Optimised optical waveforms drive efficient water window attosecond pulse generation

The ability to precisely tailor optical waveforms provides a powerful means of controlling strong-field interactions, including high-harmonic generation (HHG) for attosecond pulse production. Although numerous tailored waveforms have been proposed to extend the harmonic cut-off or enhance conversion efficiency, achieving such improvements in the soft X-ray region remains challenging because plasma formed in the medium can strongly distort the driving field during propagation. Moreover, isolated attosecond pulse (IAP) generation demands sub-cycle waveform shaping to enhance a single attosecond burst while suppressing contributions from neighbouring optical cycles. Here, we demonstrate order-of-magnitude enhancements in IAP generation efficiency up to∼450 eV, by driving HHG in helium with tailored fields spanning the 0.7 - 2.2 μm range, surpassing a 1.5-cycle driver centred at 1.8 μm. Direct waveform measurement and simulations allow us to track the propagation of optimised optical transients through the nonlinear medium and to disentangle the intertwined microscopic and macroscopic contributions that govern soft X-ray HHG. These results establish sub-cycle waveform engineering as a robust strategy for optimising soft X-ray IAP generation and provide an experimentally validated framework for next-generation attosecond sources.