• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 194
  • 81
  • 43
  • 22
  • 13
  • 11
  • 4
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 2062
  • 627
  • 593
  • 516
  • 512
  • 187
  • 175
  • 129
  • 126
  • 122
  • 106
  • 100
  • 95
  • 92
  • 89
  • 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.
111

Atomic quantum metrology with narrowband entangled and squeezed states of light

Wolfgramm, Florian 29 June 2012 (has links)
The use of light, especially of laser light, is in many cases the most sensitive way to perform measurements. However, the highest sensitivity that can be achieved with laser light as probe is bounded by the standard quantum limit (SQL). As many instruments are approaching this fundamental limit, it becomes crucial to explore ways to overcome the SQL. Quantum metrology offers the possibilities to increase the sensitivities of the most accurate measurements beyond the SQL by using photonic quantum states of light as a tool. Two well-known classes of quantum states that provide a metrological advantage and break the SQL are squeezed states and a certain class of entangled states, called NOON states. While it is of special interest to apply these quantum states to atomic systems, such as atomic vapors, this requires quantum states of the highest quality in terms of purity, fidelity, brightness, and indistinguishability. Most importantly, for the probing of atomic systems, the quantum states need to be extremely narrowband to match the atomic linewidths. As NOON states are usually generated in a broadband spontaneous parametric down-conversion (SPDC) process, they are not compatible with narrowband atomic resonances. The goal of this thesis was the generation of suitable narrowband entangled and squeezed quantum states of light and their application to atomic systems. To increase the rate of atom-resonant SPDC photons by orders of magnitude, we used a cavity-enhanced setup. Polarization-squeezed states and polarization-entangled NOON states were created. The spectral brightness of the generated NOON states is one of the highest of pairs of indistinguishable photons reported so far. The photon pairs were carefully characterized by full quantum state tomography showing high fidelities with a perfect NOON state. After filtering the photon source output by a novel filter based on the “interaction-free measurement” scheme, a cross-correlation measurement demonstrated its potential as a narrowband heralded single-photon source, needed for example in quantum information. To apply these states in a quantum metrology scheme and to show the metrological advantage, we chose an atomic magnetometer as a model system. The assembled shot-noise-limited magnetometer is based on the Faraday effect in a vapor of hot rubidium atoms. It could be demonstrated that both quantum states perform better in the magnetometer application than any classical state, i.e., they break the SQL. In the case of NOON states, this is the first use of multi-photon coherence in an atomic experiment. In addition to applications in quantum metrology, the presented techniques of quantum-light generation and filtering are also directly applicable to quantum information tasks, especially to the use in quantum memories. / El uso de la luz, en particular la luz láser es, en muchos casos, el método que permite realizar mediciones de la manera más sensible. No obstante, la mayor sensibilidad que se puede conseguir gracias a la luz láser como sistema de sondeo queda delimitada por el límite cuántico estándar (SQL). Visto que muchos instrumentos se están acercando a este límite fundamental, es crucial explorar formas de superar el SQL. La metrología cuántica ofrece la posibilidad de incrementar la sensibilidad de las medidas más precisas más allá del SQL empleando los estados cuánticos de luz como herramienta. Dos categorías conocidas de estados cuánticos que brindan una ventaja metrológica y rompen con el SQL son los estados “comprimidos” y ciertas categorías de estados entrelazados llamados estados “NOON”. Aunque es de especial interés aplicar estos estados cuánticos a los sistemas atómicos, como a los vapores atómicos, se requieren estados cuánticos de óptima calidad en términos de pureza, fidelidad, luminosidad e identidad. Lo más importante para los sistemas atómicos de investigación es que los estados cuánticos sean de banda extremadamente estrecha para que coincidan con el ancho de banda de átomos. Puesto que los estados NOON suelen ser generados en un proceso de conversión espontánea paramétrica descendente (SPDC) de banda ancha, no son compatibles con las resonancias atómicas de banda estrecha. El objeto de esta tesis fue la generación de estados cuánticos de luz apropiados de banda estrecha, entrelazados y comprimidos, y su aplicación en los sistemas atómicos. Para incrementar el número de fotones generados por SPDC resonantes con la transición atómica por órdenes de magnitud, se empleó un sistema aumentado por un resonador. Se crearon estados de polarización comprimida y estados NOON de polarización entrelazada. La luminosidad espectral de los estados NOON generada supone una de las más altas que se hayan reportado hasta el momento entre pares de fotones idénticos. Los pares de fotones fueron cuidadosamente caracterizados por medio de una tomografía completa del estado cuántico que muestra la gran fidelidad con un estado NOON perfecto. Después de filtrar la producción de la fuente de fotones por medio de un novedoso filtro basado en el esquema “interaction-free measurement”, una medida de correlación cruzada demostró su potencial como fuente de fotones individuales anunciados de banda estrecha que resulta necesaria, por ejemplo, en la información cuántica. Para aplicar estos estados en un esquema de metrología cuántica y demostrar la ventaja metrológica, elegimos un magnetómetro atómico como sistema modelo. El montaje del magnetómetro delimitado por el límite cuántico estándar se basa en el efecto Faraday en un vapor de átomos de rubidio calientes. Se podía demostrar que el comportamiento de ambos estados cuánticos es superior en la aplicación con el magnetómetro que cualquier estado clásico, es decir, que superan el SQL. En el caso de los estados NOON, este es el primer uso de la coherencia multifotónica en un experimento atómico. Además de las aplicaciones en la metrología cuántica, las técnicas presentadas de generación de luz cuántica y filtración también son directamente aplicables a las tareas de información cuántica, en especial al uso en las memorias cuánticas.
112

