VTechWorks Repository :: Browsing by Author "Heflin, James R."

Browsing by Author "Heflin, James R."

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Analysis of optical response of long period fiber gratings to nm-thick thin-film coatings
Wang, Zhiyong; Heflin, James R.; Stolen, Rogers H.; Ramachandran, Siddharth (Optical Society of America, 2005-04-01)
We have theoretically and experimentally demonstrated that the resonant wavelength of long period fiber gratings (LPG) can be shifted by a large magnitude by coating with only a nm-thickthin-film that has a refractive index higher than that of the glass cladding. The resonant wavelength shift can result from either the variation of the thickness of the film and/or the variation of its refractive index. These results demonstrate the sensitivity of LPG-based sensors can be enhanced by using a film of nm-thickness and refractive index greater than silica. This coating schematic offers an efficient platform for achieving high-performance index-modulating fiber devices and high-performance index/thickness-sensing LPG-based fiber sensors for detecting optical property variations of the thin-film coating. (C) 2005 Optical Society of America.
The Application of a Nanomaterial Optical Fiber Biosensor Assay for Identification of Brucella Nomenspecies
McCutcheon, Kelly; Bandara, Aloka B.; Zuo, Ziwei; Heflin, James R.; Inzana, Thomas J. (MDPI, 2019-05-21)
Bacteria in the genus Brucella are the cause of brucellosis in humans and many domestic and wild animals. A rapid and culture-free detection assay to detect Brucella in clinical samples would be highly valuable. Nanomaterial optical fiber biosensors (NOFS) are capable of recognizing DNA hybridization events or other analyte interactions with high specificity and sensitivity. Therefore, a NOFS assay was developed to detect Brucella DNA from cultures and in tissue samples from infected mice. An ionic self-assembled multilayer (ISAM) film was coupled to a long-period grating optical fiber, and a nucleotide probe complementary to the Brucella IS711 region and modified with biotin was bound to the ISAM by covalent conjugation. When the ISAM/probe duplex was exposed to lysate containing ≥100 killed cells of Brucella, or liver or spleen tissue extracts from Brucella-infected mice, substantial attenuation of light transmission occurred, whereas exposure of the complexed fiber to non-Brucella gram-negative bacteria or control tissue samples resulted in negligible attenuation of light transmission. Oligonucleotide probes specific for B. abortus, B. melitensis, and B. suis could also be used to detect and differentiate these three nomenspecies. In summary, the NOFS biosensor assay detected three nomenspecies of Brucella without the use of polymerase chain reaction within 30 min and could specifically detect low numbers of this bacterium in clinical samples.
The Application of Thin Film Ionic Self-assembled Multilayer (ISAM) Nanostructures in Electromechanical Bending Actuators and Micro-fabricated Gas Chromatography (uGC) Devices
Wang, Dong (Virginia Tech, 2015-01-14)
Ionic self-assembled multilayer (ISAM) thin film nanostructures, including highly porous and conductive gold nanoparticles (GNP), and highly porous and thermally stable silica nanoparticles (SNP), were fabricated via the layer-by-layer (LbL) self-assembly technique. Their application in ionic polymer-metal composite (IPMC) electromechanical bending actuators and microfabricated gas chromatography (microGC) devices were investigated and significant performance improvements of these devices were achieved. IPMC bending actuators, consisting of an ionic electroactive polymer (iEAP) membrane as backbone, ionic liquids (IL) as electrolyte, and ISAM GNP thin film as porous electrode, were fabricated and investigated. The influences of humidity, conductive network composite (CNC), and IL uptake on the bending performance were examined and discussed. An equivalent circuit model to simulate both the electrical and mechanical responses was also proposed and experimentally verified. Moreover, IPMC actuators made from other newly synthesized iEAP membranes were fabricated and tested. Some of them showed promising performance that was comparable or even better as compared to the ones made from Nafion. LbL fabricated ISAM SNPs thin film coatings were also applied in the microGC devices including micro fabricated thermal preconcentrators (microTPC) and separation columns (microSC) as adsorbent and stationary phase materials, respectively. New fabrication approaches were developed to selectively coat uniform conformal ISAM SNP coatings in these devices with different 3D microstructures. Thus, functionalized microTPCs and microSCs showed good performance, which can be further improved by using the ISAM SNPs coating as a nanotemplate for modifying additional polymer adsorbents or as the anchor sites for incorporating functional molecules for targeting detection.
