Selective enhancement of single-order and two-order harmonics from He atom via two-color and three-color laser fields

An effective method to achieve the selective enhancement of single-order and two-order harmonics from He atom has been proposed through laser waveform control of two-color and three-color laser fields. Firstly, with the optimization of omega-2 omega two-color laser waveform, not only the wavelength tunable single-order harmonic (i.e. from 182th order to 328th order) can be enhanced by 17 times compared with the neighbor harmonics, but also the enhancement of two-order harmonics can also be obtained. Theoretical analyses show that the enhancement of the specific harmonics is coming from the folded structure on the short quantum path of the harmonic emission peak. Detail analyses show that the above folded structure happens in the reversed sub-peak region of the half-cycle laser waveform, where the deceleration and further acceleration of the free electron can be found in this structure. Moreover, this structure is dependent on the pulse duration of the controlling pulse. However, it is not very sensitive to the wavelength of the controlling pulse. Further, by properly adding a 3 omega pulse, not only the efficiency of spectral continuum can be enhanced by 300 times compared with that from the two-color field, but also the generation of single-order harmonic with the intensity enhancement of 20 times can be found. Furthermore, the enhancement of the harmonic spectrum can also be achieved when using the 4 omega or 6 omega controlling pulses.