Saddle point analysis on selective enhancement of high-order harmonic generation

We theoretically demonstrate control of selective enhancement of the near-cutoff odd order harmonic generation by dual-color chirped laser field. The sharp enhancements of a single harmonic order ranging from H37 to H59 (lambda h=21.6nm-13.5nm) are achieved by adjusting the intensity of the fundamental frequency field. The saddle point method is applied to the analysis of high-order harmonic generation using the distorted-waveform laser field. A series of orbits of principal contribution to enhancement are extracted using the saddle point method, and the enhancement of the key orbits' interference is reconstructed by quantum orbital theory. Through the analysis of the interference mechanism of enhancement, we give a full explanation of the dependence of enhanced harmonic order on laser intensity.