[Paper Review] Compact realization of all-attosecond pump-probe spectroscopy

Martin Kretschmar, Evaldas Svirplys+5 more|arXiv (Cornell University)|Jun 28, 2023

Laser-Matter Interactions and Applications4 citations

TL;DR

This paper presents a compact, 1 kHz laser-based setup for all-attosecond pump-probe spectroscopy using post-compressed few-femtosecond near-infrared pulses and a transient blueshift-enhanced high-harmonic generation (HHG) geometry. It achieves near-isolated attosecond XUV pulses with peak intensities up to 1×10¹³ W/cm², enabling element-specific, high-time-resolution studies of electron dynamics in atoms and molecules.

ABSTRACT

The ability to perform attosecond-pump attosecond-probe spectroscopy (APAPS) is a longstanding goal in ultrafast science. While first pioneering experiments demonstrated the feasibility of APAPS, the low repetition rates (10-120 Hz) and the large footprints of existing setups have so far hindered the widespread exploitation of APAPS. Here we demonstrate two-color APAPS using a commercial laser system at 1 kHz, straightforward post-compression in a hollow-core fiber and a compact high-harmonic generation (HHG) setup. The latter enables the generation of intense extreme-ultraviolet (XUV) pulses by using an out-of-focus HHG geometry and by exploiting a transient blueshift of the driving laser in the HHG medium. Near-isolated attosecond pulses are generated, as demonstrated by one-color and two-color XUV-pump XUV-probe experiments. Our concept allows selective pumping and probing on extremely short timescales and permits investigations of fundamental processes that are not accessible by other pump-probe techniques.

Motivation & Objective

• Overcome the limitations of low-repetition-rate, large-footprint setups that have hindered widespread adoption of attosecond-pump attosecond-probe spectroscopy (APAPS).
• Enable high-time-resolution studies of electron dynamics by generating intense, near-isolated attosecond extreme-ultraviolet (XUV) pulses using a compact, commercial laser system.
• Achieve selective probing and pumping of core- and inner-valence electron dynamics using XUV pulses, which are more localized and element-specific than near-infrared (NIR) pulses.
• Demonstrate that APAPS with XUV pump and XUV probe can achieve signal changes of 10–100%, significantly higher than the 10⁻⁴ changes seen in XUV-NIR experiments, enabling more robust measurements.
• Develop a scalable, stable, and compact platform for attosecond science that is suitable for routine laboratory use and future applications in complex systems.

Proposed method

• Utilize a commercial 1 kHz, 36-fs, 13 mJ, 800 nm laser system as the driver for high-harmonic generation (HHG).
• Implement post-compression of the laser pulses using a 1 m long, 400 µm core hollow-core fiber filled with helium at 3.5 bar, achieving a compressed pulse duration of 3.8 fs.
• Employ a transient blueshift effect in the HHG medium by using an out-of-focus geometry to enhance the generation of intense, short XUV pulses.
• Optimize HHG efficiency using a 1 mm iris and a pulsed gas jet with krypton or xenon at 4 bar backing pressure.
• Characterize the XUV pulses using a spectrometer with a diffraction grating and microchannel plate / phosphor screen, with Al filters (100 nm and 200 nm) to suppress residual NIR light.
• Use a split-and-delay unit with spherical multilayer mirrors (reflectivity peaks at <25 eV and 33.5 eV) to control the pump-probe delay and overlap the XUV pulses spatially.

Figure 1: Experimental setup. NIR pulses with a duration of 3.8 fs are focused into a vacuum chamber using a spherical mirror with a focal length of 75 cm. A pulsed gas jet (Kr or Xe at a backing pressure of 4 bar) is placed about 3.5 cm in front of the driving laser focus, and a 100-nm-thick Al fil

Experimental results

Research questions

• RQ1 Can a compact, high-repetition-rate (1 kHz) laser system generate intense, near-isolated attosecond XUV pulses suitable for pump-probe experiments?
• RQ2 Can the transient blueshift effect in an out-of-focus HHG geometry significantly enhance XUV pulse generation efficiency and intensity?
• RQ3 Can all-attosecond pump-probe spectroscopy be realized with XUV pulses as both pump and probe, enabling higher signal-to-noise ratios than XUV-NIR schemes?
• RQ4 What are the achievable XUV pulse durations and peak intensities in a compact, stable, and commercially viable setup?
• RQ5 Can this setup resolve electron dynamics on the attosecond-to-few-femtosecond timescale with element-specific sensitivity?

Key findings

• The system generates near-isolated attosecond XUV pulses with a measured duration of approximately 100 attoseconds, confirmed by one-color and two-color XUV-pump XUV-probe experiments.
• XUV peak intensities of up to 1×10¹³ W/cm² were achieved, with pulse energies of 0.15 nJ at the sample (from mirror B at 33.5 eV), enabling strong-field interactions.
• The XUV pulse energy at the source was estimated at 10 nJ, with transmission and reflection losses reducing the final pulse energy to 0.12–0.15 nJ at the target.
• The setup achieved a signal change of 10–100% in XUV-pump XUV-probe experiments, significantly higher than the 10⁻⁴ changes typical in XUV-NIR pump-probe experiments.
• The use of a 1 kHz laser system enables a high signal-to-noise ratio and stable operation, overcoming the limitations of low-repetition-rate systems used in prior APAPS experiments.
• The transient blueshift effect in the out-of-focus HHG geometry enhanced the generation of intense XUV pulses, enabling efficient conversion even with moderate laser pulse energy (1 mJ).

Figure 2: XUV spectrum and two-photon ionization scheme. a , Measured XUV spectrum (orange curve) and simulated reflectivities of mirror A (blue curve) and mirror B (violet curve). The combination of the XUV spectrum and the XUV mirror reflectivity produces a pump spectrum that is centered around 20