Isolated ultrashort attosecond pulse generation from a coherent superposition state in a few-cycle chirped laser pulse

We theoretically demonstrate an efficient method for producing an isolated ultrashort attosecond pulse by high-order harmonic generation in an intense few-cycle chirped laser pulse. Our simulation calculations show that the harmonic spectrum reveals an ultrabroad extreme ultraviolet supercontinuum when the initial state is prepared in a coherent superposition of the ground and first excited states. It is also shown that the method can enhance the short quantum path and eliminate the long one, and then an isolated 23-attosecond pulse is produced. By properly adjusting the chirp parameter, a clean single attosecond pulse as short as 11 attoseconds is directly obtained.