Single frequency vertical external cavity surface emitting lasers

Gardner, Kyle Scot January 2007 (has links)
The thesis presents the development and implementation of single frequency vertical external cavity surface emitting lasers (VECSELs). Numerous cavity designs are reported, exploiting some unique features for single frequency operation. A small VECSEL cavity configuration is reported utilising a positive curvature mirror in reverse to create a 6mm cavity where the air gap between the mirror surface and VECSEL wafer act as an etalon, which induces single frequency operation. A 7nm tuning range has been shown with maximum output of 19.4mW. Thermal modelling was undertaken to analyse how the removal of the VECSEL's substrate could increase the thermal efficiency for high power operation. Another small cavity design of length 50mm was created, producing a high power, compact single frequency VECSEL. Using a birefringent filter and solid etalon single frequency operation was achieved. A tuning range of 1Onm was achieved with output powers of 271mW. Extensive mapping of the pump profile and eventual manipulation of this resulted in the Mp2s of the laser of 1.1 being reduced to 1.02. In addition an air etalon system was constructed to eliminate walk-off losses experienced by the solid etalon. This resulted in a 20nm tuning range. Frequency doubling of an 850nm VECSEL using KNbOb3s is reported with 1.3mW of 425nm being achieved, corresponding to an efficiency of 3.2%/W. This system also incorporates a polarisation coupled pump system delivering 3W at 670nm from a 100(So(Bm fibre. The relationship between the VECSEL's gain and frequency conversion efficiency is also analysed in detail.
113

Studies towards improved focusing methods of photo electron auto-radiography

Shapland, D. J. M. January 1956 (has links)
No description available.
114

Fourier optics approaches to enhanced depth-of-field applications in millimetre-wave imaging and microscopy

Lucotte, Bertrand M. January 2010 (has links)
In the first part of this thesis millimetre-wave interferometric imagers are considered for short-range applications such as concealed weapons detection. Compared to real aperture systems, synthetic aperture imagers at these wavelengths can provide improvements in terms of size, cost, depth-of-field (DoF) and imaging flexibility via digitalrefocusing. Mechanical scanning between the scene and the array is investigated to reduce the number of antennas and correlators which drive the cost of such imagers. The tradeoffs associated with this hardware reduction are assessed before to jointly optimise the array configuration and scanning motion. To that end, a novel metric is proposed to quantify the uniformity of the Fourier domain coverage of the array and is maximised with a genetic algorithm. The resulting array demonstrates clear improvements in imaging performances compared to a conventional power-law Y-shaped array. The DoF of antenna arrays, analysed via the Strehl ratio, is shown to be limited even for infinitely small antennas, with the exception of circular arrays. In the second part of this thesis increased DoF in optical systems with Wavefront Coding (WC) is studied. Images obtained with WC are shown to exhibit artifacts that limit the benefits of this technique. An image restoration procedure employing a metric of defocus is proposed to remove these artifacts and therefore extend the DoF beyond the limit of conventional WC systems. A transmission optical microscope was designed and implemented to operate with WC. After suppression of partial coherence effects, the proposed image restoration method was successfully applied and extended DoF images are presented.
115

