Tunneling Ionization Research Articles

Simulation of laser-induced tunnel ionization based on a curved waveguide

The problem of tunneling ionization and the associated questions of how long it takes for an electron to tunnel through the barrier, and what the tunneling rate has fascinated scientists for almost a century. In strong field physics, tunnel ionization plays an important role, and accurate knowledge of the time-dependent tunnel rate is of paramount importance. The Keldysh theory and other more advanced related theories are often used, but their accuracy is still controversial. In previous work, we suggested using a curved waveguide as a quantum simulator to simulate the tunnel ionization process. Here we implemented for the first time such a curved waveguide and observed the simulated tunneling ionization process. We compare our results with the theory.

Dynamics of laser-induced formation of color centers in a lithium fluoride crystal

The dynamics of the laser-induced formation of aggregate color centers under the action of the first (950 nm) and second (475 nm) harmonics of a femtosecond Ti:sapphire laser has been studied. During the irradiation of the samples with laser radiation, the photoluminescence spectra of the induced centers were repeatedly measured. It was found that intensity of the spectral luminescence band of F3+ centers exceeds intensity of the band of F2 centers in samples with color centers induced by the first harmonic. At the same time, the opposite ratio of the intensities of these bands was observed in the luminescence spectra of color centers produced by second harmonic radiation for the same energy of laser pulses. It has been established that the observed effect is caused by different degree of heating of the crystalline medium in the regions of passage of laser filaments. Heating accelerates the migration of kinetic particles that carry out the aggregation process. The ratio of the concentrations of F2 and F3+ centers depends on the ratio of the duration of irradiation and the lifetime of kinetic particles, which significantly depends on temperature. A multiphoton-tunneling mode of interband ionization of a crystalline substance was implemented in the experiment. It is shown that the identity of the absorption spectra of color centers induced by the first and second harmonics of femtosecond laser radiation is a new distinguishing characteristic of experimental implementation of the tunneling ionization mechanism in experiment.

Coulomb corrected nonadiabatic instantaneous ionization rate and the electron trajectory in an elliptically polarized laser field

This paper presents an analytical model that investigates the impact of Coulomb effect on tunneling ionization events in an elliptically polarized laser field. The model focuses on ionization rate and momenta, at a specific moment, based on recent experimental findings and strong-field approximation. Results show that incorporating Coulomb correction improves photoionization rate across various intensities and provides valuable insights into ionization mechanism, particularly for argon atoms at 410 and 800 nm wavelengths. This has significant implications for attosecond physics and ultrafast laser technology.

Ionization transition rates in the intermediate regime of the Keldysh parameter for a (0,1)*LG spiral amplitude modulated laser field

The mechanisms of the tunnel and multiphoton ionization transitions of hydrogen-like atoms and noble gas atoms are discussed. Atoms potassium and argon, with ionization energy of 4.34 and 15.76 eV, were chosen as the target. The atoms are exposed to Ti:Sapphire, (0,1)*LG, spiral amplitude modulated, laser beam at λ = 800 nm wavelength in a broad intensity range 1012 to 1015 W/cm2. The computational approach to describe tunnel and multiphoton processes was based on using the ADK theory. Stark and ponderomotive effects are also included to study their influence on the transition rate. Obtained results show that, for the lower γ values, the contribution of multiphoton ionization was less significant than the tunnel ionization contribution. In comparison, for higher γ values, multiphoton ionization dominated over tunnel ionization in a total transition rate. It is found that, in this particular case of spiral amplitude modulated mode, the intermediate regime, where both processes equally contribute, strongly depends on the atom selection and laser field intensity. Ionization in the intermediate regime occurs for γ ≈ 10 and 12 for low laser intensities, as γ ≈ 2 and 2.5 for the higher values, in the case of potassium and argon respectively. Our analysis indicated that the Stark and ponderomotive effects have a significant influence on the total transition rate. It is shown that these effects decrease the transition rate value and move the intermediate regime’s position toward lower values of the γ parameter, mainly in the case of higher laser field intensity.

Wavelength scaling of photoionization circular dichroism of atomic electron ring currents and its effect on photoelectron spin polarization

Circularly polarized orbitals in atoms are referred to as electron ring currents with a helical angular phase and thus manifest circular dichroism in photoionization by circularly polarized light fields. Opposite circular dichroism has been revealed in single-photon and nonadiabatic tunneling ionization regimes. The transition of the circular dichroism and its underlying mechanism are still not clear. Here, we systematically study the wavelength scaling of photoionization circular dichroism of electron ring currents. We present a universal description of the dichroic photoelectron energy spectra and the spin polarization from the infrared to the extreme ultraviolet regions. We reveal that the channel interference and competition via the intermediate states with different angular quantum numbers in a few-photon ionization regime give rise to the transition of the circular dichroism and the spin polarization of photoelectrons.

