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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
61

Development of a parallel-plate ionisation chamber for α-particles and its application to the measurement of fluorescence yields

Park, Jacob Jae Hang January 1959 (has links)
No description available.
62

Femtosecond optical parametric oscillator frequency combs for coherent pulse synthesis

McCracken, Richard A. January 2013 (has links)
Coherent pulse synthesis takes as its objective the piecewise assembly of a sequence of identical broadband pulses from two or more mutually-coherent sequences of narrowband pulses. The requirements for pulse synthesis are that the parent pulses share the same repetition frequency, are phase coherent and have low mutual timing jitter over the required observation time. The work carried out in this thesis explored the requirements for broadband coherent pulse synthesis between the multiple visible outputs of a synchronously pumped femtosecond optical parametric oscillator. A femtosecond Ti:sapphire laser was characterised and used to pump a PPKTP-based OPO that produced a number of second-harmonic and sum-frequency mixing outputs across the visible region. Using a novel lock-to-zero CEO stabilisation technique, broadband phase coherence was established between all the pulses on the optical bench, producing the broadest zero-offset frequency comb to date. Employing a common optical path for all the pulses provided common-mode rejection of noise, ensuring less than 150 attoseconds of timing jitter between the pulses over a 1 second observation window. The parent pulses were compressed and their relative delays altered in a quasi-common path prism delay line, allowing pulse synthesis at a desired reference plane.
63

Nonlinear optical properties of indium arsenide with ultrafast femtosecond radiation at mid-infrared wavelengths

Corbett, Mark Allan January 2005 (has links)
Nonlinear optical properties of the narrow band-gap semiconductor InAs have been studied using ultrashort femtosecond infrared pulses from a regeneratively amplified Ti:Sapphire pumped optical parametric amplifier (OPA) and difference frequency mixing (DFM) system. Characterisation of the laser showed good stability with 1.5 µJ pulse energy in a Gaussian spatial distribution. Wavelength spectra were broad Gaussian distributions consistent with dispersion of ultrashort femtosecond duration pulses. Sub-200 fs pulse lengths were measured with a two-photon detector based autocorrelation. The measured two-photon absorption coefficient agreed well with perturbation theory from 4 to 5.5 µm. At longer wavelengths the coefficient was smaller than expected. This was attributed to the broad, polychromatic wavelength and uncertainty in the pulse length. No additional free carrier absorption was observed, even at very high irradiance. It is hypothesized that this was due to the carriers having insufficient time to undergo intra- band relaxation on the timescale of the pulses. Beam spreading and saturation effects may also have been occurring. Nonlinear refraction was measured using the z-scan technique. A strong defocusing component dominated which was consistent with free carrier plasma effects. A weaker component was also observed that was defocusing at 4 µm and self-focusing at 5.5 µm. This was in agreement with predictions of an ultrafast, n₂ contribution, corresponding to the real part of the third order susceptibility χ⁽³⁾. Pump-probe measurements showed that the nonlinear refraction was instantaneous but the absorption had a build up time of ~100 ps. This was attributed to hot-phonon screened intra-band relaxation. Fitting of the pump-probe data produced Shockley-Read-Hall and Auger recombination lifetimes consistent with prior literature data.
64

Investigating quantum phenomena in nano- and micromechanical oscillators

Joshi, Chaitanya January 2012 (has links)
This thesis theoretically investigates quantum features in nano- and micromechanical oscillators. The thesis aims at proposing novel schemes to prepare mesoscopic mechanical systems in non-classical states including entangled states. The main emphasis of the work is to understand genuine quantum features in coupled harmonic oscillators with in nite dimensional Hilbert spaces. With the recent experimental breakthroughs in achieving the ground state of mesoscopic mechanical systems, the time is now ripe to investigate in detail a full quantum description of such mesoscopic mechanical systems. Thus, the main emphasis of the thesis is on probing salient quantum features in coupled mechanical systems that are assumed to be prepared in vibrational states close to their quantum ground states. A major part of the thesis makes use of var- ious theoretical techniques widely used in quantum optics and quantum information. The majority of the results reported in this thesis involves analytical calculations augmented with numerical investigations. We believe many of the results obtained will be of interest to researchers with background in quantum optics and quantum information and with research interest in the quantum-classical crossover in continuous variable systems.
65

