We perform a systematic analysis of how ultrafast photoelectron holography is influenced by an elliptically polarized field, with emphasis on quantum interference effects. We find that the interplay of the external field and the binding potential leads to twisted holographic patterns for low ellipticities and recover well-known angular offsets for high ellipticities. Using the Coulomb quantum-orbit strong-field approximation, we assess how the field ellipticity affects specific holographic patterns, such as the fan and the spider. The interplay of the external field and the binding potential leads to twisted holographic patterns in the fan and to loss of contrast in the spider. This behavior can be traced back to interfering electron trajectories and unequal changes in tunneling probability due to nonvanishing ellipticity. We also derive tunneling times analytically using the strong-field approximation, provide estimates for ellipticity ranges for which interference is expected to be prominent, and discuss how to construct continuous electron momentum distributions exploring the rotation symmetry around the origin.

What is the nature of tunnelling? This yet unanswered question is as pertinent today as it was at the dawn of quantum mechanics. This article presents a cross section of current perspectives on the interpretation, computational modelling, and numerical investigation of tunnelling processes in attosecond physics as debated in the Quantum Battles in Attoscience virtual workshop 2020.

For a variety of spectroscopic methods, control over the polarization of attosecond pulses is necessary to probe chiral-sensitive light-matter interactions [1] at ultrafast time scales. This is a challenge that goes beyond generating individual harmonics of particular polarization [2] , since filtering individual harmonics will lose the sub-femtosecond temporal resolution. In our work, we combine a bichromatic counter-rotating circularly polarized (BiCRCP) driving field with a spatial inhomogeneity. We show that the direction and strength of the plasmonic-enhanced field provides a knob to shape the polarization of attosecond pulses from a High Harmonic Generation (HHG) process [3] . Plasmonic-field-assisted HHG with linear polarisation yields a significant energy enhancement [4] , but relies on the photoelectron trajectory to be (dominantly) parallel to the polarization and inhomogeneity. HHG with BiCRCP driving fields can generate a spectrum consisting of doublets of harmonic orders with alternating helicity [2] , but this requires a three-fold-symmetry which is broken by the plasmonic-enhanced field.

High Harmonic Generation (HHG) creates coherent high frequency radiation via the process of strong field ionization followed by recombination. Recently, a complementary approach based on Frustrated Tunnel Ionization (FTI) was demonstrated (Nature Photonics 12, 620 (2018)). It uses spectrally separated peaks created by lower quantum number Rydberg states to produce coherent extreme ultraviolet (EUV) light. While much is understood about enhancing emission from HHG by controlling recombining electron trajectories, relatively little is known about controlling the quantum number distribution of Rydberg states. This distribution is generally believed to be determined primarily by field strength and laser frequency. We show that, in fact, it also changes significantly with the duration of the laser pulse: increasing pulse duration depletes lower-lying Rydberg states, thereby substantially decreasing EUV yield. Using electron trajectory analysis, we identify elastic recollision as the underlying cause. Our results open a door to greater control over production of coherent high frequency radiation, by combining FTI and HHG mechanisms, and also improved the interpretation of molecular imaging experiments that rely on elastic electron recollision.

Quantum tunneling time is a highly debated topic - we explain why. We discuss the attoclock technique to extracting tunneling delays with regards to the typical approximations such as the dipole approximation, non-adiabatic effects, photoelectron momenta at the tunnel exit, electron correlation and exit coordinate.

Received 18 December 2018DOI:https://doi.org/10.1103/PhysRevA.99.019901©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAtomic & molecular processes in external fieldsMultiphoton or tunneling ionization & excitationUltrafast phenomenaPhysical SystemsAtomsTechniquesAdiabatic approximationSemiclassical methodsAtomic, Molecular & Optical

We investigate numerically and analytically the intensity dependence of the fraction of electrons that end up in a Rydberg state after strong-field ionization with linearly polarized light. We find that including the intensity dependent distribution of ionization times and non-adiabatic effects leads to a better understanding of experimental results. Furthermore, we observe using Classical Trajectory Monte Carlo simulations that the intensity dependence of the Rydberg yield changes with wavelength and that the previously observed power-law dependence breaks down at longer wavelengths. Our work suggests that Rydberg yield measurements can be used as an independent test for non-adiabaticity in strong field ionization.

