High-order harmonic generation in graphene and carbon nanotubes

Tesis por compendio de publicaciones[EN]This thesis presents a comprehensive theoretical study of the process of high-order harmonic generation (HHG) induced by intense fewcycle infrared laser pulses in two different types of low dimensional carbon allotropes: 2D single layer graphene (SLG) and 1D singlewall carbon nanotubes (SWNTs). Our results show the emergence of a non-perturbative spectral plateau at large intensities but, unlike other more common systems, such as atoms, molecules or bulk solids, there is no simple law governing the scaling of the cut-off frequency. Interpreting this particular behavior allows to unveil the fundamental mechanism for HHG in those low dimensional carbon allotropic structures. Using a model for the emission dipole based on the saddlepoint approximation, we show that the first step for HHG in these carbon compounds is radically different from the tunneling ionization/excitation process found in gas systems and finite gap solids, and that is closely related to the singular geometry of their band structure. In this sense, we demonstrate the crucial role that Dirac points in graphene and van Hove singularities in SWNTs play in the creation of electron-hole pairs. We also show that the high-order harmonic response in SLG is highly anisotropic, making it possible to emit elliptically polarized harmonics from linear-polarized drivers, and linearly polarized harmonics from elliptically-polarized pulses[ES]Esta tesis presenta un estudio teórico exhaustivo del proceso de generación de armónicos de orden alto (HHG) inducidos por pulsos láser infrarrojos, ultracortos e intensos, en dos tipos diferentes de alótropos de carbono de baja dimensión: grafeno monocapa 2D (SLG) y nanotubos de carbono de pared simple 1D (SWNTs). Los resultados obtenidos muestran la aparición de una meseta espectral no perturbativa cuando la intensidad del láser es lo suficientemente elevada, aunque a diferencia de otros sistemas más conocidos, como átomos, moléculas o sólidos semiconductores, no parece existir una ley simple que gobierne el escalado de la frecuencia de corte espectral con la intensidad del pulso. La interpretación de este comportamiento particular nos permite revelar el mecanismo fundamental para la HHG en esas estructuras alotrópicas. Usando un modelo para la emisión dipolar basado en la aproximación del punto de silla, mostramos que el primer paso para la HHG en estos materiales es radicalmente diferente del proceso de ionización/excitación por efecto túnel que se observa en sistemas gaseosos y en sólidos semiconductores, y que está estrechamente relacionado con la geometría singular de su estructura de bandas. En este sentido, demostramos el papel crucial que los puntos de Dirac en el grafeno y las singularidades de van Hove en los SWNTs juegan en la creación de pares electrón-hueco. También mostramos que la respuesta armónica de orden alto en SLG es altamente anisotrópica, lo que hace posible la emisión de armónicos polarizados elípticamente a partir de pulsos láser con polarización lineal, y de armónicos polarizados linealmente a partir de pulsos polarizados elípticamente