Applications of Layer-by-Layer Films in Electrochromic Devices and Bending Actuators
Jain, Vaibhav (Virginia Tech, 2009-09-02)
This thesis presents work done to improve the switching speed and contrast performance of electrochromic devices. Layer-by-Layer (LbL) assembly was used to deposit thin electrochromic films of materials ranging from organic, inorganic, conducting polymers, etc. The focus was on developing new materials with high contrast and long lifecycles. A detailed switching-speed study of solid-state EC devices of already-developed (PEDOT (Poly(3,4-ethylenedioxythiophene)), polyviologen, inorganic) materials and some new materials (Prodot-Sultone) was performed. Work was done to achieve the optimum thickness and number of bilayers in LbL films resulting in high-contrast and fast switching. Device sizes were varied for comparison of the performance of the lab-made prototype device with the commercially available "small pixel" size displays. Symmetrical EC devices were fabricated and tested whenever conducting polymers are used as an EC material. This symmetrical configuration utilizes conducting polymers as an electroactive layer on each of two ITO-coated substrates; potential is applied to the two layers of similar conducting polymers and the device changes color from one redox state to another. This method, along with LbL film assembly, are the main factors in the improvement of switching speed results over already-published work in the literature. PEDOT results show that EC devices fabricated by LbL assembly with a switching speed of less than 30 ms make EC flat-panel displays possible by adjusting film thickness, device size, and type of material. The high contrast value (84%) for RuP suggests that its LbL films can be used for low-power consumption displays where contrast, not fastest switching, is the prime importance. In addition to the electrochromic work, this thesis also includes a section on the application of LbL assembly in fabricating electromechanical bending actuators. For bending actuators based on ionic polymer metal composites (IPMCs), a new class of conductive composite network (CNC) electrode was investigated, based on LbL self-assembled multilayers of conductive gold (Au) nanoparticles. The CNC of an electromechanical actuator fabricated with 100 bilayers of polyallylamine hydrochloride (PAH)/Au NPs exhibits high strain value of 6.8% with an actuation speed of 0.18 seconds for a 26 µm thick IPMC with 0.4 µm thick LbL CNCs under 4 volts.
Characterization of Organic and Inorganic Optoelectronic Semiconductor Devices Using Advanced Spectroscopic Methods
Schroeder, Raoul (Virginia Tech, 2001-12-14)
In this thesis, advanced spectroscopy methods are discussed and applied to gain understanding of the physical properties of organic conjugated molecules, II-VI thin film semiconductors, and vertical cavity surface emitting lasers (VCSEL). Experiments include single photon and two-photon excitation with lasers, with subsequent measurements of the absorption and photoluminescence, as well as photocurrent measurements using tungsten and xenon lamps, measuring the direct current and the alternating current of the devices. The materials are investigated in dissolved form (conjugated polymers), thin films (polymers, II-VI semiconductors), and complex layer structures (hybrid device, VCSEL). The experiments are analyzed and interpreted by newly developed or applied theories for two-photon saturation processes in semiconductors, bandgap shrinkage due to optically induced electron hole pairs, and the principle of detailed balance to describe the photoluminescence in thin film cadmium sulfide.
Construction of an Optical Quarter-Wave Stack Using the ISAM (Ionic Self-Assembled Multilayers) Technique
Papavasiliou, Kriton (Virginia Tech, 2010-07-09)
The purpose of this thesis is to make a broadband antireflection coating configuration known as a quarter-wave stack consisting of one layer of titania and of one layer of silica nanoparticles. We utilize much that is already known about silica nanoparticle deposition. The first objective of this thesis is deposition and characterization of titania nanoparticle films deposited on glass microscope slides by a technique known as Ionic Self-Assembled Multilayers or ISAM deposition. This technique takes advantage of the electrostatic attraction between oppositely charged materials and ideally results in a uniform nanoparticle film whose thickness and optical properties can be tightly controlled. Deposition of a quarter-wave stack based on ISAM deposition of silica and titania nanoparticles is significantly simpler and less expensive than alternative deposition methods. Initial attempts to deposit titania films were unsuccessful because of excess diffuse scattering due to inhomogeneities in the film. In order to reduce diffuse scattering, two approaches were considered. The first approach was to improve the deposition process itself by experimenting with different values of deposition parameters such as solution pH and solution molarity. The other approach focused on removing the large nanoparticle aggregates from the colloidal solutions of titania nanoparticles that were suspected to be responsible for rough film surfaces resulting in diffuse scattering. This approach was successful. In addition, evidence suggested that surface roughness contributed more to diffuse scattering than the bulk of the films. After minimizing diffuse scattering from titania nanoparticle films, we used known results from research on silica nanoparticle films to deposit quarter-wave stacks consisting of one layer of titania nanoparticles with high refractive index and one layer of silica nanoparticles with low refractive index. This contrast in refractive indices is a desirable characteristic of quarter-wave stacks. The thicknesses and refractive indices of the two layers in the quarter-wave stacks were measured by ellipsometry and compared to the nominal thicknesses of these layers. Finally, the reflectance was derived from a model of the quarter-wave stack and was compared to the measured reflectance. It was found that construction of a quarter-wave stack by ISAM is possible but that it will be necessary to acquire data from more experiments.
Cooperative Behavior in Driven Lattice Systems with Shifted Periodic Boundary Conditions
Anderson, Mark Jule Jr. (Virginia Tech, 1998-04-17)
We explore the nature of driven stochastic lattice systems with non-periodic boundary conditions. The systems consist of particle and holes which move by exchanges of nearest neighbor particle-hole pairs. These exchanges are controlled by the energetics associated with an internal Hamiltonian, an external drive and a stochastic coupling to a heat reservoir. The effect of the drive is to bias particle-hole exchanges along the field in such a way that a particle current can be established. Hard-core volume constraints limit the occupation of only one particle (hole) per lattice site. For certain regimes of the overall particle density and temperature, a system displays a homogeneous disordered phase. We investigate cooperative behavior in this phase by using two-point spatial correlation functions and structure factors. By varying the particle density and the temperature, the system orders into a phase separated state, consisting of particle-rich and particle-poor regions. The temperature and density for the co-existence state depend on the boundary conditions. By using Monte Carlo simulations, we establish co-existence curves for systems with shifted periodic boundary conditions.
Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion
Drees, M.; Premaratne, K.; Graupner, W.; Heflin, James R.; Davis, Ruth M.; Marciu, D.; Miller, M. (AIP Publishing, 2002-12-01)
Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C-60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk, gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface. (C) 2002 American Institute of Physics.
Demonstration Of A Cylindrically Symmetric Second-Order Nonlinear Fiber with Self-Assembled Organic Surface Layers
Daengngam, Chalongrat; Hofmann, M.; Liu, Z. W.; Wang, Anbo; Heflin, James R.; Xu, Yong (Optical Society of America, 2011-05-01)
We report the fabrication and characterization of a cylindrically symmetric fiber structure that possesses significant and thermodynamically stable second-order nonlinearity. Such fiber structure is produced through nanoscale self-assembly of nonlinear molecules on a silica fiber taper and possesses full rotational symmetry. Despite its highly symmetric configuration, we observed significant second harmonic generation (SHG) and obtained good agreement between experimental results and theoretical predictions. (C) 2011 Optical Society of America
Design and Characterization of Electrospun Mats with Tailored Morphologies for Enhanced Active Layer Performance in Energy Conversion and Energy Storage Applications
Forbey, Scott (Virginia Tech, 2014-05-15)
The goal of this research was to utilize the morphological control inherently imparted by the electrospinning process to improve the active layer performance in energy conversion devices as well as to better understand the relationship between morphology and performance in energy storage devices. Discrete control of the active layer morphology can promote exciton dissociation in organic photovoltaic cells (OPVs), whereas developing efficient ion diffusion pathways and beneficial polymer-ion interaction in polymer-gel electrolytes is demonstrated to result in enhanced battery performance. We demonstrate the ability to develop unique morphologies in Poly(3-hexafluoro propylene) (P3HT) films with energy storage applications using various electrospinning techniques. Electrospinning in a solvent-saturated atmosphere allows for the design of ribbon architectures with polymer domains on the order of 5-10 um. These ribbon structures form what appear to be bi-continuous films, which could then be filled with an acceptor / fullerene type material to create a bulk heterojucton for OPV devices. Dropping chloroform onto the electrospinning needle during the spinning process results in P3HT fibers with porous surfaces. These fibers have diameters of ~ 2 um. Using a coaxial needle to electrospin a P3HT solution in the core, and a CHCl3 sheath solution created hybrid ribbon-fiber structures. These structures have even smaller domain sizes than the ribbons created using a solvent saturated atmosphere. Cospinning P3HT with sacrificial polymers results in P3HT fiber morphologies upon removal of the sacrificial template polymer. Additionally, introducing P3HT into an established fiber matrix results in fibrous P3HT architectures after the template fibers are removed. Developing hybrid polymer-gel electrolytes using crosslinked PEO electrospun fibers results in membranes with high affinity for liquid electrolyte components. These electrospun PEO fiber mats exhibit excellent ionic conductivities at room temperature (12 mS/cm) exceeding an electrospun PVDF control. Furthermore, the PEO fiber mats can absorb nearly three times as much liquid electrolyte as the PVDF control. PEO has been show to interact with lithium salts to aid in dissociation and diffusion during battery cycling. Although the ionic conductivity data suggest PEO to be a superior electrolyte, pulsed-field-gradient NMR shows that lithium diffusion is faster in PVDF samples. From coin cell discharge experiments, PEO is believed to interact strongly with Li+ ions, inhibiting them from diffusing rapidly during fast charge/discharge rates. However, PEO/PETA fiber electrolytes show nearly 100% theoretical capacity discharge at C/100 and a capacity retention of ~ 35% at a C/5 discharge rate in contrast to a glass fiber separator which shows only a capacity that is approximately 85% of the theoretical value. The unique mechanical properties of PEO/PETA electrospun mats could lead to interesting artificial skin and wound healing applications. Upon crosslinking at elevated temperatures (~40 degrees C), the fiber mats exhibit improved tensile strength and much higher ultimate stress at break. The porous nature of the materials lend to easy oxygen diffusion for wound healing, and the hydrophilicity promotes continued adhesion to existing tissue making these mats possible adhesive-less bandages.
