The topographic distribution of the magnetic P100M to full- and half-field stimulation

Visual evoked magnetic responses were recorded to full-field and left and right half-field stimulation with three check sizes (70′, 34′ and 22′) in five normal subjects. Recordings were made sequentially on a 20-position grid (4 × 5) based on the inion, by means of a single-channel direct current-Superconducting Quantum Interference Device second-order gradiometer. The topographic maps were consistent on the same subjects recorded 2 months apart. The half-field responses produced the strongest signals in the contralateral hemisphere and were consistent with the cruciform model of the calcarine fissure. Right half fields produced upper-left-quadrant outgoing fields and lower-left-quadrant ingoing fields, while the left half field produced the opposite response. The topographic maps also varied with check size, with the larger checks producing positive or negative maximum position more anteriorly than small checks. In addition, with large checks the full-field responses could be explained as the summation of the two half fields, whereas full-field responses to smaller checks were more unpredictable and may be due to sources located at the occipital pole or lateral surface. In addition, dipole sources were located as appropriate with the use of inverse problem solutions. Topographic data will be vital to the clinical use of the visual evoked field but, in addition, provides complementary information to visual evoked potentials, allowing detailed studies of the visual cortex. © 1992 Kluwer Academic Publishers.

Advances in femtosecond micromachining and inscription of micro and nano photonic devices

This thesis has focused on three key areas of interest for femtosecond micromachining and inscription. The first area is micromachining where the work has focused on the ability to process highly repeatable, high precision machining with often extremely complex geometrical structures with little or no damage. High aspect ratio features have been demonstrated in transparent materials, metals and ceramics. Etch depth control was demonstrated especially in the work on phase mask fabrication. Practical chemical sensing and microfluidic devices were also fabricated to demonstrate the capability of the techniques developed during this work. The second area is femtosecond inscription. Here, the work has utilised the non-linear absorption mechanisms associated with femtosecond pulse-material interactions to create highly localised refractive index changes in transparent materials to create complex 3D structures. The techniques employed were then utilised in the fabrication of Phase masks and Optical Coherence Tomography (OCT) phantom calibration artefacts both of which show the potential to fill voids in the development of the fields. This especially the case for the OCT phantoms where there exists no previous artefacts of known shape, allowing for the initial specification of parameters associated with the quality of OCT machines that are being taken up across the world in industry and research. Finally the third area of focus was the combination of all of the techniques developed through work in planar samples to create a range of artefacts in optical fibres. The development of techniques and methods for compensating for the geometrical complexities associated with working with the cylindrical samples with varying refractive indices allowed for fundamental inscription parameters to be examined, structures for use as power monitors and polarisers with the optical fibres and finally the combination of femtosecond inscription and ablation techniques to create a magnetic field sensor with an optical fibre coated in Terfenol-D with directional capability. Through the development of understanding, practical techniques and equipment the work presented here demonstrates several novel pieces of research in the field of femtosecond micromachining and inscription that has provided a broad range of related fields with practical devices that were previously unavailable or that would take great cost and time to facilitate.

The visual evoked magnetic response to a pattern onset stimulus

The topography of the visual evoked magnetic response to a pattern onset stimulus was studied in four normal subjects. The topography of th CIIm component was consistent when measured on the same subject nine months apart. Full field responses were more variable than half field responses. With decreasing check size, the field pattern changes from a simple distribution with one outgoing and one ingoing area of field to a more complex pattern with in and outgoing fields over each hemisphere of the brain. The source may originate at the pole or from within the calcarine fissure.

The use of flash and pattern evoked fields in the diagnosis of Alzheimer's disease

Subjects with Alzheimer's disease (AD) exhibit normal visually evoked potentials (VEP) to pattern reversal stimuli but a delayed P2 flash response. The pattern response may originate in the primary visual cortex via the geniculo-calcarine pathway while the flash P2 may originate in the association areas via the cholinergic-tectal pathway. We now show: a) that the pathology of AD is more prominent in the visual association areas B18/19 than in B17 and b) that the magnetic signal to flash and pattern may originate from B18/19 and B17 respectively.

