficient Infrared Light Harvest In Nanocrystal-Polymer Hybrid Photovoltaic Devices
With the development of synthesis technology in semiconductor nanocrystal quantum dots (NQDs), NQDs have attracted tremendous interest in the last few years due to their excellent optical and electronic properties. NQDs can be incorporated into conjugated-polymers to create hybrid photovoltaic cells featured for efficient light harvesting in the visible and infrared regions of the solar spectrum.
This thesis reviews the current development of hybrid NQD/polymer photovoltaic devices and describes our efforts to investigate the possibilities of developing hybrid solar cells with absorption in the infrared spectral region as well as their unique photoelectric characterizations. A particular noteworthy example consists of a blend of lead selenium (PbSe) nanostructures and poly-3(hexylthiophene) (P3HT) which exhibits high solar conversion efficiency in the infrared portion of the solar spectrum. The photoelectric properties of the PbSe/P3HT photovoltaic device were measured under different conditions. Meanwhile, the NQD/polymer photovoltaic structures were tailored by varying the NQDs loadings and the polymer concentrations in the solutions prior to the deposition, and the resultant quantum efficiencies of the composite films were tested and compared in order to optimize the structure of the hybrid photovoltaic device.
Advisor: P. M. Lenahan
Committee: S. Ashok, J. Ruzyllo, J. Todd
Identification of Deep Level Defects in 4H-Silicon Carbide Metal-Oxide Semiconductor Field-Effect Transistors with Thermally Grown Insulators
Silicon carbide (SiC) is an extremely promising semiconductor which has been investigated and improved for decades. Interest in SiC stems from superior material properties which allow SiC-based electronics to function well in high-temperature, high-power and high-frequency applications, where current silicon-based devices cannot.
Perhaps the most widespread application would be in-situ monitoring and controlling of fuel combustion. The ideal locations for sensors which monitor these functions are generally too hot for current electronic devices. SiC offers the possibility of introducing sensors into such areas, including car engines and jet turbines as well as a slew of automotive, aerospace and military applications. In addition to high-temperature, several high-power applications would benefit our daily lives; these include high-voltage switching in public electric power distribution, electric/hybrid electric vehicles, and high-power lighting. Combining high-power and high-frequency capabilities creates the possibility of more powerful microwave electronics for radar and communication, such as the base stations for commercial cell phones and the upcoming 802.16 broadband wireless networking standard.
These applications are very possible with SiC, but because it is a newly developing semiconductor, many aspects of the material are not well understood. Consequently current SiC devices are far from optimized and suffer from unknown defects which limit their electrical performance (i.e. the electronics are not very fast/reliable/efficient).
This study uses very sensitive magnetic resonance techniques on SiC prototype transistors in order to examine atomic-scale defects in the semiconducting crystal. Several generations of devices are examined with the goal of achieving a better understanding of these defects. With a better understanding, it should become possible to greatly reduce or even eliminate these defects, thus unlocking the vast potential of SiC-based microelectronic devices.
Advisor: M. T. Lanagan
Committee: M. Horn, P Lenahan, V. D. Heydemann, J. Todd
Microwave Characterization of Oxide Thin Films
Dielectric thin films are important for transmission lines and components in diverse applications such as cellular phones, computers, broadband data transmission, and automotive guidance. Dielectric properties of thin films must be quantified in the GHz frequency range in order to provide accurate data for electromagnetic simulation and device design.
The purpose of this research effort was to provide an accurate method of obtaining the dielectric properties of thin films in the microwave frequency range, in specific titanium dioxide (TiO2) thin films. The techniques explored throughout this research were a split cavity resonant frequency technique, a coplanar waveguide (CPW) transmission line method, a parallel plate capacitor and short standard device, and interdigital capacitors (IDC). The characterization techniques in the microwave frequency range once established with the TiO2 thin film should be a useful tool in characterizing more complex thin film materials such as semiconductors, magnetic materials, and other exotic thin films.
The split cavity technique is a reliable method to obtain dielectric properties of substrates but this technique fails to provide accurate results for thin film materials with properties below predefined film thickness limits. With the geometric and dielectric constant limits of this technique understood, this technique can be used to compare to the other techniques for thin films with a certain thickness threshold values for thin film dimensions can be established. The split cavity measurements indicate that the loss of a 300 nm thick TiO2 thin film is only 0.68% in the microwave frequency range and that the film has a relative permittivity of 62.9.
