Phase-matched high-order-harmonic generation by guided femtosecond pulses

Summary form only given. High harmonic generation (HHG) in an ionizing medium has distinguishing characteristics among the methods of the generation of coherent extreme ultraviolet light. The shortest wavelength has reached into the water window region. The high-harmonic pulsewidth were measured to be shorter than the driving laser pulse width. Recently, a guiding technique with an Ar-filled hollow fiber has been applied to HHG in order to control the phase matching condition, resulting in 100 times enhancement around the 25th harmonic. There remains the question of whether any harmonic can be enhanced by the macroscopic phase matching in the fiber. In order to clarify these issues, we made an experiment where Ar was replaced by Ne which was expected to generate shorter wavelength. In spite of various differences of experimental condition, harmonics near the cutoff order were also enhanced by two orders of magnitude in Ne, giving the similar intensity distribution to one obtained with a Ar-filled hollow fiber. In this paper, we report the phase-matched HHG by guided intense femtosecond pulses, using a hollow fiber-guided and self-guided laser pulses. Based on the experimental results, macroscopic and microscopic phase matching of high-order harmonics are discussed.