Modification of Vibrational Parameters of a CO2 Molecule by a Laser Field: Impact on Tunnel Ionization

In this paper, we theoretically study the laser-induced modification of the vibrational parameters of a carbon dioxide molecule regarding its tunnel ionization. Our study predicts a 5% increase in the ionization rate in anti-Stokes channels that corresponds to pumping the Σu mode up to vai=10. The molecule is imparted with an additional energy from the pre-pumped vibrational states, which is absorbed during ionization. As a result, the tunneling rate increases. This amplification of tunnel ionization of the CO2 gas target can potentially be used for the laser separation of carbon isotopes.

High electric field characteristics of GaAsSb photodiodes on InP substrates

Low noise avalanche photodiodes (APDs) detecting 1550 nm wavelength play a crucial role in optical communication and LiDAR systems. These APDs utilize a separate absorption, charge, and multiplication (SACM) architecture with an absorber for 1400–1650 nm detection and a low noise, high gain multiplier that can be independently optimized for a high signal-to-noise ratio. Recently, GaAs0.5Sb0.5/Al0.85Ga0.15As0.56Sb0.44 SACM APDs have demonstrated ultra-high gain and extremely low noise, possibly improving sensitivity over Si and InGaAs/InP commercial APDs. This accomplishment was achieved using a GaAsSb absorber instead of a conventional InGaAs absorber, mitigating band discontinuities between the absorber and the multiplier. However, further optimization is required to reduce noise due to tunneling and impact ionization from the GaAsSb absorber, which occurs at a high electric field region. This paper focuses on the study of the high-field characteristics of GaAsSb photodiodes (PDs). The tunneling phenomenon is analyzed through current density-voltage measurements, and the impact ionization behavior is evaluated by measuring the multiplication of p-i-n GaAsSb PDs. The result suggests that when designing a SACM APD with a GaAsSb absorber, the electric field in the absorber can be increased to 175 kV/cm without the detrimental effects of ionization occurring in the absorber. The findings from this investigation will assist in optimizing GaAsSb-based SACM APDs and promoting further advancements in the 1550 nm APD technology.

Sub-barrier recollisions and the three classes of tunneling time delays in strong-field ionization

Tunneling ionization is characterized by a negative time delay, observed asymptotically as a specific shift of the photoelectron momentum distribution, which is caused by the interference of the sub-barrier recolliding and direct ionization paths. In contrast, a \textit{Gedankenexperiment} following the peak of the wavefunction shows a positive tunneling time delay at the tunnel exit, considering only the direct ionization path. In this paper, we investigate the effects of sub-barrier recollisions on the time delay pattern at the tunnel exit. We conclude that the interference of the direct and recolliding trajectories decreases the tunneling time delay at the exit by the value equal to the asymptotic time delay maintaining, however, its sizeable positive value. Finally, we discuss the recent experiment [Light: Science \& Applications 11, 1 (2022)] addressing the tunneling time in a modified two-color attoclock setup. The analysis of the experimental findings with our theoretical model indicates the physical necessity to introduce a new time characteristic for tunneling ionization -- the time delay describing the initiation of the tunneling wave packet.

Attosecond Real-Time Observation of Recolliding Electron Trajectories in Helium at Low Laser Intensities.

Laser-driven recollision physics is typically accessible only at field intensities high enough for tunnel ionization. Using an extreme ultraviolet pulse for ionization and a near-infrared (NIR) pulse for driving of the electron wave packet lifts this limitation. This allows us to study recollisions for a broad range of NIR intensities with transient absorption spectroscopy, making use of the reconstruction of the time-dependent dipole moment. Comparing recollision dynamics with linear vs circular NIR polarization, we find a parameter space, where the latter favors recollisions, providing evidence for the so far only theoretically predicted recolliding periodic orbits.

Nonadiabatic tunneling ionization of atoms in few-cycle elliptically polarized laser pulses

We theoretically investigate nonadiabatic effects in tunneling ionization of a hydrogen atom induced by a few-cycle elliptically polarized laser pulse. We find a deflection angle between the maximum of the photoelectron momentum distribution and the direction of the negative vector potential corresponding to the electric-field peak. This deflection angle mainly originates from the fact that the instant of the maximal ionization rate does not exactly correspond to the instant of the electric-field peak in the few-cycle elliptically polarized laser pulse, revealing an ionization time shift between those two instants. The ionization time shift is a result of the nonadiabatic effect induced by the envelope of the few-cycle elliptically polarized laser pulse, which disappears in a long-pulsed laser. Our study is useful for understanding attoclock experiments using few-cycle elliptically polarized laser pulses.

