Spectral dependence of harmonic generation efficiency at metal surfaces

Summary form only given. There is considerable interest in various fundamental aspects of second and higher order optical harmonic generation induced by intense ultrashort laser pulses at solid surfaces. Investigations have been performed over a wide range of excitation wavelengths from the ultraviolet to the infrared, which makes a direct comparison of the reported experimental observations to date difficult, especially those pertaining to general surface conditions and modeling of "higher-than-second-order" processes. This study is concerned with a study of the wavelength dependence of the harmonic generation efficiency at metal surfaces to address the validity of current theoretical models. Measurements were performed to investigate the dispersion properties of the second harmonic generation (SHG) process at a metal target surface by use of femtosecond laser pulses from a Ti:sapphire laser. Such ultrashort pulses are characterized by a broad spectral bandwidth that is inversely proportional to their pulse duration. This inherent feature permits us to acquire the spectral dependence in a single-shot experiment without actual wavelength tuning. In our case, we obtain a 30 nm wavelength range by using 40 fsec laser pulses. Therefore it is possible to obtain information of the harmonic generation efficiency by a direct comparison of spectra of the harmonic signal to that of the inducing laser pulse. This method should be also applicable for pulses with even shorter duration and with corresponding broader spectral content and for the higher order harmonics.