Development of "Core-Suction" Technique for Fabrication of Highly Doped Fibers for Optical Amplification and Characterization of Optical Fibers for Raman Amplification
Goel, Nitin Kumar (Virginia Tech, 2005-10-11)
This thesis presents a novel technique named "Core Suction" for fabricating optical fiber preforms for manufacturing highly doped fibers (HDFs) for optical amplification (Raman effect based or Erbium fiber based). The technique involves drawing the molten non-conventional core glass material into the silica cladding tube to form the preform. The developed technique is simple, inexpensive and shows great potential for fabricating preforms of highly nonlinear non-conventional multi-component glasses as the core material. Preforms were made with various core glasses such as Schott SF6, Lead-Tellurium-Germanate, Lead-Tellurium-Germanate- Neodymium -Erbium and MM2 in silica cladding tubes and then pulled into fibers. The fabricated fibers were measured for refractive index profile, loss spectrum and spontaneous Raman spectra. Elemental analysis of the fiber samples was also performed using an electron microprobe. Erbium doped fiber amplifiers (EDFAs) were setup using 30 cm, 5cm and 1 cm lengths of fabricated erbium doped fibers and their gain spectra measured. The distributed gain spectrum for an EDFA was also measured using an optical frequency domain reflectometery (OFDR) technique. Commercial dispersion compensated fiber (DCF) with very high GeO2 doping was used to setup a Raman amplifier and the gain spectrum measured. One of the needs of Raman amplification in optical fibers is to predict an accurate Raman gain, based on the fiber's refractive index profile. A method of predicting Raman gain in GeO2 doped fibers is presented and the predicted Raman gain values are compared with the measured ones in the same fibers. Raman gain issues like the dependence of the Raman gain on the GeO2 concentration, polarization dependence were taken into account for the gain calculations. An experimental setup for Raman gain measurements was made and measurement issues addressed. Polarization dependence of the Raman gain in one kilometer of polarization maintaining fiber was also measured.
Development of an Optical Fiber Biosensor with Nanoscale Self-Assembled Affinity Layer
Zuo, Ziwei (Virginia Tech, 2014-01-29)
Optical sensor systems that integrate Long-Period-Gratings (LPG) as the detection arm have been proven to be highly sensitive and reliable in many applications. With increasing public recognition of threats from bacteria-induced diseases and their potential outbreak among densely populated communities, an intrinsic, low-cost biosensor device that can perform quick and precise identification of the infection type is in high demand to respond to such challenging situations and control the damage those diseases could possibly cause. This dissertation describes the development of a biosensor platform that utilizes polymer thin films, known as ionic self-assembled multilayer (ISAM) films, to be the sensitivity- enhancing medium between an LPG fiber and specific, recognition layer. With the aid of cross- linking reactions, monoclonal antibodies (IgG) or DNA probes are immobilized onto the surface of the ISAM-coated fiber, which form the core component of the biosensor. By immersing such biosensor fiber into a sample suspension, the immobilized antibody molecules will bind the specific antigen and capture the target cells or cell fragments onto the surface of the fiber sensor, resulting in increasing the average thickness of the fiber cladding and changing the refractive index of the cladding. This change occurring at the surface of the fiber results in a decrease of optical power emerging from the LPG section of the fiber. By comparing the transmitted optical power before and after applying the sample suspension, we are able to determine whether or not certain bacterial species have attached to the surface of the fiber, and as a consequence, we are able to determine whether or not the solution contains the targeted bacteria. This platform has the potential for detection of a wide range of bacteria types. In our study, we have primarily investigated the sensitivity and specificity of the biosensor to methicillin- resistant Staphlococcus aureus (MRSA). The data we obtained have shown a sensitive threshold at as low as 102 cfu/ml with pure culture samples. A typical MRSA antibody-based biosensor assay with MRSA sample at this concentration has shown optical power reduction of 21.78%. In a detailed study involving twenty-six bacterial strains possessing the PBP2a protein that enables antibiotic resistance and sixteen strains that do not, the biosensor system was able to correctly identify every sample in pure culture samples at concentration of 104 cfu/ml. Further studies have also been conducted on infected mouse tissues and clinical swab samples from human ears, noses, and skin, and in each case, the system was in full agreement with the results of standard culture tests. However, the system is not yet able to correctly distinguish MRSA and non-MRSA infections in clinical swab samples taken from infected patient wounds. It is proposed that nonspecific binding due to insufficient blocking methods is the key issue. Other bacterial strains, such as Brucella and Francisella tularensis have also been studied using a similar biosensor platform with DNA probes and antibodies, respectively, and the outcomes are also promising. The Brucella DNA biosensor is able to reflect the existence of 3 Brucella strains at 100 cfu/ml with an average of 12.2% signal reduction, while negative control samples at 106cfu/ml generate an average signal reduction of -2.1%. Similarly, the F. tularensis antibodies biosensor has shown a 25.6% signal reduction to LVS strain samples at 100 cfu/ml, while for negative control samples at the same concentration, it only produces a signal reduction of 0.05%. In general, this biosensor platform has demonstrated the potential of detecting a wide range of bacteria in a rapid and relatively inexpensive manner.
