High-Flux 100kHz attosecond pulse source driven by a high-average power annular laser beam

Attosecond pulses are indispensable tools for time-resolved studies of electron dynamics on their natural time scale: attoseconds (10-18 second). Such studies include coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio, especially in the case of solid and molecular samples in chemistry and biology, all with an exponentially growing interest. For these cutting-edge research topics scientists need to increase the number of attosecond pulses in a certain unit of time, which can only be achieved by increasing the repetition rate of the attosecond source. To do so, a laser source of high average power and high repetition rate is necessary. However, the high average power of the driving laser source comes along with difficulties compared to conventional attosecond beamlines using lower power drivers: it is not easy to separate the attosecond pulses from the high-average-power laser beam after generation. To overcome this issue, scientists of the Extreme Light Infrastructure Attosecond Pulse Light Source (ELI ALPS) shaped the laser beam to an annular shape, and combined with proper experiment configuration they achieved the highest attosecond-pulse-train energy per shot produced by a system with a repetition rate above 10 kHz. 

These results have been achieved using one of the HR-GHHG beam lines of ELI ALPS by Dr. Peng Ye and his colleagues in a project supervised by Dr. Balázs Major and Prof. Katalin Varjú. While the idea of generating high-order harmonics using an annular beam was proposed by J. Peatross in 1994, and since then it has been used to generate attosecond pulses using low power lasers by Y. Mairesse in 2003, scaling this concept to high average had several obstacles which had to be overcome for success. The main difficulty comes from the fact that when a laser of high average power is used, the propagation of the laser pulse through free space and the ionized generation medium should be carefully and properly considered. Scientists at ELI ALPS adapted this approach to high average power systems, and as a result obtained the highest transmission of attosecond pulses from the generation point to the target position to date. 

The method relies on the strong field effect involved in the process of high harmonic generation. The focused, annular laser beam propagates to a Gaussian-like solid spot to generate attosecond pulses at the focus, and will propagate further to a perfect annular beam after that. Due to the strong electric field of the laser pulse, highly nonlinear effects happen in this light-matter interaction, and the shape of the generated attosecond beam differs from the laser beam shape. This way it can be separated spatially from the driving laser. Scientists at ELI ALPS also took advantage of free propagation to shape the probe beam to be annular, so it can be combined with the attosecond pulses with low loss. As a result, pump-probe experiments can benefit from the high energies of both the attosecond pulses and the probe laser beam. 

With the achieved 100 kHz high-energy attosecond pulse-train source many experiments which need both a high repetition rate and enough energy can be performed now in ELI ALPS.