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

Trains of attosecond pulses structured with time-ordered polarization states

[EN]Ultrafast laser pulses generated at the attosecond timescale represent a unique tool to explore the fastest dynamics in matter. An accurate control of their properties, such as polarization, is fundamental to shape three-dimensional laser-driven dynamics. We introduce a technique to generate attosecond pulse trains whose polarization state varies from pulse to pulse. This is accomplished by driving high-harmonic generation with two time-delayed bichromatic counter-rotating fields with proper orbital angular momentum (OAM) content. Our simulations show that the evolution of the polarization state along the train can be controlled via OAM, pulse duration, and time delay of the driving fields. We, thus, introduce an additional control into structured attosecond pulses that provides an alternative route to explore ultrafast dynamics with potential applications in chiral and magnetic materials.Junta de Castilla y León (SA287P18); FEDER funds; Ministerio de Ciencia, Innovación y Universidades (FIS2016-75652-P, RYC-2017-22745, PID2019-106910GBI00); Ministerio de Educación, Cultura y Deporte (FPU16/02591); EuropeanResearch Council (ERC) (851201); Barcelona Supercomputing Center (FI-2019-1-0013)

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)

Necklace-structured high-harmonic generation for low-divergence, soft x-ray harmonic combs with tunable line spacing

The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse necklace-shaped phased array of linearly polarized HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme ultraviolet and soft x-ray high-harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs—particularly in the soft x-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles, and materials.The JILA team graciously acknowledges support from the Department of Energy BES Award No. DE-FG02-99ER14982 for the experimental implementation, a MURI grant from the Air Force Office of Scientific Research under Award No. FA9550-16-1-0121 for the mid-infrared laser soft x-ray research, and a National Science Foundation Physics Frontier Center grant PHY-1734006 for theory. N.J.B. acknowledges support from National Science FoundationGraduate Research Fellowships (grant no. DGE-1650115). Q.L.D.N. acknowledges support from National Science Foundation Graduate Research Fellowships (grant no. DGE-1144083). J.S.R., L.P., and C.H.-G. acknowledge support from Ministerio de Ciencia e Innovación (FIS2016-75652-Pand PID2019-106910GB-I00). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant agreement no. 851201). J.S.R., L.P., and C.H.-G. also acknowledge support from Junta de Castilla y León FEDER funds (project no. SA287P18). L.R. acknowledges support from Ministerio de Educación, Cultura y Deporte (FPU16/02591). C.H.-G. acknowledges Ministerio de Ciencia, Innovación, y Universidades for Ramón y Cajal contract (RYC-2017-22745), cofunded by the European Social Fund. L.R., J.S.R., L.P., and C.H.-G. thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (FI-2020-3-0013)

Realization of Polarization Control in High-Order Harmonic Generation

The nature of high-order harmonic generation process limits the harmonics emission to linear polarization. In this paper, we review the recent progress to generate elliptically or circularly polarized high-harmonic EUV pulses. We further demonstrate how complete control of polarization state of isolated high-harmonic pulse can be realized today by noncollinear focusing of two driving pulses with identical ellipticity but counter-rotating helicity. This paper opens a path towards the study of the fastest dynamics--down to attosecond time scales--in circular dichroism of magnetic materials, chiral molecules, and electronic spin motion.Taiwan Ministry of Science and Technology; Academia Sinica; Junta de Castilla y León; Ministerio de Economía y Competitividad; Leonardo Grant for Researchers and Cultural Creators, BBVA Foundation; Ministerio de Ciencia, Innovación y Universidades for a Ramón y Cajal; European Social Fund; Ministerio de Educación, Cultura y Deporte

La divulgación de fenómenos ópticos como recurso didáctico

Memoria ID-124. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2019-2020.[ES]El objetivo principal del proyecto es demostrar a los estudiantes que la labor divulgativa, en particular en el ámbito de la óptica y la fotónica, además de un servicio hacia la sociedad muy enriquecedor, implica una refelexión y profundización sobre los conceptos manejados en las charlas y/o experiencias que es tremendamente útil para un mejor aprendizaje de los mismos por parte de los divulgadore

Práctica tipo "descubrimiento" sobre la propagación de la luz láser

Memoria ID-120. Ayudas de la Universidad de Salamanca para la innovación docente, curso 2018-2019

Nueva metodología y recursos online para la adaptación de las prácticas de laboratorio de Óptica a escenarios docentes con diferente grado de presencialidad

Memoria ID-040 Ayudas de la Universidad de Salamanca para la innovación docente, curso 2020-2021

High-order Nonlinear Dipole Response Characterized by Extreme-Ultraviolet Ellipsometry

Polarization engineering and characterization of coherent high-frequency radiation are essential to investigate and control the symmetry properties of light–matter interaction phenomena at their most fundamental scales. This work demonstrates that polarization control and characterization of high-harmonic generation provides an excellent ellipsometry tool that can fully retrieve both the amplitude and phase of a strong-field-driven dipole response. The polarization control of high-harmonic generation is realized by a transient nonlinear dipole grating coherently induced by two noncollinear counterrotating laser fields.By adjusting the ellipticity of the two driving pulses simultaneously, the polarization state of every high-harmonic order can be tuned from linear to highly elliptical, and it is fully characterized through an energy-resolved extreme ultraviolet polarimeter. From the analysis of the polarization state, the ellipsometry indicated that both the amplitude and phase of the high-harmonic dipole scale rapidly with the driving laser field for higher-order harmonics, and, especially, for gases with a small ionization potential. Our experimental results were corroborated by theoretical simulations. Our findings revealed a novel high-harmonic ellipsometry technique that can be used for the next generation of high-harmonic spectroscopy and attosecond metrology studies because of its ability to provide single-digit attosecond accuracy.Our work also paves the way to precisely quantify the strong-field dynamics of fundamental processes associated with the transfer of energy and angular momentum between electron/spin systems and the symmetry-dependent properties of molecules and materials.Ministry of Science and Technology (109-2634-F-007-023, 109-2636-M-007-008); Ministry of Education; Japan Society for the Promotion of Science KAKENHI (JSPS) (19H02637, 20H05670); Ministerio de Ciencia e Innovación y Universidades (FIS2016-75652-P, PID2019-106910GB-I00, RYC-2017-22745); Junta de Castilla y León FEDER (SA287P18); Ministerio de Educación, Cultura y Deporte (FPU16/02591); H2020 European Research Council (ERC) (851201)