Development of Low-power Wireless Sensor Nodes based on Assembled Nanowire Devices
Narayanan, Arvind (Virginia Tech, 2000-07-24)
Networked wireless sensor systems have the potential to play a major role in critical applications including: environmental monitoring of chemical/biological attacks; condition-based maintenance of vehicles, ships and aircraft; real-time monitoring of civil infrastructure including roads, bridges etc.; security and surveillance for homeland defense systems; and battlefield surveillance and monitoring. Such wireless sensor networks can provide remote monitoring and control of operations of large-scale systems using low-power, low-cost, "throw-away" sensor nodes. This thesis focuses on two aspects of wireless sensor node development: (1) post-IC assembly of nanosensor devices onto prefabricated complementary-metal-oxide-semiconductor (CMOS) integrated circuits using a technique called dielectrophoretic (DEP) assembly; and (2) design of a low-power SiGe BiCMOS multi-band ultra-wideband (UWB) transmitter for wireless communications with other nodes and/or a central control unit in a wireless sensor network. For the first part of this work, a DEP assembly test chip was designed and fabricated using the five-metal core CMOS platform technology of Motorola's HiP6W low-voltage 0.18_m Si/SiGe BiCMOS process. The CMOS chip size was 2.5mm x 2.5 mm. The required AC signal for assembling nanowires is provided to the bottom electrodes defined in the Metal 4 (M4) layer of the IC process. This signal is then capacitively coupled to the top/assembly electrodes defined in the top metal (M5) layer that is also interconnected to appropriate readout circuitry. The placement and alignment of the nanowires on the top electrodes are defined by dielectrophoretic forces that act on the nanowires. For proof of concept purposes, metallic rhodium nanowires ((length = 5μm and diameter = 250 nm) were used in this thesis to demonstrate assembly onto the prefabricated CMOS chip. The rhodium nanowires were manufactured using a nanotemplated electroplating technique. In general, the DEP assembly technique can be used to manipulate a wider range of nanoscale devices (nanowire sensors, nanotubes, etc.), allowing their individual assembly onto prefabricated CMOS chips and can be extended to integrate diverse functionalized nanosensors with sensor readout, data conversion and data communication functionalities in a single-chip environment. In addition, this technique provides a highly-manufacturable platform for the development of multifunctional wireless sensor nodes based on assembled nano-sensor devices. The resistances of the assembled nanowires were measured to be on the order of 110 Ω consistent with prior prototype results. Several issues involved in achieving successful assembly of nanowires and good electrical continuity between the nanowires and metal layers of IC processes are addressed in this thesis. The importance of chemical/mechanical planarization (CMP) technique in modern IC processes and considerations for electrical isolation of readout circuit from the assembly sites are discussed. For the second part of this work, a multi-band hopping ultrawideband transmitter was designed to operate in three different frequency bands namely, 4.8 GHz, 6.4 GHz and 8.0 GHz. As a part of this effort, this thesis includes the design of a CMOS phase/frequency detector (PFD), a CMOS pseudo-random code generator and an on-chip passive loop filter, which were designed for the multi-band PLL frequency synthesizer. The CMOS PFD provided phase tracking over a range of -2π to +2π radians. The on-chip passive loop filter was designed for a 62_ phase margin, 250 μA-charge pump output current and 4 MHz-PLL loop-bandwidth. The CMOS pseudorandom code generator provided a two-bit output that helped switch the frequency bands of the UWB transmitter. With all these components, along with a BiCMOS VCO, a CMOS charge pump and a CMOS frequency divider, the simulated PLL frequency synthesizer locked within a relatively short time of 700ns in all three design frequency bands. The die area for the multi-band UWB transmitter as laid out was 1.5 mm x 1.0 mm. Future work proposed by this thesis includes sequential assembly of diverse functionalized gas/chemical nanosensor elements into arrays in order to realize highly sensitive "electronic noses". With integration of such diverse functionalized nano-scale sensors with low-power read-out and data communication system, a versatile and commercially viable low-power wireless sensor system can be realized.
Development of Random Hole Optical Fiber and Crucible Technique Optical Fibers
Kominsky, Daniel (Virginia Tech, 2005-09-06)
This dissertation reports the development of two new categories of optical fibers. These are the Random Hole Optical Fiber (RHOF) and the Crucible Technique Hybrid Fiber (CTF). The RHOF is a new class of microstructure fiber which possesses air holes which vary in diameter and location along the length of the fiber. Unlike all prior microstructure fibers, these RHOF do not have continuous air holes which extend throughout the fiber. The CTF is a method for incorporating glasses with vastly differing thermal properties into a single optical fiber. Each of these two classes of fiber brings a new set of optical characteristics into being. The RHOF exhibit many of the same guidance properties as the previously researched microstructure fibers, such as reduced mode counts in a large area core. CTF fibers show great promise for integrating core materials with extremely high levels of nonlinearity or gain. The initial goal of this work was to combine the two techniques to form a fiber with exceedingly high efficiency of nonlinear interactions. Numerous methods have been endeavored in the attempt to achieve the fabrication of the RHOF. Some of the methods include the use of sol-gel glass, microbubbles, various silica powders, and silica powders with the incorporation of gas producing agents. Through careful balancing of the competing forces of surface tension and internal pressure it has been possible to produce an optical fiber which guides light successfully. The optical loss of these fibers depends strongly on the geometrical arrangement of the air holes. Fibers with a higher number of smaller holes possess a markedly lower attenuation. RHOF also possess, to at least some degree the reduced mode number which has been extensively reported in the past for ordered hole fibers. Remarkably, the RHOF are also inherently pressure sensitive. When force is applied to an RHOF either isotropically, or on an axis perpendicular to the length of the fiber, a wavelength dependent loss is observed. This loss does not come with a corresponding response to temperature, rendering the RHOF highly anomalous in the area of fiber optic sensing techniques. Furthermore an ordered hole fiber was also tested to determine that this was not merely a hitherto undisclosed property of all microstructure fibers. Crucible technique fibers have also been fabricated by constructing an extremely thick walled silica tube, which is sealed at the bottom. A piece of the glass that is desired for the core (such as Lead Indium Phosphate) is inserted into the hole which is in the center of the tube. The preform is then drawn on an fiber draw tower, resulting in a fiber with a core consisting of a material which has a coefficient of thermal expansion (CTE) or a melting temperature (Tm) which is not commonly compatible with those of silica.