High-order harmonic generation in graphene and carbon nanotubes

Tesis por compendio de publicaciones[ES] Esta tesis presenta un estudio teórico exhaustivo del proceso de generación de armónicos de orden alto (HHG) inducidos por pulsos láser infrarrojos, ultracortos e intensos, en dos tipos diferentes de alótropos de carbono de baja dimensión: grafeno monocapa 2D (SLG) y nanotubos de carbono de pared simple 1D (SWNTs). Los resultados obtenidos muestran la aparición de una meseta espectral no perturbativa cuando la intensidad del láser es lo suficientemente elevada, aunque a diferencia de otros sistemas más conocidos, como átomos, moléculas o sólidos semiconductores, no parece existir una ley simple que gobierne el escalado de la frecuencia de corte espectral con la intensidad del pulso. La interpretación de este comportamiento particular nos permite revelar el mecanismo fundamental para la HHG en esas estructuras alotrópicas. Usando un modelo para la emisión dipolar basado en la aproximación del punto de silla, mostramos que el primer paso para la HHG en estos materiales es radicalmente diferente del proceso de ionización/excitación por efecto túnel que se observa en sistemas gaseosos y en sólidos semiconductores, y que está estrechamente relacionado con la geometría singular de su estructura de bandas. En este sentido, demostramos el papel crucial que los puntos de Dirac en el grafeno y las singularidades de van Hove en los SWNTs juegan en la creación de pares electrón-hueco. También mostramos que la respuesta armónica de orden alto en SLG es altamente anisotrópica, lo que hace posible la emisión de armónicos polarizados elípticamente a partir de pulsos láser con polarización lineal, y de armónicos polarizados linealmente a partir de pulsos polarizados elípticamente. [EN] This thesis presents a comprehensive theoretical study of the process of high-order harmonic generation (HHG) induced by intense fewcycle infrared laser pulses in two different types of low dimensional carbon allotropes: 2D single layer graphene (SLG) and 1D singlewall carbon nanotubes (SWNTs). Our results show the emergence of a non-perturbative spectral plateau at large intensities but, unlike other more common systems, such as atoms, molecules or bulk solids, there is no simple law governing the scaling of the cut-off frequency. Interpreting this particular behavior allows to unveil the fundamental mechanism for HHG in those low dimensional carbon allotropic structures. Using a model for the emission dipole based on the saddlepoint approximation, we show that the first step for HHG in these carbon compounds is radically different from the tunneling ionization/excitation process found in gas systems and finite gap solids, and that is closely related to the singular geometry of their band structure. In this sense, we demonstrate the crucial role that Dirac points in graphene and van Hove singularities in SWNTs play in the creation of electron-hole pairs. We also show that the high-order harmonic response in SLG is highly anisotropic, making it possible to emit elliptically polarized harmonics from linear-polarized drivers, and linearly polarized harmonics from elliptically-polarized pulses

Theory of high-order harmonic generation for gapless graphene

[EN]We study the high-harmonic spectrum emitted by a single-layer graphene, irradiated by an ultrashort intense infrared laser pulse. We show the emergence of the typical non-perturbative spectral features, harmonic plateau and cut-off, for mid-infrared driving fields, at fluences below the damage threshold. In contrast to previous works, using THz drivings, we demonstrate that the harmonic cut-off frequency saturates with the intensity. Our results are derived from the numerical integration of the time-dependent Schrödinger equation using a nearest neighbor tight-binding description of graphene. We also develop a saddle-point analysis that reveals a mechanism for harmonic emission in graphene different from that reported in atoms, molecules and finite gap solids. In graphene, the first step is initiated by the non-diabatic crossing of the valence band electron trajectories through the Dirac points, instead of tunneling ionization/excitation. We include a complete identification of the trajectories contributing to any particular high harmonic and reproduce the harmonic cut-off scaling with the driving intensity.We acknowledge fruitful discussions with I J Sola, H Crespo, E Pinsanty, J Biegert, J M Pérez-Iglesias, R Rengel, M J Martín and C Hernández García. We acknowledge support from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2013-44174-P, FIS2016-75652-P) and European Union (FEDER). AP acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 70256

Transverse phase matching of high-order harmonic generation in single-layer graphene

[EN]The efficiency of high-harmonic generation (HHG) from a macroscopic sample is strongly linked to the proper phase matching of the contributions from the microscopic emitters. We develop a combined micro+macroscopic theoretical model that allows us to distinguish the relevance of high-order harmonic phase matching in single-layer graphene. For a Gaussian driving beam, our simulations show that the relevant HHG emission is spatially constrained to a phase-matched ring around the beam axis. This remarkable finding is a direct consequence of the non-perturbative behavior of HHG in graphene—whose harmonic efficiency scaling is similar to that already observed in gases— and bridges the gap between the microscopic and macroscopic HHG in single-layer graphene.This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 851201). We thankfully acknowledge the computer resources at MareNostrum and the technical support provided by Barcelona Supercomputing Center (FI-2020-3-0013). Junta de Castilla y León and FEDER (SA287P18); European Research Council (851201); Ministerio de Ciencia, Innovación y Universidades (FIS2016-75652-P, RYC-2017-22745, PID2019-106910GB-I00)

