Tunable coherent femtosecond X-ray source driven by an intense femtosecond laser

The wavelength tunability of high harmonic generation is presented. By applying intense femtosecond laser pulse to gaseous medium, the wavelengths of high-order harmonics could be tuned. The wavelength shift large enough to cover the interval between adjacent orders was obtained in harmonics from Ar and Ne atoms driven by 25 fs pulses, opening a way to realize a tunable coherent x-ray source.