Digital spectral analysis and adaptive processing techniques for phase modulated optical fiber sensors
Tran, Tuan A. (Virginia Tech, 1996)
The objective of this work is to investigate new signal processing techniques for optical fiber sensors that utilize the phase information of the electromagnetic field. Research concentrated on Fourier transform spectroscopy as a means for capturing wavelength encoded information from the fiber sensor. Classical spectral analysis utilizing the Fourier transform as a mathematical foundation for relating a time or space signal to its frequency-domain representation was shown to be inadequate for mitigating the bias errors caused by harmonic distortions. A modified spectral estimation algorithm is presented to overcome some of the practical issues while maintaining the high spectral resolution characteristic of the classical technique. This research also showed that unlike in free-space propagation, an optical signal propagating through a fiber waveguide, even over short distances, can experience significant phase modulation noise. A number of chromatic distortion mechanisms including modal interference, mode coupling due to periodic perturbations such as microdeformation and macrobends, and mode field diameter variations are addressed. We treated these issues by employing both theoretical simulation and experimental data. Coupled-mode formalism based upon approximated field solutions is used in the theoretical analysis. An extensive error analysis was also performed to determine how waveguide and noise distortion affect the performance of the spectral estimation algorithm.
Digital to Analog Converter Design using Single Electron Transistors
Perry, Jonathan (Virginia Tech, 2005-04-29)
CMOS Technology has advanced for decades under the rule of Moore's law. But all good things must come to an end. Researchers estimate that CMOS will reach a lower limit on feature size within the next 10 to 15 years. In order to assure further progress in the field, new computing architectures must be investigated. These nanoscale architectures are many and varied. It remains to be seen if any will become a legitimate successor to CMOS. Single electron tunneling is a process by which electrons can be trans- ported (tunnel) across a thin insulating surface. A conducting island sepa rated by a pair of quantum tunnel junctions creates a Single Electron Transistor (SET). SETs exhibit higher functionality than traditional MOSFETs, and function best at very small feature sizes, in the neighborhood of 1nm. Many circuits must be developed before SETs can be considered a viable contender to CMOS technology. One important circuit is the Digital to Analog Converter (DAC). DACs are present on many microprocessors and microcontrollers in use today and are necessary in many situations. While other SET circuits have been proposed, including ADCs, no DAC design exists in open literature. We propose three possible SET DAC designs and characterize them with an HSPICE SET simulation model. The first design is a charge scaling architecture similar to what is frequently used in CMOS. The second two designs are based on a current steering architecture, but are unique in their implementation with SETs.
Dual Spin-Cast Thermally Interdiffused Polymeric Photovoltaic Devices
Kaur, Manpreet (Virginia Tech, 2011-08-04)
An in depth study of the performance of thermally interdiffused concentration gradient polymer photovoltaic devices is carried out with particular attention to the effect of the thickness and the thermal treatments on the power conversion efficiency, short circuit current, open circuit voltage and other key electrical properties. Bilayer films of sequentially spin-cast donor and acceptor materials are exposed to various heat treatments in order to induce the interdiffusion. The depth profiles show concentration gradients in the donor and acceptor as a result of interdiffusion and these devices show an order of magnitude increase in the device performance compared to the bilayer devices. Dual spin-cast poly (3-octylthiophene-2,5-diyl) (P3OT)- [6,6] phenyl C61 butyric acid methyl ester (PCBM) and poly (3-hexylthiophene-2,5-diyl) (P3HT)-PCBM interdiffused devices are studied in detail by varying the thickness of the donor and acceptor layers as well as the annealing conditions for initial polymer layer and the time and temperature of the interdiffusion process. Auger spectroscopy and X-ray photoelectron spectroscopy along with ion beam milling are used to investigate the concentration gradient formed as a result of the interdiffusion. The sulfur signal present in the P3OT and P3HT backbone is detected to identify the concentration profiles in the P3OT-PCBM and P3HT-PCBM devices. The interdiffusion conditions and thickness of the active layers have been optimized to obtain the highest power conversion efficiency. The best device performance of the P3OT-PCBM interdiffused devices is achieved when the interdiffusion is carried out at 150°C for 20 minutes and the P3OT thickness is maintained at 70 nm and the PCBM thickness at 40-50 nm. The highest efficiency achieved for P3OT-PCBM interdiffused devices is 1.0% under AM1.5G solar simulated spectrum. In order to further increase the efficiency, P3OT