I. A. Ivanov1,∗ C. Hofmann, L. Ortmann, A. S. Landsman2,3, Chang Hee Nam1,4, Kyung Taec Kim1,4† Center for Relativistic Laser Science, Institute for Basic Science, Gwangju 61005, Korea Max Planck Institute for the Physics of Complex Systems, Noethnitzer Strasse 38, 01187 Dresden, Germany Max Planck Center for Attosecond Science/Department of Physics, Pohang University of Science and Technology, Pohang 37673, Korea Department of Physics and Photon Science, GIST, Gwangju 61005, Korea (Dated: August 4, 2018)

Due to their very nature, ultrafast phenomena are often accompanied by the occurrence of nonadiabatic effects. From a theoretical perspective, the treatment of nonadiabatic processes makes it necessary to go beyond the (quasi) static picture provided by the time-independent Schrödinger equation within the Born-Oppenheimer approximation and to find ways to tackle instead the full time-dependent electronic and nuclear quantum problem. In this review, we give an overview of different nonadiabatic processes that manifest themselves in electronic and nuclear dynamics ranging from the nonadiabatic phenomena taking place during tunnel ionization of atoms in strong laser fields to the radiationless relaxation through conical intersections and the nonadiabatic coupling of vibrational modes and discuss the computational approaches that have been developed to describe such phenomena. These methods range from the full solution of the combined nuclear-electronic quantum problem to a hierarchy of semiclassical approaches and even purely classical frameworks. The power of these simulation tools is illustrated by representative applications and the direct confrontation with experimental measurements performed in the National Centre of Competence for Molecular Ultrafast Science and Technology.

This work investigates High Harmonic Generation (HHG) in gas targets, induced by intense two-colour driving fields. We compared classical trajectory Monte Carlo simulations based on a semiclassical model of strong field tunnel ionisation of helium to experimental HHG spectra, and investigated the relative contribution of long trajectories to low harmonic orders. This phenomenon can be found even when the experimental setup is chosen to suppress long trajectories and favour phase matching for short trajectories.

The notion of the instantaneous ionization rate (IIR) is often employed in the literature for understanding the process of strong field ionization of atoms and molecules. This notion is based on the idea of the ionization event occurring at a given moment of time, which is difficult to reconcile with the conventional quantum mechanics. We describe an approach defining instantaneous ionization rate as a functional derivative of the total ionization probability. The definition is based on physical quantities, such as the total ionization probability and the waveform of an ionizing pulse, which are directly measurable. The definition is, therefore, unambiguous and does not suffer from gauge non-invariance. We compute IIR by numerically solving the time-dependent Schrödinger equation for the hydrogen atom in a strong laser field. In agreement with some previous results using attoclock methodology, the IIR we define does not show measurable delay in strong field tunnel ionization.

The use of semiclassical models based on the Strong Field Approximation (SFA) is ubiquitous in strong field multi-photon ionization and underlies many key developments in attosecond science, including the description of High Harmonic Generation (HHG). However, such models are notably lacking in streaking experiments, which use an attosecond pulse to initiate single-photon ionization and a lower frequency infrared pulse to provide timing information. Here, we introduce a classical Wigner propagation (CWP) method, which analogously to semiclassical models in strong field ionization, treats the ionization step quantum mechanically, followed subsequently by classical propagation (with initial conditions obtained from the Wigner function) in the infrared probe field. As we demonstrate, this method compares well with experimental data and full two-electron quantum simulations available for helium, includes multi-electron effects, and can be applied to molecules, where the full solution of the time-dependent Schrodinger equation is not feasible. Applying the CWP method to a many-atom molecule, like 2,3,3-trimethyl-butyl-2-iodide, we find a relatively significant directional dependence of streaking delays, indicating the importance of orientation-resolved measurements in molecules.

