Techniques Converge, Yield Attosecond Spectroscopy Milestone

Researchers at the Max Born Institute in Berlin have demonstrated attosecond-pump attosecond-probe spectroscopy (APAPS) at a repetition rate of 1 kHz. The advance opens new avenues for the investigation of extremely fast electron dynamics in the attosecond regime.

Current pump-probe setups for the investigation of attosecond phenomena typically pair a weak extreme-ultraviolet (XUV) pulse generated through high harmonic generation with intense NIR pulses in the femtosecond range, with each source contributing limitations.

In the experimental setup, NIR pulses were focused behind a pulsed gas jet, where attosecond pulses are generated. At some distance from the gas jet, spherical half-mirrors were used to spectrally select and focus the attosecond pump and the probe pulses. The generated ions are recorded using a velocity-map imaging spectrometer. Courtesy of the Max Born Institute.

In APAPS, an attosecond pump pulse initiates electron dynamics in an atom, a molecule, or a solid-state sample, and a second attosecond probe pulse interrogates the system at different time delays. The technique, long a goal in ultrafast physics, has been shown to be possible, but not practical. Bulky setups and low repetition rates have hindered exploration and application of the modality.

The current work demonstrated an approach which enables APAPS experiments with a much more compact setup. The team used a turn-key driving laser at a kilohertz repetition rate, resulting in substantially more stable operation — a key requirement for the successful implementation of APAPS.

The approach still utilizes HHG, however the setup differs in that the gas jet was placed further away from the driving laser’s focus. At some distance from the gas jet, spherical half-mirrors were used to spectrally select and focus the attosecond pump and the probe pulses. The setup allowed attosecond pulses with a relatively high pulse energy and a small virtual source size to be generated. The generated ions were recorded using a velocity-map imaging spectrometer.

The generation of Ar⁺, as initiated by a broadband attosecond pump pulse with a photon energy around 20 eV, was probed by a second pulse with a central photon energy of 33.5 eV. This is above the second ionization potential of Ar, thereby producing Ar²⁺. The increase of the Ar²⁺ ion yield around zero delay is explained by the more efficient generation of Ar²⁺ when the probe pulse follows the pump pulse. The inset shows a fit of the attosecond pulse structure. Courtesy of the Max Born Institute.

The researchers made use of these stable and intense attosecond source by performing an APAPS experiment, in which argon atoms were ionized by an attosecond pump pulse, resulting in the generation of singly-charged Ar⁺ ions. The formation of these ions was probed by an attosecond probe pulse, leading to further ionization and the formation of doubly-charged Ar²⁺ ions. The team observed an increase of the Ar²⁺ ion yield on a very fast timescale, confirming that the involved pump and probe pulses have attosecond pulse durations. The increase of the Ar²⁺ ion yield around zero delay is explained by the more efficient generation of Ar²⁺ when the probe pulse follows the pump pulse.

The modest infrared driving pulse energies used in the study open the way for performing APAPS experiments at even higher repetition rates up to the megahertz level. The required laser systems to drive these experiments are already available or under development. As a result, the novel concept may enable unprecedented insights into the world of electrons on extremely short timescales, which are not accessible by current attosecond techniques.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.adk9605).