Submicrometer focusing of isolated attosecond pulses opens a route to high-intensity attosecond science
image: Fig. 1. Spectral conditions and experimental setup for XUV focusing. (a) Spectral conditions relevant to the focusing experiments. The APT (black dashed line) and IAP (red solid line) spectra at the XUV focus are shown, along with the transmission of a 100- nm-thick Zr thin-film filter (green dotted line) and the reflectivity of the Mo/Si multilayer coating for s-polarized light (blue dashed–dotted line). The Zr-filter transmission was calculated using the optical constants reported in Refs. [34,35]. (b) Schematic of HHG beamline. The system comprises waveform synthesis, HHG in a gas cell, and XUV beam transport. (c) Enlarged view of the focusing region. The incident beam is drawn thicker than the scale for clarity.
Credit: Ultrafast Science
Researchers at RIKEN have demonstrated submicrometer focusing of isolated attosecond pulses in the extreme-ultraviolet (XUV) region. Using a custom-designed ellipsoidal mirror, they focused a 1.1-gigawatt tabletop isolated-attosecond-pulse source to a spot as small as 0.46 × 0.36 micrometers, corresponding to a peak intensity of 3.0 × 10¹⁵ W/cm².
Attosecond pulses—lasting on the order of billionths of a billionth of a second—provide a way to observe electron motion on its natural timescale. Submicrometer focusing has previously been achieved for attosecond pulse trains, which consist of repeated attosecond bursts, but isolated attosecond pulses contain a single well-defined burst and are therefore better suited to time-resolved measurements such as attosecond pump–probe experiments. Before this work, achieving sufficiently tight focusing of isolated attosecond pulses remained a major technical challenge.
To address this challenge, the team developed a focusing platform based on an ellipsoidal multilayer mirror designed for XUV light near 60 electron volts. They also stabilized the XUV focal position during knife-edge measurements and used a median-based statistical analysis to reduce errors caused by residual focal-position fluctuations, allowing the focal spot to be evaluated reliably even under realistic experimental conditions.
The smallest measured focal spots were 0.46 ± 0.13 micrometers horizontally and 0.36 ± 0.12 micrometers vertically, close to the diffraction-limited performance predicted by wave-optical simulations. For the 1.1-GW isolated attosecond pulse source, these dimensions correspond to a peak intensity of 3.0 × 10¹⁵ W/cm². To the best of the authors’ knowledge, this is the first experimental demonstration of submicrometer focusing of HHG-based isolated attosecond pulses with quantitatively validated beam-size measurements. The resulting peak intensity of 3.0 × 10¹⁵ W/cm² is the highest reported for a tabletop HHG-based isolated-attosecond-pulse source.
The results provide a promising route toward extending nonlinear attosecond optics from gaseous systems to solids and other condensed-matter systems. By reducing optical losses in the beamline, the study also identifies a realistic pathway toward a peak intensity of 2.6 × 10¹⁶ W/cm². Such intense and tightly focused isolated attosecond pulses could ultimately provide a basis for future all-XUV attosecond pump–probe experiments, combining high peak intensity with attosecond temporal resolution to investigate ultrafast electron dynamics.