Structuring high-order harmonic generation with the angular momentum of light

Tesis por compendio de publicaciones[ES] Los pulsos láser ultracortos son una herramienta única para explorar las dinámicas más rápidas de la materia. Sorprendentemente, los pulsos de láser más cortos obtenidos hasta la fecha se producen a partir del fenómeno no lineal de conversión de frecuencias de generación de armónicos de orden alto (HHG), que resulta en la emisión de pulsos con duraciones de attosegundo. Es importante destacar que estos pulsos de attosegundo pueden exhibir una propiedad muy interesante, el momento angular, que presenta dos formas diferentes, el momento angular de espín (SAM) y el momento angular orbital (OAM), y que abre nuevos escenarios para las interacciones luz-materia a escalas espaciales nanométricas y temporales ultracortas. En esta tesis desarrollamos un conjunto de esquemas para la crea- ción de armónicos de orden alto y pulsos de attosegundo con nuevas propiedades de momento angular mediante la estructuración del pro- ceso de HHG a través de las características de los haces incidentes. Para ese propósito, primero abordamos la descripción de los mecanismos físicos fundamentales de la HHG. En particular, estudiamos la ioniza- ción túnel en moléculas, descubriendo que depende de la ubicación del electrón dentro de la molécula, debido a la naturaleza extendida de estas. Esta característica deja huellas importantes en los espectros de HHG y de fotoelectrones. Por lo tanto, hemos desarrollado una receta para implementar este fenómeno en los modelos de campos intensos existentes. A continuación, predecimos y describimos teóricamente la gene- ración de haces láser en el ultravioleta extremo (XUV) con nuevas propiedades de momento angular que, en la mayoría de los casos, son también creadas y caracterizadas experimentalmente por nuestros colaboradores del grupo Kapteyn-Murnane en JILA, en la Universidad de Colorado (EE. UU.), y del grupo del Prof. M.-Ch. Chen del Instituto de Tecnologías Fotónicas de la Universidad Tsing Hua (Taiwán). Para empezar, demostramos la generación, por primera vez, de haces de luz con OAM variable en el tiempo, una propiedad que denominamos como el auto-torque de la luz. Es importante destacar que los haces con auto-torque surgen naturalmente en el régimen XUV cuando el campo incidente para la HHG está formado por dos vórtices infrarro- jos retardados en el tiempo. Bajo esta configuración, el OAM de los armónicos de orden alto cambia a lo largo del tiempo en una escala de tiempo de attosegundos, siendo la cantidad de auto-torque controlada a través de las propiedades temporales de los pulsos incidentes. Por lo tanto, creemos que los haces con auto-torque pueden servir como nuevas herramientas para la manipulación láser-materia. Además, mostramos cómo el OAM puede servir como instrumento para mani- pular las propiedades espectrales y de divergencia de los armónicos de orden alto. Empleando dos vórtices con el contenido adecuado de OAM como pulsos incidentes, obtenemos peines de frecuencias de armónicos de orden alto con un espaciado entre líneas espectrales sintonizable y baja divergencia. Este control es particularmente intere- sante para espectroscopía y formación de imagen en el XUV o incluso en los rayos X blandos. Además, presentamos varios esquemas para el control de la eliptici- dad de los pulsos de attosegundo y de los armónicos de orden alto. Utilizando la configuración no colineal contrarrotante, extraemos el escalado de la elipticidad de los armónicos de orden alto con la de los haces incidentes y desvelamos la información sobre la respuesta dipolar oculta en esa conexión. Además, mostramos la generación de vórtices polarizados circularmente a partir de la HHG usando un campo incidente bi-circular vorticial. Destacablemente, al seleccionar correctamente el OAM del campo incidente, podemos obtener, o bien pulsos de attosegundo polarizados circularmente, o bien armónicos de orden alto con baja carga topológica. Por último, demostramos teóricamente la generación de trenes de pulsos de attosegundo con estados de polarización ordenados temporalmente mediante la combi- nación de dos campos incidentes bi-circulares vorticiales retardados en el tiempo. Creemos que la generación de pulsos de attosegundo con elipticidad controlada se puede emplear para el estudio de la dinámica ultrarrápida de SAM en moléculas quirales o materiales magnéticos. [EN] Ultrashort laser pulses are a unique tool to explore the fastest dy- namics in matter. Remarkably, the shortest laser pulses to date are produced from the non-linear frequency upconversion phenomenon of high-order harmonic generation (HHG), which results in the emis- sion of pulses of attosecond durations. Importantly, such attosecond pulses can exhibit a very exciting property, the angular momentum, which presents two different forms, the spin angular momentum (SAM) and the orbital angular momentum (OAM), and that brings new sce- narios for the light-matter interactions at the nanometric spatial and ultrashort temporal scales. In this thesis work, we develop a compilation of schemes for the creation of high-order harmonics and attosecond pulses with novel angular momentum properties by structuring the HHG process through the characteristics of the driving beams. For that purpose, we first address the description of the fundamental physical mechanisms of HHG. In particular, we study the tunnel ionization in molecules, finding that it is site-specific—its rate depends on the position of the electronic wavefunction at the ion sites—, due to the extended nature of the molecules. This characteristic leaves important signatures in the HHG and photoelectron spectra. Therefore, we provide a recipe for implementing the site-specificity in the existing strong-field models. Afterwards, we theoretically predict and describe the creation of extreme-ultraviolet (XUV) beams with novel angular momentum prop- erties, which, in most of the cases, are experimentally generated and characterized by our collaborators from the Kapteyn-Murnane group in JILA, at the University of Colorado (USA) and from the group of Prof. M.-Ch. Chen at the Institute of Photonics Technologies of the Tsing Hua University (Taiwan). To begin with, we demonstrate the generation, for the first time, of light beams with time-varying OAM, a property which we denote as the self-torque of light. Importantly, self- torqued beams arise naturally in the XUV regime from HHG driven by two time-delayed infrared vortex beams. Under this configuration, the OAM of the high-order harmonics changes along time in the attosec- ond time-scale, being the amount of self-torque controlled through the temporal properties of the driving pulses. Thus, we believe that self-torqued beams can serve as unprecedented tools for laser-matter manipulation. In addition, we show how the OAM can serve as an instrument to manipulate the spectral and divergence properties of the high-order harmonics. By driving HHG with two vortex beams with properly selected OAM, we obtain high-order harmonic frequency combs with tunable line-spacing and low divergence. Such control is particularly interesting for XUV/soft-X-ray spectroscopy and imaging. Moreover, we present several schemes for the ellipticity control of the high-order harmonics and attosecond pulses. Using the non-collinear counter-rotating scheme, we extract the scaling of the ellipticity of the high-order harmonics with that of the driving beams’ and we unveil the information about the non-perturbative dipole response hidden in that connection. Also, we show the generation of circularly polarized vortex beams from HHG driven by a bi-circular vortex field. Interest- ingly, by properly selecting the OAM of the driving field we can obtain either circularly polarized attosecond pulses, or high-order harmonics with low topological charge. Finally, we theoretically demonstrate the generation of attosecond pulse trains with time-ordered polarization states by combining two time-delayed bi-circular vortex driving fields. We believe that the generation of attosecond pulses with controlled ellipticity can be employed for the study of ultrafast spin dynamics in chiral molecules or magnetic materials