Roles of laser ellipticity in attoclocks

We study ionization of atoms in strong elliptically polarized laser fields numerically and analytically. We focus on the effects of laser ellipticity on the offset angle in the photoelectron momentum distribution. This angle is considered to encode the time information of tunneling ionization in attoclock experiments. The calculated offset angle increases with the decrease of ellipticity, but the momentum along the major axis of laser polarization related to this angle changes slowly, in agreement with experiments. With a Coulomb-included strong-field model, the scaling laws for the ellipticity dependence of this angle and relevant momentum components are obtained, and the ellipticity dependence of Coulomb-induced ionization time lag encoded in this angle is also addressed.

Correlation analysis of frustrated tunneling ionization

We visualize frustrated tunneling ionization (FTI) using the correlation function analysis outlined in our preceding works [I. Ivanov and K. T. Kim, J. Phys. B 55, 055001 (2022); Sci. Rep. 12, 19533 (2022)]. We apply this technique to the hydrogen atom subjected to a strong laser field. Our analysis supports the basic premises of the theory of FTI and demonstrates its sensitive dependence on the laser pulse duration and carrier envelope phase.

Efficient ultrafast laser writing with elliptical polarization

Photosensitivity in nature is commonly associated with stronger light absorption. It is also believed that artificial optical anisotropy to be the strongest when created by light with linear polarization. Contrary to intuition, ultrafast laser direct writing with elliptical polarization in silica glass, while nonlinear absorption is about 2.5 times weaker, results in form birefringence about twice that of linearly polarized light. Moreover, a larger concentration of anisotropic nanopores created by elliptically polarized light pulses is observed. The phenomenon is interpreted in terms of enhanced interaction of circularly polarized light with a network of randomly oriented bonds and hole polarons in silica glass, as well as efficient tunneling ionization produced by circular polarization. Applications to multiplexed optical data storage and birefringence patterning in silica glass are demonstrated.

Comparative study of photoelectron momentum distributions from Kr and CO2 near a backward rescattering caustic by carrier-envelope-phase mapping

We study carrier-envelope-phase- (CEP-) dependent photoelectron momentum distributions (PEMDs) of Kr atoms and randomly oriented ${\mathrm{CO}}_{2}$ molecules exposed to sub-two-cycle intense infrared fields with linear polarization. The adiabatic theory predicts that, at fixed laser parameters (intensity, pulse duration, and CEP), the PEMDs for targets with equal ionization potentials considered across a backward rescattering caustic have identical shapes and only their absolute values depend on the target. We choose Kr and ${\mathrm{CO}}_{2}$ as targets with almost identical ionization potentials (approximately $14$ eV) to explore this behavior of PEMDs. We find that the measured PEMDs for Kr and ${\mathrm{CO}}_{2}$ have cutoff structures, which are associated with the caustics, with identical decaying profiles. We also reveal that CEP-dependent PEMDs around the cutoff exhibit target dependence, reflecting the differences in the differential cross sections (DCSs) of elastic rescattering and tunneling ionization rates. Furthermore, we extract the DCSs from the measured PEMDs around the cutoffs at different CEPs based on the adiabatic theory. The extracted DCSs agree with ones calculated by using the single-active-electron (SAE) scattering model, which includes all multiple-scattering processes, with orientation averaging for ${\mathrm{CO}}_{2}$. In contrast, the independent-atom model, which only considers single-scattering processes, cannot reproduce the extracted DCSs of ${\mathrm{CO}}_{2}$ at low scattering momenta. This finding indicates that multiple scattering is not negligible in the momentum range of 1.5--2.5 a.u. The extracted DCSs also show interference features assigned to multicenter scattering of O-O and C-O. These results confirm that the signature of electron interference is retained after orientation averaging, indicating that the internuclear distances of the constituent atoms can be retrieved from the CEP-dependent PEMDs for randomly oriented molecules using the SAE model.

Effect of transmission characteristics of femtosecond laser focusing on micro-/nanostructure morphology of titanium alloy

Various types of micro-/nanostructures can be fabricated by femtosecond laser irradiation on metal surfaces. It has received extensive attention and research because of its potential application value. In this paper, the effect of laser focusing position on the micro-/nanostructure formation on the surface of a titanium alloy (TC4) was systematically researched. The experimental results show that, in the process of femtosecond laser focusing, a spatial distribution of laser energy with a narrower beam center and a ring around the beam is formed in front of the geometric focus and the distribution of laser energy changes along the laser transmission direction. This property can affect the morphology of the micro-/nanostructure on the surface of irradiated materials. It is believed that the new spatial distribution of laser fluence caused by focusing is due to the nonlinear process of plasma defocus and laser self-focusing caused by multi-photon or tunnel ionization in front of the target by focusing the femtosecond laser. The discovery of the nonlinear characteristics of the focused femtosecond laser and the application of the unique energy spatial distribution to the surface morphology of the fabricated surface put forward a new idea for the direct fabrication of micro-/nanostructures via femtosecond laser, which has certain significance for the development of micro-/nanofabrication technology of femtosecond lasers.

