A method to resolve quantum interference between photoionization pathways with attosecond resolution

The field of attosecond physics was created with the mission of exploring light-matter interactions at unprecedented temporal resolutions. Recent advances in this field have allowed physicists to shed new light on the quantum dynamics of charge carriers in atoms and molecules.

One technique that has proven particularly useful for conducting research in this area is RABBITT (i.e., attosecond beat reconstruction by interference of two-photon transitions). This promising tool was initially used to characterize ultrashort laser pulses, part of a research effort that won this year’s Nobel Prize, but it has since also been used to measure other ultrafast physical phenomena.

Researchers from East China Normal University and Queen’s University Belfast recently used the RABBITT technique to distinctly measure individual contributions to photoionization. Their article, published in Physical Examination Lettersintroduces a very promising new method for conducting research in attosecond physics.

“The RABBITT technique essentially provides an ultra-fast stopwatch for electronic processes, so we can measure (for example) the time delay between the ionization of different electrons in an atom,” said Andrew C. Brown, co-author of the article, in Phys. .org.

“One of the difficulties of these experiments, however, is that when there are multiple interfering processes, the picture becomes considerably more complex and we can no longer make concrete claims about the timing of different mechanisms. In essence, you have too many variables and not enough equations to solve them.

“The real genius of Xiaochun and Jian’s experiment was that it provided more equations, or more precisely, more distinct measurements, which allowed us to tease out the different mechanisms.”

In their experiments, Xiaochun Gong and Jian Wu, the authors who led the project, used two laser pulses, which is standard practice when implementing the RABBITT technique. However, they changed the polarization (i.e. tilt angle) of these pulses in order to better control the measurements collected.

First, the researchers sought to resolve photoionization delays for different emission angles. In other words, they wanted to determine whether an electron behaves differently when emitted in different directions relative to the laser field. However, once they started looking at the data collected from their experiments, they realized it painted a much more complex picture than they had anticipated.

“Our current work is also a step forward from our previous work on partial-wave atomic counters,” Gong said. “Our dream is to push attosecond photoionization measurement to the partial-wave level, which is the original definition of scattering phase shift.”

The researchers carried out their measurements on samples of helium, neon and argon. Examining helium is simple because it only has two electrons and there is really only one method to ionize it, while neon and argon are much more complex systems.

“Specifically, when you ionize helium, there is only one possible residual ionic state,” Brown said. “For neon and argon, however, things are significantly more complicated. On the one hand, there are more electrons to worry about, and on the other hand, there are multiple residual ionic states , all of which contribute in a (previously) unknown way to the measured signal. The way we interpreted/explained this was to think of the classic “Young’s double slit” experiment, in which light passes through two openings before being “measured” on a screen.

In a classic Young double-slit experiment, light passing through two apertures produces an interference pattern on a screen. Indeed, the waves passing through each opening arrive at the same place by different routes, which results in what are called “fringes” of constructive or destructive interference.

“The key to this experiment, and the reason it formed such a compelling metaphor, especially for quantum theorists, is that you can’t tell which slit the light passed through, because that can’t be measured.” , Brown said. “All you can measure is interference, and ‘which way information’ is inaccessible.”

In the experiments performed by Brown, Gong, and co-workers, the two apertures of Young’s classic double-slit experiments were two different residual ion states in neon. In contrast, the interference pattern they measured was the angular distribution of photoelectrons produced by the two asymmetric laser pulses.

“By making the measurement for two different tilt angles, and then determining all the different paths the electrons could take to get to a final state, we could then solve the equations to give us both the amplitude and the phase for each different path.” » said Brown. “In other words, we determined which slit the electron passed through and how.”

Most studies in experimental attosecond physics use light theoretical calculations to explain their results after the fact. However, this project required much more detailed simulations to account for the complex dynamics at play and, essentially, provide a prediction that the experiment needed to confirm.

“The method we used to reconstruct the different pathways of the experiment has a solid theoretical basis, but the dynamics are so complex that it would be difficult to prove conclusively that the numbers we extract from the experiment are reliable,” Brown said. “We ran simulations with the R-matrix with Time-dependence (RMT) code, which can handle all these dynamics from first principles, and from there we were able to directly extract the amplitudes and phases.”

When they compared their experimental results with those from the simulation, they found that they were closely aligned. This suggests that their experiment actually measured what they theoretically claimed.

“In summary, we are trying to use the laser field to attach an extra phase to the intermediate D wave,” Gong said. “We can identify the S wave and the D wave, but we can perturb their phase property and observe their final interference property. For example, we can open the box to find out whether the ‘quantum cat’ is alive or no, but we can add some disturbance and check if the box has a response or not, the responses being essential depending on the reaction of the cat inside.

The researchers consider the proposed experimental method a “part-wave meter,” or in other words, a tool capable of efficiently measuring individual contributions to photoionization. Notably, the proposed method relies on two distinct experimental techniques, namely changing the laser polarization and measuring the coincidence of photoelectrons and ions, which were not used together before.

“Our work combined these techniques in a way that made this new measurement possible,” Brown said. “This is not to say that the measurements were simple, but it would not be surprising to see this same combination of techniques used to make more interesting measurements of ultrafast dynamics in years to come.”

Another unique aspect of this recent study is the simulation used to validate the team’s experimental results. For a long time, scientists tried to interpret experimental data using theoretical models, but Brown, Gong and their colleagues decided to use a simulation instead.

“The results provided by RMT are less intuitive because the model is far from simple, explained Brown. “However, by including a description of all the interesting multielectronic effects and doing it in a general way so as not to be limited to specific effects, atoms or specific laser parameters, we can actually start to conduct experiments in this area in a way that simply hasn’t been possible in the thirty or so years of attoscience so far.

The recent work of this team of researchers offers new insight into the fundamental dynamics of photoionization. While Brown, Gong and their collaborators focus primarily on the physics of this phenomenon, their efforts in the future could help identify new strategies for controlling electrons using light. This could inform the development of ultra-fast electronic circuits and photovoltaic technologies (solar panels), or perhaps even help design medical tools that prevent damage to cells from radiation.

“We are working to develop a more complete theory of higher-order processes in photoemission,” Brown said. “In other words, we are trying to theoretically describe what happens when you absorb many (more than two) photons in these RABBITT-like experiments. Although we have this RMT code that can simulate the dynamics from first principles “If you want to interpret the results, you also need a relatively simple model to explain the different pathways.”

While working on a theoretical model that can explain the data collected in their experiments, the researchers plan to continue conducting experiments and running simulations at increasingly higher intensity regimes. They hope this will allow them to examine in more detail the transitions from few-photon to multi-photon systems and, ultimately, to strong-field physics.

“The development of strong field physics is moving away from traditional scattering theory, and there is a significant gap between them,” Gong added. “It is necessary to build an intermediate bridge to enable a smooth understanding from a photon to a multiphoton.”

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