is replaced by (P3HT) which has higher hole mobility. P3HT- PCBM based concentration gradient devices show improved device performance over P3OT-PCBM devices. Power conversion efficiency of the order of ~3.0% is obtained for P3HT-PCBM interdiffused devices when the interdiffusion is carried out at 150°C for 20 minutes. For both P3OT:PCBM and P3HT:PCBM devices, the optimum performance occurs when the concentration gradient extends across the entire film and is correlated with an increase in the short circuit current density and fill factor as well as a decrease in the series resistance. The results demonstrate that an interdiffused bilayer fabrication approach is a novel and efficient approach for fabrication of polymer solar cell devices. In addition, porphyrin derivative 5, 10, 15, 20-Tetraphenyl-21H, 23H-porphine zinc (ZnTPP) is studied as a new donor material for organic solar cells. ZnTPP: PCBM blend devices are investigated in detail by varying the weight ratio of the donor and acceptor materials in blend devices. The devices with ZnTPP: PCBM in 1:9 ratios showed the best device performance and the efficiency of the order of 0.2% is achieved under AM1.5G solar simulated conditions. Trimetallic Nitride Tempelated (TNT) endohedral fullerenes are also examined in this thesis as the novel acceptor materials. Bulk heterojunction or blend devices are fabricated with P3HT as the donor material and several TNT endohedral fullerenes as the acceptor material. Y3N@C₈₀PCBH based devices which are annealed both before and after the electrode deposition show improvement in the device performance compared to devices that are only annealed before the electrode deposition. The highest power conversion efficiency achieved for TNT endohedral fullerene devices is only 0.06%, suggesting that substantial additional work must be done to optimize the compatibility of the donor and acceptor as well as the device fabrication parameters.
Effects of Electric Fields on Forces between Dielectric Particles in Air
Chiu, Ching-Wen (Virginia Tech, 2013-05-01)
We developed a quantitative measurement technique using atomic force microscopy (AFM) to study the effects of both DC and AC external electric fields on the forces between two dielectric microspheres. In this work we measured the DC and AC electric field-induced forces and adhesion force between two barium titanate (BaTiO?) glass microspheres in a low humidity environment by this technique. The objective here is to find out the correlation between these measured forces and applied field strength, frequency, and the separation distance between the two spheres was studied. Since the spheres would oscillate under an AC field, the AC field-induced force was divided into dynamic component (i.e., time-varying term) and static component (i.e., time-averaged term) to investigate. The oscillatory response occurs at a frequency that is twice the drive frequency since the field-induced force is theoretically proportional to the square of the applied field. This behavior can be observed in the fast Fourier transformation (FFT) spectra of the time series of the deflection signal. The magnitude of the vibration response increases when the frequency of the drive force is near resonant frequency of the particle-cantilever probe. The amplitude of this vibration increases with proximity of the two particles, and ultimately causes the particles to repeatedly hit each other as in tapping mode AFM. The effect of the Maxwell-Wagner interfacial relaxation on the DC electric field-induced force was discovered by monitoring the variation of the field-induced force with time. The static component of the AC electric field-induced force does not vary with the applied frequency in the range from 1 to 100 kHz, suggesting that the crossover frequency may equal to or less than 1 kHz and the permittivities of the BaTiO? glass microspheres and medium dominate the field-3 induced force. The AC field-induced force is proportional to the square of the applied electric field strength. This relationship persists even when the separation between the spheres is much smaller than the diameter of the microspheres. The large magnitude of the force at small separations suggests that the local field is distorted by the presence of a second particle, and the continued dependence on the square of the field but the measured force is much larger than the theoretical results, suggesting that the local electric field around the closely spaced spheres is distorted and enhanced but the effects of the local field distortion may have not much to with the applied electric field. Compared with the calculated results from different models, our results demonstrate that the field-induced force is much more long-range than expected in theory. In addition, the DC field-induced adhesion force is larger than the AC field-induced one due to the interfacial charge accumulation, agreeing with the discovery of the Maxwell-Wagner interfacial relaxation effect on the DC field-induced force. No obvious correlation between the field-induced adhesion and the applied frequency is found. However, both the DC and AC field-induced adhesion forces display the linearity with the square of the applied electric field strength as well.
