Tunable coherent femtosecond soft X-ray source based on high-harmonic generation

With the technical advancement of intense femtosecond lasers, experiments on high-harmonic generation (HHG) have greatly extended its emission wavelength and efficiency. Soft X-rays generated by high-harmonics contain properties similar to a driving laser, like femtosecond time duration and temporal and spatial coherence. The frequencies of soft x-rays generated by high harmonics, however are limited to odd integer times the laser frequency. In this paper we present an experimental result that tunable high harmonics can be generated by changing the intensity of a driving femtosecond laser and show that a theoretical analysis based on a semiclassical calculation can give a good explanation of the observed blueshift. This is a non-adiabatic effect observed for the first time in HHG.