Selective Enhancement of a Single Harmonic Emission in a Driving Laser Field with Subcycle Waveform Control

We experimentally demonstrate a robust scheme to select a single high-order harmonic among the harmonic comb by using a driving laser field with subcycle waveform control, which is synthesized by the fundamental 800 nm laser pulse and two controlling laser pulses at 400 and 267 nm with perpendicular polarizations. By controlling the relative phase among the pulses of different colors, a single high-order harmonic is selectively enhanced while the adjacent harmonics are greatly suppressed with the intensity contrast increased by more than 1 order of magnitude and the peak intensity enhanced simultaneously by more than 2 orders of magnitude compared to the case by using only the fundamental 800 nm laser pulse. Such phenomena can be mainly attributed to the intra-atomic phase matching realized with the sub-cycle waveform controlled field.

Figure 1

(a) Schematic of the experimental setup. (b) Harmonic spectra obtained with different gas pressures.

Figure 2

Harmonic spectra driven by (a) the two-color laser field and (b) the three-color laser field as a function of the time delay, respectively.

Figure 3

(a) Comparison of the maximum harmonic yields driven by different laser fields. (b) Comparison of the harmonic yields driven by the three-color laser field with two different relative phases. (c) Comparison of the harmonic yields driven by the three-color laser field with different intensity and density conditions.

Figure 4

(a) The phase of the 18th harmonic in different laser fields (upper panel) and the comparison with the adjacent 16th and 17th harmonics (lower panel). (b) The harmonic spectra in two polarization directions (${\mathrm{HHG}}_x$, ${\mathrm{HHG}}_y$).