Highest Attosecond-Pulse-Train energy source reached

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Attosecond pulses are essential tools for time-resolved studies of the dynamics of electrons on their natural time scale: attoseconds (10−18 seconds). Coincidence spectroscopy and high expectation experiments on statistics or signal-to-noise ratio, especially in the case of solid and molecular samples in chemistry and biology, are examples of such studies, all of which are growing in popularity.

The propagation of a ring beam in space. The highlighted region indicates where a mirror should be placed to effectively separate the attosecond pulses from the generator laser beam. Image Credit: Superfast Science.

Researchers need to increase the number of attosecond pulses in a given time unit for these advanced research subjects, which can only be done by increasing the repetition rate of the attosecond source.

A laser source with a high average power and a high repetition rate is required for this. However, compared to traditional attosecond beamlines using lower power drivers, the high average power of the driving laser source poses a challenge: it is difficult to separate the attosecond pulses from the high average power laser beam after generation.

To solve this problem, researchers at the Extreme Light Infrastructure Attosecond Pulse Light Source (ELI ALPS) shaped the laser beam into a ring shape, which they combined with an appropriate experiment setup to produce the train energy. number of attosecond pulses per shot produced by a system with a repetition rate greater than 10 kHz.

Dr. Peng Ye and his colleagues used one of ELI ALPS’ HR-GHHG beamlines to achieve these results in a project supervised by Dr. Balázs Major and Prof. Katalin Varjú. While J. Peatross put forward the idea of ​​producing high order harmonics using a ring beam in 1994, and Y. Mairesse used it to produce attosecond pulses with low power lasers in 2003, scaling this concept to a high medium posed many challenges that had to be overcome.

The main challenge is that when a high average power laser is used, the propagation of the laser pulse through free space and the ionized generation medium must be carefully considered. ELI ALPS researchers modified this method for high average power systems, resulting in the highest transmission of attosecond pulses from point of generation to target position ever achieved.

The method is based on the strong field effect that occurs during the process of generating high harmonics. The focused annular laser beam propagates to a Gaussian-like solid point, where attosecond pulses are generated, and then propagates to a perfect annular beam.

Highly nonlinear effects occur in this light-matter interaction due to the strong electric field of the laser pulse, and the shape of the attosecond beam produced varies from the shape of the laser beam.

This allows it to be spatially separated from the driving laser. Free propagation was also used by ELI ALPS scientists to shape the probe beam to be annular, allowing it to be combined with attosecond pulses with minimal loss. As a result, the high energies of the probe’s attosecond pulses and laser beam can benefit pump-probe experiments.

Many experiments that require both a high repetition rate and enough energy can now be performed in ELI ALPS thanks to the realized 100 kHz high energy attosecond pulse train source.

Journal reference:

Yeah., et al. (2022) 100 kHz high flux attosecond pulse source driven by a high average power annular laser beam. Ultra-fast science. doi.org/10.34133/2022/9823783.

Source: https://spj.sciencemag.org/journals/ultrafastscience/

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