Topography of the visual evoked magnetic response and hemispherical specialization

The visual evoked magnetic response CIIm component to a pattern onset stimulus presented half field produced a consistent scalp topography in 15 normal subjects. The major response was seen over the contralateral hemisphere, suggesting a dipole with current flowing away from the medial surface of the brain. Full field responses were more unpredictable. The reponses of five subjects were studied to the onset of a full, left half and right half checkerboard stimuli of 38 x 27 min arc checks appearing for 200 ms. In two subjects the full field CIIm topography was consistent with that of the mathematical summation of their relevant half field distribution. The remaining subjects had unpredictable full field topographies, showing little or no relationship to their half or summated half fields. In each of these subjects, a distribution matching that of the summated half field CIIm distribution appears at an earlier latency than that of the predominant full field waveform peak. By examining the topography of the full and half field responses at 5 ms intervals along the waveform for one such subject, the CIIm topography of the right hemisphere develops 10 ms before that of the left hemisphere, and is replaced by the following CIIIm component 20 ms earlier. Hence, the large peak seen in full field results from a combination of the CIIm component of the left hemisphere plus that of the CIIIm from the right. The earlier peak results from the CIIm generated in both hemispheres, at a latency where both show similar amplitudes. As the relative amplitudes of these two peaks alter with check and field size, topographic studies would be required for accurate CIIm identification. In addition. the CIIm-CIIIm complex lasts for 80 ms in the right hemisphere and 135 ms in the left, suggesting hemispherical apecialization in the visual processing of the pattern onset response.

Influence of blur on the visual evoked magnetic response to a pattern reversal stimulus

Blurring a pattern reversal stimulus increases the latency and decreases the amplitude of the visual evoked potential (VEP) P100 peak. Recording the visual evoked magnetic response (VEMR) is some subjects may therefore be difficult because their spectacles create excessive magnetic noise. Hence, the effect of varying degrees of blur (-5 to +5 D) on the VEMR was investigated in three subjects with 6/6 vision to determine whether refraction with non-magnetic frames and lenses was necessary before magnetic recording. Small (32') and larger (70') checks were studied since there is evidence that blurring small checks has a more significant effect on the VEP compared with large checks. The VEMR was recorded using a single channel dc-SQUID, second order gradiometer in an unshielded laboratory. The latency (ms) and amplitude (fT) of the most prominant positive peak within the first 130 ms (P100M) were measured. Blurring the 32' checks significantly increased latency aand reduced the amplitude of the P100M peak. The resulting response curves were parabolic with minimum latency and maximum amplitude recorded at 0 D. Blurring the 70' check had no significant effect on latency or amplitude. Hence, the magnetic P100M responds similarly to the electrical P100 in response to blur. It would be essential when recording the VEMR that vision is corrected with non-magnetic spectacles especially when small checks are used.

Enzymatic production of oligosaccharides in centrifugal fields

The aims of this work have been to identify an enzymatic reaction system suitable to investigate and develop the high-speed centrifuge as a novel reaction system for performing such reactions. The production of galacto-oligosaccharides by the trans-galactosyl activity of the enzyme β-galactosidase on lactose monohydrate was identified as a model enzymatic system to elucidate the principles of this type of process. Galacto-oligosaccharides have attracted considerable commercial interest as food additives which have been shown to be beneficial to the health of the human gastrointestinal tract. The development of a single unit operation capable of controlling the biosynthesis of galacto-oligosaccharides whilst simultaneously separating the enzyme from the reaction products would reduce downstream processing costs. This thesis shows for the first time that by using a combination of (a) immobilised or insolubilised β-galactosidase , (b) a rate-zonal centrifugation technique, and (c) various applied centrifugal fields, that a high-speed centrifuge could be used to control the formation of galacto-oligosaccharides whilst removing the enzyme from the reaction products. By layering a suspension of insolubilised β-galactosidase on top of a lactose monohydrate density gradient and centrifuging, the applied centrifugal fields generated produced sedimentation of the enzyme particles through the substrate. The higher sedimentation rate of the enzyme compared to those of the reaction products allowed for separation to take place. Complete sedimentation, or pelleting of the enzyme permits the possible recovery and re-use. Insolubilisation of the enzyme allowed it to be sedimented through the substrate gradient using much lower applied centrifugal fields than that required to sediment free soluble enzyme and this allowed for less expensive centrifugation equipment to be used. Using free soluble and insolubilised β-galactosidase stirred-batch reactions were performed to investigate the kinetics of lactose monohydrate hydrolysis and galacto-oligosaccharide formation. Based on these results a preliminary mathematical model based on Michaelis-Menten kinetics was produced. It was found that the enzyme insolubilisation process using a chemical cross-linking agent did not affect the process of galacto-oligosaccharide formation. Centrifugation experiments were performed and it was found that by varying the applied centrifugal fields that the yield of galacto-oligosaccharides could be controlled. The higher the applied centrifugal fields the lower the yield of galacto-oligosaccharides. By increasing the applied centrifugal fields the 'contact time' between the sedimenting enzyme and the substrate was reduced, which produced lower yields. A novel technique involving pulsing the insolubilised enzyme through the substrate gradient was developed and this was found to produce higher yields of galacto-oligosaccharide compared to using a single enzyme loading equivalent to the total combined activity of the pulses. Comparison of the galacto-oligosaccharide yields between stirred-batch and centrifugation reactions showed that the applied centrifugal fields did not adversely affect the transgalactosyl activity of the insolubilised enzyme.