The CPW technique is not a recommended technique for low microwave frequencies to due the errors associated with the results caused by stray inductances and capacitances. The errors arise due to the metallization issues in the microwave frequency range. These metallization issues are due to skin depth effects present that produce internal inductances that theoretical models presently do not account for.
The research indicated that the parallel plate capacitor and short standards device is the most promising microwave characterization technique. The measurements from the capacitor and short standards for a 300 nm thick TiO2 thin film indicate that the average permittivity value is 67.2 in the frequency range of 1 to 3 GHz. The permittivity value has a percent difference of 6.79% when compared to the permittivity measured with the split cavity of 62.9. For a 100 nm thick TiO2 thin film, the relative permittivity was found to be 65.3 with the capacitor and short standards method. The Q of the capacitor device ranged from 27 to 4 over the 1 to 3 GHz frequency range.
Advisor: M. W. Horn
Committee: A. Lakhtakia, J. Xu, B. Shaw, J. Todd
Fabrication And Characterization of TiO2 Sculptured Thin Films Exhibiting The Circular Bragg Phenomenon
Everyday in the newspapers and other media outlets, one can find headlines regarding the newest nanotechnology: nano-things, nano-mabobs, or nano-stuff. Many nanotechnology companies sell themselves to investors by promising huge gains from a technology that has the potential to be bigger than silicon. However, the majority of these companies only reference initial work on topics semi-related to their breakthrough nanotechnology. For this reason, investing in nanotechnology has been the talk of Wall Street for some time. Sorting through all of the nanotechnology business ideas to find the true nanoscience can be very difficult. That is why it was a breath of fresh air when news regarding Sculptured Thin Films, or STFs as they are more commonly known, was made available through the recent Nanotech 50 awards.
STF technology is essentially a blending of material science with physics and engineering. Material, in the form of thin films, is actually engineered on the nanoscale. Building anything nanoscale is very difficult; however, researchers have an enormous amount of numerical data as well as pictures to prove this point. The films are produced through a deposition method that uses a modified evaporation or sputtering system similar to the kinds that are used to deposit chrome on the plastic hood ornaments of most cars. Simply by correctly positioning and rotating the film during deposition, a seemingly infinite variety of nanoscale structures can be built into free-standing nanowires and microcolumns within the films. These nano-engineered structures are responsible for novel properties that make many new applications feasible.
The first example of this nanotechnology was discovered in 1959. Researchers deposited a chiral STF and observed its optical rotation properties. These properties were representative of a chiral STF with correct structure. However, that development became obscure, and was rediscovered in 1998 after the formulation and enunciation of the STF concept at Penn State in 1995. STF technology was born again through theoretical predictions. Soon following was experimental verification. This pattern has continued through the present time.
STFs have many properties that are attractive to current engineers and scientists. The films have an enormous surface area due to the unique nanoscale structure. This property makes the films useful for thermal barriers, microsieves, and biological applications. Research has been done in academia to realize these applications. Because of the film’s nanostructure, they interact selectively with light. The films can be engineered for a large number of optical applications where the selective reflection or filtering of light is important. This property is useful for sensing applications where slight changes in the film brought about by a chemical or biological agent can be detected with the optical response of the films. While many proof-of-concept experiments have been done to verify these properties, a STF device has not yet been commercialized. A large amount of successful theory and experiment has been published regarding STFs and a marketable device is not far away.
For societal benefits of STFs to be realized, much research needs to be done to correlate all of the experimental data on different materials and fabrication processes with well formulated theory. The purpose of this research was to fabricate TiO2 (known as titania) chiral STFs to be used as optical sensors or filters and take initial steps in identifying important experimental parameters needed for these applications. Emphasis was placed on those applications, such as sensors, which make use of the special optical properties of chial STFs. High quality TiO2 STFs were fabricated and characterized. Within this thesis, the morphology and optical property changes resulting from variations in deposition parameters are presented. It was determined that various methods can be used to produce the desired optical response characteristics. These methods would be useful as STFs become commercialized. As devices were fabricated experimental parameters such as substrate roughness were identified and studied. Three devices were fabricated using the newly gained deposition knowledge; a dye sensitized solar cell, resonator cavity, and spectral-hole filter. These initial devices were successful and each have spawned new lines of research. Post-deposition annealing and etching were studied and shown to significantly affect the optical properties of chiral STFs. These techniques can be used to tune the optical response of chiral STFs for any application. The results also showed that chemical etching is a suitable effect to be measured in an optical sensor.
