Control of the supercontinuum spectrum bandwidth and emission efficiency for intense-isolated sub 40-as pulse generation by using a coherent superposition of states

► Numerical study for generation of intense and short single as pulse in pre-excited He-ionic medium. ► Quantum path control scheme. ► After optimizing the same-frequency laser pulses, the broadband XUV supercontinuum can be obtained. ► Generation of a strong isolated sub-40-as pulse without phase compensation.