Direct laser-written polymer structures for guided-wave optical interconnects

Suyal, Himanshu January 2006 (has links)
This thesis describes the developments of guided-wave optical interconnects suitable for integration with printed circuit boards. The technology is based around direct laser writing of waveguides and other features in a newly developed multifunctional acrylate polymer system, using a He-Cd (325 nm) laser. It was demonstrated that, by writing with a laser spot having top-hat intensity profile, more sharply defined vertical and angled sidewalls could be achieved, compared to conventional methods using a Gaussian beam. Typical dimensions of the multimode waveguides were 50 x 50 J.lm, written with 50 J.lW of optical power with 100 J.lm/s scanning speed. The waveguide losses were measured, using the cut-back technique, to be -0.6 dB/em. A novel oil-immersion technique was developed to.overcome the limitations of refraction of the laser beam at the air / polymer inte~face and hence directly write 45° angled structures in the polymer. Metallised 45° out-of-plane mirrors were fabricated using these angled polymer structures and losses were measured to be ~ 0.8 dB per reflection. Successful coupling of optical signals between waveguides in different layers was also demonstrated in a double layer structure, in which the out-ofplane 45° mirrors provided the necessary optical connectivity. Direct laser writing was also employed to fabricate ~50 J.lm wide and 100 J.lm high polymer bumps for use in flip-chip bonding. Electroless gold plating was used to selectively metallise the polymer bumps and to produce electrical tracks on the substrate. Electrical resistances between the top of the bump and a lower metal pad were measured as less than ~5 ohms.
116

Infrared time-correlated single-photon counting

Warbuton, Ryan Ellis January 2008 (has links)
This Thesis investigates near infrared ( ~ 1550 nm) time-correlated singlephoton counting, studying the single-photon detectors and some of the potential application areas. Custom designed and fabricated InGaAs/InP single-photon avalanche diode detectors were characterised. Our devices yielded single-photon detection efficiencies of ~10 %, timing jitter of 200 ps, and noise equivalent power comparable to the best commercially available avalanche photodiodes operated in Geiger-mode. The afterpulsing phenomenon which limits the maximum count rate of InGaAs/InP single-photon avalanche diodes has been investigated in detail and activation energies calculated for the traps that cause this problem. This was found to be ~250 meV for all the devices tested, despite their differing structures and growth conditions, and points to the InP multiplication region as the likely location of the traps. Ways of reducing the effects caused by the afterpulsing phenomenon were investigated and sub-Geiger mode operation was studied in detail. This approach enabled freerunning, afterpulsing-free operation at room temperature of an InGaAs/InP singlephoton avalanche diode detector for the first time. Finally, time-of-flight photon counting laser ranging was performed using both singlephoton avalanche diodes and superconducting nanowire single-photon detectors. The use of the latter resulted in a surface to surface depth resolution of 4 mm being achieved at low average laser power at an eye-safe wavelength of 1550 nm.
117

Geometric and photometric affine invariant image registration

Cuesta Contreras, Angel January 2009 (has links)
This thesis aims to present a solution to the correspondence problem for the registration of wide-baseline images taken from uncalibrated cameras. We propose an affine invariant descriptor that combines the geometry and photometry of the scene to find correspondences between both views. The geometric affine invariant component of the descriptor is based on the affine arc-length metric, whereas the photometry is analysed by invariant colour moments. A graph structure represents the spatial distribution of the primitive features; i.e. nodes correspond to detected high-curvature points, whereas arcs represent connectivities by extracted contours. After matching, we refine the search for correspondences by using a maximum likelihood robust algorithm. We have evaluated the system over synthetic and real data. The method is endemic to propagation of errors introduced by approximations in the system.
118

Efficient, high-power operation of CW Yb:YAG and Q-switched Nd:YAG planar waveguide lasers

