Broadband isolated attosecond pulse with high spatiotemporal quality in pre-excited medium by multi-cycle two-color fields

We propose a scheme that combines multi-cycle two-color pump pulses and pre-excited medium to simultaneously control the ionization and acceleration steps of electron wave packet. Intense two-plateau supercontinua are observed at microscopic level, which support isolated attosecond pulse with tunable central wavelength. We further investigate the propagation effects and find that the macroscopic response is mainly associated with the initial population of the excited state. Large initial population can distort the pump field and weaken the phase-matching of the supercontinua due to the high density of free electrons. On the contrary, small initial population of 3% can generate well phase-matched supercontinua, and an efficient isolated attosecond pulse with gaussian-like spatial distribution is directly achieved. The divergence angle of the generated pulse is less than 0.2 mrad, which indicates its beam quality is good. This pulse benefits for some potential applications of ultrafast measurements with high spatiotemporal resolutions.