Identification of tunneling and multiphoton ionization in intermediate Keldysh parameter regime

Strong-Field Ionization Phenomena Revealed by Quantum Trajectories

We investigate the photoionization dynamics of atoms subjected to intense, ultrashort laser pulses through the use of quantum trajectories. This method provides a unique and consistent framework for examining electron dynamics within a time-dependent potential barrier. Our findings demonstrate that quantum trajectories offer additional insights into several key aspects of strong-field ionization, including the transition between ionization regimes, nonadiabatic effects under the barrier, the impact of the shape of the electronic potential, and the efficiency of over-the-barrier ionization.

Ultrafast dynamics of fluorene initiated by highly intense laser fields

We present an investigation of the ultrafast dynamics of the polycyclic aromatic hydrocarbon fluorene initiated by an intense femtosecond near-infrared laser pulse (810 nm) and probed by a weak visible pulse (405 nm). Using a multichannel detection scheme (mass spectra, electron and ion velocity-map imaging), we provide a full disentanglement of the complex dynamics of the vibronically excited parent molecule, its excited ionic states, and fragments. We observed various channels resulting from the strong-field ionization regime. In particular, we observed the formation of the unstable tetracation of fluorene, above-threshold ionization features in the photoelectron spectra, and evidence of ubiquitous secondary fragmentation. We produced a global fit of all observed time-dependent photoelectron and photoion channels. This global fit includes four parent ions extracted from the mass spectra, 15 kinetic-energy-resolved ionic fragments extracted from ion velocity map imaging, and five photoelectron channels obtained from electron velocity map imaging. The fit allowed for the extraction of 60 lifetimes of various metastable photoinduced intermediates.

The symmetry in the model of two coupled Kerr oscillators leads to simultaneous multi-photon transitions

We consider the model of two coupled oscillators with Kerr nonlinearities in the rotating-wave approximation. We demonstrate that for a certain set of parameters of the model, the multi-photon transitions occur between many pairs of the oscillator states simultaneously. Also, the position of the multi-photon resonances does not depend on the coupling strength between two oscillators. We prove rigorously that this is a consequence of a certain symmetry of the perturbation theory series for the model. In addition, we analyse the model in the quasi-classical limit by considering the dynamics of the pseudo-angular momentum. We identify the multi-photon transitions with the tunnelling transitions between the degenerate classical trajectories on the Bloch sphere.

Reconciling Conflicting Approaches for the Tunneling Time Delay in Strong Field Ionization

Several recent attoclock experiments have investigated the fundamental question of a quantum mechanically induced time delay in tunneling ionization via extremely precise photoelectron momentum spectroscopy. The interpretations of those attoclock experimental results were controversially discussed, because the entanglement of the laser and Coulomb field did not allow for theoretical treatments without undisputed approximations. The method of semiclassical propagation matched with the tunneled wave function, the quasistatic Wigner theory, the analytical R-matrix theory, the backpropagation method, and the under-the-barrier recollision theory are the leading conceptual approaches put forward to treat this problem, however, with seemingly conflicting conclusions on the existence of a tunneling time delay. To resolve the contradicting conclusions of the different approaches, we consider a very simple tunneling scenario which is not plagued with complications stemming from the Coulomb potential of the atomic core, avoids consequent controversial approximations and, therefore, allows us to unequivocally identify the origin of the tunneling time delay.

Photoionization of Electrons in Degenerate Energy Level of Hydrogen Atom Induced by Strong Laser Pulses

Photoionization dynamics of bounded electrons in the ground state, the first and second excited states of a hydrogen atom, triggered by ultrashort near-infrared laser pulses, have been investigated in a transition regime (γ∼1) that offers both multiphoton and tunneling features. Significant differences in spectral characteristics are found between the three low-energy states. The H(2s) ionization probability is larger than the H(2p) value with a special oscillating structure, but both are much greater than the ground state H(1s) in a wide range of laser intensities. By comparing the momentum spectrum and angular distributions of low-energy photoelectrons released from these degenerate states, we find the H(2p) state shows a stronger long-range Coulomb attraction force than the H(2s) state on account of the difference in the initial electron wave packet. Furthermore, analysis of the photoelectron momentum distributions sheds light on both the first and second excited states with a symmetrical intercycle interference structure in a multicycle field but an intracycle interference of an asymmetric left-handed or right-handed rotating spectrum in a few-cycle field. By analyzing photoelectron spectroscopy, we identify the parity characteristics of photoelectrons in different energy intervals and their corresponding above-threshold single-photon ionization (ATSI) or above-threshold double-photon ionization (ATDI) processes. We finally present the momentum distributions of the electrons ionized by laser pulses with different profiles and find the carrier-envelope phase (CEP) is a strong factor in deciding the rotating structure of the emission spectrum, which provides a new method to distinguish the CEP of few-cycle pulses.

Nonadiabatic Nano-optical Tunneling of Photoelectrons in Plasmonic Near-Fields

Nonadiabatic nano-optical electron tunneling in the transition region between multiphoton-induced emission and adiabatic tunnel emission is explored in the near-field of plasmonic nanostructures. For Keldysh γ values between ∼1.3 and ∼2.2, measured photoemission spectra show strong-field recollision driven by the nanoscale near-field. At the same time, the photoemission yield shows an intensity scaling with a constant nonlinearity, which is characteristic for multiphoton-induced emission. Our observations in this transition region were well reproduced with the numerical solution of Schrödinger's equation, mimicking the nanoscale geometry of the field. This way, we determined the boundaries and nature of nonadiabatic tunneling photoemission, building on a key advantage of a nanoplasmonic system, namely, that high-field-driven recollision events and their signature in the photoemission spectrum can be observed more efficiently due to significant nanoplasmonic field enhancement factors.