Design and fabrication technologies for single-mode quantum cascade lasers

Kennedy, Kenneth January 2010 (has links)
Quantum Cascade Lasers (QCL's) are unipolar semiconductor lasers offering the potential for low cost, high power, laser sources emitting in the mid- and far- infrared. Single-mode devices are required for spectroscopic and imaging applications because of their narrow emission linewidth and are achieved through distributed feedback (DFB) designs where gratings are incorporated in or close to the active region. This thesis describes design and fabrication technologies for single-mode yield improvements and improved wavelength targeting of QC lasers. Mid-infrared, single-mode laser designs are developed utilising novel fabrication processes and designs in the indium phosphide (InP) based material system. Lasers without facet coatings, with single-mode yields up to 80% and side mode suppression ratios (SMSR) greater than 25dB are observed. Metalised surface gratings, buried (overgrown) gratings and lateral gratings are defined using inductively coupled plasma (rCP) etching to produce DFB lasers operating above room temperature for wavelengths near to lOllm. A deep etched lateral grating quantum cascade DFB is demonstrated for the first time in the InP material system. Waveguide modelling demonstrates accurate methods to predict the grating coupling strength of the DFB lasers with good agreement to experimental results. The emission wavelength (A. -IOllm) is found to be highly dependent on the laser ridge width with the experimental shift in wavelength found to be much greater than that predicted. A ridge width and temperature dependence on emission wavelength is utilised in an array device where a continuous tuning range in excess of 230nm is observed. ii Finally, an increase in thermal conductance and a symmetric far-field profile are observed for lasers (A. - 4J.lm) with narrow ridge widths. The lasers were designed for improved heat extraction from the active region and a beam quality factor of M2 :: 1. Almost identical lateral and vertical far-field profiles with high duty cycle operation at thermoelectric temperatures are observed for a ridge waveguide laser that is approximately 5 J.lm wide.
66

Exploitation of microchannel plate optics

Martin, Adrian Peter January 2000 (has links)
This thesis contains work on microchannel plate (MCP) optics as used for X-ray focusing, and can be split into two sections; research and applications.;Research into improving the reflectivity of MCPs is presented which includes results obtained at the Daresbury Synchrotron, and electron microscope analysis. Different treatments performed on Nova Scientific channel plates were shown only to make a improvement to reflectivity in the case of annealing. Evidence for a 300A layer of silica on the surfaces of the microchannels, a result of the acid etching process, was discovered.;The method of bending, or slumping MCPs to a spherical form by Photonis and Nova has been assessed, and X-ray images using slumped plates are presented. The accuracy and reproducibility of the process was not found to be excellent (within 10% of the target radius), but were acceptable for the plates slumped to date.;A comprehensive report is given of the application of channel plates as the imaging device in an Imaging X-ray Fluorescence Spectrometer, firstly at the Rutherford Appleton Laboratory and subsequently in the laboratory in Leicester. The spectrometer successfully imaged a multi-element target, resolving both elementally (down to Fluorine, Z=9) and spatially (to under 2mm) in a 34 hour integration. The concept of Bragg reflection imaging is examined as another use of the spectrometer.
67

