Above-threshold ionization of argon with ultrashort orbital-angular-momentum beams

Light-matter interaction with laser pulses endowed with orbital angular momentum (OAM) raises a fundamental question about the nature of the transfer of this property of light to matter. In this work, a “reaction microscope” is used for precise measurement of the momentum of ionized photoions and photoelectrons from the interaction of Ar atoms with a linearly polarized, ultrashort ($\sim 25$ fs), moderately intense (${10}^{13}–{10}^{14}\mathrm{W}/{\mathrm{cm}}^2$) OAM carrying laser pulses. The angle and energy-resolved photoelectron spectrum is compared with the photoelectron spectrum obtained from the interaction with the laser beams with no OAM at similar intensities. No evidence of angular momentum transfer to the electrons from light is observed in our experiment.

Figure 1

2D photoelectron momentum distribution with horizontal axis $p_z$ being parallel to the laser polarization direction and the vertical axis being the transverse momentum in a plane perpendicular ($p_r$) to the laser polarization direction. The photoelecteons emerge from interaction of argon with ${\mathrm{HG}}_{\text{0,0}}$ beams at (a) 90 $\mathrm{TW}/{\mathrm{cm}}^2$ and (b) 105 $\mathrm{TW}/{\mathrm{cm}}^2$ intensities. Vertical cuts (near $\sim \pm$ 0.6 a.u.) show regions where the spectrometer has no resolution in the transverse direction. The lower panels (c, d) are projections of the above momentum distributions along $p_z$. The vertical lines are to identify the start of the plateau region electrons.

Figure 2

Same as Fig. 1 showing the photoelectrons emerging from the interaction of argon atoms with ${\mathrm{LG}}_{\text{0,1}}$ beams at (a, c) 90 $\mathrm{TW}/{\mathrm{cm}}^2$ and (b, d) 105 $\mathrm{TW}/{\mathrm{cm}}^2$.

Figure 3

Normalized (total counts) photoelectron energy spectra at different intensities of 90 $\mathrm{TW}/{\mathrm{cm}}^2$, 95 $\mathrm{TW}/{\mathrm{cm}}^2$,100 $\mathrm{TW}/{\mathrm{cm}}^2$, and 105 $\mathrm{TW}/{\mathrm{cm}}^2$. The tilted solid lines indicate the expected position of the ATI peak with a 15-photon absorption at the peak intensity. The tilted dotted line shows the expected position of the ATI peak including the focal volume effect with 13-photon absorption. The cross hairs with error bars indicate the peak positions of the nonresonant ATI peaks in the photoelectron spectra. Higher-lying Rydberg states are shown with solid horizontal lines in the negative energy scale to highlight the possible resonant ionization processes. The intensity-independent structures from resonant ionization through Rydberg states are marked with dashed-dot lines.

Figure 4

Photoelectron angular distributions (PADs) of the resonant structures (a) ($0.75\pm 0.05\mathrm{eV}$) and (b) ($0.93\pm 0.05\mathrm{eV}$) at peak intensity $\sim 105$ $\mathrm{TW}/{\mathrm{cm}}^2$ are fitted with Legendre polynomial up to order $l=6$, and the contribution of individual terms are is shown in (c) and (d) as a function of peak intensity of the ${\mathrm{HG}}_{\text{0,0}}$ beam, respectively.

Figure 5

(a) Same as Fig. 4, but for near-threshold photoelectrons as a function of peak intensity of the Gaussian beam. (b) The dominant angular momentum of the near-threshold photoelectrons as a function of peak intensity of the Gaussian beam. The black solid line indicates the semiclassical result. The dashed red and the dashed-dot blue lines highlight the uncertainty region due to quantum discreteness.

Figure 6

The photoelectron angular distribution of the near-threshold photoelectrons from ${\mathrm{HG}}_{\text{0,0}}$ (black) and ${\mathrm{LG}}_{\text{0,1}}$ (red) beams is shown at different peak intensities [(a) 90 $\mathrm{TW}/{\mathrm{cm}}^2$, (b) 95 $\mathrm{TW}/{\mathrm{cm}}^2$, (c) 100 $\mathrm{TW}/{\mathrm{cm}}^2$, (d) 105 $\mathrm{TW}/{\mathrm{cm}}^2$]. Angular distributions of resonant photoelectrons with energy $\sim 0.7\mathrm{eV}$ [(e)–(h)] and $\sim 0.9\mathrm{eV}$ [(i)–(l)] are shown at the same intensities.

Figure 7

Same as Fig. 6 but for the nonresonant photoelectrons at 1.85 eV and 3.4 eV at various peak intensities [(a, e) 90 $\mathrm{TW}/{\mathrm{cm}}^2$, (b, f) 95 $\mathrm{TW}/{\mathrm{cm}}^2$, (c, g)100 $\mathrm{TW}/{\mathrm{cm}}^2$, and (d, h)105 $\mathrm{TW}/{\mathrm{cm}}^2$].