Researchers manage to generate attosecond pulses at a repetition rate of 100 kHz

Experimental setup. Our in-house OPCPA system provides 7 fs pulses at a repetition rate of 100 kHz. These pulses are shortened to a duration of 3.3 fs via hollow fiber pulse compression. Attosecond streak experiments are performed in a specially designed beamline. Credit: MBI

Attosecond laser pulses in the extreme ultraviolet (XUV) are a unique tool for observing and controlling the dynamics of electrons in atoms, molecules and solids. Most attosecond laser sources operate at a pulse repetition rate of 1 kHz (1000 shots per second), which limits their usefulness in complex experiments. Using a high-power laser system developed at MBI, we managed to generate attosecond pulses at a repetition rate of 100 kHz. This enables new types of experiments in attosecond science.

Light pulses in the extreme ultraviolet (XUV) region of the electromagnetic spectrum, with durations on the order of 100 s of attoseconds (1 as = 10-18 s) allow scientists to study the ultrafast dynamics of electrons in atoms, molecules and solids. Usually, experiments are performed using a sequence of two laser pulses with a controllable delay between them. The first pulse excites the system, and the second pulse takes a snapshot of the evolving system, recording an appropriate observable. Usually, the momentum distributions of ions or electrons or the transient absorption spectrum of the XUV pulse are measured as a function of the delay between the two pulses. By repeating the experiment for different times between the two pulses, a movie of the dynamics under study can be created.

In order to obtain the most detailed information on the dynamics of the studied system, it is advantageous to measure as completely as possible the information available on the temporal evolution. In experiments with atomic and molecular targets, it can be advantageous to measure the three-dimensional momenta of all charged particles. This can be done with a so-called reaction microscope (REMI) apparatus. The scheme works by ensuring unique ionization events for each laser shot and detecting coincidence electrons and ions. This, however, has the disadvantage that the detection rate is limited to a fraction (usually 10-20%) of the laser pulse repetition rate. Meaningful pump-probe experiments in a REMI are not possible with 1 kHz class attosecond pulse sources.

At MBI, we have developed a laser system based on Optical Parametric Pulse Amplification (OPCPA). In parametric amplification, no energy is stored inside the amplification medium; therefore, very little heat is generated. This allows laser pulses to be amplified to much higher average powers than with the current “workhorse” Ti:Sapphire laser, which is most often used in attosecond labs around the world. The second advantage of OPCPA technology is the ability to amplify very broad spectra. Our OPCPA laser system directly amplifies short-cycle laser pulses with durations of 7 fs at average powers of 20 W. This is a pulse energy of 200 uJ at a repetition rate of 100 kHz. With this laser system, we have previously successfully generated trains of attosecond pulses.

In many attosecond experiments it is advantageous to have single attosecond pulses instead of a train of multiple attosecond pulses. To enable efficient generation of isolated attosecond pulses, the laser pulses driving the generation process should have pulse durations as close to a single light cycle as possible. In this way, the emission of attosecond pulses is confined to a point in time, leading to isolated attosecond pulses. In order to obtain quasi single-cycle laser pulses, we used the hollow fiber pulse compression technique. The 7 fs pulses are sent through a 1 m long hollow waveguide filled with neon gas for spectral broadening. Using specially designed chirped mirrors, pulses can be compressed to pulse widths as short as 3.3 fs. These pulses consist of only 1.3 optical cycles.

Attosecond pulses: 100 times more

Results of attosecond streaks. (a) Trace of measured photoelectron streaks. (b) Intensity envelope of the recovered isolated attosecond pulse (inset: the intensity profile on the logarithmic scale) (c) Recovered spectral intensity and spectral phase. Credit: MBI

The 1.3-cycle pulses are sent into an attosecond beamline developed at MBI. Most of the energy is used to generate isolated attosecond XUV pulses in a target gas cell. After removing the high power NIR beam, spectral filtering and focusing, about 106 photons per laser shot (corresponding to an unprecedented photon flux of 1011 photons per second) are available for experiments.

In order to characterize the generated attosecond XUV pulses, we performed an attosecond streak experiment. Essentially, the XUV pulse is used to ionize an atomic gaseous medium (neon in our case), while a strong NIR pulse is used to modulate the photoelectron wave packets generated by XUV. Depending on the exact timing of the XUV and NIR pulses, the photoelectrons are accelerated (gain energy) or decelerated (lose energy) leading to a characteristic “streak trail”. From this data matrix, the exact shapes of the NIR pulse, as well as the XUV pulse, can be determined. Attosecond pulse shapes were retrieved using a global optimization algorithm developed for this project. Our careful analysis shows that the main attosecond pulses have a duration of 124 ± 3 as. The main pulse is accompanied by two adjacent satellite pulses. These come from the generation of attosecond pulses half a NIR cycle before and after the generation of main attosecond pulses. The pre- and post-pulse satellites have a relative intensity of only 1 × 10-3 and 6×10-4respectively.

These high-flux isolated attosecond pulses open the door to studies of attosecond pump-probe spectroscopy at a repetition rate 1 or 2 orders of magnitude above current implementations. We are currently starting experiments with these pulses in a reaction microscope (REMI).

The research is published in Optical.

100 kHz high flux attosecond pulse source driven by high average power annular laser beam

More information:
Tobias Witting et al, Generation and characterization of isolated attosecond pulses at a repetition rate of 100 kHz, Optical (2021). DOI: 10.1364/OPTICA.443521

Provided by Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI)

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