A tailored laser field to manipulate quantum pathways

Have you ever asked yourself what is the ultimate limit to image a molecule? To “see” where all the atoms of one molecule are located in space, one needs picometer space resolution. To also “watch” how the molecule starts undergoing a chemical reaction, one needs attosecond temporal resolution as well. Laser-induced electron diffraction (LIED) is the only technique that combines picometer with sub-cycle resolution, and it can image one isolated molecule with only one of its own electrons. Quantum mechanically, even one electron can be at different locations at the same time. Thus, to achieve the ultimate limit, one would need to eliminate the electron’s different quantum paths to achieve the physical limit.

Manipulating the photoelectron quantum trajectories

This is exactly what was achieved in the experiment, published in Optica, by ICFO researchers Aurelien Sanchez, Kasra Amini and Tobias Steinle, led by ICREA Prof. at ICFO Jens Biegert, in collaboration with researchers at the University of Rostock in Germany, the Weizmann Institute of Science and Technion in Israel.

In the article, the team uses a phase-controlled and tailored two-color laser field to manipulate the rescattering pathways, enabling, on one hand, the selection of specific quantum pathways and on the other hand, the precise mapping of the electron momentum to rescattering time.

Researchers first conducted near-infrared (NIR) and mid-infrared (MIR) experiments using two-color laser fields and manipulated the laser parameters to control the relative phases and intensities. The wavelengths used were 788 nm in NIR and 3.2 µm in MIR, with a pulse duration of 30 fs and 50 fs respectively.

Then, they detected and measured the emitted photoelectrons by using velocity-map-imaging (VMI) and reaction microscopes, and saw that in the MIR experiments, there was a clear separation between the trajectories. To be able to further understand the underlying quantum dynamics, they analyzed the time-resolved spectra using theoretical simulations, differentiating between the even- and odd-order rescattering returns.

Finally, they could isolate the single electron trajectories removing the unwanted interferences, and were able to unambiguously map the energy and the rescattering time. By tuning the different parameters – such as the relative intensity ratio two-color field or the phase difference-, researchers were able to control the contributions of long and short electron trajectories.

By combining experimental approaches and theoretical simulations (time-dependent Schrödinger equation), the article demonstrates that a phase-controlled two-color laser field can shape quantum trajectories in LIED measurements. Being able to control and select specific ionizing quantum trajectories can impact several applications in ultrafast physics and imaging space and time.