Developments and extensions of Roth's method of double Fourier series with applications to the solution of electromagnetic field problems

The first part of the thesis compares Roth's method with other methods, in particular the method of separation of variables and the finite cosine transform method, for solving certain elliptic partial differential equations arising in practice. In particular we consider the solution of steady state problems associated with insulated conductors in rectangular slots. Roth's method has two main disadvantages namely the slow rate of con­vergence of the double Fourier series and the restrictive form of the allowable boundary conditions. A combined Roth-separation of variables method is derived to remove the restrictions on the form of the boundary conditions and various Chebyshev approximations are used to try to improve the rate of convergence of the series. All the techniques are then applied to the Neumann problem arising from balanced rectangular windings in a transformer window. Roth's method is then extended to deal with problems other than those resulting from static fields. First we consider a rectangular insulated conductor in a rectangular slot when the current is varying sinusoidally with time. An approximate method is also developed and compared with the exact method.The approximation is then used to consider the problem of an insulated conductor in a slot facing an air gap. We also consider the exact method applied to the determination of the eddy-current loss produced in an isolated rectangular conductor by a transverse magnetic field varying sinusoidally with time. The results obtained using Roth's method are critically compared with those obtained by other authors using different methods. The final part of the thesis investigates further the application of Chebyshdev methods to the solution of elliptic partial differential equations; an area where Chebyshev approximations have rarely been used. A poisson equation with a polynomial term is treated first followed by a slot problem in cylindrical geometry.

Visual evoked magnetic responses (VEMR) to flash and pattern reversal stimulation

The problems of using a single channel magnetometer (BTi, Model 601) in an unshielded clinical environment to measure visual evoked magnetic responses (VEMR) were studied. VEMR to flash and pattern reversal stimuli were measured in 100 normal subjects. Two components, the P100M to pattern reversal and P2M to flash, were measured successfully in the majority of patients. The mean latencies of these components in different decades of life were more variable than the visual evoked potentials (VEP) that have been recorded to these stimuli. The latency of the P100M appeared to increase significantly after about 55 years of age whereas little change occurred for the flash P2M. The effects of blur, check size, stimulus size and luminance intensity on the latency and amplitude of the VEMR were studied. Blurring a small (32') check significantly increased latency whereas blurring a large (70') check had little effect on latency. Increasing check size significantly reduced latency of the P100M but had little effect on amplitude. Increasing the field size decreases the latency and increases the amplitude of the P100M. Within a normal subject, most of the temporal variability of the P100M appeared to be associated with run to run variation rather than between recording sessions on the same day or between days. Reproducibility of the P100M was improved to a degree by employing a magnetically shielded room. Increasing flash intensity decreases the latency and increases the amplitude of the P2M component. The magnitude of the effects of varying stimulus parameters on the VEMR were frequently greater than is normally seen in the VEP. The topography of the P100M and P2M varied over the scalp in normal subjects. Full field responses to a large check could be explained as approximately the sum of the half field responses and were consistent with the cruciform model of the visual cortex. Preliminary source localisation data suggested a shallower source in the visual cortex for the flash P2M compared with the P100M. The data suggest that suitable protocols could be devised to obtain normative data of sufficient quality to use the VEMR to flash and pattern clinically.