Relativistic analytical R -matrix theory for strong-field ionization

The analytical R-matrix (ARM) theory has been known for an efficient description of the Coulomb effects of the atomic core in strong-field ionization in the nonrelativistic regime. We generalize the ARM theory into the relativistic domain aiming at the application to strong-field ionization of highly charged ions in ultrastrong laser fields. Comparison with the relativistic Coulomb-corrected strong-field approximations (SFA) is provided, highlighting the advantages and disadvantages. The weakly relativistic asymptotics and its accordance with the nondipole Coulomb-corrected SFA are examined. As an example of a physical application of the relativistic ARM, the Coulomb enhancement of tunneling ionization probability for highly charged ions at the cutoff of the direct channel is discussed.

Controlling Magnetic and Electric Nondipole Effects with Synthesized Two Perpendicularly Propagating Laser Fields

Nondipole effects are ubiquitous and crucial in light-matter interaction. However, they are too weak to be directly observed. In strong-field physics, motion of electrons is mainly confined in transverse plane of light fields, which suppresses the significance of nondipole effects. Here, we present a theoretical study on enhancing and controlling the nondipole effect by using the synthesized two perpendicularly propagating laser fields. We calculate the three-dimensional photoelectron momentum distributions of strong-field tunneling ionization of hydrogen atoms using the classical trajectory Monte Carlo model and show that the nondipole effects are noticeably enhanced in such laser fields due to their remarkable influences on the sub-cycle photoelectron dynamics. In particular, we reveal that the magnitudes of the magnetic and electric components of nondipole effects can be separately controlled by modulating the ellipticity and amplitude of driving laser fields. This novel scenario holds promising applications for future studies with ultrafast structured light fields.

Simulation of the electron trajectories by ultrashort high intensity linearly polarized two lasers to improve the harmonics generation in plasmas

A numerical simulation based on the one and two-dimensional semi-classical model and the tunnelling ionization process was investigated. The influence of the laser electric field on the paths of an electron and the laser magnetic field on the longitudinal drift of the electron was calculated. The electron trajectories in ultrahigh intensity laser were studied in this simulation, the laser field was polarized in the x-direction and propagated in the z-direction. The spreading of electron wave-packet in a continuum was taken into account, and the energy of the generated harmonic photon was estimated by this simulation. A new method to optimize the harmonic photons by controlling the tunnelling ionization and correcting the longitudinal drift of an electron was suggested. This new method utilized two orthogonal linearly-polarized laser beams with a suitable phase difference. It was found that; two counter-propagating laser beams can also decrease the longitudinal drift of the electron and increase the probability of harmonic photons generation.

Relativistic Tunneling Ionization in Crossed Petawatt Laser Beams

A scheme is proposed for a two-beam experiment on multi-petawatt laser systems aimed at searching for relativistic effects in electron tunneling from deep levels of multiply charged ions. It is shown that the most efficient relativistic ionization will occur in the field of a standing wave that arises in the case of counter-propagating beams. Estimates are given showing that the observation of relativistic effects during the ionization of s-states of hydrogen-like ions with charge Z = 80{-} 90 is potentially possible at laser intensity in the beam overlap domain simeq {kern 1pt} 1025 W/cm2.

Quantum control of optoelectronic and thermodynamic properties of dopamine molecule in external electric field : A DFT and TD-DFT study

The effect of external electric field on molecular properties of dopamine is investigated using DFT and TD-DFT methods with B3LYP functional and 6-311++G(d,p) basis set. Optoelectronic properties (energy, polarizability, molecular orbitals, HOMO-LUMO gap, optical gap, electron–hole binding energy, UV-visible and electroabsorption spectra), and thermodynamic properties (zero-point vibrational energy, thermal energy, entropy, and specific heat capacity) were investigated. These properties are useful in developing molecular sensing devices, and understanding molecule receptor binding interaction. Our results using TD-DFT for excited states show that when an external electric field of magnitude F = 0.020 a.u. is applied along the x-axis, there is a shift in the absorption spectra of dopamine from the ultraviolet to the visible region which will be useful in optogenetic stimulation using visible light. The tunneling ionization rate increases and tunneling ionization time decreases with increase in electric field. These studies have potential applications in many of the emerging technologies in neurobiology such as deep brain stimulation and iontophoresis.