Tunable Few- to Many-Cycle Source for High-Order Harmonic Generation and Time-Resolved Spectroscopy

The temporal confinement of laser light pulses to durations approaching the optical period, and the subsequent conversion of these pulses into extreme ultraviolet and x-ray wavelengths through the process of high-order harmonic generation (HHG), has enabled measurement and control of ultrafast processes spanning picosecond to attosecond timescales. Typically achieved by nonlinear compression of multi-cycle pulses in gas-filled hollow-core fibers, compression to single-and even sub-cycle durations is now becoming routine due to the availability of state-of-the-art Ti:sapphire laser amplifiers outputting millijoule level pulses with pulse durations below ten cycles. Even so, reliance on mJ-level Ti:sapphire lasers has in most cases limited repetition rates to the few kilohertz regime, therefore restricting their application to time-resolved spectroscopies for which high repetition rates are needed. Toward this end, nonlinear compression of Yb-doped solid state and fiber sources, for which small quantum defect allows for high average powers, has garnered considerable attention in recent years. In this dissertation, I investigate the spectral broadening and temporal compression of sub-millijoule, 280 femtosecond pulses from a high average power Yb-doped laser amplifier by nonlinear compression in gas and solid media. The application of these pulses to high-repetition rate time-resolved studies is further established through their use in both HHG and time- and angle-resolved- photoemission spectroscopy. Moreover, I demonstrate the ability to harness the delayed nonlinearity of molecular gases to obtain multi-octave spectral broadening from pulses with long input durations and achieve compression to sub-two cycle durations. The fidelity of the sub-two cycle pulses is demonstrated through the generation of a high-order harmonic XUV continuum, suggesting a path to perform attosecond measurements with commercial laser systems. Finally, I investigate the potential to extend this technique to high average powers by studying the effects of nonequilibrium rotational state distributions in the repetitively laser-heated molecular gas on the supercontinuum spectrum

Transcontinental lives: intersections of homophobia and xenophobia in South Africa

This thesis focuses on prejudice located at the intersections of sexuality and nationality. Drawing on mixed qualitative research sessions involving men who are ‘LGBTI migrants’ from African countries, and who are living in South Africa, the thesis offers three overarching points of focus. Firstly, it contextualises and critiques historical state structures and attitudes which shape understandings of identity in South Africa. Secondly, it analyses everyday experiences of xenophobia and homophobia, as experienced by ‘LGBTI’ people who have migrated to the country for a variety of reasons. Finally, it locates these experiences within the structures identified and, based on participant-led discussions, offers a framework for understanding and suggestions for meaningful intervention. Using an overarching critical perspective of intersectionality and queer necropolitics I argue that contemporary South Africa fosters an image of inclusivity and exceptionalism that is vastly at odds with reality. In everyday spaces ‘LGBTI migrants’ are often forced to ‘switch’ between being either African or LGBTI. However, I argue that through journey-derived questioning of both Africanness and Queerness these processes of switching foreground hope and action. These are rooted in values of solidarity and community which extend, for fleeting moments, beyond labels and beyond geographic boundaries. Through a reconciled merging of these seemingly opposed subjectivities I argue that insight is offered into life beyond, yet within, national structures. In this way the participants exhibit an ‘African Queerness/Queer Africanness’ which shifts them beyond necropolitical death and towards transcontinental life. I ultimately argue that this may be harnessed as a tool to intellectually, and practically, render Africa as a site of (African) queer potentiality. I suggest that LGBTI migrants, through their embodiment of a specific transcontinental future, are pioneers in revealing this potentiality

