Orbital angular momentum control of strong-field ionization in atoms and molecules

Abstract

Tunnel ionization, the fundamental process in strong field physics and attosecond science, along with the subsequent electron dynamics are typically governed by the polarization and carrier envelope phase of the incident laser pulse. Moreover, most light-matter interactions involve Gaussian beams and rely primarily on dipole-active transitions. In this article, we reveal that Orbital Angular Momentum (OAM) carrying beams enable to control tunnel ionization in atoms and molecules. The ionization process is manipulated by the sign and value of the OAM and by displacing the phase singularity. We show that the helical phase and field gradients inherent in the higher-order multipole expansion of the tunneling process cause ionization to depend on OAM. Simulations indicate that, in contrast to Gaussian beams, the ponderomotive effects can also be controlled with OAM and the asymmetry in the optical vortex. Our findings have an impact on attosecond science, spectroscopy, and super-resolution microscopy.

Fig. 1. OAM-dependent ionization in atoms.

a Distortion of atomic potential at peak intensity for a vertically polarized Gaussian beam (black dashed line) and, for l = − 1 (blue) and l = + 1 (red) LG beams. The potentials were evaluated with the beam coordinates at ($\frac{w_0}{3}$,0) μm for δ =−w₀/3. b Normalized transverse profiles of ionization probability for symmetric (δ = 0μm) LG beam with l = − 1 (left panel) and l = + 1 (right panel). c Schematic of the experimental layout showing the optics used in the generation of OAM beams and the measurement of ion yields using time-of-flight mass spectrometer. d Normalized transverse profiles of ionization probability for asymmetric LG beam when the singularity is displaced to either side of the center by $\delta =\pm \frac{w_0}{3}$ for l = + 1 (top panels) and l = − 1 (bottom panels). Ionization probabilities were evaluated at an intensity of 10¹⁴ W/cm², and the x and y-axis are given in terms of the beam waist, w₀ = 3 μm.

Fig. 2. OAM control of ionization of argon atom.

a Single ionization yields of argon for l = + 1 (black) and l = − 1 (red). b Difference in ionization between left- and right-handed helical light (l = 1). c Single ionization yields of argon for l = + 3 (black) and l = −3 (red). d Difference in ionization between left- and right-handed helical light (l = 3). The laser polarization was linear (s = 0) and the intensity for l = 1 was 10¹⁴ W/cm² and l = 3 was 1.5 × 10¹⁴ W/cm². The x and y-axis are given in terms of the beam waist, w₀ = 3 μm. The error bars represent the standard error of multiple measurements (n = 3).

Fig. 3. OAM-dependent ionization in molecules.

Single ionization yields for l = + 1 and l = − 1 as a function of displacement of the singularity in (a) R(+)-limonene, (c) acetone. (b), (d) shows the difference in the ionization yields between the left- and right-helical light for the respective molecules. The laser polarization was linear (s = 0), and the intensity was 8 × 10¹³ W/cm² and 9 × 10¹³ W/cm² for limonene and acetone, respectively. The x and y-axis are given in terms of the beam waist, w₀ = 3 μm. The error bars represent the standard error of multiple measurements (n = 3).

Fig. 4. Simulated differential ionization probability of argon.

a Simulated difference in ionization probabilities of argon between left- and right-helical light for l = 1 in black and l = 3 in red, respectively. b Single ionization probability of argon for l = +1 (l = +3) in blue (black) and l = −1 (l = −3) in magenta (red). They were obtained by solving Eq. (6) integrated over the beam cross-section for vertical polarization and intensity of 10¹⁴W/cm² for l = 1 and 1.2 × 10¹⁴ W/cm² for l = 3. The x and y-axis are given in terms of the beam waist, w₀ = 3 μm.

Fig. 5. Tuning the ponderomotive energy and force.

Simulation of (a) peak ponderomotive energy, and (b) average ponderomotive force, at an intensity of 10¹⁴ W/cm², for different positions of the singularity in a linearly polarized OAM beam and for different l-values. The open circles correspond to a Gaussian beam at the same laser intensity. The ponderomotive force was integrated over the beam cross-section at the laser focus. c Electron recollision dynamics in argon for δ = w₀/3. The dash (solid) lines represent the long (short) trajectories created when the electron is ionized before (after) the peak located at ${\omega }_0{t}^{\prime }=0.05\times 2\pi =18^{\circ }$. The electron will return to the parent ion at ω₀t = 0.7 × 2π = 252°. The x and y-axis are given in terms of the beam waist (w₀ = 3 μm) and the polarization in terms of ellipticity (ε).

Fig. 6. Field localization to achieve super-resolution.

a–d Transverse intensity profiles of a) Gaussian, and b–d) asymmetric LG beams with l = 1, 2, 3 for w₀ = λ and δ = w₀/3. The dimensions of the intense central region in (b) represent a threshold intensity of 90%. e shows the x-dimension (black) and y-dimension (blue) of the intense region as a function of the OAM-value along with the Gaussian beam (red dot).