Advisor: P. M. Lenahan
Committee: S. Ashok, J. Ruzyllo, J. Todd
Electron Spin Resonance Observations of Defect Centers in The Near Silicon/Dielectric Interfacial Layer of Hafnium Oxide Based Metal-Oxide-Silicon Structures
Integrated circuits are the lifeblood of our modern society. Since their conception in the 1950's their performance has increased exponentially. Most high performance integrated circuits (computer microprocessors for example) are constructed of metal-oxide-silicon-field-effect-transistors (MOSFETs). Performance increases arise from being able to make the MOSFETs smaller and to pack more of them into a given area, known as scaling. Current state-of-the-art microprocessors can easily contain 100 million transistors fabricated on a piece of silicon just a few square centimeters in area.
The heart and soul of a MOSFET is the gate dielectric. The gate is designed to apply a voltage to the underlying silicon which essentially switches the transistor on and off. The gate dielectric electrically insulates the gate so no current will flow through it. For the last 40+ years the gate dielectric material has been silicon dioxide. Today, the gate dielectrics have been scaled down to their fundamental physical limits. Being only a few atoms thick, the silicon dioxide gate dielectric can no longer effectively stop current flow through the gate.
The only solution to this problem is to replace the silicon dioxide with a new dielectric material. This would allow for a physically thicker gate dielectric without sacrificing transistor performance. Unfortunately, replacing the silicon dioxide gate dielectric is no easy task. It may be the most challenging problem the microelectronics community ever undertook. Silicon dioxide grown on silicon is the perfect match, it posses a very low number of defects and is quite reliable. All the potential replacements are not nearly as good as the silicon/silicon dioxide system. They tend to form an unacceptable number of defects which limit device performance and reliability.
This thesis utilizes a magnetic resonance technique (electron spin resonance) to examine the physical and chemical nature of these performance and reliability limiting defects in the leading replacement candidate; hafnium oxide.
Advisor: M. C. Demirel
Committee: A. Lakhtakia, J. Bond, J. Todd
Paracyclophane Chiral Sculptured Thin Films as Substrates For The Growth of HEK-293 Human Kidney Cells
Cells are the building blocks of all living organisms forming tissues with specific functions. The surface upon which cells grow, termed the substrate and in the human body is known as the basement membrane, has a large effect on the behavior of the cells. A set of experiments was performed with the aim of mapping these behavioral modulations by observing cells grown on substrates engineered with different features and properties. The substrates used for cell growth were structured thin films (STFs) made of a polymer, which can be fabricated with micro- and nanoscale features, mimicking that of the basement membrane. Careful engineering of STFs can possibly lead to the manipulation of the growth and development of cellular tissues.
Advisor: J. L. Rose
Committee: B. R. Tittmann, C. J. Lissenden, J. Todd
In situ Analysis of Viscoelastic Coatings on Hollow Cylinders for Enhanced Guided-Wave Inspection
Millions of people in the United States and other countries depend upon complex albeit virtually unseen pipeline infrastructures to provide, in a reliable fashion, the energy needed to maintain the modern lifestyle. Through an intricate network of subterranean piping, said infrastructures transport and distribute the fuel needed to heat and cool homes, to cook hot meals, and to power appliances and vehicles. When a pipeline fails, lost revenue, environmental damage, and human injury and casualty may ensue. For this reason, significant efforts are put forth for the prevention, location, monitoring, and repair of material defects which may lead to catastrophic pipeline failure.
Corrosion is one of the most common degenerative material defects that occur in subterranean pipeline. A common practice for the prevention of pipeline corrosion is the application of a protective coating to the outside surface of the pipe. Protective pipeline coatings come in many forms with several of the most popular types being asphalt-based tapes and paints and polymer powder coatings. Protective coatings, though effective, are not perfect and corrosion areas can still form in the pipe wall. Therefore, in addition to prevention, it is necessary to be able to detect and monitor regions of corroding pipe.
