Mastering Compact Realization of All-Attosecond Pump-Probe Spectroscopy

Researchers from the Max Born Institute in Berlin have, for the first time, shown attosecond-pump attosecond-probe spectroscopy (APAPS) operating at a 1 kilohertz repetition rate.

This achievement was facilitated by the development of a compact, intense attosecond source using an out-of-focus generation geometry. This method offers new opportunities for exploring ultrafast electron dynamics in the attosecond regime.

The introduction of the first attosecond pulses around the start of the 21st century marked a groundbreaking advancement in understanding electron behavior. This development paved the way for remarkable insights into the microscopic world. In recognition of their groundbreaking work, Anne L’Huillier, Pierre Agostini, and Ferenc Krausz were awarded the Nobel Prize in Physics in 2023 for their pioneering contributions that led to the demonstration of attosecond pulses in 2001.

Despite these achievements, current attosecond techniques face a significant limitation. To capture a dynamic sequence of events in a pump-probe experiment, an attosecond pulse must typically be paired with a femtosecond pulse. The optical cycles of the femtosecond pulse, which are a few femtoseconds long, serve as a clock with attosecond precision. This requirement constrains the investigation of electron dynamics on attosecond timescales.

Since the initial demonstration of attosecond pulses, many scientists have aspired to conduct experiments where an initial attosecond pump pulse triggers electron dynamics in an atom, molecule, or solid-state sample, followed by a second attosecond probe pulse that interrogates the system at various time delays.

However, realizing this goal has proven extremely challenging due to the requirement for intense attosecond pulses. The high-harmonic generation (HHG) process that underlies attosecond pulse generation is inherently inefficient. As a result, only a few proof-of-principle demonstrations of attosecond-pump attosecond-probe spectroscopy (APAPS) have been achieved, and these typically involved large setups and specialized laser systems operating at low repetition rates (10-120 Hertz).

A team of researchers from the Max Born Institute (MBI) in Berlin has introduced a novel approach that enables them to conduct attosecond-pump attosecond-probe spectroscopy (APAPS) experiments using a much more compact setup. They utilized a turn-key driving laser operating at a kilohertz repetition rate to achieve this. This innovation led to significantly enhanced stability, a crucial factor in successfully executing APAPS experiments.

In their method, the researchers employed infrared laser pulses for attosecond pulse generation within a gas jet. Unlike conventional approaches, they positioned the gas jet not in close proximity to the driving laser focus but at a distance from it. This strategic placement resulted in the generation of attosecond pulses characterized by relatively high pulse energy and small virtual source size. Subsequent refocusing of these pulses allowed the researchers to obtain high-intensity attosecond pulses.

The researchers utilized the stable and intense attosecond source to conduct an attosecond-pump attosecond-probe spectroscopy (APAPS) experiment. In this experiment, argon atoms were ionized by an attosecond pump pulse, resulting in the generation of singly-charged Ar⁺ ions. These ions were then probed by an attosecond probe pulse, resulting in further ionization and the formation of doubly-charged Ar²⁺ ions.

The findings of this experiment reveal a rapid increase in the yield of Ar²⁺ ions, indicating ultrafast dynamics. This observation confirms that the pump and probe pulses have attosecond pulse durations, as intended.

The use of relatively low infrared driving pulse energies in this study suggests the potential for conducting attosecond-pump attosecond-probe spectroscopy (APAPS) experiments at even higher repetition rates, potentially reaching the megahertz level. Laser systems capable of driving such experiments are either already available or currently in development. This novel approach could, therefore, provide unparalleled insights into electron behavior on incredibly short timescales, which are currently beyond the reach of existing attosecond techniques.

Journal Reference:

Kretschmar, M., et al. (2024) Compact realization of all-attosecond pump-probe spectroscopy. Science Advances. doi.org/10.1126/sciadv.adk9605