Isolated attosecond pulse generation in a semiinfinite gas cell driven by time-gated phase matching

Isolated attosecond pulse(IAP)generation usually involves the use of short-medium gas cells operated at high pressures.In contrast,long-medium schemes at low pressures are commonly perceived as inherently unsuitable for IAP generation due to the nonlinear phenomena that challenge favourable phase-matching conditions.Here we provide clear experimental evidence on the generation of isolated extreme-ultraviolet attosecond pulses in a semiinfinite gas cell,demonstrating the use of extended-medium geometries for effective production of IAPs.To gain a deeper understanding we develop a simulation method for high-order harmonic generation(HHG),which combines nonlinear propagation with macroscopic HHG solving the 3D time-dependent Schrödinger equation at the singleatom level.Our simulations reveal that the nonlinear spatio-temporal reshaping of the drivingfield,observed in the experiment as a bright plasma channel,acts as a self-regulating mechanism boosting the phase-matching conditions for the generation of IAPs.