Thomson, Ian James January 2010 (has links)
The planar waveguide gain medium o ers a good approach to creating a high- power lasers. Excellent thermal properties allow for lasers with good beam-quality and high-e ciency. A Neodymium doped YAG planar waveguide laser was actively Q-switched using a hybrid unstable resonator and acousto-optic modulator. A Yt- terbium doped YAG planar waveguide laser was side-pumped using two laser diode stacks with a total of 800 W incident pump power. The Nd:YAG planar waveguide laser has a core 200 um thick and was face pumped with ten laser diode bars. Q-switching was achieved through the use of a new hybrid unstable resonator con guration providing high-speed modulation of the laser cavity. The laser produced 100 W of average power with a pulse repetition rate of 100 kHz, giving 1 mJ pulses with 50 ns duration. Shorter 15 ns pulses were achieved with 4.5 mJ pulse energy at lower pulse repetition rates. The Yb:YAG planar waveguide laser has a 150 um core thickness, side pumped by two 430 W, six-bar diode laser stacks. Each diode stack has a custom phase- plate, correcting for collimation errors and lens aberrations. These gave a line focus matching the dimensions of the waveguide core with an incident intensity on each side of approximately 22 kWcm-2. Ray tracing of spontaneous emission within the waveguide core shows side facet angles of 7 and 20 were required to minimise ASE and parasitic oscillations within the waveguide core. A technique to image the uorescence from the Yb:YAG waveguide core has been developed. This technique allows for easy alignment of the pump lasers to the waveg- uide core and measurement of the pump intensity pro le throughout the planar waveg- uide. This technique showed single-sided pumping to follow Beer's law, and with double-sided pumping considerably improved the pump uniformity. A diode laser probe was used to measure the gain present within the waveguide. The small signal gain coe cient was shown to be 1 cm-1 with a uniform pro le across the width of the waveguide when pumped to saturation. This highlights the potential of operating the planar waveguide laser as an e cient ampli er. High-power high-e ciency operation of the Yb:YAG planar waveguide laser was achieved through the use of a hybrid stable waveguide resonator. Output power of 400 W was extracted in a multi-mode beam with a slope e ciency of 75%. This result represents the highest output power from a diode-pumped solid-state planar waveguide laser to date.
119

Schrodinger, Dirac and beyond : non-Abelian gauge fields in ultracold quantum gases

Merkl, Michael January 2010 (has links)
In this thesis, optically induced non-Abelian gauge fields in ultracold atoms are investigated. Gauge fields can emerge in the equation of motion for the optically addressed ultracold atoms. To this end, spacially dependent dark states are necessary for the internal states of the atoms. A tripod level scheme yields two degenerate dark states which leads to a non-Abelian SU(2) gauge field. These fields can be understood in terms of spin-orbit coupling between the momentum of the atom and its internal pseudo-spin formed by the dark state manifold. On a ring trap, this spin-orbit coupling leads to internal and spacial Josephson oscillations, persistent currents and grey solitons. The influence of interatomic interactions is crucial for solitons and leads to modifications of the groundstate in a harmonically trapped scenario. Similarities between ultracold atoms and high energy physics are drawn in the low momentum limit as the dispersion relation offers a widely controllable Dirac cone there. With an induced mass gap Zitterbewegung and bright solitons are investigated. Moreover, non-linear Dirac equations, adapted from high energy physics, lead to new self confining solutions for ultracold gases. Further analogues between ultracold atoms and quantum chromodynamics, like chiral symmetry breaking are discussed
120

Design and characterisation of nanostructured gradient index lenses

Hudelist, Florian January 2010 (has links)
The design and characterisation of nanostructured gradient index lenses is investigated in this thesis. Nanostructured gradient index materials achieve their refractive index pro le by creating a pattern with feature sizes of =5 and smaller from two glasses with di erent refractive indices. These structures are fabricated by the stack-anddraw technology generally used for photonic crystal bres. The rigorous theoretical analysis is performed with the Fourier modal method or the nite di erence time domain algorithm. A comprehensive introduction of the Fourier modal method for one and two dimensional gratings is given. Due to the inherit periodicity of the Fourier modal method, an algorithm to calculate the transmitted eld of isolated non-periodic lamellar gratings is developed and tested experimentally with a multi layer lens grating in the microwave regime. Furthermore, the eld stitching method for the analysis of large two dimensional gratings with very small feature sizes is developed. The numerical performance is tested with a di ractive element consisting of 32 32 pixels and shown to reduce the required memory as well as the computation time by more than an order of magnitude in certain con gurations. Considerations of symmetries in the grating structure are also included in the derivation of the eld stitching method. The e ective medium theory for nanostructured gradient index materials is introduced which allows to describe nanostructured materials with the equations for standard gradient index lenses. The stack-and-draw fabrication process is described including the choice of glass types, assembly and drawing of the preforms. For the design of the required binary pattern, the simulated annealing algorithm is used in conjunction with the e ective medium theory. In order to provide experimental evidence of the simulations, two lenses were assembled from PTFE rods with a diameter of 6mm and characterised in the microwave regime at = 3 cm. It is shown that with this wavelength to feature size ratio, the nanostructured gradient index lenses can have properties nearly identical to conventional gradient index lenses. Finally, a spherical and an elliptical nanostructured microlens are characterised in the optical regime. On the elliptical microlens, phase and intensity measurements are performed and compared to simulations obtained with the Fourier modal method.

Page generated in 0.0337 seconds