Ultrasonic guided-wave inspection is an efficient method for the nondestructive testing of long sections of pipe for corrosion and other critical material defects. Ultrasonic waves are sound waves with frequencies exceeding that of the audible spectrum. When ultrasonic waves reflect from boundaries, such as pipe walls, they form interference patterns. If the interference is constructive in nature, propagating wave packets called "modes" are formed. By understanding the mechanics of the propagating wave modes, it is possible to predict defect location, type, and size.
The presence of a protective coating on the outer surface of a pipe can alter wave propagation characteristics by varying degrees. For some coating types, it is acceptable to ignore the presence of the coating and for other coating types it is not. Since it is uncertain how significantly any one coating will effect wave propagation, it would be considered best practice to always include the effects in calculations, not matter how miniscule. To do this, it is necessary to measure the material properties of the coating as it exists in its natural state on the pipe under study.
This work deals with the development of a device and protocol for the nondestructive measurement of coating material properties. It is demonstrated how the predicted properties can be used to enhance inspection routines by improving inspection distance. Tests are performed on seven differently coated pipe specimens. It is shown that the developed property measurement technique accurately predicts the ultrasonic damping induced by each of the seven coatings. Field test results are presented for the developed coating evaluation instrument and protocol.
An investigation of the effects of the coating/pipe interface condition on wave propagation is presented. In addition to a well bonded interface condition, a wetted and a soiled interface are experimentally simulated. It is shown that interface condition can be a contributing factor to wave attenuation.
By loading a pipe with ultrasonic energy in specific loading patterns, the ultrasonic energy can be made to constructively interfere within some concentrated area. This technique is referred to as guided-wave focusing and its benefits include increased inspection distances, increased sensitivity to defects, and decreased false-call rates. A study of the effects of coating presence on focusing ability in pipe is presented. It is shown that for several coating types and focal distances, it is acceptable to ignore the presence of the coating. Additionally, it is shown that for some coating conditions and focal distances it is unacceptable to assume bare pipe conditions and focusing routines must be modified to account for coating effects.
Advisors: S. M. Copley, E. S. Venstel
Committee: J. Todd
Mechanical Behavior and Optimization of Laser Fabricated Metallic Cellular Sandwich Panels
The maritime industry has shown a growing interest in the utilization of sandwich panels for watertight doors, platforms, and other structures due to weight savings and reduced maintenance costs. As a result of advances in laser technology, laser-welded metallic sandwich panels have been recognized as an economical, lightweight alternative to conventional structures. Corrugated core sandwich panels are recognized to embrace high strength and stiffness to weight ratios.
The panels being developed at the Applied Research Laboratory at the Pennsylvania State University integrate the application of a laser to engineer the closed cellular metallic sandwich panel. The objective of this investigation was to evaluate the potential of square cellular sandwich panels in watertight doors for the Navy. Cellular core sandwich panels consist of interlocking metallic stiffeners that are positioned between two thin face sheets. The stiffeners of the cellular sandwich panels are laser cut and then welded to the face sheets at high speeds by a carbon dioxide laser. This is accomplished without major distortion to the panels' form as well as providing beneficial prestressing to the panel.
In this investigation, a series of numerical calculations and experimental analyses have been conducted on such structures that prove it to be capable of withstanding the predetermined conditions of a Navy watertight door. During the course of the research, preliminary linear stress analyses were carried out on this innovative cellular structure. Mechanical testing of the panels combined with this analysis suggested the development of thermally induced residual stresses during welding of panels that are restrained to assure flatness. These stresses are beneficial because they increase resistance of the panel to buckling. Additional experiments utilizing strain gages confirmed that beneficial prestresses do exist as predicted. An optimization analysis was also performed on cellular structures to aid in its evaluation for future applications.
Advisor: A. Segall
Committee: I. Smid, J. Singh, T. Eden, J. Todd
Development of Ni-Based Self-Lubricating Composite Coatings For Ti-6Al-4V Dovetail Joints Using The Cold Spray Process
Dovetail joints are used to attach Titanium turbine blades to Titanium disks. Despite close tolerances, there is still relative motion of between the mating parts in the joint. Such a situation leads to a material degradation phenomenon known as fretting wear. Several types of surface treatments and coatings have been investigated in the past to protect the dovetail joints from the harmful effects of fretting wear. Generally, this is accomplished by increasing the wear resistance or lubricity of the contacting surfaces. However, the lubricants typically lose their ability to reduce friction at the high temperatures and pressures encountered in the turbine engine.