Step and flash imprint lithography: Fabrication of patterned media for extremely high density magnetic data storage devices

Currently the data storage industry is facing huge challenges with respect to the conventional method of recording data known as longitudinal magnetic recording. This technology is fast approaching a fundamental physical limit, known as the superparamagnetic limit. A unique way of deferring the superparamagnetic limit incorporates the patterning of magnetic media. This method exploits the use of lithography tools to predetermine the areal density. Various nanofabrication schemes are employed to pattern the magnetic material are Focus Ion Beam (FIB), E-beam Lithography (EBL), UV-Optical Lithography (UVL), Self-assembled Media Synthesis and Nanoimprint Lithography (NIL). Although there are many challenges to manufacturing patterned media, the large potential gains offered in terms of areal density make it one of the most promising new technologies on the horizon for future hard disk drives. Thus, this dissertation contributes to the development of future alternative data storage devices and deferring the superparamagnetic limit by designing and characterizing patterned magnetic media using a novel nanoimprint replication process called "Step and Flash Imprint lithography". As opposed to hot embossing and other high temperature-low pressure processes, SFIL can be performed at low pressure and room temperature. Initial experiments carried out, consisted of process flow design for the patterned structures on sputtered Ni-Fe thin films. The main one being the defectivity analysis for the SFIL process conducted by fabricating and testing devices of varying feature sizes (50 nm to 1 μm) and inspecting them optically as well as testing them electrically. Once the SFIL process was optimized, a number of Ni-Fe coated wafers were imprinted with a template having the patterned topography. A minimum feature size of 40 nm was obtained with varying pitch (1:1, 1:1.5, 1:2, and 1:3). The Characterization steps involved extensive SEM study at each processing step as well as Atomic Force Microscopy (AFM) and Magnetic Force Microscopy (MFM) analysis.

Optical Control of Magnetic Feshbach Resonances by Closed-Channel Electromagnetically Induced Transparency

Optical control of interactions in ultracold gases opens new fields of research by creating ``designer" interactions with high spatial and temporal resolution. However, previous optical methods using single optical fields generally suffer from atom loss due to spontaneous scattering. This thesis reports new optical methods, employing two optical fields to control interactions in ultracold gases, while suppressing spontaneous scattering by quantum interference. In this dissertation, I will discuss the experimental demonstration of two optical field methods to control narrow and broad magnetic Feshbach resonances in an ultracold gas of $^6$Li atoms. The narrow Feshbach resonance is shifted by $30$ times its width and atom loss suppressed by destructive quantum interference. Near the broad Feshbach resonance, the spontaneous lifetime of the atoms is increased from $0.5$ ms for single field methods to $400$ ms using our two optical field method. Furthermore, I report on a new theoretical model, the continuum-dressed state model, that calculates the optically induced scattering phase shift for both the broad and narrow Feshbach resonances by treating them in a unified manner. The continuum-dressed state model fits the experimental data both in shape and magnitude using only one free parameter. Using the continuum-dressed state model, I illustrate the advantages of our two optical field method over single-field optical methods.

Tailoring Magnetic, Mechanical and Dielectric Properties of Magnetorheological Elastomers based on Natural Rubber and Iron by Magnetic Field Assisted Curing for Possible Applications