Effects of Thickness, Morphology and Molecular Structure of Donor and Acceptor Layers in Thermally Interdiffused Polymer Photovoltaics
Gopal, Anamika (Virginia Tech, 2007-04-10)
An in-depth study of concentration gradients in thermally-interdiffused polymer – fullerene photovoltaic devices, with a focus on thickness and heat treatments, is presented in this thesis. Device performance is improved from the bilayer by the creation of a concentration gradient of the donor and acceptor materials throughout the active layer of the device. Concentration gradients are expected to improve device performance by optimizing the charge transfer, transport and collection processes. This is achieved through heat-induced interdiffusion of the two materials at temperatures above the glass transition temperature of the polymer. Investigation of the poly(3-octylthiophene) (P3OT) – C₆₀ system show a three-fold improvement in the external quantum efficiencies (EQE) as compared with bilayer devices. Auger spectroscopy, combined with argon-ion beam milling, serves to record the concentration depth profile and identify concentration gradients in the device through detection of the sulfur in the P3OT backbone. Concentration gradients are optimized to yield the best devices through a thickness variation study conducted on the P3OT – C₆₀ system for fixed thermal interdiffusion conditions at 118 °C for 5 minutes. An optimum thickness of 40 to 60 nm is obtained for the two materials that yields the ideal morphology of a concentration gradient as recorded by Auger spectroscopy. For such devices, the concentration gradient is seen to extend through the device, ending in a thin layer of pure material at each electrode. A monochromatic power conversion efficiency of 2.05% is obtained for 5.3 mW/cm²⁺ illumination at 470 nm. A brief study is also presented to optimize the concentration gradient profile through variations of the thermal parameters. The dependence of the concentration gradient on the interdiffusion time and temperature is investigated. The merits of heat treatment on the crystallinity of P3OT and the overall device performance are also discussed. It is shown in some case that devices with annealed P3OT layers show almost twice the EQE as non-annealed P3OT layer devices. Potential alternatives for C₆₀ in interdiffused devices with P3OT have been presented. [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a well-investigated acceptor for blend devices, is studied as an acceptor in concentration gradient devices. A method for spin-coating uniform bilayers of P3OT and PCBM, without solution damage to either layer, is presented. A thermal variation study of the interdiffusion conditions on this system indicated higher interdiffusion temperatures and times are preferred for P3OT – PCBM systems. For interdiffusion at 150 °C for ten minutes, EQE values approaching 35 % at 500 nm are obtained. Auger spectroscopy studies on this system yielded the same conclusions about the concentration gradient device morphology that gives optimum device output. 1:1 and 1:2 blends of P3OT – PCBM are also studied. The influence of various thermal treatments on these devices is described. The endohedral fullerene Sc₃N@C₈₀ is introduced as a new acceptor material. The endohedral fullerene consists of Sc₃N cluster enclosed in a C₈₀ cage. An order of magnitude increase is seen in device performance upon sublimation of these molecules on a P3OT layer confirming its effectiveness as an acceptor. Preliminary studies done on this system indicated the need for greater thermal treatment to produce optimum concentration gradients. An in depth study for varying temperatures and times is presented. The best device performance was seen for interdiffusion at 160 °C for 25 minutes. The endohedral fullerene devices also show a long-term deterioration and so best result are presented from a set of devices fabricated within the same time period. The study of these three donor-acceptor systems confirms that the conclusions on the thickness dependence and device performance study conducted for the P3OT – C₆₀ system extend to other acceptors. A model of EQE for varying thicknesses based on absorption in the interdiffused concentration gradient regions is also presented. This model effectively highlights the influence of P3OT layer thickness on the trends observed in the EQE. It did not, however, reproduce the experimental thickness variation results for varying C₆₀ thicknesses. Incorporation of the effects of the electric field intensity distribution is expected to correct for this. Suggestions have been given on how this might be achieved.
Electrical injection and detection of spin polarization in InSb/ferromagnet nanostructures
Kim, Yong-Jae (Virginia Tech, 2012-07-30)
We present studies of the electical detection of spin injection and transport in InSb/CoFe heterostructures. As a narrow gap semiconductor, InSb has a high mobility and strong spin-orbit interaction. Using ferromagnetic CoFe, lateral InSb/CoFe devices are fabricated by semiconductor processing techniques. The saturation magnetizations of various CoFe electrodes with different widths are calculated from Hall measurements in which the fringing fields of the CoFe electrodes are detected. A magnetic model provides reasonable estimation of the saturation magnetization for micrometer scale geometries. The interface magnetoresistance measurements of InSb/CoFe thin film layered structures present a unique peak at low field, having a symmetric behavior in magnetic field with a critical field Hc and a strong temperature dependence. We attribute our signal to a ferromagnetic phase in the InSb induced by spin injection. In a non-local lateral spin valve measurement, we observed the following. Firstly, Hc of the lateral spin valve signals is identical to Hc of interface magnetoresistance signals. Secondly, the non-local lateral spin valve signals are strongly dependent on temperature, which is also a unique characteristic magnetoresistance. Thirdly, the signals are tunable in response to an applied injector bias. Lastly, the signals are dependent on the exact interfaces. Based on these observations, the detected signals may be considered as spin current signals. The Hall and magnetoresistance signals are measured locally and non-locally in InSb/CoFe Hall devices. The non-local magnetoresistance signals exhibit asymmetric behavior in applied magnetic field which are considered as signatures of spin phenomena. The non-local Hall signals present switching behavior with the CoFe magnetization switching at the coercive field. The non-local Hall signals in a perpendicular field show Hc, similarly seen in non-local lateral spin valves. Inverse spin Hall effect measurements with tilted magnetic fields show an in-plane magnetic field dependence in non-local type Hall signal and a perpendicular magnetic field dependence in the local Hall measurement. We have found that the signal can have its origin in a spin current from our observation of Hc and hysteresis in the magnetization traces. As yet, the spin current transport mechanism is unknown.

Browsing by Author "Heflin, James R."

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