49 research outputs found
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 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
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
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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 , 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
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
The imaginary part of the high-harmonic cutoff
High-harmonic generation - the emission of high-frequency radiation by the
ionization and subsequent recombination of an atomic electron driven by a
strong laser field - is widely understood using a quasiclassical trajectory
formalism, derived from a saddle-point approximation, where each saddle
corresponds to a complex-valued trajectory whose recombination contributes to
the harmonic emission. However, the classification of these saddle-points into
individual quantum orbits remains a high-friction part of the formalism. Here
we present a scheme to classify these trajectories, based on a natural
identification of the (complex) time that corresponds to the harmonic cutoff.
This identification also provides a natural complex value for the cutoff
energy, whose imaginary part controls the strength of quantum-path interference
between the quantum orbits that meet at the cutoff. Our construction gives an
efficient method to evaluate the location and brightness of the cutoff for a
wide class of driver waveforms by solving a single saddle-point equation. It
also allows us to explore the intricate topologies of the Riemann surfaces
formed by the quantum orbits induced by nontrivial waveforms.Comment: Supplementary Material is available at
https://imaginary-harmonic-cutoff.github.io with a stable version at
https://doi.org/10.5281/zenodo.369256
High-harmonic generation: taking control of polarization
The ability to control the polarization of short-wavelength radiation generated by high-harmonic generation is useful not only for applications but also for testing conservation laws in physics
Principal frequency of an ultrashort laser pulse
We introduce an alternative definition of the main frequency of an ultrashort
laser pulse, the principal frequency . This parameter is
complementary to the most accepted and widely used carrier frequency
. Given the fact that these ultrashort pulses, also known as
transients, have a temporal width comprising only few cycles of the carrier
wave, corresponding to a spectral bandwidth covering several
octaves, describes, in a more precise way, the dynamics driven by
these sources. We present examples where, for instance, is able to
correctly predict the high-order harmonic cutoff independently of the carrier
envelope phase. This is confirmed by solving the time-dependent Schr\"odinger
equation in reduced dimensions, supplemented with the time-analysis of the
quantum spectra, where it is possible to observe how the sub-cycle electron
dynamics is better described using . The concept of ,
however, can be applied to a large variety of scenarios, not only within the
strong field physics domain.Comment: 11 pages, 6 figures, accepted in PR