Towards isolated attosecond pulses at megahertz repetition rates

The strong-field process of high-harmonic generation is the foundation for generating isolated attosecond pulses(1), which are the fastest controllable events ever induced. This coherent extreme-ultraviolet radiation has become an indispensable tool for resolving ultrafast motion in atoms and molecules(2,3). Despite numerous spectacular developments in the new field of attoscience(2-4), the low data-acquisition rates imposed by low-repetition-rate (maximum of 3 kHz) laser systems(5) hamper the advancement of these sophisticated experiments. Consequently, the availability of high-repetition-rate sources will overcome a major obstacle in this young field. Here, we present the first megahertz-level source of extreme-ultraviolet continua with evidence of isolated attosecond pulses using a fibre laser-pumped optical parametric amplifier(6) for high-harmonic generation at 0.6 MHz. This 200-fold increase in repetition rate will enable and promote a vast variety of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy(7), or even video-rate acquisition for spatially resolved pump-probe measurements.