Coherent femtosecond X-ray source based on high-order harmonics

High-order harmonics generated by intense laser pulse have become a promising way of providing coherent ultrashort radiation in the extreme ultraviolet range. High-order harmonics possess unique properties, such as good coherence and low beam divergence as well as extremely short pulse duration, inherited from the driving laser. For efficient harmonic generation in macroscopic gas medium, the phase-matching conditions have to be fulfilled and also the propagation of pumping laser pulse has to be well controlled to deliver sufficient laser energy to as many atoms as possible. Though high-order harmonics themselves can provide numerous wavelengths, the wavelength tunability is needed to generate harmonics at arbitrary wavelengths. In this presentation, methods to generate bright harmonics using a long gas jet and to continuously tune harmonic wavelengths without losing spectral sharpness are explained