A Jones Calculus Approach to High-Order Harmonic Generation in Solids

High-order harmonics from bulk solids were first observed in 2011 by focusing an intense mid-infrared laser through a bulk crystal and detecting the harmonics in a transmission geometry. Due to birefringence and possible nonlinear effects in bulk crystal, the polarization state of the laser can change as it propagates through the crystal in this transmission geometry. This can result in harmonic signal generated with an unknown polarization of light, disrupting the signal. Alternatives to bulk crystal, such as a reflection geometry or thin films, are not always ideal – reflection geometry can introduce nonlinear reflection coefficients, while crystalline thin films can be difficult to produce and are not available for all materials. We propose Jones calculus as a new method to analyze high-order harmonics from bulk solids in a transmission geometry. We predict the laser’s polarization changes due to propagation through a bulk crystal and we show that these changes can be accounted for using a combination of wave plates. Our results indicate that linear birefringence dominates the polarization change in bulk ZnO crystals driven in the mid-IR, which allows us to neglect the effect of nonlinear propagation effects on the polarization state. After compensating for the birefringence, we observe ellipticity-dependent, rotationally sensitive features in the harmonic signal which differ from those observed in previous transmission-geometry experiments. This method increases confidence in and control of HHG measurements in bulk crystal

The asymmetric Otto engine: frictional effects on performance bounds and operational modes

We present a detailed study of an asymmetrically driven quantum Otto engine with a time-dependent harmonic oscillator as its working medium. We obtain analytic expressions for the upper bounds on the efficiency of the engine for two different driving schemes having asymmetry in the expansion and compression work strokes. We show that the Otto cycle under consideration cannot operate as a heat engine in the low-temperature regime. Then, we show that the friction in the expansion stroke is significantly more detrimental to the performance of the engine as compared to the friction in the compression stroke. Further, by comparing the performance of the engine with sudden expansion, sudden compression, and both sudden strokes, we uncover a pattern of connections between the operational points, and we indicate the optimal operation regime for each case. Finally, we analytically characterize the complete phase diagram of the Otto cycle for both driving schemes and highlight the different operational modes of the cycle as a heat engine, refrigerator, accelerator, and heater

Diagnóstico general e importancia de las pequeñas y medianas empresas en el desarrollo económico y social de la ciudad de Cartagena en el período de 1994-2004

El presente proyecto de grado se desarrolla, en base a la necesidad de entregarle a la comunidad universitaria y gremial de Cartagena, un diagnostico de la importancia de la micro, pequeña y mediana empresa MIPYMES en Cartagena durante 1994-2004,en razón que existe muy poca disponibilidad de información con respecto a los últimos años; Sin embargo se manejan algunos indicadores complementarios que muestra la situación económica y social entre 2004-2006 en base a estudios realizados por el Observatorio del Caribe Colombiano y la Cámara de Comercio de Cartagena. La investigación de carácter eminentemente descriptivo-analítico se realiza a través de fuentes secundarias que se apoyan en visitas a algunos establecimientos; observación directa y entrevistas con algunos empresarios para determinar su situación actual y plantear posibles alternativas de desarrollo.Incluye bibliografí

La Lucha de los Pueblos y Comunidades Indígenas de Guerrero - México: por el Reconocimiento de sus Derechos a Elegir Autoridades Municipales Por usos y Costumbres una Mirada al Derecho comparado

