Dependence of the Generation Efficiency of High-Order Sum and Difference Frequencies in the Extreme Ultraviolet Region on the Wavelength of an Added Tunable Laser Field

We investigate the dependence of the generation efficiency of sum and difference frequencies in the extreme ultraviolet (xuv) region on the wavelength of an added tunable laser field. The wavelength of the added field ranges from 600 to 1500 nm. The generation efficiency of sum and difference frequencies is dramatically enhanced when the wavelength of the added field is longer than that of the fundamental field for pure harmonics. The discussions are held to the added field with perturbative intensity first, and they are further extended to that with nonperturbative intensity.