The validity of the adiabatic approximation in strong field ionization under typical experimental conditions has recently become a topic of great interest. Experimental results have been inconclusive, in part, due to the uncertainty in experimental calibration of intensity. Here we turn to the time dependent Schrodinger equation, where all the laser parameters are known exactly. We find that the centre of the electron momentum distribution (typically used for calibration of elliptically and circularly polarized light) is sensitive to non-adiabatic effects, leading to intensity shifts in experimental data that can significantly affect the interpretation of results. On the other hand, the transverse momentum spread in the plane of polarization is relatively insensitive to such effects, even in the Keldysh parameter regime approaching gamma approximate to 3. This suggests the transverse momentum spread in the plane of polarization as a good alternative to the usual calibration method, particularly for experimental investigation of non-adiabatic effects using circularly polarized light.

We investigate multi-electron effects in strong-field ionization of helium using a semi-classical model that, unlike other commonly used theoretical approaches, takes into account electron-electron correlation. Our approach has an additional advantage of allowing to selectively switch off different contributions from the parent ion (such as the remaining electron or the nuclear charge) and thereby investigate in detail how the final electron angle in the attoclock experiment is influenced by these contributions. We find that the bound electron exerts a significant effect on the final electron momenta distribution that can, however, be accounted for by an appropriately selected mean field. Our results show excellent agreement with other widely used theoretical models done within a single active electron approximation.

In strong-field tunnel ionization of Helium, both adiabatic and fully non-adiabatic theoretical descriptions predict smaller final longitudinal electron momentum distributions than measured experimentally. Semiclassical simulations including an initial longitudinal momentum spread reproduce experimental values.

Strong field ionization momentum distribution contains much information. The angular distribution indicates real tunneling delay time, the center shows ellipticity dependent Coulomb effects, and the longitudinal momentum spread challenges the assumption of zero initial momentum.

We investigate whether nonadiabatic effects, rather than an initial longitudinal momentum spread, can explain the additional final momentum spread measured in strong-field ionization experiments with ultrafast laser pulses. We find that, when used consistently, a well-known nonadiabatic theory which includes an initial velocity offset yields results similar to adiabatic theory. By "consistent use" we mean that nonadiabatic theory is used also for field strength calibration of the experiment. The additional momentum spread can be accounted for by including an initial longitudinal momentum spread, as was done previously in the adiabatic case. Interestingly, when the experimental intensity is calibrated using a common in situ calibration method based on adiabatic assumptions, the nonadiabatic theory improves upon the adiabatic theory. This result highlights the uncertainty associated with using theory-based calibration methods, which are the most common way of calibrating experimental data in attosecond science.

We present experimental data that show significant deviations from theoretical predictions for the location of the center of the electron momenta distribution at low values of ellipticity ε of laser light. We show that these deviations are caused by significant Coulomb focusing along the minor axis of polarization, something that is normally neglected in the analysis of electron dynamics, even in cases where the Coulomb correction is otherwise taken into account. By investigating ellipticity-resolved electron momenta distributions in the plane of polarization, we show that Coulomb focusing predominates at lower values of ellipticity of laser light, while Coulomb asymmetry becomes important at higher values, showing that these two complementary phenomena can be used to probe long-range Coulomb interaction at all polarizations of laser light. Our results suggest that both the breakdown of Coulomb focusing and the onset of Coulomb asymmetry are linked to the disappearance of Rydberg states with increasing ellipticity.

We introduce a method to investigate the longitudinal momentum spread resulting from strong-field tunnel ionization of helium which, unlike other methods, is valid for all ellipticities of laser pulse. Semiclassical models consisting of tunnel ionization followed by classical propagation in the combined ion and laser field reproduce the experimental results if an initial longitudinal spread at the tunnel exit is included. The values for this spread are found to be of the order of twice the transverse momentum spread.