Descriptive study that aims to publicize the rights that the indigenous peoples and communities of the state of Guerrero-Mexico have based on the uses and customs as a form of internal life, which are recognized by treaties, international agreements and the Political Constitution. of the United Mexican States. The problem that is exposed essentially lies in the difficulties or obstacles that do not allow the full exercise of this right; ranging from the lack of regulation in the legal framework, the resistance to the recognition of these rights by the authorities and society, generating discrimination, indifference, social exclusion and violation of human rights. Likewise, emphasis is placed on the process of struggle of the indigenous peoples and communities in Mexico, particularly in the state of Guerrero, which have been giving recognition to their uses and customs as a different or non-conventional form of the pre-established legal system. when electing their authorities (municipal president). For this reason, the electoral behavior of two indigenous municipalities of the state of Guerrero is analyzed, which began a process for the recognition of their uses and customs as a right from the year 2015, achieving a movement that forced the electoral authorities of the state of Guerrero, to recognize and generate the legal conditions and mechanisms so that later the first election oriented by uses and customs will be held in the municipality of Ayutla de los Libres Guerrero.Investigación descriptiva que tiene como objetivo dar a conocer los derechos que los pueblos y comunidades indígenas del Estado de Guerrero-México tienen a partir de los usos y costumbres como forma de vida interna, mismos que son reconocidos por tratados, convenios internacionales y la Constitución Política de los Estados Unidos Mexicanos. El problema que se expone radica esencialmente en las dificultades u obstáculos que no permiten el ejercicio pleno de este derecho; que van desde la falta de regulación en el marco jurídico, la resistencia al reconocimiento de estos derechos por las autoridades y la sociedad, generando discriminación, indiferencia, exclusión social y violación de derechos humanos. Así mismo, se pone énfasis en el proceso de lucha de los pueblos y comunidades indígenas en México, particularmente en el Estado de Guerrero, que han venido dando para que fueran reconocidos sus usos y costumbres como una forma distinta o no convencional al sistema jurídico preestablecido al momento de elegir a sus autoridades (presidente municipal). Por tal razón se analiza el comportamiento electoral de dos municipios indígenas del Estado de Guerrero, que iniciaron un proceso por el reconocimiento de sus usos y costumbres como un derecho a partir del año 2015, logrando un movimiento que obligó a las autoridades en materia electoral del Estado de Guerrero, a que reconocieran y generarán las condiciones y mecanismos jurídicos para que posteriormente se realizará la primera elección orientada por usos y costumbres en el municipio de Ayutla de los Libres Guerrero

Extreme ultraviolet time- and angle-resolved photoemission setup with 21.5 meV resolution using high-order harmonic generation from a turn-key Yb:KGW amplifier

Characterizing and controlling electronic properties of quantum materials require direct measurements of nonequilibrium electronic band structures over large regions of momentum space. Here, we demonstrate an experimental apparatus for time- and angle-resolved photoemission spectroscopy using high-order harmonic probe pulses generated by a robust, moderately high power (20 W) Yb:KGW amplifier with a tunable repetition rate between 50 and 150 kHz. By driving high-order harmonic generation (HHG) with the second harmonic of the fundamental 1025 nm laser pulses, we show that single-harmonic probe pulses at 21.8 eV photon energy can be effectively isolated without the use of a monochromator. The on-target photon flux can reach 5 × 1010 photons/s at 50 kHz, and the time resolution is measured to be 320 fs. The relatively long pulse duration of the Yb-driven HHG source allows us to reach an excellent energy resolution of 21.5 meV, which is achieved by suppressing the space-charge broadening using a low photon flux of 1.5 × 108 photons/s at a higher repetition rate of 150 kHz. The capabilities of the setup are demonstrated through measurements in the topological semimetal ZrSiS and the topological insulator Sb2−xGdxTe3

Artificial Neural Networks as a Probe of Many-Body Localization in Novel Topologies

We attempt to show that artificial neural networks may be used as a tool for universal probing of many-body localization in quantum graphs. We produce an artificial neural network, training it on the entanglement spectra of the nearest-neighbour Heisenberg spin1/2 chain in the presence of extremal (definitely ergodic/localizing) disorder values and show that this artificial neural network successfully qualitatively classifies the entanglement spectra at both extremal and intermediate disorder values as being in either the ergodic regime or in the many-body-localizing regime, based on known results. To this network, we then present the entanglement spectra of systems having different topological structures for classification. The entanglement spectra of next-to-nearest-neighbour (J1 − J2, and, in particular, Majumdar-Ghosh) models, star models, and bicycle wheel models - without any further training of the artificial neural network - are classified. We find that the results of these classifications - in particular how the mobility edge is affected - are in agreement with heuristic expectations. This we use as a proof of concept that neural networks and, more generally, machine learning algorithms, endow physicists with powerful tools for the study of many-body localization and potentially other many-body physics problems