Generation and Applications of Extreme-Ultraviolet Vortices

Vortex light beams are structures of the electromagnetic field with a spiral phase ramp around a point-phase singularity. These vortices have many applications in the optical regime, ranging from optical trapping and quantum information to spectroscopy and microscopy. The extension of vortices into the extreme-ultraviolet (XUV)/X-ray regime constitutes a significant step forward to bring those applications to the nanometer or even atomic scale. The recent development of a new generation of X-ray sources, and the refinement of other techniques, such as harmonic generation, have boosted the interest of producing vortex beams at short wavelengths. In this manuscript, we review the recent studies in the subject, and we collect the major prospects of this emerging field. We also focus on the unique and promising applications of ultrashort XUV/X-ray vortex pulsesA.P. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 702565. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007-2013), under REA grant Agreement No 328334. We acknowledge support and from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2013-44174-P, FIS2015-71933-REDT, FIS2016-75652-P)

Roadmap of ultrafast x-ray atomic and molecular physics

X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Angstrom, and HHG provides unprecedented time resolution (∼50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ∼280 eV (44 Angstroms) and the bond length in methane of ∼1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science

Roadmap of ultrafast x-ray atomic and molecular physics

X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ~1 Angstrom, and HHG provides unprecedented time resolution (∼50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ∼280 eV (44 Angstroms) and the bond length in methane of ∼1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science

Roadmap of ultrafast x-ray atomic and molecular physics

X-ray free-electron lasers (XFELs) and table-top sources of x-rays based upon high harmonic generation (HHG) have revolutionized the field of ultrafast x-ray atomic and molecular physics, largely due to an explosive growth in capabilities in the past decade. XFELs now provide unprecedented intensity (1020 W cm−2) of x-rays at wavelengths down to ∼1 Ångstrom, and HHG provides unprecedented time resolution (∼50 attoseconds) and a correspondingly large coherent bandwidth at longer wavelengths. For context, timescales can be referenced to the Bohr orbital period in hydrogen atom of 150 attoseconds and the hydrogen-molecule vibrational period of 8 femtoseconds; wavelength scales can be referenced to the chemically significant carbon K-edge at a photon energy of ∼280 eV (44 Ångstroms) and the bond length in methane of ∼1 Ångstrom. With these modern x-ray sources one now has the ability to focus on individual atoms, even when embedded in a complex molecule, and view electronic and nuclear motion on their intrinsic scales (attoseconds and Ångstroms). These sources have enabled coherent diffractive imaging, where one can image non-crystalline objects in three dimensions on ultrafast timescales, potentially with atomic resolution. The unprecedented intensity available with XFELs has opened new fields of multiphoton and nonlinear x-ray physics where behavior of matter under extreme conditions can be explored. The unprecedented time resolution and pulse synchronization provided by HHG sources has kindled fundamental investigations of time delays in photoionization, charge migration in molecules, and dynamics near conical intersections that are foundational to AMO physics and chemistry. This roadmap coincides with the year when three new XFEL facilities, operating at Ångstrom wavelengths, opened for users (European XFEL, Swiss-FEL and PAL-FEL in Korea) almost doubling the present worldwide number of XFELs, and documents the remarkable progress in HHG capabilities since its discovery roughly 30 years ago, showcasing experiments in AMO physics and other applications. Here we capture the perspectives of 17 leading groups and organize the contributions into four categories: ultrafast molecular dynamics, multidimensional x-ray spectroscopies; high-intensity x-ray phenomena; attosecond x-ray science