Controlling photoionization using attosecond time-slit interferences

Abstract

When small quantum systems, atoms or molecules, absorb a high-energy photon, electrons are emitted with a well-defined energy and a highly symmetric angular distribution, ruled by energy quantization and parity conservation. These rules are based on approximations and symmetries which may break down when atoms are exposed to ultrashort and intense optical pulses. This raises the question of their universality for the simplest case of the photoelectric effect. Here we investigate photoionization of helium by a sequence of attosecond pulses in the presence of a weak infrared laser field. We continuously control the energy of the photoelectrons and introduce an asymmetry in their emission direction, at variance with the idealized rules mentioned above. This control, made possible by the extreme temporal confinement of the light-matter interaction, opens a road in attosecond science, namely, the manipulation of ultrafast processes with a tailored sequence of attosecond pulses.

Keywords: attosecond pulses; electron momentum spectroscopy; photoelectric effect; photoionization.

Fig. 1.

Principle of the experiment: Helium atoms are exposed to (A) two or (B) three XUV attosecond pulses (blue) in the presence of a weak IR laser field (red) at a fixed delay. EWPs (violet) are emitted with an up–down asymmetry relative to the direction of polarization, resulting in different spectra (brown and green) when recording electrons emitted in the two opposite directions. In the case of two pulses (in A), the photoelectron spectrum is shifted toward higher or lower energies, while, for three pulses (in B), peaks at different frequencies, called sidebands, are observed, mostly in the up direction.

Fig. 2.

Experimental setup. (A) The 200-kHz 6-fs IR laser pulses with vertical polarization are sent through a wedge pair for CEP control and focused with an achromatic lens into a high-pressure argon gas jet. A tailored sequence of a few XUV attosecond pulses is then generated and focused by a gold-coated toroidal mirror into a 3D momentum spectrometer, where it intersects an effusive helium jet. An Al filter can be introduced to eliminate the copropagating IR field. (B) A 3D representation of electron momentum distribution as a function of azimuthal angle $\varphi$ and angle $\theta$ for XUV-only radiation.

Fig. 3.

XUV attosecond pulse trains and angular-resolved spectrograms. (A and B) XUV (blue) and IR (red) electric fields with (A) CEP = $\pi /2$ and (B) CEP = 0. (C–F) Color plots representing the photoelectron angular distributions as function of energy. The experimental results are shown in C and D, while corresponding simulated photoelectron spectra are shown in E and F. The red dashed lines indicate the photoelectron kinetic energies after absorption of XUV radiation. When two attosecond pulses are used, the electron distribution shifts in energy, in opposite ways for the up and down emission directions (in C and E). In the three-pulse case, sidebands appear, but only in the up direction (in D and F).

Fig. 4.

Interference through multiple temporal slits. (A) The interference of two EWPs separated by half of a laser cycle with a $\pi$ phase difference (Left, 1) leads to a modulation in the energy (frequency) domain, with maxima at the energies corresponding to excitation by odd harmonics (Right, 1, blue curve). A phase change, $\phi$, of one EWP shifts the interference fringes (1, green curve). A momentum-dependent phase change, $2{\eta }_p$ (2), leads to an energy-dependent shift of the interference fringes, as well as to a temporal shift ($\delta t$) of one EWP relative to the other. (B) The interference of three EWPs separated by half of a laser cycle with a $\pi$ phase difference (Left, 1) leads to interferences with maxima at the energies corresponding to excitation by odd harmonics, and weak “secondary” maxima at the SB position (Right, 1, blue curve). A phase change between the side and central EWPs ($\phi$) enhances the SB relative to the main peak (1, green curve). A momentum-dependent phase change ($2{\eta }_p$) leads to energy-dependent sideband amplitudes, but no energy shift (Right, 2). The spectral phase difference between consecutive attosecond pulses ($s$) enhances (yellow curve) or reduces (red curve) this effect depending on the direction of emission (3).

Fig. 5.

Attosecond time domain control: By manipulating the phase and amplitude of a sequence of attosecond pulses, photoionization of atoms and molecules can be controlled in the frequency domain. The addition of a weak IR pulse allows for additional phase control.