Slalom in complex time: emergence of low-energy structures in tunnel ionization via complex time contours

The ionization of atoms by strong, low-frequency fields can generally be described well by assuming that the photoelectron is, after the ionization step, completely at the mercy of the laser field. However, certain phenomena, like the recent discovery of low-energy structures in the long-wavelength regime, require the inclusion of the Coulomb interaction with the ion once the electron is in the continuum. We explore the first-principles inclusion of this interaction, known as analytical R-matrix theory, and its consequences on the corresponding quantum orbits. We show that the trajectory must have an imaginary component, and that this causes branch cuts in the complex time plane when the real trajectory revisits the neighbourhood of the ionic core. We provide a framework for consistently navigating these branch cuts based on closest-approach times, which satisfy the equation $\mathbf{r}(t) \cdot \mathbf{v}(t) = 0$ in the complex plane. We explore the geometry of these roots and describe the geometrical structures underlying the emergence of LES in both the classical and quantum domains.Comment: Supplementary information at http://episanty.github.io/Slalom-in-complex-time

Momentum transfers in correlation-assisted tunnelling

We consider correlation-assisted tunnel ionization of a small molecule by an intense low-frequency laser pulse. In this mechanism, the departing electron excites the state of the ion via a Coulomb interaction. We show that the angular distribution for this process has significant qualitative differences compared to direct tunnelling of an electron from a deeper orbital. These differences could be used to distinguish the two contributions, and give rise to interference effects when the contributions are comparable. The saddle-point approximation is also shown to require special attention in this geometric analysis.Comment: 6 pages, 4 figure

Relativity and Magnetism in Ni-Pd and Ni-Pt Alloys

We show that the differences in the magnetic properties of Ni-Pd and Ni-Pt alloys arise mainly due to relativity. In particular, we find that the local magnetic moment of Ni increases with the addition of Pd in Ni-Pd while it decreases with the addition of Pt in Ni-Pt, as found experimentally, only if relativity is present. Our analysis is based on the effects of relativity on (i) the spin-polarized densities of states of Ni, (ii) the splitting of majority and minority spin d-band centers of Ni, and (iii) the separation between s-d band centers of Pd and Pt in Ni-Pd and Ni-Pt alloys.Comment: one figure added, a paragraph added in discussio

Electron dynamics in complex time and complex space

This thesis investigates the dynamics of electrons ionized by strong low frequency laser fields, from a semiclassical perspective, developing a trajectory-based formalism to describe the interactions of the outgoing electron with the remaining ion. Trajectory models for photoionization generally arise in the regime known as optical tunnelling, where the atom is subjected to a strong, slow field, which tilts the potential landscape around the ion, forming a potential energy barrier that electrons can then tunnel through. There are multiple approaches that enable the description of the ionized electron, but they are generally limited or models derived by analogy, and the status of the trajectories is unclear. This thesis analyses this trajectory language in the context of the Analytical R-Matrix theory of photoionization, deriving a trajectory model from the fundamentals, and showing that this requires both the time and the position of the trajectory to be complex. I analyse this complex component of the position and I show that it requires careful handling: of the potentials where it appears, and of the paths in the complex plane that the trajectory is taken through. In this connection, I show that the Coulomb potential of the ion induces branch cuts in the complex time plane that the integration path needs to avoid, and I show how to navigate these branch cuts. I then use this formalism to uncover a kinematic mechanism for the recently discovered (Near-)Zero Energy Structures of above-threshold ionization. In addition, I analyse the generation of high-order harmonics of the driving laser that are emitted when the photoelectron recollides with the ion, using a pair of counter-rotating circularly polarized pulses to drive the emission, both in the context of the conservation of spin angular momentum and as a probe of the long-wavelength breakdown of the dipole approximation.Open Acces

Kinematic origin for near-zero energy structures in mid-IR strong field ionization

We propose and discuss a kinematic mechanism underlying the recently discovered 'near-zero energy structure' in the photoionization of atoms in strong mid-infrared laser fields, based on trajectories which revisit the ion at low velocities exactly analogous to the series responsible for low-energy structures. The different scaling of the new series, as $E\sim {I}_{p}^{2}/{U}_{p}$, suggests that the near-zero energy structure can be lifted to higher energies, where it can be better resolved and studied, using harder targets with higher ionization potential