In the current research, a low temperature coating method known as Cold Spray has been modified for the development of coatings which have a slippery component imbedded into them. During cold spray, powder particles are accelerated to very high velocity so that they deform and bond to the substrate surfaces. Due to the low temperatures and absence of other chemical reactions, thermally vulnerable materials can be deposited without significant material degradation. For the alloy component, Nickel has been selected due to its high hardness and wear resistance. Molybdenum-disufide (MoS2) and Boron Nitride (BN) have been employed as the lubricants because of their excellent solid lubricating properties. The developed coatings were examined for the distribution of lubricant and presence of porosity using microscopy. Furthermore, coatings with the most homogeneous lubricant distributions were analyzed for hardness, bond-strength, and fretting wear. Based on the above analysis, it was concluded that cold spray process can be used for the development of Ni-BN composite coatings which possess better hardness and comparable bond strength relative to pure Ni coatings.
Advisor: B. R. Tittmann
Committee: I. Smid, A. E. Segall, J. Todd
Acoustic Waveguided Transmission Matching to Impedance Discontinuities: Simulations and Experiments
During the recent years ultrasonic waves have been used more and more widely. We can use ultrasonic waves not only in Nondestructive Evaluation (NDE) of flaws but also in many other applications, such as remotely evaluating material properties and changing parameters, such as the temperature in harsh environments. With the aid of Waveguides ultrasonic waves can travel over long distances and still keep their characteristic properties such as frequency and wave structures. But if the application requires several connected waveguides reflection can occur several times. Sometimes special materials such as Remendur are required, for example in the use of magnetostriction for transmitting and receiving ultrasonic signals. Then the connections between different samples must be considered. There applications lead to the important problem of how to connect the wave guides for example wires or rods in order to get efficient transmission, especially if the waveguides have different elastic properties and physical dimensions?
This thesis treats the problem of designing efficient connections between waveguides by wave-mechanical analysis and finite element methods. One approach studied involves the use connecting sleeves to provide not only good transmission but also a sturdy mechanical connection. By using computer simulation it was possible to rapidly change the thickness of the sleeve and the length of the sleeve to get different energy pass ratio. Half-wave-length canceling methods were developed to get more wave energy to pass through the connection. In order to study the connection between wires or rods and plates two kinds of connections were simulated: a trapezoidal prism connection and a cylindrical connection. Elastic wave analysis and finite element method were used together to compare these two approaches. In the prism connection situation the length of the prism was changed to optimize the energy pass ratios. In the cylindrical connection situation the thickness and length of the cylinder were changed to get different energy pass ratios. Quarter-wave length canceling approaches were developed to get more wave energy to pass. The pros and cons of several different connection were been presented in this thesis. Finite element methods were found to be useful to successfully model the guided wave propagation in the impedance discontinuity media. Finite element software can be used to solve the wave propagation problems. At last several experiments was employed to compare with the simulation results.
Advisors: A. Lakhtakia
Committee: M. Horn, J. Xu, J. Todd
Theoretical Study of a One-Dimensional Dual-Periodicity Superlattice as a Diffraction Grating
A diffraction grating is a device used to redirect optical energy, and is thus a common component in many modern devices. A diffraction grating is capable of reflecting and transmitting different intensities of light at many scattering angles, depending on the wavelength of the illuminating light in relation to the geometric and constitutive properties of the grating. Gratings have periodic morphologies.
In this thesis, the chosen grating is a superlattice with two different periodicities, but only one overall period. This type of grating has never been examined either experimentally or theoretically. The objective of this research is to theoretically identify unique and novel optical characteristics.
Each unit cell of the chosen superlattice has two regions: a region occupied by a homogeneous dielectric material and a region occupied by a material that has sinusoidally varying dielectric properties. A rigorous method, the rigorous coupled-wave approach, is used to computationally solve for the reflection and transmission efficiencies of the chosen superlattice as functions of the wavelength of the incident light.
Two parametric studies are undertaken. The first study is performed to examine the effects of a wide variety of physical parameters on the diffraction efficiencies. This study reveals the existence of anomalous features. The second study is performed to further identify the characteristics and creation mechanisms underlying these anomalous characteristics. Both studies assist in identifiyng trends to predict the occurence of anomalous features with respect to various physical parameters. These identified trends are expected to be useful in designing gratings with complex performance characteristics.
This page was last updated on October 22, 2007.