A spherical Fizeau interferometer for the testing of optical surfaces

Cobb, L. M. January 1978 (has links)
No description available.
68

The electron microscopy of crystalline polymers

Grubb, D. T. January 1970 (has links)
This thesis describes the micro-structure of crystalline polymers, particularly as revealed by electron microscopy, and also the effect of the electron beam on the radiation sensitive specimens under various conditions. Polymer single crystals grown from dilute solution were used for the radiation damage studies, as they are thin (100 andAring;), and a single preparation may contain 10<sup>8</sup> crystals of uniform properties. Preparations of polyethylene (PE), polyoxymethylene (POM), poly-4-methylpentene-1 (P4MP), nylon 6 and polyethylene oxide were characterized in the transmission electron microscope, confirming previous work for the most part. The scanning electron microscope, operating in its normal mode, showed even the thinnest single crystals clearly, when they were sedimented onto a metal coated substrate. Attempts to observe the 3-D pyramidal structure of PE single crystals were unsuccessful. The crumpling seen when crystals floating on glycerol, or freely suspended from a fine mesh, are irradiated in an electron microscope indicates that even if the structure were preserved, by freeze drying or however, it would be difficult to observe. Thicker, complex, crystals seemed more stable in the scanning electron microscope, and cannot be seen well in the transmission microscope. A chart recorder, picoammeter and special faraday cup were fitted to a Siemens Elmiskop 1 for accurate beam current measurements, and dynamic recording of the varying intensity of polymer crystal images. Reproducible curves for the variation of intensity of one spot in the diffraction pattern were obtained for POM and PE. There was a period, p, of very variable intensity, followed by an exponential decay. The very variable intensity is due to the diffraction contrast of detail, so after p no information could be gained from the crystal, although the diffraction pattern shows it to be largely crystalline at that point. For the particular preparations described, at 100 kV,</p> <table> <tr> <td> </td> <th>PE</th> <th>POM</th> <td> </td> </tr> <tr> <th>p</td> <td>43 ± 5</td> <td>34 ± 4</td> <td rowspan="2">values in Coulombs/sq. m.</td> </tr> <tr> <th>Diff. Pattern Decayed</td> <td>92 ± 10</td> <td>70 ± 7</td> </tr> </table> <p>It is shown that in the electron microscope, heating and ionic bombardment are negligible effects compared to the electron irradiation. An energy loss analysing microscope applied to PE crystals showed that</p> <ol type="1"> <li>1 Coulomb/sq.m. is equivalent to 42 Mrads. at 100 kV in agreement with theory.</li> <li>The density of PE increases sharply on irradiation, and the final product seems much like amorphous carbon.</li> </ol> <p>Radiation damage mechanisms are discussed, with special reference to polymers and to conditions in an electron microscope. For POM, PE and P4MP the variation in radiation damage rate with beam voltage between 30 kV and 100 kV was found to be accurately andprop; (electron velocity)<sup>-2</sup> as predicted by theory, both simple and complex. With exactly similar samples of PE and POM in the 1 MV electron microscope at U.S. Steel, the trend was the same up to 1 MV. Between 700 kV and 1 MV there was a deviation from proportionality towards even slower damage. Absolute measurements differed by 30-50%. Adapting the scanning electron microscope to transmission, the damage of POM could be studied down to 3 kV. There was a steady increase of damage rate as the voltage was reduced, but it was not possible to compare the results with those at higher voltages. The effect of specimen temperature on damage rate was investigated with a liquid helium cooled specimen stage. In going from room temperature to < 30°K the rate was reduced by a factor of 2 (2.2 ± 0.8). This is not enough to outweigh practical difficulties. There is a fundamental noise limit to resolution with a decaying crystal. This has been calculated, and found to be well below anything practically realized. For example random structure 50 andAring; across, and periodic structure of 20 andAring; should be resolvable in a POM crystal, 85 andAring; thick imaged in bright field at 100 kV and 13,000 times magnification. The real limits are image instability and focus, and there is no good way round either. There is no advantage to be gained from higher beam voltages. Ease of rapid operation of the microscope is the most important factor. Image intensifiers, if easy to use, may be a great help, allowing more manipulation of specimens, and presenting more of the information that is obtained. Thin films of PE and POM were cast from slurry and observed in the electron microscope. PE gives complex crystallographic diffraction contrast which rapidly disappears but another, stronger, contrast builds up showing another structure, and this is retained however large the dose. This beam-induced image looks just like what one might expect from undamaged material, in this case, banded spherulites. Detailed study of the changes, combined with optical microscopy and experiments on single crystals leads to the following conclusion: The beam-induced contrast is due to anisotropic deformation caused by radiation damage. Each crystalline unit expands along <strong>a</strong>, and contracts along <strong>c</strong>, while the third dimension, along <strong>b</strong>, remains constant. This explanation supports the current view of spherulite structure, a collection of radiating twisting lamellae, and may well apply to other polymers and structures. Thin films of PE were drawn at room temperature and annealed at 120°C. On irradiation, the drawn regions retracted by around 30%a, but this is not a measure of contraction along <strong>c</strong> (predicted as > 25%) because of the unknown behaviour of interlamellar material. The fibrils seen in drawn PE after irradiation were found to be the radial lamellae, homogeneously deformed, whenever the spherulite structure persisted enough for measurements to be made. An ultra-thin PE film which became in the limit, an open network, was produced in an uncontrolled way. Normal spherulite structure was not observed. Areas seemingly undrawn gave a single crystal type of diffraction pattern, and drawn ribbons looked like a 'shish-kebab', transverse platelets on a fine thread. The plates were 200 ± 25 andAring; thick, separated by 200 ± 75 andAring;. The centre to centre period was 380 ± 40 andAring;. Diffraction indicated that the orientation was very good, with <strong>b</strong> and <strong>c</strong> in the specimen plane, and <strong>c</strong> along the ribbon. The film was quite stable, and diffracting units could be identified with individual platelets. This was a fascinating demonstration of an ideal drawn PE, but it could not be made to order, or investigated systematically. The POM films degraded to a featureless film in the electron microscope with < 20% of the mass remaining. Optical microscopy showed coarse textured spherulites, beginning to organise into banded spherulites with radial period > 10andmu;. Initial contrast in the electron microscope showed a radiating structure from well spaced centres. The radiating units gave a single crystal type of diffraction pattern, with a {101-0} growth direction. It seems that because of the coarse texture of POM spherulites, a thicker film is needed to give information about the bulk. This would need a high voltage microscope.
69

Observations on MsTh2, using a double β-ray spectrometer

Campbell, C. G. January 1952 (has links)
No description available.
70

The diffraction of light by ultrasonic waves

Noble, W. J. January 1952 (has links)
No description available.

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