High harmonic generation in armchair carbon nanotubes

[EN]We study high-order harmonic generation (HHG) in armchair-type single-wall carbon nanotubes (SWNTs) driven by ultrashort, mid-infrared laser pulses. For a SWNT with chiral indices (n, n), we demonstrate that HHG is dominated by bands |m| = n − 1 and that the cut-off frequency saturates with intensity, as it occurs in the case of single layer graphene. As a consequence, HHG in SWNTs can be described effectively as a one-dimensional periodic system, whose high-frequency emission can be modified through the proper control of the structural parameters. Additionally, we show that the HHG mechanism in nanotubes has some similarities to that previously reported in single layer graphene. However, as a main difference, the electron-hole pair excitation in SWNTs is connected to the non-adiabatic crossing through the first van Hove singularity of the |m| = n − 1 bands, instead of the crossing through the Dirac point that takes place in graphene.Junta de Castilla y León, European Regional Development Fund(SA287P18); Ministerio de Ciencia, Innovación y Universidades (FIS2016-75652-P, RYC-2017-22745, PID2019-106910GB-I00); European Research Council (851201

High-order harmonic spectroscopy of polycrystalline graphene

European Research Council (851201); Ministerio de Educación y Formación Profesional (FPU18/03348); Junta de Castilla y León (SA287P18); Ministerio de Ciencia, Innovación y Universidades (PID2019-106910GB-100, RYC-2017-22745).Present mass production of large-area single-layer graphene relies fundamentally on chemical vapor deposition methods. The generation of grain boundaries, which divides the sample into a set of crystalline domains, is inherent to these fabrication methods. Recent studies have demonstrated a strong anisotropy in the ultrafast non-linear response of single-layer graphene when subjected to non-perturbative, intense laser fields below the damage threshold. We propose to exploit this anisotropy to characterize the size distribution of graphene domains in polycrystals via high-order harmonic polarimetry. Our simulation results demonstrate the sensitivity of the harmonic polarization state to details of the polycrystal grain distribution. In particular, we show that the rotation in the polarization tilt of the highest-order harmonics holds information about the grain distribution in the polycrystal. As a proof-of-concept, we propose a method to determine the standard deviation of the grain size distribution from the values of the most frequent grain size and the standard deviation of the harmonic tilt rotation from a set of hypothetical measurements on different polycrystal realizations. Our work reveals the capability of high-order harmonic polarimetry to characterize polycrystalline two-dimensional materials

Optical anisotropy of non-perturbative high-order harmonic generation in gapless graphene

[EN]High harmonic generation in atomic or molecular targets stands as a robust mechanism to produce coherent ultrashort pulses with controllable polarization in the extreme-ultraviolet. However, the production of elliptically or circularly-polarized harmonics is not straightforward, demanding complex combinations of elliptically or circularly-polarized drivers, or the use of molecular alignment techniques. Nevertheless, recent studies show the feasibility of high-harmonic generation in solids. In contrast with atoms and molecules, solids are high-density targets and therefore more efficient radiation sources. Among solid targets, 2D materials are of special interest due to their particular electronic structure, which conveys special optical properties. In this paper, we present theoretical calculations that demonstrate an extraordinary complex light-spin conversion in single-layer graphene irradiated at non perturbative intensities. Linearly-polarized drivings result in the emission of elliptically-polarized harmonics, and elliptically-polarized drivings may result in linearly-polarized or ellipticity-reversed harmonics. In addition, we demonstrate the ultrafast temporal modulation of the harmonic ellipticity.Junta de Castilla y León (SA287P18); Agencia Estatal de Innovación (FIS2016-75652-P, EQC2018-004117-P); Comunidad de Madrid (2017-T1/IND-5432); Agencia Estatal de Investigación (RYC-2017-22745)

Non-classical high harmonic generation in graphene driven by linearly-polarized laser pulses