Kinematic origin for near-zero energy structures in mid-IR strong field ionization

We propose and discuss a kinematic mechanism underlying the recently discovered 'near-zero energy structure' in the photoionization of atoms in strong mid-infrared laser fields, based on trajectories which revisit the ion at low velocities exactly analogous to the series responsible for low-energy structures. The different scaling of the new series, as $E\sim {I}_{p}^{2}/{U}_{p}$, suggests that the near-zero energy structure can be lifted to higher energies, where it can be better resolved and studied, using harder targets with higher ionization potential

Desnutrición crónica en escolares: itinerarios de desatención nutricional y programas oficiales en comunidades indígenas de Guerrero, México

En este texto se analizan diversos elementos de la dinámica nutricional de un municipio predominantemente indígena de Guerrero, México, con el propósito de explorar la pertinencia del concepto de itinerarios de desatención nutricional para ampliar la comprensión del daño evitable a la salud. En el marco de un proceso de acompañamiento a la Comisión de Salud de la Coordinadora Regional de Autoridades Comunitarias - Policía Comunitaria, en el transcurso del año 2015, se aplicaron los siguientes recursos metodológicos: a) revisión de programas oficiales mediante consulta de documentos técnicos y entrevistas con personal sanitario; b) obtención de narrativas locales en familias de niños desnutridos, mediante entrevistas y observación participante; c) realización de somatometría en 151 menores en tres escuelas primarias de la región. Se resalta la inadecuación técnica y cultural de los programas oficiales de atención a la desnutrición y la existencia de distintas escalas (individual, familiar, comunitaria, estatal, estructural) y dimensiones (económica, ambiental, cultural, institucional, afectivo-emocional) de desatención, materializadas en altos índices de desnutrición crónica.Through the analysis of different elements of the nutritional dynamics in a predominantly indigenous municipality in Guerrero, Mexico, we explore the relevance of the concept of itineraries of nutritional neglect in order to broaden the understanding of avoidable health damage. In the framework of a process of accompaniment of the Health Commission of the Regional Coordination of Community Authorities - Community Police, the following methodological strategies were applied throughout the year 2015: a) a review of official programs by means of the analysis of technical documents and interviews with health personnel; b) the compilation of local narratives from families of undernourished children through interviews and participatory observation; c) the somatometric measurement of 151 children in three elementary schools. We highlight the technical and cultural inadequacy of official nutrition programs and the existence of different scales (individual, familial, community, state, structural) and dimensions (economic, environmental, cultural, institutional, affective-emotional) of neglect, materialized in high rates of chronic undernourishment

Spin conservation in high-order-harmonic generation using bicircular fields

We present an alternative theoretical model for a recent experiment [A.Fleischer et al., Nature Photon. 8, 543 (2014)] which used bichromatic, counter-rotating high intensity laser pulses to probe the conservation of spin angular momentum in high harmonic generation. We separate elliptical polarizations into independent circular fields with definite angular momentum, instead of using the expectation value of spin for each photon in the conservation equation, and we find good agreement with the experimental results. In our description the generation of each individual harmonic conserves spin angular momentum, in contrast to the model proposed by Fleischer et al. Our model also correctly describes analogous processes in standard perturbative optics.Comment: Final changes from published version. Updated license to CC-BY-NC-S

Quantum tunnelling without a barrier

Tunnelling is a renowned concept in modern physics that highlights the peculiarity of non-classical dynamics. Despite its ubiquity questions remain. We focus on tunnelling through the barrier created by a strong laser field that illuminates an atomic target, which is essential to the creation of attosecond pulses and ultimately all attosecond processes. Here, we present an optical tunnelling event that, unexpectedly, happens at a time when the instantaneous electric field is zero and there is no barrier. We discover this strong-field ionisation event by introducing the colour-switchover technique $-$ the gradual replacement of a laser field with its second harmonic $-$ within which the zero-field tunnelling appears when the two amplitudes are equal. This event is a topologically stable feature and it appears at all Keldysh parameters. The tunnelling without a barrier highlights the disconnect between the standard intuition built on the picture of a quasi-static barrier, and the nonadiabatic nature of the process. Our findings provide a key ingredient to the understanding of strong-field processes, such as high-harmonic generation and laser-induced electron diffraction, driven by the increasingly accessible class of strongly polychromatic light fields.Comment: 7 pages, 5 figure