Wavelength dependence of harmonic generation efficiency at metal surfaces induced by femtosecond Ti:Sapphire laser pulses

Second and third order harmonics of high intensity (similar to 10(10) W/cm(2)), femtosecond Ti:sapphire laser pulses were generated at a gold surface. The spectral dependence of the second harmonic generation efficiency was measured as a function of the fundamental excitation wavelengths (780-850 nm). The efficiency is found to decrease significantly toward the longer wavelengths. This behavior is consistent with the recently observed very low efficiency of the high order harmonic generation at infrared wavelengths (1-3 mu m), while contradicting the single particle perturbative theoretical descriptions of the harmonic generation process at the vacuum/metal interface.