Thomson and collisional regimes of in-phase coherent microwave scattering off gaseous microplasmas

The total number of electrons in a classical microplasma can be non-intrusively measured through elastic in-phase coherent microwave scattering (CMS). Here, we establish a theoretical basis for the CMS diagnostic technique with an emphasis on Thomson and collisional scattering in short, thin unmagnetized plasma media. Experimental validation of the diagnostic is subsequently performed via linearly polarized, variable frequency (10.5-12 GHz) microwave scattering off laser induced 1-760 Torr air-based microplasmas (287.5 nm O₂ resonant photoionization by ~ 5 ns, < 3 mJ pulses) with diverse ionization and collisional features. Namely, conducted studies include a verification of short-dipole-like radiation behavior, plasma volume imaging via ICCD photography, and measurements of relative phases, total scattering cross-sections, and total number of electrons [Formula: see text] in the generated plasma filaments following absolute calibration using a dielectric scattering sample. Findings of the paper suggest an ideality of CMS in the Thomson "free-electron" regime-where a detailed knowledge of plasma and collisional properties (which are often difficult to accurately characterize due to the potential influence of inhomogeneities, local temperatures and densities, present species, and so on) is unnecessary to extract [Formula: see text] from the scattered signal. The Thomson scattering regime of microwaves is further experimentally verified via measurements of the relative phase between the incident electric field and electron displacement.

Immunocytochemical Localization of XRCC1 and γH2AX Foci Induced by Tightly Focused Femtosecond Laser Radiation in Cultured Human Cells

To assess the prospects for using intense femtosecond laser radiation in biomedicine, it is necessary to understand the mechanisms of its action on biological macromolecules, especially on the informational macromolecule-DNA. The aim of this work was to study the immunocytochemical localization of DNA repair protein foci (XRCC1 and γH2AX) induced by tightly focused femtosecond laser radiation in human cancer A549 cells. The results showed that no XRCC1 or γH2AX foci tracks were observed 30 min after cell irradiation with femtosecond pulses of 10¹¹ W∙cm^-2 peak power density. An increase in the pulse power density to 2 × 10¹¹ W∙cm^-2 led to the formation of linear tracks consisting both of XRCC1 and γH2AX protein foci localized in the places where the laser beam passed through the cell nuclei. A further increase in the pulse power density to 4 × 10¹¹ W∙cm^-2 led to the appearance of nuclei with total immunocytochemical staining for XRCC1 and γH2AX on the path of the laser beam. Thus, femtosecond laser radiation can be considered as a tool for local ionization of biological material, and this ionization will lead to similar effects obtained using ionizing radiation.

Control of the polarization direction of isolated attosecond pulses using inhomogeneous two-color fields

We theoretically demonstrate the control of the polarization direction of isolated attosecond pulses (IAPs) with inhomogeneous two-color fields synthesized by an 800-nm fundamental pulse and a 2000-nm control pulse having crossed linear polarizations. The results show that by using the temporally and spatially shaped field, the high-order harmonic generation (HHG) process can be efficiently controlled. An ultra-broad supercontinuum ranging from 150th to 400th harmonics which covers the water window region is generated. Such a supercontinuum supports the generation of a 64-as linearly polarized IAP, whose polarization direction is at about 45° with respect to the x axis. Moreover, we analyze the influence of the inhomogeneity parameters and the relative angle of the fundamental and control pulses on the IAP generation. It is shown that the polarization direction of the IAP can rotate in a wide range approximately from 8° to 90° relative to the x axis when the inhomogeneity parameters and the relative angle vary.

Reflection Enhancement and Giant Lateral Shift in Defective Photonic Crystals with Graphene

This study investigates the reflectance of the defective mode (DM) and the lateral shift of reflected beam in defective photonic crystals incorporated with single-layer graphene by the transfer matrix method (TMM). Graphene, treated as an equivalent dielectric with a thickness of 0.34 nm, was embedded in the center of a defect layer. The reflectance of the DM was greatly enhanced as the intraband transition of electrons was converted to an interband transition in graphene. The reflectance of the DM could be further enhanced by increasing the Bragg periodic number. Furthermore, a large lateral shift of the reflected beam could also be induced around the DM. This study may find great applications in highly sensitive sensors.

Photoelectron Holographic Interferometry to Probe the Longitudinal Momentum Offset at the Tunnel Exit

Laser-induced electron tunneling underlies numerous emerging spectroscopic techniques to probe attosecond electron dynamics in atoms and molecules. The improvement of those techniques requires an accurate knowledge of the exit momentum for the tunneling wave packet. Here we demonstrate a photoelectron interferometric scheme to probe the electron momentum longitudinal to the tunnel direction at the tunnel exit by measuring the photoelectron holographic pattern in an orthogonally polarized two-color laser pulse. In this scheme, we use a perturbative 400-nm laser field to modulate the photoelectron holographic fringes generated by a strong 800-nm pulse. The fringe shift offers direct experimental access to the intermediate canonical momentum of the rescattering electron, allowing us to reconstruct the momentum offset at the tunnel exit with high accuracy. Our result unambiguously proves the existence of nonzero initial longitudinal momentum at the tunnel exit and provides fundamental insights into the nonquasistatic nature of the strong-field tunneling.