If magnetism is universal in nature, magnetic materials are ubiquitous. A life without magnetism is unthinkable and a day without the influence of a magnetic material is unimaginable. They find innumerable applications in the form of many passive and active devices namely, compass, electric motor, generator, microphone, loud speaker, maglev train, magnetic resonance imaging, data recording and reading, hadron collider etc. The list is endless. Such is the influence of magnetism and magnetic materials in ones day to day life. With the advent of nanoscience and nanotechnology, along with the emergence of new areas/fields such as spintronics, multiferroics and magnetic refrigeration, the importance of magnetism is ever increasing and attracting the attention of researchers worldwide. The search for a fluid which exhibits magnetism has been on for quite some time. However nature has not bestowed us with a magnetic fluid and hence it has been the dream of many researchers to synthesize a magnetic fluid which is thought to revolutionize many applications based on magnetism. The discovery of a magnetic fluid by Jacob Rabinow in the year 1952 paved the way for a new branch of Physics/Engineering which later became magnetic fluids. This gave birth to a new class of material called magnetorheological materials. Magnetorheological materials are considered superior to electrorheological materials in that magnetorheology is a contactless operation and often inexpensive.Most of the studies in the past on magnetorheological materials were based on magnetic fluids. Recently the focus has been on the solid state analogue of magnetic fluids which are called Magnetorheological Elastomers (MREs). The very word magnetorheological elastomer implies that the rheological properties of these materials can be altered by the influence of an external applied magnetic field and this process is reversible. If the application of an external magnetic field modifies the viscosity of a magnetic fluid, the effect of external magnetic stimuli on a magnetorheological elastomer is in the modification of its stiffness. They are reversible too. Magnetorheological materials exhibit variable stiffness and find applications in adaptive structures of aerospace, automotive civil and electrical engineering applications. The major advantage of MRE is that the particles are not able to settle with time and hence there is no need of a vessel to hold it. The possibility of hazardous waste leakage is no more with a solid MRE. Moreover, the particles in a solid MRE will not affect the performance and durability of the equipment. Usually MR solids work only in the pre yield region while MR fluids, typically work in the post yield state. The application of an external magnetic field modifies the stiffness constant, shear modulus and loss modulus which are complex quantities. In viscoelastic materials a part of the input energy is stored and released during each cycle and a part is dissipated as heat. The storage modulus G′ represents the capacity of the material to store energy of deformation, which contribute to material stiffness. The loss modulusG′′ represents the ability of the material to dissipate the energy of deformation. Such materials can find applications in the form of adaptive vibration absorbers (ATVAs), stiffness tunable mounts and variable impedance surfaces. MREs are an important material for automobile giants and became the focus of this research for eventual automatic vibration control, sound isolation, brakes, clutches and suspension systems

Investigations on the structural, optical and magnetic properties of nanostructured cerium oxide in pure and doped forms and its polymer nanocomposites

In recent years, nanoscience and nanotechnology has emerged as one of the most important and exciting frontier areas of research interest in almost all fields of science and technology. This technology provides the path of many breakthrough changes in the near future in many areas of advanced technological applications. Nanotechnology is an interdisciplinary area of research and development. The advent of nanotechnology in the modern times and the beginning of its systematic study can be thought of to have begun with a lecture by the famous physicist Richard Feynman. In 1960 he presented a visionary and prophetic lecture at the meeting of the American Physical Society entitled “there is plenty of room at the bottom” where he speculated on the possibility and potential of nanosized materials. Synthesis of nanomaterials and nanostructures are the essential aspects of nanotechnology. Studies on new physical properties and applications of nanomaterials are possible only when materials are made available with desired size, morphology, crystal structure and chemical composition. Cerium oxide (ceria) is one of the important functional materials with high mechanical strength, thermal stability, excellent optical properties, appreciable oxygen ion conductivity and oxygen storage capacity. Ceria finds a variety of applications in mechanical polishing of microelectronic devices, as catalysts for three-way automatic exhaust systems and as additives in ceramics and phosphors. The doped ceria usually has enhanced catalytic and electrical properties, which depend on a series of factors such as the particle size, the structural characteristics, morphology etc. Ceria based solid solutions have been widely identified as promising electrolytes for intermediate temperature solid oxide fuel cells (SOFC). The success of many promising device technologies depends on the suitable powder synthesis techniques. The challenge for introducing new nanopowder synthesis techniques is to preserve high material quality while attaining the desired composition. The method adopted should give reproducible powder properties, high yield and must be time and energy effective. The use of a variety of new materials in many technological applications has been realized through the use of thin films of these materials. Thus the development of any new material will have good application potential if it can be deposited in thin film form with the same properties. The advantageous properties of thin films include the possibility of tailoring the properties according to film thickness, small mass of the materials involved and high surface to volume ratio. The synthesis of polymer nanocomposites is an integral aspect of polymer nanotechnology. By inserting the nanometric inorganic compounds, the properties of polymers can be improved and this has a lot of applications depending upon the inorganic filler material present in the polymer.