[EN]Recent studies in high-order harmonic generation (HHG) in solid targets reveal new scenarios of extraordinary rich electronic dynamics, in comparison to the atomic and molecular cases. For the later, the main aspects of the process can be described semiclassically in terms of electrons that recombine when the trajectories revisit the parent ion. HHG in solids has been described by an analogous mechanism, in this case involving electron-hole pair recombinations. However, it has been recently reported that a substantial part of the HHG emission corresponds to situations where the electron and hole trajectories do not overlap in space. According to the present knowledge, HHG from this imperfect recollisions reflects the quantum nature of the process, arising in systems with large Berry curvatures or for elliptically polarized driving fields. In this work, we demonstrate that imperfect recollisions are also relevant in the more general case. We show the signature of such recollisions in the HHG spectrum from monolayer graphene —a system with null Berry curvature— irradiated by linearly polarized driving fields. Our calculations also reveal that imperfect multiple-order recollisions contribute to the harmonic emission when electron-hole excursion times exceed one cycle of the driving field. We believe that our work adds a substantial contribution to the full understanding of the sub-femtosecond dynamics of HHG in solid systems.European Research Council (851201); Ministerio de Educación, Cultura y Deporte (FPU18/03348); FEDER funds (SA287P18); Junta de Castilla y León (SA287P18); Ramón y Cajal (RYC-2017-22745); Ministerio de Ciencia, Innovación y Universidades (PID2019-106910GB-100)

Topological high-harmonic spectroscopy.

[EN]Linearly polarized vector beams are structured lasers whose topology is characterized by a well-defined Poincar index, which is a topological invariant during high-order harmonic generation. As such, harmonics are produced as extreme-ultraviolet vector beams that inherit the topology of the driver. This holds for isotropic targets such as oble gases, but analogous behaviour in crystalline solids is still open to discussion. Here, we demonstrate that this conservation rule breaks in crystalline solids, in virtue of their anisotropic non-linear susceptibility. We identify the topological properties of the harmonic field as unique probes, sensitive to both the microscopic and macroscopic features of the target’s complex non-linear response. Our simulations, performed in single-layer graphene, show that the harmonic field is split into a multi-beam structure whose topology encodes information about laser-driven electronic dynamics. Our work promotes the topological analysis of the high-order harmonic field as a spectroscopic tool to reveal the nonlinearities in the coupling of light and target symmetries

Use of International Criticality Safety Benchmark Evaluation Project benchmarks for the validation of CSAS26 (Scale 4.4a) for configurations of low-enriched uranium

[ES]El artículo presenta la validación del módulo de control CSAS26 de SCALE 4.4a para el cálculo en ordenador personal, como un ejemplo de la aplicación del proyecto ICSBEP. Como resultado el módulo es considerado como un método calculacional bastante adecuado para la aplicación del análisis criticidad- seguridad al uranio de enriquecimiento bajo.[EN]As an example of the application of the lntemational Criticality Safety Benchmark Evaluation Project (ICSBEP) work to the nuclear industry, the validation of the control module CSAS26 of SCALE 4.4a for criticality calculations on a personal computer platform is presented. This work has been done using the models of critical experiments being compiled by the Organisation for Economic Co-operation and Development/Nuclear Energy Agency (OECD/NEA) since 1992. The description and results of this compilation were first presented during the Fifth lntemational Conference on Nuclear Criticality Safety(ICNC'95). Out of 2881 critical configurations included in the latest edition (September 2002) of the ICSBEP "lntemational Handbook of Evaluated Criticality Safety Benchmark Experiments." NEA/NSC/ DOC(95)03, OECD/NEA, 131 have been selectedfor the CSAS26 validation. The selected critical experiments have characteristics similar to the systems to be simulated with CSAS26 for low-enriched uranium (LEU) fuel fabrication applications. They represent both homogeneous configurations and hexagonallypitched rod lattices of low-enriched (from 1.60 to 5.00 wt% 235U) U02 with several absorbers. The statistical uncertainties related to the application of CSAS26 for criticality calculations are also evaluated. The great number of cases involved allows an exhaustive statistical treatment of the data, including the analysis of correlations related to the type of system being simulated. The statistical uncertainties found are very small. As a result, the module CSAS26 is considered as a quite suitable calculationalmethod for application to criticality safety analysis at LEU facilities